Comets, Asteroids, and Sun

Overview

The planets are large and interesting, but over 99% of the solar system's mass lies elsewhere. The sun is the most important body that affects our lives, yet there are still many mysteries about it. One of the missions that has been launched to study the sun is NASA's Genesis probe, which collected particles from the Sun and returned to Earth in 2004.

The sun is one star. The planets number at eight (nine if Pluto is included). However, there are hundreds of thousands of known asteroids, and the number of actual ones is probably in the millions. They are believed to represent original material from the solar system's birth. These important pieces of rubble will be explored by Japan's ISAS mission Muses-C.

Besides asteroids, comets also abound in the solar system. NASA had a probe, CONTOUR, that was going to study two or three comets in close proximity, but the probe was destroyed during a maneuver to head towards the sun from Earth orbit. Now, the probe Stardust has the objective of encountering a comet, collecting material, and returning it to Earth for study. The ESA's craft Rosetta is also now en route to study a comet and plant a lander on its surface. And NASA smashed a probe into a comet with the Deep Impact mission in July 2005.

Genesis

Overview

The basic mission profile of the Genesis project is to collect samples of the sun's solar wind - the charged particles that stream from the sun - and to return them to Earth for study. A project like this has never been done, and scientists hope to use the solar material to learn more about stars, star formation, and solar system formation. Between 10 and 20 micrograms (3-5 millionths of a pound) are expected to be returned.

Genesis was launched on August 8, 2001. About an hour after launch, the craft started to journey for three months to a point approximately 0.01 A.U. from Earth (L1). This point is beyond any significant influences of Earth's magnetic field and the particles that it traps. It reached this point on November 16, 2001, and deployed its collector arrays on December 3, 2001.

The craft continued to collect solar wind samples until April of 2004, when it began the journey back to Earth. After five months travel, the sample return canister containing the solar wind matter was sealed in a contamination-proof container. It was dropped and supposed to parachute down over the Utah desert where it was to be caught in mid-air by an airplane, but the parachutes never opened and the canister crashed in the desert.

Even though much of the samples were contaminated from the crash, NASA scientists were able to recover several uncontaminated isolated samples and ship them off to scientists for study. Genesis was the fifth launch in NASA's Discovery program. The total cost of the project was $164 million for spacecraft development and science instruments and $45 million for operations and science data analysis.

Craft Data

CONTOUR: COmet Nucleus TOUR

This craft has been destroyed. On August 15, 2002, it was supposed to fire its rocket to move out of Earth orbit and towards the sun. However, since the firing, no contact has been made with the craft. Telescope surveys show three main small objects near where CONTOUR was supposed to end up, and these are presumed to be the pieces of the craft (circled in green in the picture at right). The reason for the breakup was found to be most likely caused by overheating of the craft by the solid rocket motor's exhaust plume. The mission was given up for loss on December 20, 2002.

The CONTOUR mission was supposed to study the two comets Encke and Schwassmann-Wachmann-3 in close proximity. Also planned was the possibility to study comet d'Arrest, although it was hoped that a newly found comet between 2006 and 2008 would be able to be studied instead. The goals were to take optical pictures at a resolution of 4 m (13 feet), and spectral images at a resolution of 100-200 m (330-660 feet) of the comets' nuclei. Also, readings of the composition of gas and dust in the near-nucleus region were going to be taken.

CONTOUR was launched on July 3, 2002 from Cape Canaveral Air Force Station. After several maneuvering orbits, the rocket fired on August 15, 2002 to put it into a more heliocentric orbit. Planned was for this to put it on a trajectory that would rendezvous with comet Encke on November 12, 2003, at a distance of 100-160 km (62-100 miles). June 18, 2006 was to be the date for the flyby of Schwassmann-Wachmann-3, and August 16, 2008 of d'Arrest. All of the encounters would have been near the peak of the activity cycle for the comet.

Stardust

Overview

Stardust is a comet sample return mission. Its goal is to fly by the comet P/Wild 2 (pronounced "Vilt") and to collect samples of dust and gas from the comet's coma and return them to Earth for study. Mission designers hope to collect at least 1000 analyzable particles larger than 15 µm in diameter.

