Mission Design & Timeline

Image: NASA/LMSAL
Image: NASA/LMSAL

IRIS is planned to perform a two-year nominal mission in a sun-synchronous orbit after being launched by a Pegasus XL rocket and completing in-orbit commissioning. Based on spacecraft performance and science demands, the primary mission will be extended.

The orbit of the vehicle will decay within 25 years after primary mission termination in accordance to new guidelines associated with orbital debris mitigation.

During its mission, IRIS measures the flow of energy and matter from the solar surface to the corona with simultaneous UV image and spectra acquisition. More about the scientific goals and techniques of the IRIS mission, visit the dedicated pages.

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Orbit Requirements

Image: NASA
Image: NASA

IRIS is launched into a sun-synchronous orbit, also called twilight orbit. Requirements that went into the orbit selection process included sun-exposure, debris mitigation and downlink data volume.

The spacecraft operates from a 596 by 666-Kilometer Orbit at an inclination of 97.91 degrees and a period of 97.3 minutes. The local ascending node (equator crossing) is at 17:36 local time. This orbit allows IRIS to observe the sun for at least 8 months during the first two years during its mission. Also, for the first several months, IRIS will be constantly exposed to the sun to complete the most critical scientific observations. A 98-day eclipse season with eclipses in the southernmost part of the orbit occur centered on the summer solstice with a maximum eclipse time of 22 minutes.

In years 1&2, 8.8 months per year will be eclipse free. Even with longer eclipse periods later in the mission, IRIS is expected to continue scientific observations on the non-eclipsed part of its orbit with available power being sufficient to continue nominal operations.

The elliptical orbit is also in accordance with the 25-year decay requirement. Because IRIS has no onboard propulsion system capable of terminating the mission via targeted re-entry, the initial orbit of the vehicle has to be at an altitude that allows a natural decay within the given time period.

The final requirement was data downlink volume via the only ground station of the mission. The chosen orbit allows sufficient data downlink with margins over the course of the entire mission.

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Image: NASA/LMSAL
Image: NASA/LMSAL

 

Launch and Acquisition Phase

IRIS is launched aboard a Pegasus XL launch vehicle which is an air-launched rocket that is deployed via the Stargazer/L-1011 Aircraft at an altitude of above 12,000 meters. A detailed launch vehicle overview is available here.

Once the launcher is dropped by the carrier aircraft and five seconds have passed, the all solid-fueled vehicle ignites its first stage. The point at which the Rocket is released is predetermined and the Aircraft has to be in an exact position concerning location, altitude, speed and velocity vector.

Photo: Orbital Science Corporation
Photo: Orbital Science Corporation

Shortly after Orion 50S ignition, the Pegasus launcher uses its delta wing and actuated fins to perform a pitch-up maneuver. After the 69-second burn of the first stage and a very short coast, the solid rocket motor separates along with the tail fins and delta wing.

Second stage ignition follows. Attitude control is provided by Thrust Vector Control of the Orion 50 Engine for Pitch and Yaw Control and Nitrogen Thrusters on the third stage for Roll Control. Second Stage Burn time is 1 Minute and 9 Seconds. While being powered by the Second Stage, the Vehicle initiates Payload Fairing Separation when the Spacecraft has left the Atmosphere. After second stage burnout, the vehicle enters a six-minute coast phase still holding onto the second stage. Once the vehicle has reached a certain altitude, the second stage is jettisoned and the Orion 38 Solid Rocket Motor of the third stage fires for 68.5 seconds to boost the stack into its planned orbit.

After burnout, the vehicle re-orients for spacecraft separation about 12 minutes into the mission.

Once IRIS is released, the third stage performs collision avoidance maneuvers and completes the launch vehicle’s job of delivering the payload to its sun-synchronous orbit. Separation occurs with IRIS sun-pointed with its Z-Axis facing the sun. IRIS can tolerate up to 4deg/s angular rates, but release is planned with zero angular rates.

Once released, IRIS starts a busy initial on-orbit sequence that will span about three orbits. At Separation +93 seconds, the spacecraft initiates solar array deployment. Also, IRIS powers up its Attitude Control Sensors and actuator suite. Solar Array deployment occurs without live telemetry link to the ground.

The spacecraft initiates a sequence to acquire a stable attitude which can take up to 133 minutes in a worst-case separation scenario.

