STARDUST
Mission Description In Detail
STARDUST
Mission Description
In Detail
 |
| 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
- Launch February 1999
- Encounter January 2004
- Earth Return January 2006
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
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
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
Trajectory Loop 2
Trajectory Loop 3
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.
- Far Encounter: E-100 to E-1 days
- Near Encounter: E-1 d to E-5 hr
- Close Encounter: E-5 hr to E+5 hr
- Closest Encounter: E-240 sec to E+240 sec
- Post Encounter: E-0 d to E+150 days
Encounter Requirements
Science
- Collect Dust Particles
- Images
- E-100 Days to E-12 Hours
- 2 Sets of Images Through Each of the Narrow Band Filters
- One Set Long Exposure, Second Set Factor of Ten Shorter in Duration
- Close Encounter (Pre-Encounter) - As Many Image Sets as Practical for Real
Time Transmission Prior to Encounter
- Closest Encounter
- Transmit Real Time One Wide Band Filter Image as Near as Possible to Nucleus
- Image Size is 150x150 pixels
- 4 Color Images (Blue, Yellow, Red, Infrared) Just Before Roll
- 4 Color Images (Blue, Yellow, Red, Infrared) Just After Roll
- 48 Wideband Filter Images 5 Seconds apart Centered on Time of Closest Approach
- 4 Color Images (Blue, Yellow, Red, Infrared) Just Before Roll
- 4 Color Images (Blue, Yellow, Red, Infrared) Just After Roll
- Close Encounter (Post-Encounter) - As Many Images as Possible Until Memory
is filled
- CIDA and DFMI - Store Science Data
- Dynamics Science - Provide Engineering and Doppler Data
Navigation
- Far Encounter
- E-100 to E-50 Days
- Provide One Wideband Filter Image Weekly
- E-50 to E-7 Days
- Provide Two Wideband Filter Images Weekly
- E-7 to E-1 Day
- Provide One Wideband Filter Image Daily
- E-24 to E-12 Hours
- Provide One Wideband Filter Image Hourly
- Provide Sufficient Tracking Coverage to Support the Design of TCMs
Spacecraft
- Deploy the Aerogel Prior to Close Encounter
- Perform a Roll About the X-Axis to Maintain Nav Camera Field of View (Pre
and Post)
- Command the Mirror to Maintain the Comet Nucleus Within the Nav Camera
Field of View
Navigation Plan
Encounter Geometry
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
- L+1691 to L+1841 (E-100 to E+50 Days)
- Exiting Longest Solar Conjunction (~60 Days) at Start
- 4 TCMs
- Optical Navigation
- Deploy Aerogel Collector
- CIDA and DFMI Power During Entire Phase
- Encounter Training Excerise and Readiness Review
- Sub-Phases
- Far Encounter
- Near Encounter
- Close Encounter
- Closest Encounter
- Post Encounter
Far Encounter
Far Encounter Details (E-100 to E-1 days)
- Nominal Attitude +Z to Sun ±15° Deadband
- Image Attitude Deadband is ±0.5°
- Deploy Aerogel ~9 Days Before Encounter
- HGA Communications
- Downlink Allocation is 50% Images / 50% Other
- 25% for Stored Engineering Telemetry and CIDA/DFMI Science Data
- 25% for Real Time Engineering Telemetry
- 50% for Images
- Data Rate is 1975 BPS (DSN 34 Meter Antenna)
- At E-2 Days Use DSN 70 Meter Antenna (7900 BPS)
- MGA Communications
- 100 BPS (DSN 34 Meter Antenna)
- Real Time Engineering
- Stored Engineering Data When Possible
- Science Data
- Images
- 12 Bit/Pixel Images
- Windowed and Compressed to Fit Bandwidth
- CIDA and DFMI Available for Data Collection
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
- Navigation Images are 12 Bits/Pixel (Windowed and Compressed)
- Image Attitude Deadband is ±0.5°, Otherwise ±2° HGA
on Earth
- Continuous Communications Using HGA and DSN 70 Meter Antenna
- Telemetry Data Rate is 7900 BPS
- Close Encounter Block Loaded at E-24 Hours
- CIDA and DFMI Available for Data Collection
- Final Uplink Contains TCM and Updated Close Encounter Timing
- After TCM, ACS is in Encounter Mode (±0.3°, 1lb Thrusters Enabled)
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
Close Encounter Details
- Data Rate is 7900 BPS (DSN 70 Meter Antenna)
- Using 50% of Bandwidth for Science Images
- ~160 Minutes Available to Transmit Images Before Encounter
- Fixed Number of Science Image Bits That Can Be Downlinked
- 37.9 MBits Can Be Downlinked
- 67.1 MBits per Image Set (8 Images) (No Compression)
- Science
- Images
- Images are Hardware Compressed (8 Bits/Pixel)
- Flexible Number of Image Sets Prior to Centroiding
- All Images Prior to Centroiding Are Downlinked, Not Stored On-Board
- Post Closest Encounter Will Take Additional 6 Images to Fill Memory
- CIDA and DFMI Data is Stored and Downlinked as Bandwidth is Available
- Dust Collection Happens
- Centroiding Starts at E-20 Minutes
- Image Every 10 Seconds
- Flight S/W Calculates Projected Mirror Position and Commands Mirror
- Centroiding Ends at E+5 Minutes
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
- Images Are 8 Bits/Pixel
- Giotto Quality Image (1500 KM) Will be Windowed/Compressed in FSW
- Images Are Taken 5 Seconds Apart
- 64 Images Stored In Memory
- No Images Taken During Roll
- CIDA and DFMI Data is Stored
Post Encounter
- Stow Aerogel Collector
- Transmit Science Data
- Image Data
- 600 MBits of Image Data
- Approximately 21 Hours at 7950 BPS using HGA and 70m Antenna
- Approximately 84 Hours at 1975 BPS using HGA and 34m Antenna
- CIDA Data
- 100 MBits of Data
- Approximately 3.5 Hours at 7950 BPS using HGA and 70m Antenna
- Approximately 14 Hours at 1975 BPS using HGA and 34m Antenna
- DFMI Data
- 16 MBits of Data
- Approximately 34 Minutes at 7950 BPS using HGA and 70m Antenna
- Approximately 2.3 Hours at 1975 BPS using HGA and 34m Antenna
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
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
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
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 Approach
- Entry
- Terminal Descent
- Recovery
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.
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.
Landing Site
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.
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.
Appendix
Background Papers
STARDUST: Discovery's InterStellar Dust and Cometary Sample Return Mission, by Kenneth L. Atkins
STARDUST Webmaster Tom Meyer,
meyert@colorado.edu
Last Update
Wednesday, 26-Nov-2003 12:46:43 PST
