The New Horizons spacecraft is outfitted with six primary science instruments and one student-operated payload, driven by the power and mass requirements of the mission.
The instruments are installed fixed to the spacecraft structure, requiring the craft itself to change its orientation for pointing of the instruments. New Horizons is equipped with instruments covering optical imaging, spectroscopy in multiple bands, as well as plasma, particle and dust sensing to obtain a detailed picture of Pluto, its composition both on the surface and its atmosphere, its moons and its environment.
These are New Horizons’ instruments:
[Alice and Ralph are also collectively referred to as PERSI – Pluto Exploration Remote Sensing Investigation.]
The fundamental (Group 1) science objectives of the New Horizons mission can be achieved with the core science payload comprised of Alice, REX and Ralph.
The supplemental payload deepens and broadens the mission science, but is not required to achieve the minimum criteria for a mission success. The boresights of the Ralph, LORRI, and Alice airglow channel are aligned with the spacecraft –X axis allowing them to operate simultaneously:
Alice – Ultraviolet Imaging Spectrometer
The ALICE instrument of the New Horizons spacecraft is an imaging spectrometer sensitive for wavelengths in the ultraviolet range. It is an improved version of the ALICE instrument of the Rosetta comet exploration craft, featuring the addition of a second channel and different bandpass characteristics than the system flown on Rosetta. As an imaging spectrometer, ALICE separates the different wavelengths of ultraviolet radiation while simultaneously capturing an image of the target so that the finished data product is an image that, within each of its pixels, contains a full spectrum of that pixel.
The primary task of ALICE is studying the atmosphere of Pluto, determining the abundance of different atomic and molecular constituents of the atmosphere, but also delivering information on the atmospheric structure. UV Spectroscopy has become a powerful tool for the investigation of physical and chemical properties in the field of astrophysics as the ultraviolet range of the spectrum can be used to extract a wealth of information on atmospheric constituents and all interplanetary spacecraft making a first journey to a planet carried ultraviolet sensors, highlighting ALICE’s role on New Horizons as one of the core instruments.
ALICE will address the New Horizons mission objective of characterizing the neutral atmosphere of Pluto and its escape rate. Specifically, the instrument will determine the mixing ratios of major atmospheric constituents including nitrogen, carbon monoxide, methane, hydrogen and noble gases. Also, ALICE can study the vertical density and thermal structure of the upper layers of the atmosphere, look at the hydrocarbon and nitrile photochemistry ongoing in the upper reaches of the atmosphere and determine whether hydrodynamic escape occurs on Pluto.
The instrument makes its measurements in part by observing a solar occultation, that is, observing the sun through the Plutonian atmosphere and recording the UV spectrum. At wavelengths smaller than 100nm, nitrogen is responsible for the majority of opacity allowing sampling of the uppermost atmosphere while methane dominates between 100 and 150nm providing a look at the middle atmosphere from 300 to around 1200 Kilometers. At higher wavelengths, hydrocarbons with strong Far-UV absorption bands can expected to be optically important as well as hazes which will deliver information about the lowest part of the atmosphere down to about 100 Kilometers.
ALICE can also shed light on the thermal structure of the atmosphere that is suspected to be dominated by a steep temperature increase from 10 Kilometers up due to absorption of infrared radiation by methane bands. Further heating by Lyman-Alpha Photodissociation could increase temperatures to 120K at 600km in altitude. At even higher altitudes, absorption of solar extreme UV radiation by Nitrogen may be at work to deliver additional heating, but atmospheric temperature likely decreases above 600km due to cooling associated with hydrodynamic escape.
The ALICE instrument can also look at the mixing ratio profiles and their variation with altitude to examine winds within the atmosphere which is a crucial piece of information when assessing the planet’s escaping wind. Furthermore, studies will be made to determine condensable species within the atmosphere and possible precipitation of these photochemical reaction products to the surface. Another open question is the abundance of Argon within Pluto’s atmosphere which can be easily answered by ALICE.
ALICE uses a common UV spectrometer design utilizing a Rowland circle. Overall, the instrument weighs around 4.5 Kilograms and fits within an envelope of 20 by 41 by 12 centimeters with a low power consumption of 4.4 Watts. The instrument covers a spectral range of 46.5 to 188 nanometers, covering the far and extreme UV spectral ranges. ALICE achieves a spectral resolution up to 3.6 Angstroms and a spatial resolution of 0.05 by 0.6 degrees.
The ALICE-P instrument differs to the ALICE instrument on Rosetta in a number of characteristics, the most notable being the use of two separate entrance apertures that feed light to the telescope, the main aperture known as the Airglow Channel AGC and the Solar Occultation Channel SOC.
Light entering the telescope section of the instrument through the AGC passes through an aperture of 40 by 40 millimeters while the SOC uses a one-millimeter diameter opening perpendicular to the telescope section of the instrument, requiring an additional relay mirror to direct the radiation collected through SOC into the telescope. Light entering either aperture is collected and focused by an off-axis paraboloidal mirror.
The light is focused on the entrance slit of the spectrograph from where it reaches the dispersive element, a toroidal holographic grating, before entering the microchannel plate detector. The slit, grating and detector are arranged on a 0.15-meter rowland circle.
Light entering the telescope section of ALICE first passes through the aperture that is equipped with a door that is opened after launch using a limited angle torque motor that allows the door to be closed and opened on command to protect the instrument during thruster operation and lengthy periods of cruising. ALICE uses a variety of baffles and low-scatter materials inside the instrument to reduce stray light within the optics section.
The off-axis parabolic mirror of the telescope section of the instrument has a clear aperture of 41 by 65 millimeters and reflects the incoming radiation to the spectrometer section of ALICE. The mirror and its mounting fixture is made of a monolithic piece of Aluminum that is coated with electroless Nickel and polished using a low-scatter technique. The optical surface of the mirror is coated with Silicon Carbide for optimized reflectivity in the Extreme and Far-UV range. Heaters are installed on the OAP mirror to avoid cold-trapping of contaminants during flight.
The focused light from the mirror is passed onto the Spectrograph Entrance Slit that is composed of two sections – one for the Airglow Channel and one for the Solar Occultation Channel. The AGC slit is an actual slit with a field of view of 0.1 by 4.0° while the SOC uses a square with a field of view of 2.0 by 2.0 degrees. This slit design was driven for the Airglow Channel by the combination of spectral resolution and stray light minimization, encompassing the center boresight and providing and extended source spectral resolution of around 9 Angstroms. The choice of a rather large square for the Solar Occultation Channel was driven by the requirement to have the sun within the instrument field of view during the very short measurement window of solar occultations by Pluto and Charon which also occur nearly simultaneously with the occultation observation of the Radio Science Experiment.
ALICE-P is aligned on the spacecraft with a 2° tilt on the instrument’s spatial axis so that the sun is centered within the SOC field of view when the High Gain Antenna is pointing to Earth for radio science. The large SOC field of view will allow misalignments up to +/-0.9 degrees between the SOC FOV and the antenna boresight.
From the slit, the light is passed to the toroidal holographic grating that has low-scatter and near-zero line ghost problems. The grating also uses a Silicon Carbide coating and heaters. The ALICE spectrograph uses the first diffraction order throughout the 520 – 1,870-Angstrom passband, although the lower half of the first order wavelength coverage also appears in second order between the first order wavelengths.