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Orbital Plot of initial mission phase - Image: NASA/LMSAL
Orbital Plot of initial mission phase – Image: NASA/LMSAL

During the first three orbits, IRIS uses the NASA Ground Network to maximize communications coverage. The first station to pick up IRIS is McMurdo, Antarctica, which acquires IRIS’ signal 21 minutes after spacecraft separation. During 12-minute pass, telemetry data is acquired to confirm that IRIS has deployed its solar arrays and that the vehicle is power-positive. Battery state of charge data is collected and the status of attitude acquisition is determined.

Should any commanding be required, teams can use any subsequent pass over one of the ground stations.

The primary ground station for the entire IRIS mission, Svalbard, Norway, picks up the signal 70 minutes after separation for a 9-minute pass. The Poker Flats station, Alaska, has an 8-minute pass 80 minutes after S/C Sep.

For the following orbits, all these stations are available again to perform any troubleshooting or contingency procedures if required. NASA’s Wallops tracking station is available starting on the fifth orbit.

IRIS launches with a pre-programmed command sequence for the first several orbits. The spacecraft does not rely on ground commanding and automatically initiates a downlink of all stored telemetry following spacecraft separation whenever it is over a ground station. Onboard data is not released until a verification command is sent by a ground station via S-Band.

The initial mission phase ends after Orbit 3 in a nominal mission, but is extended in case of any anomalies.

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IRIS Launch & Deploy Animation

In-Orbit Checkout Phase

The IOC Phase begins after the third orbit and ends around 30 days into the mission.

The Mission Operations Center is staffed 24/7 for the first seven days of the flight to perform initial commissioning activities and monitor spacecraft telemetry as it is received. During the mission phase, all spacecraft systems are powered-up and checked out.

When Spacecraft Attitude Acquisition is verified complete, the CCD decontamination heaters are activated. All four CCDs of the spacecraft are being heated to +30°C until Day 10 of the mission to prevent any contamination build-up on the detectors while outgassing.

All activities completed during IOC are completed with real time insight via ground station coverage.

The first weeks of the mission are dedicated to spacecraft checkouts. The attitude sensor and antenna orientations are being measured pre-launch so that those components can complete their mission without on-orbit calibrations. During the first three Days if IOC, the vehicle is put through a range of attitude maneuvers to improve pointing knowledge. Antenna functionality is checked during regular data downlinks and command uplinks.

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Image: LMSAL

The two star trackers are activated and tracking data is collected while IRIS continues to fly in a sun-pointed attitude. Data is compared with ground simulations, magnetometer data and sun sensor data in order to validate the star trackers for use. Relative star tracker pointing is verified by using sun sensor data.

When star tracker performance is established, IRIS is commanded to Inertial Sunpoint Mode in which the vehicle uses the star trackers for roll attitude reference to leave the magnetometer as backup. Subsequently, IRIS is put through slew maneuvers on all axes to gather a range of attitude, sun sensor and star tracker data that can be compared to each other. This also enables teams to perform reaction wheel characterization. Finally, IRIS is commanded to set its total momentum to a variety of values to measure the influence of the torque rods.

Throughout these operations, data provided by temperature sensors located on all elements of the spacecraft are monitored and thermal control performance is verified. The same is performed for main bus voltages, battery state of charge and circuit current and voltages to ensure good power system performance.

The main instrument of the spacecraft begins checkouts around flight day 4 with power-up and initial health monitoring. Functionality of as many components is confirmed via telemetry and dark images with the front door closed are taken.

Alignment between the Guide Telescope and the Attitude Control System starts as soon as the ACS is commanded to its Fine Sunpoint Mode and initial targeting and response time is determined. The CCD cameras, mechanisms and operational heaters are all commanded during that time to verify these components are in proper condition. Around the tenth day of the flight, the CCD heaters are turned off to allow the detectors to reach their operational temperature of -50 to -70°C.

Once the CCDs and Cameras are declared operational, more dark images will be acquired over several orbits with all amplifiers as calibration data. These dark images will be taken while the vehicle is passing over all areas of its orbit – of special interest are the South Atlantic Anomaly and the high latitude zones due to their increased radiation background. Noise data and dark current/offset variations are characterized during this time in the mission and the amount of particle hits is established.

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Image: NASA/LMSAL
Image: NASA/LMSAL

Flight Day 14 will mark a mission critical day as the Front Door is opened – a one-time mission event. The door opening using the redundant wax actuator assembly is performed during a real-time telemetry pass. Additionally, the slit-jaw imager is taking data with its highest frequency to monitor light levels. After the door is opened, IRIS is left alone for some time to reach thermal equilibrium in this new configuration. Initial focus checks are run later in the mission.

