Artist's impression of MUSES-C orbiter with rover on asteroid surface. Photo: ISAS

A joint Japanese-U.S. mission will be headed to a different asteroid for in-depth studies in the next few years. The change also means a different launch period for the spacecraft in question -- the Mu Space Engineering Spacecraft-C (MUSES-C).

MUSES-C is managed by the Japanese Institute of Space and Astronautical Sciences (ISAS) and NASA.

NASA officials say that the cause of the switch lies with the M-5 launch vehicle -- the rocket that will boost the spacecraft to space. The M-5 last launched in February, but failed to loft the ASTRO-E X-ray observatory into orbit on that mission. ASTRO-E was also a joint project between the United States and Japan. After delays with preparing the M-5 for future mission, NASA and ISAS were forced to change the target from the asteroid 4660 Nereus because of weight and positioning constraints.

The new target of the mission is the asteroid 1998 SF36. It will make two close approaches to Earth in 2001 and 2004, which will allow astronomers and other scientists to take ground-based measurements to try and determine important characteristics of the asteroid, such as size, shape, rotation state and some surface characteristics, if possible.

With the new choice for a destination, MUSES-C is now set for launch from Kagoshima, Japan, on an M-5 rocket in November or December 2002. After a nearly three-year long journey through space, the craft will arrive in a low orbit around the asteroid, where it will spend around a month mapping and studying the space rock.

Rover mockup. Photo: NASA/JPL

After that period, ground controllers will command the MUSES-C to fire its small braking thrusters to descend to the surface. Moments before touchdown, the spacecraft will drop a tiny NASA-built rover that was developed at the Jet Propulsion Laboratory (JPL) in California. The rover weighs just over two pounds and will operate on the surface of the asteroid for around a month while studying the surface composition and other characteristics. The rover will also return the first pictures from the surface of an asteroid.

After the rover is released and as the main MUSES-C probe touches down, the probe will fire a small pellet into the surface of the asteroid, which will cause fragments to gather inside a small funnel on the exterior of the spacecraft. All told, the MUSES-C mission will attempt this maneuver several times. Collected fragments will be stored inside a small capsule that will be returned to Earth.

After MUSES-C is finished mapping and collecting samples from the asteroid -- a few months in all -- the space probe will return to Earth and drop a small capsule containing the material gathered from the surface. The capsule will enter the Earth's atmosphere for the samples to be analyzed, and the main spacecraft will be steered away from Earth.