ALICE uses a 2D imaging photon-counting detector utilizing a microchannel plate Z-Stack that feeds the readout array which utilizes double-delay readout. The MCP front surface is coated with opaque photocathodes of Potassium Bromide for the 520-1180Å range and Caesium Iodide for the 1250-1870Å range. The detector tube is a lightweight brazed alumina-Kovar structure that is welded to a housing. The entire tube body is enclosed in a vacuum chamber housing using stainless steel and aluminum.
This chamber is used to protect the photocathodes against damage from moisture exposure during ground processing and outgassing once in space. The housing is equipped with a door to be opened early in flight and allow light to enter the detector. This door includes a Magnesium Fluoride window that allows UV radiation of >1200Å to pass for ground testing and in case the window failed to open. Opening the door is accomplished by using a dual-redundant torsion spring.
The MCP detector has an effective area of 35 by 20 millimeters in the dispersion and spatial dimensions with a pixel format of 1024 by 32 pixels (dispersion direction by spatial dimension) – the 6-degree spatial field of view is imaged onto the central 22 of the detector’s 32 channels to be able to use the remaining channels for dark current measurements.
The MCP Z-Stack uses three 80:1 length-to-diameter microchannel plates that are curved with a radius of 75 centimeters to match the Rowland circle geometry to ensure an optimum focus. The MCPs are 46 by 30 millimeters in size with 12-micrometer pores on 15μm centers.
A repeller grid located above the MCP is biased at a negative voltage (-900V) to reflect electrons that may be liberated in the interstitial regions of the MCP Z-Stack to improve the efficiency of the detector. The MCP Z-Stack itself requires a high negative voltage bias of –3kV and an additional –600V is needed between the MCP stack and the anode array.
One concern for the instrument is the saturation of the detector at the Lyman-Alpha emission which requires physical attenuation that is achieved by masking the MCP detector where the emission comes to focus. The bare MCP glass exposed in this area has a quantum efficiency around ten times less than that of KBr at 1216 Angstroms. This masking approach has been flown successfully on Rosetta and other Space-based UV instruments.
Signals generated by the MCPs are passed to the detector electronics that are located on three 63.5 by 76.2-millimeter circuit boards mounted inside an aluminum housing installed behind the detector vacuum chamber.
Using pre-amplifiers, the analog MCP output is amplified and the electronics also convert the output pulses to pixel address locations. To be processed by ALICE, the signal pulses need to have an intensity above a set threshold level. For each event that meets the minimum intensity, a 10-bit x address and 5-bit y address is generated by the electronics for transmission to the data handling electronics. In addition to the address, the digitized amplitude of each event is sent to the command & data system. A pulse simulator can be used to test the pixel location read-out and data transfer path to allow testing of the entire ALICE detector and data system without activating the high-voltage power supply of the detector.
The ALICE support electronics include the Power Controller Electronics, Command & Data Handling Electronics, telemetry and command interface electronics, the decontamination heaters and a High-Voltage Power Supply for the detector. The ALICE instrument is controlled by an Intel 8052 microprocessor that has 32KB of local program RAM and 128KB of acquisition RAM as well as 32KB of SRAM and 128KB of EEPROM.
The Power Controller Electronics include DC-to-DC converters that interface with the spacecraft power bus to convert it to a stable 5-Volt ALICE instrument bus that is used by the various electronics of ALICE and the High-Voltage Supply. The PCE also includes the switching circuit that controls the heaters as well as circuitry to command the limited angle torque motor of the front aperture door.
The Command and Data Handling electronics are responsible for the execution of commands sent to ALICE from the spacecraft, data acquisition & handling from the detector, formatting of telemetry and science data, control & monitoring of the high-voltage power supply and control of the aperture door. A 4MHz Intel 8052 microprocessor is used to build the interface with the spacecraft for data transmission and command receipt. Housekeeping telemetry is delivered to the C&DH system of the spacecraft via an RS-422 analog bus.
ALICE uses two decontamination heaters – one installed behind the off-axis parabolic mirror and one behind the grating. These heaters are 1 Watt resistive heaters and are accompanied by two redundant thermistors to provide feedback control of the heaters. The heaters can be separately activated by the ALICE command system.
The High-Voltage Power Supply for the MCP detector is located in a bay behind the OAP mirror. It conditions the –4.5kV required for the operation of the detector.
The voltage of the Z-Stack is fully commandable over a range of 0 to –6.1kV in 25V steps. The HVPS consumes 0.6W of power during operations. ALICE can be operated in three different modes – image histogram, pixel list and count rate mode. Each mode uses a 32k x 16-bit acquisition memory.
In the Image Histogram Mode, the acquisition memory is used as a two-dimensional array in its size corresponding to the spectral and spatial dimensions of the detector array. A read-increment-write sequence is performed for each event as the x and y values are used as an address in a 16-bit cell in the 1024 by 32 element histogram memory. During a programmed integration time, the events are accumulated one at a time to create a 2D image. In the pixel list mode, the acquisition memory is used as a one dimensional array to allow the sequential collection of the x,y event address into the linear pixel list memory. Time-binning of events is accomplished by inserting a time marker in the array at specified intervals. The Count Rate Mode uses the memory as a linear array and periodically collects the total detector array count rate sequentially in the linear memory array. ALICE also includes a feature that allows certain areas of the detector to be excluded in suppression of hot pixels and other defects – this filtering is performed ahead of data processing to avoid large fractions of acquisition memory to be taken up by erroneous data.
Ralph – Imaging Telescope
Ralph is a visible/near infrared multispectral imaging and short wave infrared spectral instrument that delivers the primary imagery of Pluto and Charon for the study of their geology, morphology and composition.
The instrument is basically comprised of two sub-instruments, the Multispectral Visible Imaging Component (MVIC) covering four bands in the visible spectral range and the Linear Etalon Imaging Spectral Array (LEISA) that includes three detectors sensitive for infrared radiation. Ralph is a collaboration between NASA’s Goddard Spaceflight Center, the Southwest Research Institute and Ball Aerospace. The two components of Ralph share a single optical telescope bringing the total instrument weight to 10.5 Kilograms. Ralph requires a peak power of 7.1 Watts.
The MVIC part of Ralph is in charge of delivering full-color images of Pluto and Charon at a resolution of up to 1 Kilometer per pixel. Imagery includes stereo images and nighttime acquisitions to provide data for the refinement of Pluto and Charon’s radii and aid the search for clouds and hazes in Pluto’s atmosphere, help in the search for rings and additional moons in orbit around Pluto.
First and foremost, MVIC imagery will deliver the first color photos of a new world, unlocking the mystery of what Pluto – once the ninth planet in our solar system – actually looks like when resolved beyond a few pixels. Geological maps can be generated from MVIC data which is the primary objective of this part of the Ralph instrument.
LEISA is responsible for the mapping of water, methane, carbon dioxide, nitrogen ice and other materials on the sunlit face of Pluto and Charon. Infrared imagery can also provide insights into surface temperatures across Pluto and Charon.
Visible and infrared imagery of previously unknown bodies can deliver a wealth of critical data such as cratering history, surface structures, spatial variability of the surface, volatile transport and many more. Furthermore, Ralph looks at the distribution of the main species on Pluto/Charon, examines the areas of pure ices and mixed areas, studying seasonal transport, searching for complex species, and it assesses the connection between geology and composition.