Two weeks after front door opening, the first light checkout phase begins which includes initial optical checkouts during real-time passes. The objectives of these tests include focus stability, camera tests with light, gain, linearity, CCD plate scale as well as Image Stabilization System jitter data acquisition, PZT actuator range, scale & bias tuning (secondary mirror), Guide Telescope and ACS alignment, flat field images, image quality checks, throughput/scattered light characterization, compression verifications, solar rotation tracking, roll angle calibration and spectrograph calibration operations.

The Mission Operations Center transitions to a two shift operations schedule after 7 days and after 14 days, there is only one working shift per day with science data dumps occurring overnight via ground station passes.

SvalSat Facility - Image: Wikimedia - User: Bjoertvedt
SvalSat Facility – Image: Wikimedia – User: Bjoertvedt

Ground Station Coverage

Except for the initial mission phase and in-flight contingencies, the IRIS mission is only using the Svalbard Ground Station located in Norway. Because of its high latitude, Svalbard can ‘see’ the spacecraft on all of its ~15 orbits per day, but only an average of 12.3 passes are favorable for data downlink due to IRIS elevation and pass duration.

During Svalbard passes, IRIS downlinks acquired science data via X-Band at a bandwidth of 0.7Mbit/s on average. S-Band downlink is used to dump housekeeping and telemetry data. During nominal mission operations, only one pass per day is used to uplink commands to the spacecraft including a 24-hour observation plan.

If data volume requires the use of an additional ground station, the IRIS mission has the option of requesting additional coverage by a NASA or ESA station.

Science Verification Phase

This mission phase lasts until about day 60 of the mission and is implemented to verify crucial science requirements. Science timelines and data analysis after IRIS checkouts are complete provide guidance for the first scientific observations.

The first part of this phase begins at about Flight Day 21 with two mission phases overlapping. The first 30 days of science observations are dedicated to optimizing observing sequences, finalizing instrument calibrations as well as data validations, and carrying out the most crucial science operations to meet some critical mission objectives. To provide further calibration and characterization data, these first observations are coordinated with other solar observatories such as Hinode and SDO, and ground based facilities.

For initial optimization of observation sequences, IRIS performs a number of observations, gradually working its way from quiet-sun areas at the disk center to more active regions above the solar limb at the poles and mid-latitudes before monitoring coronal holes and prominences. These observations will be performed as small area scans with different exposures and obtain a full picture of available data. Full disk imaging instruments such as SDO will provide data needed to fine-tune the co-alignment between the instrument and the Attitude System and to gain fine-pointing knowledge.

IRIS Data Products - Image: NASA/LMSAL
IRIS Data Products – Image: NASA/LMSAL

The next step is the completion of Calibration Observations which include pole-to-pole central-meridian and limb-to-limb equatorial spectral rasters, off disk pointings and wavelength calibrations. The rasters are generated in sets of exposures and durations to achieve the optimal range for strong and weak spectral lines. The data can also be used by the scientific community to examine the center-to-limb behavior of intensities, Doppler shifts and line broadening that can help understand non-thermal energy deposition.

The next part of the SV phase is an exploration driven period during which IRIS takes advantage of the fact that it is the first spacecraft to observe the sun in UV at these temporal and spatial resolutions. IRIS explores a variety of solar targets at this point in the mission – on and off disk, to compare signals from solar regions viewed from different angles as they rotate across the sun, as seen from IRIS. Sampling as many solar targets is the goal of this phase. IRIS stays on one target for several hours to collect compact rasters of high cadence.

Sustained observations are implemented once a sufficient number of exploration targets has been mapped.

Staying on a single target for several days has proven worthy on a number of solar observation missions as it allows to discover phenomena in the context of their evolution – from before their appearance until well after their eventual disappearance.

Should IRIS find such targets, sustained observations of several days are performed – focusing on a few select regions. Using information from the initial observation phases, this period will utilize different raster sizes that were established earlier as well as different balances between spectral and slit-jaw data.

Nominal Mission Operations

Observing Strategy

To fulfill its mission objectives and science goals (outlined here), IRIS has to perform a wealth of observations that explore specific spatial, temporal, temperature and velocity domains. These requirements translate into specific observing sequences – each using one of the mission’s observing modes.

The IRIS science team is led by Principal Investigator Dr. Alan Title who is supported by mission scientists, engineers, managers and guest scientists to establish the scientific priorities based on previous findings and observations. During nominal mission operations, the science team provides an observing schedule on a monthly basis taking into account opportunities of coordinated observations with other space and ground based solar observatories to increase science data return.

