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STARDUST
Mission Description In Detail



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STARDUST
Mission Description In Detail
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STARDUST Encounter with Wild 2
 

Introduction

STARDUST is a comet sample return mission which will also be returning interstellar dust grains. These samples will be returned to Earth for analysis. A mass spectrometer derived from instruments flown on Giotto and Vega Halley missions will also be included on the payload to provide both complementary and corroborative data to the sample return results. For the comet Wild 2 encounter, the objective is to recover more than one thousand particles larger than 15 microns in diameter as well as volatile molecules on the same capture medium. The sample return objective for fresh interstellar grains is to collect over 100 particles in the 0.1 micron to 1 micron size range. They will be collected in a manner designed to preserve, at minimum, the elemental and isotopic composition for major elements in individual submicron particles.


 

Mission Flight Plan

An orbital design using one Earth gravity assist allows STARDUST to capture cometary dust intact and parent volatiles as well, at a low relative speed of 6.1 km/s (as a comparison, Giotto encounter Comet Halley at a relative speed about 10 times higher). With the aid of onboard optical navigation, the flyby will take place at an encounter distance as close as 150 km from the comet's nucleus, permitting the capture of the freshest samples from within the coma parent molecule zone. This rare trajectory imposes a very low post-launch fuel requirement and enables launch by a Med-Lite version of the Delta II launch vehicle.  

Mission Schedule in Brief

The STARDUST spacecraft will be launched in February 1999. The first orbital loop is a 2-year VEGA path with a 171 m/s delta-V trajectory correction maneuver (TCM) near aphelion. This delta-V will set up the Earth swingby that will pump the orbit up to the 2.5-year loop, which the spacecraft will fly twice. At 160 days before encounter, a small delta-V of 66 m/s will set up the Wild 2 flyby. This will occur on 1 Jan 2004, at 1.86 AU and 97.5 days past Wild 2 perihelion passage. The spacecraft will approach the comet at 6.2 km/s from sunside with a 70º phase angle. Coma fly-through will be on the sun side at a planned miss distance of 150 km. Flyby is five years after launch, and Earth return, two years later.

Altogether, three orbits will be made around the sun to minimize the delta-V requirements for the mission so that a Med-Lite version of the Delta II launch vehicle can be utilized. Also, the three orbits will maximize the time for favorable collection of interstellar dust.

 

Launch Ascent Profile

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The STARDUST spacecraft will be launched aboard a Med-Lite version of the Delta II launch vehicle. At about 10 minutes after launch the spacecraft will reach an altitude of 100 nautical miles and be 100 nautical miles down range.

Upper Stage Launch Ascent Profile

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At about 21 minutes after launch the second stage will restart. Following separation at about 24 minutes into the flight the third stage will ignite and burn for about 2 minutes after which the third stage will separate and the spacecraft will commence its cruise for the first Earth swingby.

 

 

Trajectory

STARDUST's seven-year, three-loop, VEGA (Earth gravity assist) trajectory is designed (1) to fly by Wild 2 at a low velocity while it is active, (2) to maximize the time for favorable collection of interstellar dust, and (3) to minimize the C3 (escape energy from Earth) and V requirements for the mission so that a smaller launch vehicle may be used. Figure 5 shows the spacecraft trajectory and the location of Earth, Wild 2, and the deep space maneuvers on orbit.

   * Full Screen View of the Complete Flight Plan

 

Trajectory Loop 1 

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Trajectory Loop 2

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Trajectory Loop 3

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Current Location of Wild 2 and STARDUST



 

Encounter with Comet Wild 2

 

Encounter Timeline

The encounter of the STARDUST spacecraft with Wild 2 spans a period 100 days before to 150 days after the actual fly-by of the comet. This is divided into five time periods during which various mission activities are planned.

 

Encounter Requirements

    Science


    Navigation

 
    Spacecraft
 

 

Navigation Plan

 

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Encounter Geometry

 encountr.gif The spacecraft will encounter Wild 2 at 97.5 days past perihelion at 1.86 AU from the Sun when Wild 2 is far from its peak active period and relatively safe for a close flyby. The spacecraft will approach Wild 2 from above its orbital plane, then dip slightly below it. Figure 6 shows the geometry of the flyby, which will be at 150 km on the sun side.

Orbital Geometry at Closest Encounter, where RS is the orbital plane, R=radial direction, Sun to Wild 2, S=orthogonal to R along Wild 2 velocity direction.