The Ralph instrument is comprised of a single optical telescope that feeds the two focal planes of MVIC and LEISA. Ralph’s telescope uses an unobscured, three-mirror anastigmatic design that was chosen as it provides a larger field of view than the conventional Cassegrain or Ritchey-Chrétien systems.
Light enters the telescope through a 75-millimeter aperture and falls onto the primary mirror that directs the radiation to the secondary and tertiary mirrors which focus it onto the focal plane. In between the tertiary mirror and the focal plane assembly is a dichroic beam splitter that transmits infrared radiation at a wavelength greater than 1.1 micrometers to the LEISA focal plane and reflects shorter wavelengths to the MVIC focal plane located perpendicular to the LEISA detectors. The overall focal length created by the three-mirror design is 658 millimeters.
The entire telescope assembly is manufactured from grain aligned 6061-T6 aluminum and so are the three mirror assemblies. This creates a lightweight, athermal and thermally conductive design ensuring the optical performance of the system is minimally influenced by temperature as thermal gradients are largely eliminated. The telescope entrance is heavily baffled for stray light rejection and additional measures are taken within the telescope in the form of a field baffle at an intermediate focus between the secondary and tertiary mirrors plus a Lyot stop at the exit pupil of the optics after the tertiary mirror.
A protective door in front of the instrument aperture protects the instrument from contamination during ground processing and accidental exposure to direct sunlight during the early mission phase. The door has a 20% throughput and is opened in a one-time mission event.
Ralph is a scanning instrument, requiring the New Horizons spacecraft to move the instrument field of view across its target while the detectors are read-out as part of a pushbroom design, forming the image swath as the spacecraft sweeps out the targeted image.
The MVIC Focal Plane Assembly consists of seven independent Charged Coupled Device arrays mounted on a single thermally controlled substrate, each CCD array equipped with its own specific bandpass filter for the generation of multi-band imagery with the filters mounted 700 microns above the detector surface.
Two of the 32 by 5024-pixel arrays are operated in Time-Delay Integration mode to deliver panchromatic imagery in the 400 to 975-nanometer range. Four 32 x 5024-pixel arrays are used for multi-band imaging covering a blue band (400-550nm), a red band (540-780nm) and the near infrared region (780-975nm) as well as a narrow-band methane channel at 860 to 910 nanometers.
The Time Delay Integration TDI is accomplished by synchronizing the parallel transfer rate of each of the 32 CCD rows (each 5024 pixels wide) to the relative motion of the image across the detector surface. TDI is suitable for the generation of large-format images acquired as the spacecraft scans across the target. The presence of 32 rows increases the integration time by the same factor and thus allows for high signal-to-noise measurements. Using two detector arrays for the panchromatic imagery yields a double sampled spatial resolution to be used in the processing of data into hemispheric maps of Pluto and Charon.
The normal body rates required for MVIC imaging are 1600 microrad/sec for panchromatic and 1000microrad/sec for multispectral imaging which correspond to integration times of 0.4 and 0.6 seconds, respectively. Spacecraft attitude control is sufficient to keep image smear within a quarter of one pixel over a 0.7sec integration, eliminating any smear-related concerns.
Each of the TDI arrays has a static field of view of 5.7 by 0.037°. When acquiring images of Pluto, 4600 pixels will be filled by the object, the rest being margin for pointing errors while 12 pixels on either side are dark pixels used as reference and for injected charge. All pixels used on MVIC are 13 by 13 micrometers in size.
The remaining detector elements of MVIC measuring 128 by 5024 pixels are operated in staring mode with a 0.15 by 5.7° field of view. This framing array is to be used in optical navigation of the spacecraft.
The LEISA instrument is a wedged infrared spectral imager capable of generating spectral maps in the short wave infrared spectral region from 1.25 to 2.5 micrometers. It uses a linear variable filter that is placed around 100 micrometers above the detector array. LEISA features a 256 by 256-pixel Mercury Cadmium Telluride array detector (40 by 40 micrometer pixels) operated as a push-broom sensor just like the MVIC instrument.
Employing Time Delay Integration, LEISA reads out its detector at a speed that is synchronized to the rate of the scan and automatically creates a spectral map as the image is swept out due to the use of a linear variable filter. This LVF is manufactured so that the transmit wavelength varies along the in-scan direction only so that the row-to-row image motion builds up a spectrum (analog to TDI increasing the signal over a single spectral interval on MVIC). The LEISA instrument field of view is 0.9 by 0.9 degrees.
LEISA requires spacecraft rotations of 120 microrad/sec for a frame rate of 2Hz, however, frame rate can be varied from 0.25 to 8Hz to accommodate imaging in various spacecraft rotation modes. Read-out is accomplished through two ribbon cables and a multilayer fan-out board fabricated into a single element.
The filter is comprised of two segments, the first covers wavelengths from 1.25 to 2.5 microns at an average spectral resolving power of 240. This creates the composition maps to be obtained from LEISA. The second segment of the filter covers a narrow range from 2.1 to 2.25 microns at a spectral resolving power of 560 to gather compositional information, but also collect surface temperature maps by measuring the characteristic spectral shape of frozen Nitrogen.
Both of the focal plane assemblies, in particular LEISA’s, require active cooling to limit dark currents especially in the long wavelength range of the instrument. A radiator on the top of the Ralph instrument is directly exposed to space and thermally coupled to the focal plane and the Telescope Detector Assembly. The entire telescope structure is kept at 220K in order to limit the conductive and radiative load on the focal planes. MVIC is further cooled to around 175K at the CCDs while LEISA is operated at a focal plane temperature under 130K.
For calibration of MVIC and LEISA, the Ralph instrument includes a second radiation input whose field of view is offset by 90 degrees to the instrument, aligned with the spacecraft antenna pointing to allow solar radiation to provide diffuse illumination within the telescope assembly. When the spacecraft is in its nominal Earth/Sun-pointing orientation, sunlight can enter the instrument through the Solar Illumination Assembly aperture that is 4 millimeters in diameter.
A small fused silica lens with a focal length of 10mm is part of SIA and images the light onto the input end of a 125-micrometer core fiber, 10cm in length with the end of the fiber illuminating a pair of lenses directly under the Lyot stop behind the tertiary mirror and just 10cm from the two Focal Plane Assemblies.
The overall goal of the Solar Illumination Assembly is to create a repeatable pattern that can be used for tracking the stability of the pixel-to-pixel response (flat-fielding) during the long mission duration. SIA illuminates the entire LEISA detector and about 3000 pixels of each MVIC array. Even though the sun will only measure 50 microns in diameter when imaged onto the fiber at Pluto distance, it will still underfill the fiber. A second fiber with a high attenuation can be used for flat-fielding when New Horizons is still close to the sun.
Another use of SIA is its alignment with the Solar Occultation Channel of ALICE, so that Ralph could also be used to create an atmospheric spectrum during the occultation when the Plutonian atmosphere is placed between the entrance of the instrument and the sun. Although SIA only delivers diffuse radiation into the instrument’s telescope, a vertical spectral profile can still be acquired by summing the spectra from all rows into a single spectrum.