Weekly planning meetings refine the schedule and observing plan as near-future solar activity is factored into the decision making process to be able to examine targets of opportunity.

Commands uploaded to the spacecraft include the observing plan for IRIS which is transmitted to the vehicle once per day, five days a week.

Once the initial observation campaigns are complete, IRIS teams will ask for input by the entire solar community to refine observation plans based on scientific demand.

Photo: Lockheed Martin
Photo: Lockheed Martin

Observing Modes

IRIS provides a number of different observation modes which present trade-offs between field of view, cadence, spatial resolution, spectral resolution, spectral coverage and data quality. The observation program is under the control of the instrument computer and can be modified and adjusted for the different scientific requirements each observation has, e.g. higher data rates.

The IRIS telescope/spectrograph is designed to provide rapid, time-resolved spectroscopy data. Raster scanning with adjustable step size provides dense spatial coverage and sparsely spaced spectral samples. Nominal observations will focus on six spectral lines with a cadence of 1 seconds, although the instrument can achieve a cadence of 0.5 seconds when only looking at the brightest lines. Full spectral cadence is considerably lower. IRIS uses lossless RICE data compression which is also used on SDO. The UV slit jaw imager provides full spatial coverage at high cadence for a variety of photometric and morphological studies and to provide context for spectrograph data.

Data Analysis and Data Products

IRIS will use the existing ground-based data handling architecture that has been established for the Hinode and SDO missions as IRIS data is similar in structure, but much smaller in volume. About 2,500 images and spectra are generated on each nominal observing day for a total data volume of 10GB using normal compression.

Over the 2-year baseline mission, IRIS generates about 8TB of data. For comparison, SDO AIA gathers about 70,000 16-Megaixel images per day for a total archive volume of about 2PB (2048TB). Archiving data within the existing SDO system at Stanford University and Lockheed Martin only adds 0.4% of archive size. IRIS data will also be made available via the established SDO web archive,

Once IRIS is operational and performs 12 or 13 downlink sessions per day, the received data is sent from the Svalbard Ground Station to Palo Alto, California, where the raw data is being reconstructed to images and spectra that are then cataloged in the Stanford JSOC. IRIS imaging processing re-uses a lot of techniques developed for the SDO mission.

IRIS Level 0 data represents the raw data that is reconstructed into images which are stored as lossless compressed FITS Files. Level 1 products will be images and spectra after dark subtraction, gain correction, correction for radiation hits, and normalization for exposure. This data is also stored as FITS files of floating point images in units that still need calibration.

This calibration is done to achieve Level 2 Data products. These are images and spectra with calibrated intensities and velocities. Level 2 products also include processed movies and observation-based numerical models.

To provide easy-access information on data sets, metadata including target and purpose of observations as well as information about quality, cadence and instrument performance will be part of the data products. An event processing and validation software is used to incorporate elements of the evolving heliophysics knowledge base.

IRIS data will be mirrored by the University of Oslo to provide a second access point for international users.

After a two-year nominal mission, IRIS can perform an extended mission which is proposed to be at least two additional years in duration. IRIS and all of its systems are designed to operate for more than 4 years. With IRIS using a lot of heritage hardware, it would not be a surprise to see the spacecraft in operational conditions after ten years of flight.

The IRIS orbit meets the baseline requirement of 8 months per year of eclipse free observing for the first 11 years of the flight after which a second eclipse season is encountered. If the science value is high, the IRIS teams will continue to propose mission extensions.

Mission Termination

Image of a spacecraft Re-Entry - Photo: NASA
Image of a spacecraft Re-Entry – Photo: NASA

Once IRIS approaches the end of its flight, either due to lack of funding, problems with the spacecraft or imminent orbital decay, a termination plan will be executed.

Spacecraft do not have an off switch, which means that all that can be done is place the spacecraft into Attitude Control System Safe Mode, allow its batteries to drop to minimum charge and stop communicating with the vehicle.

Because of its orbital design, the IRIS spacecraft re-enters Earth’s atmosphere within 25 years to minimize the time it spends in orbit as space debris.

At some point, most likely years after its mission ends, IRIS will re-enter the atmosphere anywhere along its orbital ground track and disintegrate and burn up upon re-entry. With IRIS being only a 167-Kilograms spacecraft, no debris are expected to survive re-entry to present no danger to life or property.