 

 

Encounter Phase Sequence of Events


 

 

Far Encounter

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  Far Encounter Details (E-100 to E-1 days) Far Encounter Subphase (E-100 d to E-1 d)
OPNAV will begin at about E-150 d when Wild 2 becomes detectable. The coma will be the focus of the imaging science during this period. Coma images acquired during this period will have resolutions of 32 to 6000 km per pixel. All eight filters will be used at each imaging episode and will be sent back at designated OPNAV telemetry time. Approximately thirty 4-hr passes of downlink time will be available during this period. At 1 kbps (50% link capability, 34-m dish), a data volume amounting to 75 frames of 2:1 compressed images may be sent back. More can be accomplished by combining the onboard "windowing" process. This, in essence, offers an opportunity to obtain full color movies of the evolving coma. At E-1 d, the coma image begins to fill the FOV of the camera, and the focus of the imaging will be on the finer details.
 

 

Near Encounter

 
nearenc.gif Near Encounter Subphase (E-1 d to E-5 hr)
STARDUST enters the terminal navigation phase with increased OPNAV activities. Continuous communication with Earth (70-m stations) will be established. At E-1 d the OPNAV picture rate will be increased to one per hour. All data acquired since the previous TCM (E-2 d) will be processed on the ground as each image is received for image location extraction, orbit determination, and the final TCM computation. We expect to obtain finer details of the coma when we image Wild 2 during this period. The Wild 2 nucleus will still be a pinpoint until the end of this phase when it begins to occupy about a pixel. Assuming a 50% link capability of the spacecraft, a real-time data volume transmission of 34 image frames with 2:1 compression is possible. Full-color images of Wild 2 with resolutions ranging from 5 to 32 km per pixel will be obtained during this period.

 

Close Encounter

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Close Encounter Details Close Encounter Subphase (E-5 hr to E+5 hr)
This is the core science period of the mission. At E-5 hr the spacecraft will begin to enter the coma (100,000 km from Wild 2) and the nucleus will start to emerge as an extended body in the camera FOV. All comet science will be on. Continuous tracking of the spacecraft with the 70-m station is planned until the end of this mission subphase. Dust collection will begin with the deployment of the dust collector after the last TCM at E-6 hr. The spacecraft dust shield and the collector array will orient perpendicular to the dust stream (spacecraft-comet relative velocity) to protect the spacecraft from the dust hazard while maximizing the collection area. CIDA will provide information on comet particle composition during the fly-through. Data from up to 10,000 CIDA events will be compressed and stored on board. The data volume allocated is about 200 Mbits. Continuous imaging and real-time transmission of data will be made from E-5 hr to E-4 min and again from E+4 min to E+5 hr. At E-4 min when the nucleus occupies 60x60 pixels, a final black and white picture surrounding the nucleus will be sent in real time. This will take no longer than 27 s. Any images taken after E-4 min will be stored on board. Figure 8 shows details of mission activities occurring from E-5 min to E+5 min. Due to the uncertainty in delivery, the image of the nucleus may spill out of the FOV of the camera beginning at about E-2 min. Although the scanning mirror can compensate for down-track and in-plane errors, only banking the spacecraft (by providing the second axis to the mirror) can correct out-of-plane errors. Because of this, temporary loss of high-gain lock to Earth during the ±3 min of the encounter is expected. The medium gain antenna will take over the critical communications function during this time.

Figure 8. Timeline During Closest Encounter, E-4 min to E+4 min

 

Closest Encounter

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Post Encounter

postent.gif Post-Encounter Subphase (E+5 hr to E+50 d)
Post-encounter spacecraft health check, reconstruction of flyby conditions and downlink of recorded data will constitute the activities of this mission phase. DSN tracking similar to cruise-phase mode will resume.


 


 

Data Collection

 

Coma and Nucleus Imaging

 The raison d'être for STARDUST is its ability to provide unique new knowledge of comets and IS particles. The mission will also provide excellent imaging, however, both optical navigation in support of the dust collection and for the study of nucleus morphology. The proposed camera will have the ability to investigate the large scale distribution of dust and associated gases in the general coma as well as in jets. It also will permit observation of the areas on the nucleus which are the source of the dust. There are advantages in studying a comet somewhat less active than Halley in that the spacecraft can fly closer to the nucleus and not be so readily overwhelmed optically or physically by the outflowing dust. The imaging results also will determine the size, shape, and albedo of the nucleus, and quite possibly its rotation. Close to the nucleus, STARDUST will provide detailed nucleus morphology at 10 times better resolution than the Giotto pictures of Halley.
 