The Ralph electronics assembly contains three boards – the detector electronics, the command and data handling board and a low voltage power supply. The electronics box of the instrument is installed directly to the spacecraft below the Telescope Detector Assembly to be able to operate at the spacecraft surface temperature. The detector board delivers biases and timing signals to both focal planes, amplifies the signals received from MVIC and LEISA and performs the analog-to-digital conversion of the imaging data. The science data uses 12-bits per pixel. The command and data handling system executes the spacecraft commands, converts low-rate engineering data from the analog to the digital format and delivers the high-speed imaging data interface to the Instrument Card within the Integrated Electronics Assembly and the low-rate engineering feed directly to the spacecraft C&DH processor. The power supply is in charge of converting the 30V spacecraft bus to the various low-voltages required by the Ralph electronics.
The entire electronics assembly of Ralph is fully redundant in architecture with two strings of components that feature abundant cross-strapping to flexibly bypass any failed components and ensure Ralph can fulfill its function as part of the core instrument suite of New Horizons. Redundancy within the MVIC instrument can not be guaranteed but at least some functionality of the instrument could be preserved in case of a failure by grouping the CCD arrays in two segments with two color and one panchromatic CCDs so that at least some data is still available in case of a single-point failure. LEISA has four independent outputs from the 128 by 128 pixel frames so that science can still be completed in case one quadrant stops working.
REX – Radio Science Experiment
The New Horizons Radio Science Experiment makes use of the spacecraft’s high-gain antenna and associated signals processors to obtain temperature and pressure profiles of Pluto’s tenuous atmosphere by measuring radiometric temperature, gravitational moments and ionospheric structure.
The instrument can also look for an ionosphere around Pluto or an atmosphere around Charon and conduct a bistatic surface scattering study on Pluto. REX functions by receiving a 4.2-centimeter wavelength radio signal (7.2GHz) from Earth (transmitted at high power) and recording the signal attenuation (or changes in the signals caused by the radio waves traveling through the atmosphere) as the New Horizons spacecraft passes behind Pluto so that the atmospheric layers are placed in between the signal source and receiver, causing an effect on the structure of the signal that allows the 4.2cm thermal emission to be deduced.
Normally, such measurements are made with the spacecraft sending the signal and a ground station capturing it and recording it, reducing resources needed on the craft in terms of power, memory and mass. However, this was not possible on New Horizons due to the large distance between the Earth and Pluto.
A combination of occultation measurements and two-way tracking can be used to determine the total mass of the Pluto system to about 0.01% and also improve the accuracy of the Pluto-Charon mass ratio.
The dedicated REX hardware weighs just 160 grams as the only addition to the operational communication system is an additional Uplink Receiver/Decoder card that can process the REX signal into science data.
On approach to Pluto, REX measures the radiometric temperature of Pluto and Charon and completes an occultation measurement on Charon to search for a discernible atmosphere. During the close portion of the flyby, REX is in charge of measuring the spacecraft’s velocity vector with high accuracy so that the masses of Pluto and Charon can be separated. After closest approach come the critical occultation measurements on Pluto and Charon to obtain the profile of the refractivity of Pluto’s atmosphere. The precision reached by REX for atmospheric pressure and temperature is 0.1Pa and 3K. It is also possible for REX to be used for the study of the solar wind, the interplanetary plasma and the solar corona when being used during cruise.
The requirement to use a ground station to transmit the signal instead of the spacecraft arose from the desired signal to noise ratio of the measurements, the large distance to Pluto and the high flyby speed of the spacecraft. Large transmitter powers are needed to support an accurate measurement of the tenuous atmosphere of Pluto which is hardly possible with the energy constraints of an RTG powered spacecraft. Additionally, the flyby velocity limits the occultation observation to minutes for the upper atmosphere and mere seconds for the lower layers of the atmosphere, increasing the required Signal to Noise Ratio.
New Horizons, within its radio system, incorporates an Ultra-Stable Oscillator USO as an inherent component of the design, both of the REX experiment and the Doppler-Tracking technique employed for navigation. The spacecraft transmitter is always referenced to the USO frequency which is entirely independent of the received uplink signals. Normally, systems use the uplink signal transmitted from the ground to form the frequency of the downlink signal for doppler tracking which creates a direct relationship between the frequency of the received uplink and the transmitted downlink which makes a calculation of the spacecraft radial velocity possible through the comparison of uplink and downlink.
New Horizons does not implement the formation of the downlink signal directly from the uplink signal. Instead, it analyzes the uplink and then uses the USO reference frequency to calculate the difference between the number of radio cycles arriving at the spacecraft and the number of USO cycles in the same period.
The observed frequency difference is sent back to the ground as part of telemetry data and makes possible the determination of the Doppler shift and the USO frequency by taking into account the inherent difference in frequency between the ground transmitter and the USO. This brings the advantage of a simpler radio system on the spacecraft, the increased stability of the downlink and the increased flexibility in using the radio link for scientific studies.
Within the receiving system of New Horizons, the noise performance has been improved by the placement of the leading Low-Noise Amplifier closer to the antenna to reduce the physical temperature of the X-Band waveguide connecting the amplifier to the high-gain antenna. The REX system follows the 4.5MHz buffer and the anti-phasing filter includes an analog-to-digital converter feeding a triple-redundant Field Programmable Gate Array. Within the FPGA, the two core functions of REX are handled – the calculation of the total power within the 4.5MHz bandwidth from the uplink signal entering the antenna that is being put through a total power integrator, and processing of the 4.5 MHz data in a digital filter to isolate the 1kHz portion of the frequency spectrum that contains the occultation signals relevant for the experiment. These signals are then processed into digital science data and routed to the data recorder to be sent to the ground for further processing and analysis.
LORRI – Long-Range Reconnaissance Imager
LORRI, the Long Range Reconnaissance Imager, is the high-resolution imaging instrument of the New Horizons Spacecraft tasked with the observation of Pluto, its giant satellite Charon and the smaller moons Nix and Hydra as well as other Kuiper Belt objects. The instrument is a narrow-angle telescope that can acquire high-resolution imagery of objects even at great distances.
Its primary purpose is the acquisition of imagery to provide information on Pluto’s geology and surface morphology as well as collisional history, atmosphere-surface interactions and any signs of activity such as plumes or cryovolcanoes, surface layering, atmospheric haze, and other phenomena occurring on the surface or within the atmosphere. Imagery acquired during the flyby will show features as small as 100 meters on the surface of Pluto and 260 meters on the surface of Charon.
LORRI is a panchromatic imager sensitive for the visible wavelengths. Imagery from the instrument finds application in optical navigation, the determination of the orbits of Pluto’s satellites before delivering the sharpest images ever obtained of Pluto and its moons. The instrument was developed and manufactured at Johns Hopkins University and SSG Precision Optronics Inc.
The LORRI instrument consists of four principal components, the Optical Telescope Assembly, the Aperture Door, the Associated Support Electronics and the Focal Plane Unit. All components are interconnected by an electrical harness and the instrument includes no moving parts except for the aperture door. All instrument components aside from the door are installed on the Central Deck of the New Horizons spacecraft while the door is mounted to an external spacecraft panel.
Thermal considerations were an important aspect in the development of LORRI since the telescope views cold space while residing within the spacecraft body that is kept well above freezing at all times. To optimize optical performance, a material with high thermal conductivity and low coefficient of thermal expansion was needed for the construction of the optical system.
As a result, the Optical Telescope structure, the primary and secondary mirrors and a metering structure were all manufactured from silicon-impregnated silicon-carbide which offers favorable thermal characteristics.