The secondary goals are to collect at least 100 interstellar particles larger than 0.1 µm in diameter, obtain 65 images of the P/Wild 2 nucleus at resolutions of at least 67 µrad/px and images of the comet's coma, as well as perform compositional analysis of the comet particles within the coma.

The tertiary objectives include performing compositional analysis of interstellar grains, interplanetary dust and other cosmic particles, collect Wild 2 coma gas, determine Wild 2 coma dust flux and size distribution, measure integrated dust fluence, measure large particle momentum, estimate an upper limit of the comet's mass, and obtain dust flux profiles through Wild 2's coma.

Once the samples are returned to Earth, the samples will undergo detailed analyses of the elemental, isotopic, mineralogical, chemical, and any biogenic properties.

In addition to the comet mission, Stardust also flew by and imaged the asteroid 5535 Annefrank.

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

Mission

After a one day delay Stardust was launched on February 7, 1999, at 21:04:15.238. The launch took place from Pad A, Launch Complex 17 at Cape Canaveral Air Station aboard a Delta 7426. 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 re-ignition of the 2^nd stage for 2 minutes. The 2^nd stage separated at 24:27 MET and at 25:04 MET the 3^rd stage ignited and burned for about 2 minutes. Stardust separated from the 3^rd 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 February 22 through May 1, 2000. After one solar orbit, an Earth flyby on January 15, 2001 at 6008 km altitude was used to boost the spacecraft aphelion to 2.7 A.U. and the inclination to 3.6°. Another period of interstellar dust collection opened July to December 2002. On November 2, 2002 at 04:50 UT, Stardust flew within 3300 km of asteroid 5535 Annefrank at a 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 and the comet P/Wild 2 encounter took place on January 2, 2004. The fly-by had a closest approach of approximately 300 km at 19:40:35 UT at a relative velocity of about 6.1 km/s and will take place 1.85 A.U. from the sun and 2.6 A.U. from Earth. The sample collector was deployed on December 24, 2003, and it was retracted, stowed, and sealed in the sample vault of the sample reentry capsule after the fly-by. Images of the comet nucleus were also obtained, with predicted coverage of the entire sunlit side at a resolution of 30 m or better.

On January 15, 2006, the capsule will separate from the main craft and return to Earth. A parachute will be deployed and the descending capsule will be recovered by a chase aircraft over the U.S. Air Force Test and Training Range in the Utah desert at roughly 3 A.M. local time.

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) will be used for collection of cometary samples and the other (B) 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. Each particle is expected to be less than a micron in size. The number of particles should be small and the impacts will leave tracks in the aerogel, so no confusion of samples collected on the A and B sides is expected. After all collections are complete, the aerogel will be sealed in the sample vault of the sample reentry capsule, and the samples will be 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.

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 A.U.), its orbit was altered by a close pass by Jupiter on September 10, 1974. It now orbits between Mars and Jupiter (1.58 to 5.2 A.U.). The comet is approximately 5.4 km across.

Craft

The Stardust spacecraft consists of a box-shaped main bus 1.7 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 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 front 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 another communication antenna, an optical navigation camera, 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, and the expected data rate at time of encounter is 7.9 kbits/sec using the 70 m Deep Space Network antennas on Earth.

Craft Data

Muses-C; Renamed Hayabusa (Falcon)

An ISAS mission, the basic goal of Muses-C is to land on an asteroid, collect samples, and return them to Earth for study.

The craft had an original launch date of December of 2002, but that was changed to May 9, 2003, due to a faulty O-ring in the attitude control system. It was launched that day at 04:29:25 UT (1:29 P.M. local time, 12:29 A.M. EDT) on an M-5 solid fuel booster from the Kagoshima launch center. Following launch, the name Muses-C was changed to Hayabusa (the Japanese word for falcon) and the spacecraft was put into a transfer orbit to bring it to asteroid 25143 Itokawa (1998 SF36), a 0.3x0.7 km near-Earth object.

A large solar flare in late 2003 degraded the solar panels. The loss of power available to Hayabusa's ion engines forced the originally planned early summer 2005 rendezvous with Itokawa to be moved back to September. Hayabusa flew by Earth on May 19, 2004, at an altitude of 3725 km at 6:23 UT. On July 31, 2004, the X-axis reaction wheel failed.

