Stardust

Description

Stardust has been chosen for an extended mission to fly by Comet Tempel 1 in 2011.

The primary objective of the Discovery class Stardust mission is to fly by the comet P/Wild 2 and collect samples (at least 1000 analyzable particles of diameter >15 microns) of dust and volatiles from the coma of the comet. It will then return these samples to Earth for detailed study. The secondary objectives are to collect and return to Earth at least 100 interstellar particles of diameter >0.1 micron, to obtain 65 images of the Wild 2 nucleus at resolutions of at least 67 microradians/pixel as well as images of the Wild 2 coma, and to perform in situ compositional analysis of cometary particles within the coma. The tertiary objectives are to perform in situ compositional analysis of interstellar grains, interplanetary dust and other cosmic particles, to collect Wild 2 coma volatiles, to determine Wild 2 coma dust flux and size distribution, to measure integrated dust fluence, large particle momentum and opportunistic estimate of the upper limit of the comet's mass, and to obtain dust flux profiles through Wild 2's coma. On Earth the cometary samples, representing primitive substances from the formation of the solar system, will undergo detailed analyses of the elemental, isotopic, mineralogical, chemical, and biogenic properties. The samples of interstellar dust, material from outside our solar system, will be carefully examined on Earth in regards to composition and size and velocity distribution. Stardust also flew by and imaged asteroid 5535 Annefrank.

Spacecraft and Subsystems

The Stardust spacecraft consists of a box-shaped main bus 1.6 m long, 0.66 m wide and 0.66 m deep with a high gain dish antenna attached to one face of the bus. The total mass of the spacecraft including the return capsule and 85 kg of propellant is 385 kg. The bus is made of flat panels fabricated with thin sheets of graphite fibers in polycyanate resin covering a lightweight aluminum honeycomb core. Two long (4.8 m tip-to-tip) rectangular solar arrays are connected by struts extending to the opposite sides of the spacecraft extending along the long axis of the spacecraft with their surfaces in the same plane as this face, extending parallel to each other in their long directions. The short cone-shaped 0.8 m diameter, 0.5 m high, 46 kg sample reentry capsule is attached at its narrow end to the back face of the bus. A paddle shaped sample collection disc can be extended from the capsule during periods of sampling, and stored inside the capsule enclosed by a cover when not in use. The propulsion units are on the rear face of the craft. A Whipple dust shield on the front of the craft protects the main core bus and is equipped with dust flux monitors, vibro-acoustic sensors which will be able to detect particle impacts on the shield. Two smaller shields protect the solar arrays.

The spacecraft is also equipped with an optical navigation camera, a dust flux monitor, and a dust spectrometer/particle impact analyzer. There are no scan platforms, all science instruments are body-mounted. Propulsion is provided by a monopropellant hydrazine system. Attitude control is maintained by eight 4.4 N thrusters and eight 0.9 N thrusters, all mounted on the bottom of the spacecraft away from the sample collector to avoid contamination. Three axis attitude knowledge is given by a star camera and gyroscopic inertial measurement unit. Power is supplied to the craft by the silicon solar arrays which provide from 170 to 800 W depending on distance from the Sun. At the Wild-2 encounter about 330 W will be generated. Telecommunications are via X-band through a low gain antenna, medium gain horn, and 0.6 m diameter high gain dish. The system power is 15 W, the expected data rate at time of encounter is 7.9 kbits/sec using the 70 m Deep Space Network antennas on Earth.

Sample Collection

Sample collection was achieved with the use of aerogel, a low-density (0.02 gm/cc) inert microporous silica-based substance which will allow capture of high-relative-speed particles with minimal physical and chemical alteration. The aerogel is in the form of a single disc-shaped sheet held by modular aluminum cells and deployed on a paddle. The aerogel is simply exposed to space during sample collection periods and stowed in the sample vault at other times. One side of the aerogel (the A side, 3 cm thick) will be used for collection of cometary samples and the other (B, 1 cm thick) side for interstellar dust. The appropriate side will be oriented toward the expected particle flux and particles striking the aerogel will be slowed down and trapped within. The number of particles should be small and the impacts will leave tracks in the aerogel. After all collections were completed, the aerogel was sealed in the sample vault of the sample reentry capsule, and the samples were recovered on Earth for study. The sample reentry capsule also contains an aeroshield/basecover, navigation recovery aids (GPS translator and emergency S-band beacon), an event sequencer, and a parachute system.