LORRI employs a telescope with a 20.8-centimeter aperture diameter using a Ritchey-Chretien design consisting of a hyperbolic primary mirror and a hyperbolic secondary mirror to eliminate third-order coma and spherical aberration. The telescope has a focal length of 263 centimeters and a narrow field of view of 0.29 by 0.29 degrees.
In total, LORRI has a mass of 8.6 Kilograms, 5.6kg of which are the optical telescope. The instrument requires 5 Watts of electrical power plus up to 10W of heater power.
The telescope was designed to have a high light throughput given the low light level at Pluto that is about 1/1000 of that found at Earth. This is also required because LORRI is limited to short exposure times given the stability of the spacecraft that only uses thrusters for attitude control.
The entire telescope structure is monolithic consisting of the primary mirror bulkhead, a short cylindrical section, and the three-blade spider hosting the secondary mirror. A field flattener assembly is installed on the primary mirror mounting plate protruding through the mirror and facilitating fused silica lenses which are the only refractive elements of the LORRI telescope. The telescope structure is mated to the composite baffle tube via three titanium feet that provide vibration isolation. The baffle is attached to the spacecraft structure itself using six glass-epoxy legs that provide thermal isolation.
Multilayer insulation is used to cover the entire Optical Telescope Assembly for thermal protection while thermal gradients are reduced through the choice of materials. The silicon-carbide structures of the Optical Telescope Assembly have a very low expansion with temperature and Invar 36 was chosen for all inserts that allow bolting together of the assembly given its comparable thermal characteristics in the expected temperature range. All Invar inserts and the secondary mirror feet are epoxy-bonded to the Optical Telescope Assembly.
The telescope itself is installed within the telescope baffle tube that consists of highly conductive graphite epoxy and forms a uniform cold sink around the entire structure to help reduce thermal gradients.
The interior of the telescope is protected from contamination and solar illumination by a door mechanism that is opened after launch. The door is mounted external to the spacecraft and interfaces with the baffle to build a contamination seal.
The door is machined from a single piece of aluminum and is covered in multilayer insulation for thermal control while the door is closed. In a one-time event, the door is opened using redundant sets of loaded springs and paraffin actuators to guarantee a successful deployment.
Baffling within the telescope assembly is accomplished using graphite composite baffle vanes to suppress stray light and reduce image ghosting. A second inner baffle is extending out from the hole in the primary mirror with inner and outer vanes plus threading.
The Focal Plane Assembly of the LORRI instrument features a temperature-controlled Charged Coupled Device detector installed on a bracket that is mounted on the Optical Telescope Assembly via titanium flexures while the bracket itself is attached to a gold-coated beryllium conduction bar that interfaces with a radiator installed on an outside spacecraft panel. Because the radiator is installed separate from the telescope, a highly-conductive aluminum alloy S-link is used to connect the radiator to the Focal Plane Assembly to allow for some motion between the two.
The Focal Plane Unit hosts a back-illuminated, high-quantum efficiency CCD detector supplied by E2V Technologies, 1024 by 1028 pixels in size, with four dark columns to create usable imagery of 1024 by 1024, using the standard 13-micrometer pixel size. The instrument has a passband of 350 to 850 nanometers, covering the visible wavelengths. The CCD is highly sensitive and employs anti-blooming technologies.
The charge level within each pixel of the CCD is represented by a 12-bit binary word and the entire CCD has a frame transfer time of 13 milliseconds. LORRI supports exposure times from 1 millisecond to 29.9 seconds, however, typical exposures are 50 to 200 milliseconds optimized for the spacecraft pointing capabilities. The Focal Plane Assembly includes a switchable 4×4 on-chip binning option to deliver 256 by 256-pixel images. For calibration, the Optical Telescope Assembly uses two incandescent bulbs that can illuminate the CCD through light scattered throughout the OTA.
Located within the Focal Plane Unit is an AD9807 analog integrated circuit that is in charge of double sampling of the CCD, amplification of the read-out signals and analog to digital conversion to the digitized 12-bit data format. This conversion occurs at a maximum rate of 6MHz, well above the pixel read out speed at 1.5Mhz.
A dedicated latch-up protection circuit is in place to avoid radiation-related latch-up of the analog device. The signal delivered to the double sampler is already amplified using a low-noise, wide-band amplifier located between the CCD and sampler to avoid having to run the analog amplifier within the sampler at high gain. Clocking signals for the CCD are provided by dedicated MIC4427 drivers delivering phase, image zone and memory zone clocking.
The digitized signals are delivered from the Focal Plane Unit to the Associated Support Electronics that are comprised of three components – a Low-Voltage Power Supply, an Event Processor Unit and an Input/Output slice.
The Event Processing Unit communicates with the spacecraft via an RS-422 link to receive commands and transmit engineering data. EPU hosts a RTX2010RH Field Programmable Gate Array as Central Processor.
The main function of the Input/Output board is the reception of serial image data from the Focal Plane Unit and and transmission of that data to one of the Instrument Interface cards of the two Integrated Electronics Modules of the spacecraft for storage within the onboard memory. Data transmission is possible on an RS-422 bus and an LVDS link, both links exist separately to either of the IEMs.
Further tasks of the Input/Output slice are to store/transmit the image header, to receive commands from the RTX processor, command the Focal Plane Unit mode and set exposure times based on inputs from RTX. The Input/Output board contains two Field Programmable Gate Arrays(FPGA) – an imager-interface and and RTX-bus interface. The first reads the images from the FPU and transmits them to the IEM, but it also generates test pattern images for transmission to the IEM. The imager-interface can also receive data from the RTX to be sent to the Focal Plane Unit setting the FPU mode and exposure time and to write the 408-bit image header that is written over the first 34 image pixels. The header information is used to match engineering data with image data. The RTX-bus calculates the 32-bin histogram of the FPU image currently being transmitted to then calculate future exposure times in a dynamic scheme to avoid overexposed images. It also gathers FPU status and temperature parameters that are made available to RTX.
The Low-Voltage Power Supply of the instrument is comprised of a redundant set of DC-to-DC converters and delivers 2.5, 6, and 15V power as required by the other electronics within LORRI.
Also, LVPS actuates the instrument heaters and delivers power bus health data back to the Power Distribution Unit.
LORRI begins Pluto observations 90 days prior to the encounter with Pluto and Charon already resolved as separate objects. These initial observations are used to refine the orbits of Pluto and Charon and the smaller moons, Nix and Hydra. Single frames and 2×1 mosaics are acquired to cover ten full orbits until about 14 days before encounter. Imagery provided in the week leading up to encounter are used for the search for librations of Pluto and Charon. The last full frame of Pluto comes ten hours prior to closest approach and two 3×3 global mosaics are taken during closest approach showing the illuminated disk of the dwarf planet. Additional images taken around the time of closest approach show a smaller area but at a great resolution for morphological studies of the surface and atmosphere. The full disk of Charon is imaged with 3×3 mosaics.
SWAP – Solar Wind at Pluto
All major interplanetary missions carry a Solar Wind Analyzer and New Horizons is no exception as it offers the first opportunity to study the interaction of Pluto with the solar wind. SWAP, the Solar Wind at Pluto instrument, is the largest-aperture instrument ever built to measure solar wind particles given the large distance of Pluto to the sun and the low intensity of the solar wind.