Rendezvous with the asteroid occurred in September 2005 with the spacecraft coming to rest relative to the asteroid at a distance of 20 km at 1:17 UT on September 12. Note that the spacecraft did not go into orbit around the asteroid, but remained in a station-keeping heliocentric orbit close by. On October 3, 2005, Hayabusa lost the use of the Y-axis reaction wheel and was using one reaction wheel and two chemical thrusters to maintain attitude control.

Hayabusa initially surveyed the asteroid's surface from a distance of about 20 km in the "home position", a region roughly on a line connecting the Earth with the asteroid on the sunward side. This was global mapping phase 1, the phase angle during this phase was small, no greater than 20°-25°. Global mapping phase 2, which lasted about a week, began on October 4, 2005, when the spacecraft reached a position near the terminator at a distance of 7 km, affording high phase angle views of the asteroid. Following this, the spacecraft moved back to the home position and then moved close to the surface in November for a "rehearsal" touchdown. This touchdown was attempted on November 4, but it was aborted due to an anomalous signal at 700 meters above the asteroid's surface.

On November 12, 2005, a second rehearsal touchdown was attempted. The spacecraft began its descent from 1.4 km altitude at 3 cm/sec to an altitude of 55 meters. The small lander/hopper, Minerva, was deployed at 6:34 UT, but unfortunately Hayabusa had already reached the 55 meter level and had begun an automatic ascent so the release was at a higher altitude than planned. Contact was lost and it is believed Minerva floated off into space without landing.

At 12:00 UT on November 19, Hayabusa began its descent towards the asteroid from an altitude of 1 km. At 19:33 UT, the final approach was commanded and the descent began from an altitude of about 450 meters at 12 cm/sec. The target marker was released at 20:30 UT about 40 meters above the asteroid and Hayabusa's descent was slowed to 3 cm/sec to allow the marker to fall ahead of it. The spacecraft reduced its speed to zero and then began free-fall at an altitude of 17 meters at which point contact was lost. Later telemetry indicated that Hayabusa hit the surface at 20:40 UT at roughly 10 cm/sec and bounced. It bounced again at 21:10 and then landed at 21:30 within about 30 meters of the target marker. At 21:58, it was commanded to make an emergency ascent. The craft remained on the surface for about half an hour but did not collect a sample. This was the first ever controlled landing on an asteroid and first ascent from any other solar system body except the Moon.

A second touchdown and sampling run was made on November 24; early telemetry indicates the spacecraft touched down at 10 cm/sec and two sampling bullets were fired 0.2 seconds apart at 22:07 UT November 24, but examination of later telemetry indicates it is not clear that the bullets were fired. More details will be available when further telemetry is returned and analyzed. On December 9, 2005, contact was lost with the spacecraft, presumably because of torques caused by a thruster leak which altered the pointing of the antenna. As of December 14, attempts to restore communication are ongoing, but may take several months. It has been decided to reschedule the Earth return to June of 2010.

The samples, with a total mass of less than one gram, are held inside a separate re-entry capsule. In early December the spacecraft was scheduled to fire its engines to begin its cruise back to Earth, but problems with the thrusters and communications have resulted in a new plan which will use the ion engines to bring the spacecraft back to Earth in 2010. The re-entry capsule will be detached from the main spacecraft at a distance of about 300,000 to 400,000 km from the Earth, and the capsule will coast on a ballistic trajectory, re-entering the Earth's atmosphere in June 2010. The capsule will experience peak decelerations of about 25 G and heating rates approximately 30 times those experienced by the Apollo spacecraft. It will land via parachute near Woomera, Australia. (This scenario is a change from the original plan to launch in July 2002 to the asteroid Nereus.)

The probe features a cutting-edge ion propulsion thruster that saves propellant, as well as a high-technology autonomous control system that will govern most of the final approach during the sample collection runs due to the communication lags between 1998 SF36 and Earth. Initially, a rover vehicle was to be included, sponsored by NASA, but it was cancelled due to budget cuts.

Rosetta

This probe was designed to rendezvous with comet 46 P/Wirtanen and to perform remote sensing investigations, as well as to land a probe on the nucleus to perform detailed measurements. However, due to problems with the launch program, the probe's launch has been postponed for at least a year, and so has missed its opportunity to visit this comet. Also planned had been flybys of asteroids 4979 Otawara and 140 Siwa en route to the comet.