It is proposed that the Voyger filter wheel with 8 positions carry three gas filters for CN, C2, and 6300 O (to monitor water distribution), three moderately wide dust filters, a polarizing filter, and a clear filter. The gas filters will permit study of the dust jets as possible sources of gas as well as a comparison of the sources of gas and dust on the nucleus surface. The three dust filters and the polarizing filter will allow study of the color and scattering properties of the dust. The clear filter will be used for the study of general nucleus morphology and albedo. A source will be provided for calibration of the imaging system to permit absolute photometry, with calibration images at least every hour before and after closest approach.

A six kilometer nucleus would fill one pixel at 100,000 km, about 4.6 hours before closest approach. The nucleus may be even larger. Information relative to size, shape, and jet location should be determined well before closest approach. In sum, the slow flyby speed and close nucleus approach distance of STARDUST provide superior imaging, an order of magnitude better in resolution and an order of magnitude greater in number of images than any prior cometary mission, without in any way compromising the primary goal of collecting cometary samples and interstellar dust.

 

Dust Collection - Spacecraft Encounter Configuration

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Comet Dust Collection

 The comet samples will be collected during a 6.1 km/s flyby of Comet Wild 2. At this extraordinarily low flyby speed, coma dust in the 1 to 100 micron size range will be captured by impact into ultra-low density aerogel and similar microporous materials. Particle collection at this speed has been extensively demonstrated in laboratory simulations and Shuttle flights [Tsou 1993] and we have shown that the comet dust collection can be done with acceptable levels of sample alteration.

Spacecraft Configuration for Comet Particle Collection

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Interstellar Dust Collection

Interstellar Grain Impact Profile
Based on recent studies [3], IGs are assumed to enter the heliosphere with a velocity of 30 km/s from the upstream direction of 7.7°±5°, 259°±15° ecliptic latitude and longitude. The flight paths of the IGs are modified by solar gravity, solar pressure, electromagnetic interaction with the interplanetary magnetic field, and various other complex processes not well or easily formulated. If one considers only the simple effects of solar gravity and solar pressure, the velocities of IGs of various sizes can be calculated as a function of , where is the ratio of solar pressure to solar gravity.

Interstellar Grain Collection Strategy
The strategy of IG collection is (1) to collect at the part of the spacecraft orbit where IG impact velocity is relatively low (<25 km/s), (2) to orient the collector in a specific direction so that the area for the desired IGs is maximized and the IG tracks indicating normal incidence may be tagged as the desired particle, and (3) to avoid pointing toward the sun in order not to intercept particles of interplanetary origin. Total duration of IG collection will be about two years.

Mission operation during the IG collection period is similar to cruise phase due the passive nature of the collector design. Although the IG collector will need to be steered in specific directions to maximize the area for intercepting desired IGs, tight attitude control is not required because the uncertainty in the IG radiant direction may be as large as 30°.

The exploratory aspect of the STARDUST mission is the collection of contemporary IS grains. During selected portions of its cruise phase, STARDUST will use the opposite sides of its comet dust collection modules to collect fresh interstellar grains.

Spacecraft Configuration for Interstellar Particle Collection

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Cometary Volatiles Measurements

Although the dust/volatiles ratio varies greatly from comet to comet, the volatile material is a significant fraction of the mass of every comet nucleus. Because the volatile and refractory components of comets may have condensed in very different locations and environments, complete knowledge of the composition of a comet requires study of both phases. The objectives of the volatile collection experiment are to determine the elemental and isotopic compositions of cometary volatiles. Of special interest are the biogenic elements (C,H,N,O,P and S) and their molecules. Some molecular bonds in large molecules can remain unbroken in a 6 km/s impact, as shown by laboratory experiment. At the very least, the obtainable information on gaseous components will be elemental and isotopic. In addition, the time-of-flight mass spectrometer carried on STARDUST will provide direct measurements of volatile species in the impacting dust samples and is expected to obtain much more information on complex molecules than for the Halley flybys because impacts with coma particles are less than 100 times as energetic. The instrument on board the STARDUST spacecraft that will analyze the dust is the Cometary and Interstellar Dust Analyzer (CIDA).