Despite its distance to the sun, Pluto is not safe from solar wind interaction. In fact, scientists believe that, due to its minute gravity, Pluto is losing a significant amount of material through escape processes driven by the solar wind as atmospheric gas molecules or atoms are stripped away by interactions with the solar wind. The SWAP instrument sets out to quantify the loss encountered by Pluto and also look
at the underlying loss mechanisms to compare them with other planets. Learning about atmospheric loss also provides valuable information on the structure of the atmosphere itself.
The SWAP instrument combines a Retarding Potential Analyzer (RPA) with an Electrostatic Analyzer (ESA) to make extremely fine and accurate energy measurements of the solar wind, capable of detecting even minute changes in solar wind speed. SAP was developed by the Southwest Research Institute like many other electrostatic instruments flown to all kinds of places throughout the solar system. The instrument weighs 3.4 Kilograms and draws 2.8 Watts of power.
The electro-optics of SWAP are comprised of the RPA, a deflector and the ESA which work together to select the angles and energies of the solar wind to be measured. Energetic ions selected by the electro optics based on the voltage settings are then registered with a coincidence detector system ahead of signal digitization and storage. The RPA rejects an ions with energy per charge values that are under the voltage setpoint of the system. Ions arriving at angles can be deflected into the subsequent electro-optics by applying a voltage to the deflector ring. The final selection of ions occurs within the ESA that rejects ions outside its set Energy/charge range and also eliminates UV light and neutrals. The ion passband can be solely determined by ESA in case the RPA is turned off, but high-resolution differential measurements of the incident ion beam can only be made by differentiating adjacent RPA/ESA combinations.
The ions that are selected by the analyzers then enter the detector section which features an ultra-thin carbon foil to create secondary ions and two channel electron multipliers (CEMs) to generate a measurable electron signal. Both, the primary particle and the secondary electrons are measured as part of the coincidence measurement by Charge Amplifiers that service the CEMs.
Overall, SWAP has a field of view of 276 by 10 degrees that is deflectable by over 15 degrees. Ion energies of 35 electron-volt to 7.5 kilo-electron-volt are supported by the instrument. The instrument can acquire either full energy and detailed peak measurements or deliver additional full energy sweep data when stepping through 128 discrete voltage steps with 0.39-second step lengths to deliver full energy spectra. SWAP is installed on the –Z corner of the spacecraft where its field of view is free of any structure. This location also allows the spacecraft to point its imaging instruments on the +Z axis and still permit SWAP to acquire measurements.
To account for the low density of the solar wind at Pluto distance, the SWAP instrument had to utilize a unique design with a very large aperture. Nevertheless, the SWAP instrument is similar in its overall architecture to various top-hat electrostatic analyzers that have flown before. To be able to detect fine-changes in solar wind speed, ESA was coupled with RPA.
The RPA consists of four concentric aluminum cylinders/screens with 90,000 close-packed holes to create a grid structure that is self-supported and has a 65% transmission rate. The grid is 0.25 millimeters thick and the holes 0.34mm in diameter drilled into 0.38mm thick nickel. The four cylinders have diameters of 17.44, 16.96, 16.64 and 16.16 centimeters. The outside and innermost cylinder are not biased and are kept at ground potential while the two central cylinders are biased between 0 and –2000 Volts in 0.49V steps. Ceramic insulators are used to isolate these cylinders from the rest of the system.
The RPA provides a low-pass filter with a sharp energy cutoff, allowing SWAP to make fine sweeps across the solar wind beam once it is located with a coarse ESA scan. Ions need to have sufficient energy to pass the grids. In the process of passing, the ions are decelerated which requires SWAP to re-accelerate the ions to their original energy which is completed in the path from the inner RPA grids to the final grounded grid.
The deflector used by SWAP is used to deflect particles from further out in –Z direction into the central plane of the instrument. It is located just inboard of the RPA and operates at a voltage of 0 to +4000 Volts applied to a metal ring. It can deflect ions of up to 7000eV/q to an angle of up to 15 degrees. Deflections for lower energies are higher.
The Electrostatic Analyzer provides a coarse energy selection and protects the detectors from UV radiation. The outer sphere of the top hat is blackened with an Ebanol coating to reduce scattering of light and particles while the inner surface is blackened but not serrated. A grounded cone completes the ESA design by providing a field-free region through which particles enter the detector area. The ESA can be operated at voltages of 0 to –4000V.
After passing through the field free region, the selected particles are accelerated towards a Focus Ring which holds the ultra-thin carbon foil with a thickness of 1 micrometer. The foil is held in place by a grid with 66% transmission. When striking the grid, the particle generates secondary electrons which, along with the primary particle, continue onward to the Primary Channel Electron Multiplier, accelerated in a 100V potential on the PCEM strip. Electrons scattered backwards are directed to a Secondary Channel Electron Multiplier to be collected.
Counts from the two CEMs are registered by the CHAMP (Charge Amplifier) and the associated electronics. A pulse from one of the CEMs starts a 100-nanosecond anti-coincidence counter during which the other CEM has to provide a pulse as well for the signal to be taken into account to rule out dark counts, UV noise and other erroneous measurements.
There are two detectors within SWAP to provide redundancy over the course of the long mission duration. A second focus ring was added to the SCEM to draw back scattered electrons for measurements without PCEM. SWAP features a two-segment door that is opened once in flight using a tensioned spring assembly. The doors are coated with back nickel and have grounding wires to keep them at spacecraft potential.
The SWAP instrument contains within it all electronics needed for the operation of the instrument, specifically, the High-Voltage Power Supply (a HVPS Driver and High Voltage Boards) and Control Board, located in an electronics volume below the SWAP instrument. Separately, the CHAMP boards are located closer to the sensor to prevent excessive noise.
The Charge Amplifiers convert a charge pulse from the CEMs to a TTL pulse that can be accepted by the Control Board for subsequent processing. The CHAMPs are located in enclosures to the top of the instrument Strong Back to keep them close to the detectors. The pulses from the CEMs are delivered to the CHAMPs through short coaxial cables. A threshold voltage for the CHAMPS can be set by a Digital to Analog Converter on the Control Board through commanding. A resistor sets the output pulse width to 70 nanoseconds and also controls the 100ns amplifier dead time. The output pulses are buffered by two Schmitt trigger buffers before being transmitted through a back-terminated series resistor to the interface cable leading to the Control Board.
The High Voltage Power Supply sets the voltage on all optical surfaces, the RPA, the deflector, the ESA and the Focus Rings. It also supplies power to the CEMs requiring six different voltage to be generated and adjusted based on the instrument mode of operation or external commands. HVPS delivers a 0 to –4500 Voltage to PCEM, 0 to +4500 to SCEM, 0 to 100 to the focus ring, 0 to +2000 to RPA, 0 to +4000 to the deflector, and 0 to –4000 to ESA. Accuracy for the deflector and ESA are 4V, the CEMs 5V and the RPA just 0.5V. The HVPS system is fully redundant to ensure reliable generation of bias voltages.
The Control Board provides the data interface between the SWAP instrument and the New Horizons spacecraft, receiving and executing commands from the spacecraft by using an 8051 microprocessor that responds to commands, controls the operation of the instrument, sets the power sequences and collects the data, also formatting the housekeeping data stream. The Control Board is connected to the two Integrated Electronics Modules of the spacecraft via an RS-422 data bus. DC-to-DC converters on the Control Board deliver the 5-Volt instrument power bus.