Mission Profile

After a one day delay Stardust was launched on 7 February at 21:04:15.238 (4:04 p.m. EST). The launch took place from Pad A, Launch Complex 17 at Cape Canaveral Air Station aboard a Delta 7426 (a Delta II Lite launch vehicle with four strap-on solid-rocket boosters and a Star 37FM third stage). The four boosters fell away one minute and 6 seconds after liftoff (1:06 MET, mission elapsed time), the first stage shut down and was ejected at 4:30 MET. The second stage ignited a few seconds later, burned until 9:55 MET, followed by an ~11 minute coast and a reignition of the 2nd stage for 2 minutes. The 2nd stage separated at 24:27 MET and at 25:04 MET the 3rd stage ignited and burned for about 2 minutes. Stardust separated from the 3rd stage at 27:19 MET and opened its solar arrays 4 minutes later. The spacecraft is now coasting in an elliptical heliocentric orbit.

The first interstellar dust collection took place from 22 February to 1 May 2000. After one solar orbit, an Earth flyby on 15 January 2001 at 6008 km altitude was used to boost the spacecraft aphelion to 2.7 AU and the inclination to 3.6 degrees. Another period of interstellar dust collection opened July to December 2002. On 2 November 2002 at 04:50 UT (1 Nov at 11:50 p.m. EST) Stardust flew within 3300 km of asteroid 5535 Annefrank. The relative velocity was 7 km/s. The dust collectors remained open throughout the flyby and images of the asteroid were taken. A second orbit of the sun was completed in mid-2003.

The spacecraft entered the coma of comet P/Wild 2 on 31 December 2003. Close encounter took place on 2 January 2004. The fly-by had a closest approach within 250 km at roughly 19:45 UT (2:45 p.m. EST) at a relative velocity of about 6.1 km/s and took place 1.85 AU from the Sun and 2.6 AU from Earth. The sample collector was deployed on 24 December 2003 and was retracted, stowed, and sealed in the sample vault of the sample reentry capsule 6 hours after the fly-by. 72 images of the comet nucleus were also obtained, with predicted coverage of the entire sunlit side at a resolution of 30 m or better. The first (bumper) layer of the Whipple shield was breached by particle impacts at least ten times during the flyby.

On 15 January 2006 the capsule separated from the main craft (with a stabilizing spin of 1.5 rpm) at 5:57 UT (12:57 a.m. EST) and entered the atmosphere four hours later at 9:57 UT (4:57 a.m. EST). An aeroshell slowed the capsule down initially for about ten minutes, the drogue parachute was deployed at 10:00 UT and the main parachute 5 minutes later at an altitude of roughly 3 km. The capsule landed at 10:10 UT (5:10 a.m. EST, 3:10 a.m. local Mountain Standard Time) within a 30 x 84 km landing ellipse at the U.S. Air Force Test and Training Range in the Utah desert. High winds caused the capsule to drift about 5 miles north of its entry ground track, but with the aid of a locator beacon the capsule was found at 10:54 UT and was later transported by helicopter to a clean room, arriving at about 13:00 UT. The main spacecraft was diverted so as not to reenter Earth's atmosphere and is now in orbit around the Sun. It was put into hibernation on 29 January with only its solar panels and receiver active to save it for possible future missions. The spacecraft will fly by Earth again on 14 January 2009.

Stardust has been funded for an extended mission to fly by Comet Tempel 1 on 14 February 2011. Tempel 1 was the target of the Deep Impact mission, which delivered an impactor into the surface of Tempel 1 on 4 July of 2005. Stardust will revisit the comet to look for signs of modification of the crater and extend the mapping of Tempel 1. Tempel 1 will be the first comet revisited and will have the most completely mapped nucleus.

Comet P/Wild 2

Comet P/Wild 2 is a newcomer to the inner solar system and therefore represents a relatively 'fresh' comet which has not been overly heated and degassed by the Sun. Originally in an orbit in the region between Jupiter and Uranus (4.9 to 25 AU), its orbit was altered by a close pass by Jupiter on 10 September 1974. It now orbits between Mars and Jupiter (1.58 to 5.2 AU). The comet is approximately 5.4 km across. Comets were formed at the same time as the solar system and are made up of primitive condensates and grains incorporated into them at this time. The samples from the coma are expected to provide insights into the composition and dynamics of the early solar system.The interstellar dust grains represent a flux of fresh material entering the solar system from the direction of the constellation Scorpio. These particles are smaller and will impact at a higher velocity than the cometary particles. The size distribution, velocity profile, and compositional make-up of these particles are important to the study of processes taking place outside our solar system.

The total mission cost of Stardust was approximately $199.6 million, of which roughly $128.4 million was the cost of development and construction of the spacecraft and $40 million was for mission operations.