The launch date was planned to be January 12, 2003; it didn't launch until 07:17 GMT on March 2, 2004, aboard a European Ariane 5 launch vehicle launched from the Guiana Space Centre in Kourou, French Guiana. The craft separated about 2.25 hours later. The cost of the delay is estimated at 70 million Euros.

Rosetta will go through a series of planetary flybys to modify its trajectory towards its target. The first flyby was on March 4, 2005, when it passed by Earth at an altitude of 1954.74 km with a velocity of 38,000 km/hr; two years later in February 2007, Rosetta will flyby Mars, and then it will head back to Earth for a pass in November 2007. A third flyby of Earth in November 2009 will send Rosetta towards the comet Churyumov-Gerasimenko, the craft's primary target. By mid-2011 at a distance of 800 million km from the sun, Rosetta will use its engine to maneuver it to an interception trajectory with the comet. Finally, around mid-2014, the craft will encounter the comet, far from the sun and hopefully inactive.

Besides the planetary flybys, two asteroid flybys are planned for Rosetta. The targets are Steins and Lutetia. The former is small and will be visited on September 5, 2008 at a distance of 1700 km at a speed of 9 km/s. The latter is much larger - about 100 km in diameter - and Rosetta will pass it on July 10, 2010 at a speed of 15 km/s at a minimum distance of about 3000 km.

In August 2014, Rosetta will orbit the nucleus, conducting detailed mapping and select a landing site for Philae, the 100 kg lander. Philae will be dropped from an altitude of about 1 km onto the nucleus, after which the lander will have to anchor itself to the nucleus with two harpoons to avoid bouncing back. The lander should operate for several weeks, relaying data to Earth through the orbiter. Rosetta should continue its observations of the nucleus through December 2015, when the comet starts to melt due to its closer proximity to the sun.

Deep Impact

This program was designed to have a craft rendezvous with comet 9P/Tempel 1 and launch a projectile into the comet nucleus. Observations were then made of the matter that was ejected (much of which will represent pristine material from the interior of the comet), the crater formation process, resulting crater, and any outgassing from the nucleus, particularly from the newly exposed surface.

Deep Impact launched on January 12, 2005, at 18:47:08.574 UT (1:47:08 P.M. EST) on a Delta II rocket. The spacecraft transferred into a heliocentric orbit and rendezvoused with comet P/Tempel 1 in July 2005. Deep Impact was about 880,000 km from the comet on July 3, 2005, moving at a velocity of 10.2 km/s relative to the comet.

The projectile was released at this point and shortly after release the flyby spacecraft executed a maneuver to slow down relative to the impactor by 120 m/s and divert by 6 m/s. On July 4, the impactor struck the sunlit side of the comet nucleus 24 hours after release, at roughly 6:00-6:30 UT. At 10.2 km/s velocity, the impactor had an impact energy of about 19 gigajoules, and it should have formed a crater roughly 25 meters deep and 100 meters wide, although this estimate is based on models of comet structure and is subject to large uncertainty. Material from the nucleus was ejected into space and the impactor and much of the ejecta was vaporized.

The flyby spacecraft was approximately 10,000 km away at the time of impact and began imaging 60 seconds before impact. At 600 seconds after impact the spacecraft was about 4000 km from the nucleus and observations of the crater began and continued up to a range of about 700 km, about 50 seconds before closest approach. At this point (about 961 seconds after impact) imaging ended as the spacecraft reoriented itself by 45° to optimize protection from dust damage as it flew by the nucleus. Closest approach to the nucleus was at a distance of about 500 km.

At 1270 seconds, the crossing of the inner coma was complete and the spacecraft oriented itself to look back at the comet and begin imaging again. At 3000 seconds the spacecraft began playback of data to Earth at 20 to 200 kilobits per second. The comet and craft will be about 0.89 A.U. from Earth and 1.5 A.U. from the Sun during the encounter. Real time return of selected impactor images and flyby images and spectra were returned to Earth during the encounter. Primary data return was over the first day after encounter, with a 28 day supplemental data return period. Earth-based observatories also studied the impact. The spacecraft ranged over a distance of 0.93 to 1.56 A.U. from the Sun during the mission.

The mission ended in August of 2005. The total budget for the mission was $240 million.