The instrument microcontroller runs at 4.9 MHz and 0.4 MIPS (Million Instructions Per Second). The instrument boot code is stored in 32KB PROM, with two 128KB EEPROMs providing redundant storage, 64KB bit storage for the program code and 64KB of Look-Up Tables. A 128KB SRAM memory provides code and data memory space. All instrument memory allocations are controlled by a dedicated Field Programmable Gate Array.
The Control Board receives all CHAMP pulses once a data acquisition window is opened which only occurs when all voltages have been set and initial settling is complete. From that point on, all pulses are registered and those with signals from both CEMs within a 100ns interval are converted to digital science data. Commanding the HVPS board via an interboard connection, the Control Board sets all voltages and receives feedback on current and voltage that is input into housekeeping telemetry.
CEM health monitoring is also performed by the Control Board which can take action to keep the instrument safe in case count rates or any health parameters exceed specified limits.
SWAP can operate in different modes – a simple BOOT mode in which the instrument is booted from the PROM image enabling the upload of new code, a LVENG (Low Voltage Engineering Mode) running from an EEPROM image considered the instrument safe mode, a LVSCI (Low Voltage Science Mode) used to verify instrument performance through CHAMP test pulses, a HVENG Mode when high voltage is set through commands for calibration and checkout, and HVSCI, the main science mode of the instrument running ESA/RPA/DFL voltages according to science run tables.
In its nominal mode of operation, SWAP completes 64-second runs to acquire data, starting with a 32-second coarse sweep across the complete instrument range in 32 steps taking 0.5 seconds each. Then, the Control Board calculates the peak that is then set as the center of the fine sweep which covers a narrow energy range and again employs 32 energy steps to 0.5 seconds each.
PEPSSI – Pluto Energetic Particle Spectrometer Science Investigation
PEPSSI is the most compact directional energetic particle spectrometer flown on a space mission and complements the SWAP instrument by covering electrons and ions at high energies to deliver additional data on solar wind interactions of the Plutonian atmosphere. Built at Johns Hopkins University’s Applied Physics Laboratory, the instrument studies the density, composition, and nature of energetic particles and plasmas that are the result of escape processes ongoing at Pluto. As a spectrometer, the instrument can identify species escaping from the atmosphere which also provides valuable information on the structure of the atmosphere itself.
PEPSSI weighs 1.5 Kilograms and requires 2.3 Watts of power. It is a classic time of flight particle spectrometer and measures electrons from 25 kilo-electronvolt to 500keV, ions from 700eV to 1MeV, CNO ions from 15 keV to 1.2MeV, and protons from 40keV to 1MeV. PEPSSI has a field of view of 160 by 12 degrees and measures 19.7 by 14.7 by 21.6 centimeters in size in its launch configuration.
The instrument is mounted on a bracket that provides the proper angular offset of the fan-shaped field of view from any of the spacecraft decks. The viewing geometry was optimized for the study of freshly ionized pick-up ions in the vicinity of Pluto caused by charge exchange from Pluto’s atmosphere. The bracket allows the spacecraft to be mounted on the spacecraft deck while looking past the large High Gain Antenna and not being obscured by the dish structure.
PEPSSI Installation Location & Field of View
Alignment control for PEPSSI was kept within 1.5 degrees. The installation of PEPSSI allows the instrument to detect solar ions when the spacecraft is in its nominal communications attitude while not directly looking into the sun.
The instrument is installed on the top deck of New Horizons with four stainless steel bolts while thermal isolation from the bracket is provided by thermal washers between the bracket and PEPSSI’s base plate.
The instrument consists of a collimator and sensor assembly known as the Sensor Module sitting atop an Electronics Board Stack facilitating six electronics boards within a box. Contained within the Sensor Head is the time of flight section about 6 centimeters in length feeding an array of Silicon Solid State Detectors that measures energy and delivers timing signals for the calculation of the time of of flight of the particles. Event energy and time of flight (velocity) can be combined to calculate the particle mass (E=0.5mv²) to determine the particle species.
The PEPSSI instrument was launched with the deployable door mechanism, consisting of two door segments that provided protection from contamination during ground operations as well as acoustic environments occurring during launch that could have damaged the ultra-thin foils within the Sensor Module.
Each half of the door covers half of the 160° aperture and swings outward on deployment that is driven by torsion springs initiated by firing an actuator that retracts a retaining pin. Tension of the springs keeps the doors open for the remainder of the mission.
For the examination of ions, PEPSSI uses an approach known as Time-of-Flight (TOF) by Energy and TOF by Microchannel Plate Pulse Height to determine the energy and velocity of ions which allow their mass to be calculated, allowing an identification of the ion species. Electrons are detected by solid state detectors that sense energy and directional distribution.
The Sensor Module consists of an aperture opening, electron deflectors, start foils and anodes, a microchannel plate detector, stop anodes and foils, solid state detectors and pre-amplifiers as well as supporting electronics. The PEPSSI Sensor Module includes TOF sections 6 centimeters across that feed the silicon solid-state detectors. The SSD array and the individual pre-amplifiers are connected to an Event Board that determines particle energies.
The direction of an incoming particle is determined as a function of the solid state detector that is struck by the particle, with six different viewing directions along the 160° fan represented by six physical Solid State Detector Elements covering 25° with 2° of spacing in between individual elements providing an accuracy sufficient to estimate the overall direction of particle inflow. Sectors 1, 3, 6 consist of two SSD detectors, one for ions and one for electrons. The electron detectors are covered with a 1-micron Aluminum layer to reject low-energy protons and heavy ions. Sections 2, 4, 5 are pure ion detectors. For ions, the directionality is determined by the detection of the entrance position on the microchannel plate time-delay anode nearest to the start foil.
As an ion enters the instrument, it first passes through a thin foil in the collimator before reaching the start foil (aluminum-polyamide-aluminum) and generating secondary electrons. These electrons are then directed from the primary particle path to the microchannel plate detector where the Start Signal is generated for the Time of Flight measurement.
A 500-Volt potential between the foil and the MCP directs the secondary electrons to the TOF detector with high accuracy (0.4ns dispersion in transit time). The segmented MCP anodes with one start anode for each of the six angular segments provide data on the direction of travel of the ion.
Secondary electrons that are created as a result of the ion passing through the stop foil (palladium-polyimide-palladium) are again directed to the MCP and cause a Stop Signal. The time-difference between the two signals represents the time it took the ion to pass through the 6-centimeter TOF instrument. Both foils, start and stop, are installed to a high-transmittance stainless steel grid for structural support.
After the stop foil, ions impact the Solid State Detectors that either consist of electron and ion pixels or are pure ion pixels. The SSD determines ion energy which coupled with the TOF measurement delivers ion mass and particle species data.
Electrons entering the instrument are first decelerated by a 2.6kV potential which is part of the TOF system for ion measurements. After passing the stop foil, the electrons are again accelerated by a 2.6kV potential. Reaching the SSD detectors, the electrons are detected in the electron pixels that can measure electrons at energies of 25 keV to 0.5 MeV.
The electron detectors are covered with 1-micrometer aluminum metal flashing that rejects protons with energies under 100keV. Light ions are blocked as well, but for heavy ions with energies over 100keV coincident TOF measurements are needed to discriminate between ions and electrons to only register electrons.
Particle energy can be measured for protons starting at 40keV and heavy ions (such as the CNO group) starting at 150keV up to over 1MeV. Lower-energy ion fluxes are measured via TOF only while the MCP pulse height can only point to a coarse indication of low-energy particles.
