Hayabusa (Muses-C)

Description

The ion engines will be used until November 2007 to put Hayabusa on a ballistic trajectory towards the Earth. The return to Earth is scheduled for June 2010.

The primary scientific objective of the Hayabusa (formerly Muses-C) mission is to collect a surface sample of material from the small (550 x 180 meter) asteroid 25143 Itokawa (1998 SF36) and return the sample to Earth for analysis. It is also a technology demonstration mission. Other scientific objectives of the mission include detailed studies of the asteroid's shape, spin state, topography, color, composition, density, photometric and polarimetric properties, interior and history.

Mission Profile

The spacecraft was launched on 9 May 2003 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 0.3 x 0.7 km near-Earth object. The ion engines were successfully test-fired starting on 27 May to the middle of June, 2003. 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 19 May 2004 at an altitude of 3725 km at 6:23 UT. On 31 July the X-axis reaction wheel failed. Rendezvous with the asteroid occured in September 2005 with the spacecraft coming to rest relative to the asteroid at a distance of 20 km at 1:17 UT on 12 September. Note that the spacecraft will not go into orbit around the asteroid, but will remain in a station-keeping heliocentric orbit close by. On 3 October 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 is global mapping phase 1, the phase angle during this phase was small, no greater than 20 - 25 degrees. Global mapping phase 2, which lasted about a week, began on 4 October 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 4 November but was aborted due to an anomalous signal at 700 meters above the asteroid's surface.

On 12 November 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 (3:34 p.m. JST) 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 19 November (9:00 p.m. JST, 7:00 a.m. EST) Hayabusa began its descent towards the asteroid from an altitude of 1 km. At 19:33 UT (4:33 a.m. JST 20 November) 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 19 November (5:30 a.m. JST 20 November) 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 19 November (5:40 a.m. JST 20 November) 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 (6:58 a.m. JST 20 November) 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 sytem body except the Moon.

A second touchdown and sampling run was made on 25 November, 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 24 November (7:07 a.m. JST 25 November) but examination of later telemetry indicates it is not clear that the bullets were fired. On 9 December contact was lost with the spacecraft, presumably because of torques caused by a thruster leak which altered the pointing of the antenna. Communications with the spacecraft were regained in early March 2006. It appears that the chemical fuel has been lost due to the leak, two of three reaction wheels are also inoperable and 4 of the 11 lithium-ion battery cells are not functioning. Ground controllers are using the solar batteries to run the ion engine, which will be used in place of the chemical thrusters to maintain attitude control. The ion engine will run until November 2007, at which time it will be turned off and the spacecraft will go into hibernation mode and continue on a ballistic trajectory. There is still a large margin of xenon left to run the thrusters for propulsion and attitude control. Earth return is now scheduled for June of 2010.

The samples, if any were collected, will have a total mass of less than one gram. It is hoped that even if the projectiles did not fire that a small number of particles may have been kicked up into the collection area during the touchdown due to the extremely low surface gravity. They are held inside a separate re-entry capsule. 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 decellerations 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.)

Spacecraft and Subsystems

The Hayabusa spacecraft has a box-shaped main body 1.5 m along each side and 1.05 m high. The launch mass is 530 kg, including 50 kg of chemical propellant and 65 kg of xenon gas. Two solar panel wings with a total array area of 12 square meters protrude from the side and a 1.5 m diameter high-gain parabolic antenna is mounted on top on a two-axis gimbal. A cylindrical sampler horn, deployed shortly after launch, protrudes from the bottom of the spacecraft. The Minerva lander was also mounted on th spacecraft near the bottom panel. Hayabusa is propelled during cruise phases by two microwave ion thruster engines, which use a microwave discharge to ionize xenon gas. The ionized plasma is accelerated by high-voltage electrodes through four thruster heads which protrude from one side of the spacecraft body to provide a peak thrust of 20 mN using 1 kW power. A nitrogen tetroxide/hydrazine propulsion system with a peak thrust of 22 N will be used for maneuvering. The spacecraft is powered by gallium-arsenide solar cells and a 15 A-hr rechargeable nickel-metal hydride (Ni-MH) battery. Communications are via X- and S-band low gain antennas and the high gain dish antenna (X-band) with a transmitted power of 20 W. The mission is also be equipped with a camera, used for imaging, visible-polarimetry studies, and optical navigation near the asteroid, a laser ranging device (LIDAR), and near-IR and X-ray spectrometers. The insulated and cushioned re-entry capsule, 40 cm in diameter and 25 cm deep with a mass of about 20 kg, is attached to the body of the spacecraft near the sample collection horn. The capsule has a convex nose covered with a 3 cm thick ablative heat shield to protect the samples from the high velocity (~13 km/s) re-entry. Cost of the Hayabusa spacecraft is roughly 12 billion yen ($100 million U.S.)

Surface Sample Collection

The lander will be equipped with a universal sample collection device which will gather roughly one gram of surface samples taken from the landings at 3 different locations. The device consists of a funnel-shaped collection horn, 40 cm in diameter at the end, which is to be placed over the sampling area. A pyrotechnic device fires a 10 gram metal projectile down the barrel of the horn at 200 - 300 m/sec. The projectile strikes the surface producing a small impact crater in the surface of the asteroid and propelling ejecta fragments back up the horn, where some of it is funnelled into a sample collection chamber. Prior to each sampling run, the spacecraft will drop a small target plate onto the surface from about 30 m altitude to use as a landmark to ensure the relative horizontal velocity between the spacecraft and asteroid surface is zero during the sampling. After sampling the samples will be stored in the re-entry capsule for return to Earth.

Minerva

The Minerva lander is a small (591 gram) cylinder about the size of a coffee can, designed to be released from the spacecraft on the first rehearsal touchdown run. It has the ability to "hop" on the surface of the asteroid and has full autonomy. It is equipped with an imaging system comprising three miniature cameras and temperature measuring devices. Data will be relayed to Hayabusa and then to Earth.

SSV Rover

The rover, or Small Science Vehicle (SSV), was to have been a NASA contribution to the mission but was cancelled due to budget contraints. The SSV would have been dropped onto the surface of the asteroid by the Hayabusa spacecraft. The rover goals were to make texture, composition and morphology measurements of the surface layer at scales smaller than 1 cm, investigations of lateral heterogeneity at small scales, investigation of vertical regolith structure by taking advantage of disturbances of the surface layer by microrover operations, and to measure constraints on the mechanical and thermal properties of the surface layer. The rover would weigh about 1 kg and would be capable of rolling, climbing, or hopping around on the surface of the asteroid. It would have run on solar power and carry a multi-band imaging camera, a near-infrared point spectrometer, and an alpha/X-ray spectrometer (AXS).