 

Earth Return

 

Earth Return Sequence of Events

Sample Earth Return Phase Design
This phase of the STARDUST mission begins two weeks before Earth re-entry and ends when the SRC is transferred to its ground-handling team. The planned landing site is the Utah Test and Training Range (UTTR) as shown in Figure 9. There are two optional landing zones accessible by targeting the approach trajectory for either a posigrade position with respect to Earth or retrograde. Figure 9. UTTR Footprints Following touchdown, the SRC will be recovered by helicopter or ground vehicles and transported to a staging area at UTTR for retrieval of the sample canister. The canister will then be transported to the planetary materials curatorial facility at Johnson Space Center. The Earth Return is divided into four subphases:

 

Earth Return Timeline

 Time           Event Description 
E - 88 days     Begin semi-weekly DSN Nav pass of 4 hrs
E - 74 days     Begin daily DSN Nav pass of 4 hrs
E - 67 days     Navigation cutoff for TCM at E-60d
E - 60 days     TCM to perform preliminary entry targeting
E - 46 days     Begin semi-weekly DSN Nav pass of 4 hrs
E - 27 days     Begin daily DSN Nav pass of 4 hrs
E - 17 days     Navigation cutoff for TCM at E-13d
E - 14 days     Begin daily DSN Nav pass of 16 hrs
E - 13 days     TCM to refine entry targeting
E - 2 days      Navigation cutoff for TCM at E-1d
E - 1 day       Continue DSN Nav tracking
E - 1 day       TCM to perform final entry targeting
E - 12 hrs      Navigation provide dispersed entry conditions
E - 5 hrs       Go/No-go Decision for SRC release
E - 4 hrs       SRC spin release (spin rate of 14 to 20 rpm)
E - 3 hrs       Spacecraft divert maneuver (prevent spacecraft entry)

 

Earth Approach

Earth Approach Subphase
Earth Approach begins with an increased tracking frequency of one 8-hour pass per day. During this period three TCMs are involved: at ER (Earth reentry) -13 d, ER-3 d and ER-3 hr. The SRC will be released soon after the last TCM and will enter the atmosphere at a nominal entry angle of -8°. The approach velocity to Earth will be approximately 6.4 km/s with a right ascension of 205.7°, a declination of 11.1°, and velocity at entry (assumed to be at an altitude of 125 km) of 12.8 km/s. The entry corridor control accuracy (3 ) attainable, based on the Navigation Plan, is 0.08°.

The spacecraft will perform a divert maneuver subsequent to the SRC release to avoid entering the atmosphere.

 

Entry

Entry Subphase
Entry begins when the spacecraft reorients for SRC release from the spacecraft bus and ends with parachute deployment. The SRC will be released from the spacecraft bus approximately 3 hours before entry. Significant activities during these 3 hours include slewing the spacecraft bus to the proper release attitude, settling and verifying spacecraft attitude, initiating the SRC on-board timer/sequencer, turning off spacecraft-bus-provided heater power to the SRC, and releasing the SRC.

The SRC will perform a direct entry at Earth. After entry the SRC will continue to free-fall until approximately 3 km, at which point the parachute deployment sequence will initiate. Elapsed time from entry to parachute deploy will be approximately 10 minutes.
 

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Terminal Descent - SRC with Parachute

Terminal Descent Subphase
Descent begins when the parachute deployment sequence initiates and continues until the SRC/parachute system has descended into the recovery zone, the UTTR.

The velocity of the SRC must be reduced from the initial entry velocity of 12.8 km/s to a level that permits soft landing.

The aeroshell removes over 99% of the initial kinetic energy of the vehicle to protect the sample canister against the resultant extreme aerodynamic heating. The heatshield is a 60° half-angle blunt cone made of a graphite/epoxy composite covered with a thermal protection system. Ablative material on the backshell protects the lander from the effects of recirculation flow around the entry vehicle.

Taking into account SRC release and entry corridor uncertainties, vehicle aerodynamics uncertainties and atmospheric dispersions, the landing footprint ellipse for the SRC has been determined to be approximately 30 km by 84 km. The SRC will approach the UTTR on a heading of approximately 122° on a north-west to south-east trajectory. Local time of landing will be approximately 3:00 am.
 

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Landing Site

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Ground Recovery

Recovery Subphase
Recovery begins a few hours before the SRC touches down. Retrieval is via ground transportation or helicopter. Given the small size and mass of the SRC, it is not expected that its recovery and transportation will require extraordinary handling measures or hardware other than a specialized handling fixture to cradle the capsule during transport. Transportation of the SRC to a staging area at the UTTR for extraction of the sample canister will follow. The sample canister then will be transported to its final destination, the planetary material curatorial facility at Johnson Space Center.

 
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Drop Test

In order to test the design and integrity of the Sample Return Capsule a drop test was performed to simulate the expected impact upon its arrival by parachute back on Earth.
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Appendix

Background Papers

* STARDUST: Discovery's InterStellar Dust and Cometary Sample Return Mission, by Kenneth L. Atkins

 

Acronyms


 

Beginning Contents

 


STARDUST Webmaster Tom Meyer, meyert@colorado.edu
Last Update Wednesday, 26-Nov-2003 12:46:43 PST