Housed within the electronics box below the Sensor Module are six electronics boards that provide all functionality to the PEPSSI instrument and build the interface between instrument and spacecraft. The Energy Board accepts the SSD event signals on 12 channels with amplitudes proportional to particle energy.
These analog signals are converted to 10-bit digital numbers by processing through a charge sensitive amplifier and peak shaping algorithm before being put through an Analog to Digital Converter. A dedicated TOF board processes the time of flight data – amplifying the start and stop signals, computing the time of flight duration between the two signals and converting the measurement into a digital format.
The High Voltage Power Supply generates the voltage bias on the entry/exit foils (-2600V), the microchannel plates (-2100 and –100V), the deflector plates (-2900V) and the SSD bias of –100V. A Digital Interface Board is in charge of running the event validation logic monitoring energy and TOF event counters and running interface functions. The Events Processor board receives energy and TOF data from the other boards that is processed into science data transmitted to the spacecraft via RS-422. It also delivers instrument status telemetry and accepts spacecraft commands that are then executed by the instrument. Finally, the Low Voltage Power Supply delivers the 15 and 5-Volt instrument buses to the electronics boards, generated from the 30V main spacecraft bus.
Operational modes supported by PEPSSI are a TOF-plus-Energy mode, TOF-Only and Energy-Only. All of them are made concurrently based on the data that is available. TOF-Plus-Energy is determined using TOF and SSD data from the same particle. TOF-Only occurs when TOF data can be gathered from secondary electron pulses but no SSD response is registered which is the case for light particles such as protons, although heavy ions at low energies can also lack an SSD signal when below the given energy threshold. Energy-Only measurements occur when ions fail to generate secondary electrons and when the TOF time is sufficiently close to zero.
VB-SDC – Venetia Burney Student Dust Counter
SDC is a student-developed instrument designed at the Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder. Its purpose is to detect microscopic dust grains in the solar system from 1AU to at least 30 Astronomical Units. Dust particles can be released by asteroids, comets, and Kuiper Belt objects, for example as the result of collisions. Obtaining an accurate count and size distribution of dust particles can provide insight into the collision rate of such bodies in the outer solar system. SDC can also search for dust in the Pluto system to look at the impact rate of tiny impactors on Pluto’s moons.
The measurement of the spatial and size distribution of interplanetary dust particles is needed to verify the existence of predicted structures within the
Zodiacal cloud. Five previous deep space missions have carried dust sensors into deep space – Pioneers 10 and 11, Ulysses, Galileo and Cassini. None of these missions was able to conduct dust measurements beyond 18 AU which means SDC, despite being a student-built instrument, will deliver a truly unique data set. The instrument is expected to continue working after the Pluto flyby to examine the dust environment within the Kuiper Belt which has not yet been explored what so ever. These measurements can advance the current understanding of the formation and evolution of the solar system and deliver new data for models of planet formation out of dust disks in other planetary systems.
The SDC instrument is comprised of two major pieces – a detector assembly with an active dust-sensing system that is exposed to the space environment, and an electronics box residing within the spacecraft. Overall, the instrument weighs 1.9 Kilograms and requires up to five Watts of electrical power. It is the first student-built payload to fly on a NASA planetary mission.
The SDC instrument features a set of polyvinylidene fluoride PVDF film impact sensors that are mounted on a detector support panel that is installed on the exterior of the New Horizons spacecraft and facing the ram direction when the spacecraft is in is nominal Earth-pointing attitude to maximize the probability of dust impacts. Signals from the sensors are relayed via an intra-harness to the instrument Electronics Box facilitated within the warm spacecraft body.
The instrument was designed to provide a spatial resolution of 0.1 Astronomical Units to be able to resolve expected resonance structures.
SDC is capable of detecting the mass of particles between one picogram and one nanogram which corresponds to grain sizes of one to 10 micrometers in radius. Heavier particles can still be detected, though their masses can not be determined.
The dust impact detector has an active area of around 0.1 square meter and uses permanently polarized PVDF films. When a particle hits, a depolarization change is caused as it impacts the film which can be measured by relatively inexpensive sensors that are stable even in extreme thermal, mechanical, electrical and radiation environments.
The magnitude of the depolarization change within the PVDF depends on the momentum of the particle and whether it fully penetrates the film. SDC uses a film 28 micrometers in thickness which is known to stop particles with a mass of 10 nanograms at a speed of 20 Kilometers per second.
The SDC sensor element consists of 12 sensor patches each 14.2 by 6.5 centimeters in size plus two detector patches on the backside of the detector assembly used as a reference to monitor the background noise level caused by mechanical vibration or cosmic ray hits to the electronics. The detector elements are installed atop a one-centimeter thick aluminum honeycomb panel which itself is attached to the exterior of the spacecraft via a three-point compliant mount consisting of titanium flexures for thermal expansion flexibility.
To protect the PVDF film from overheating, the honeycomb panel is directly attached to a high-emissivity polyimide tape that is radiatively coupled to the support panel below to spread out the heat from below the detectors. The top surface is covered with Teflon tape capable of reflecting 90% of incident solar energy.
The PVDF film on the sensors features a thin (1000 Angstroms) Aluminum-Nickel electrode material on the top and bottom surfaces. The detecting element is bonded between a pair of fiberglass frames that have built-in electrical contact wires to the two electrode surfaces bonded to the electrodes with conductive silver filled epoxy. The small signal wires run to the connection tabs where they interface with a coaxial cable. The wire is harnessed so that is can withstand dust impacts.
The electronics of the SDC instrument are facilitated on two printed wiring assemblies housed inside the electronics board. Signals from the detector are delivered through the harness to the analog Printed Wiring Assembly where amplification occurs followed by signal conditioning and conversion to the 16-bit digital regime. The digitized data is collected by registers of the Field Programmable Gate Array of the digital Printed Wiring Assembly and then directed to the microprocessor of the instrument that adds time-stamps and stores the data frames in a long-term non-volatile memory. The digital PWA also hosts the instrument power supply, health monitoring system and the interface between the instrument and the spacecraft.
The SDC Digital Board hosts an Actel RT54SX72S FPGA that completes address decoding functions, conditions the housekeeping data stream, facilitates an interrupt controller and watchdog timer for instrument safety functions and it collects the science data from the analog board.
It is also in charge of delivering housekeeping and science data one of the two Integrated Electronics Modules of New Horizons and accepts commands from the spacecraft. It watches over the performance of the instrument microcontroller and has the authority to reset it. The FGPA has access to 32KB of PROM holding the boot code, 32KB of SRAM and 4MB of Flash RAM to hold science data.
The Amtel 80C32E Microcontroller handles the communications with the spacecraft, executes commands, manages the flash memory and handles the science data, all through the FPGA.
The SDC instrument has been designed for stand alone operations to be able to keep recording dust events even when the New Horizons spacecraft is in hibernation mode. It can manage itself for up to 500 days without the need to communicate with the spacecraft or the ground. A number of autonomy rules are stored within the instrument to provide adequate response for any number of anomalies that could be encountered during the cruise to Pluto. Daily checks of the memory health are part of the instrument’s routine and the number of interrupts on each channel are measured to allow the system to take action by switching channels for lower sensitivities or block them altogether to keep the instrument in a good configuration for actual science collection.