|Number 22: 30/10/2003
A scientific publication by SGF and NEODyS
An interview to Andy Cheng, NASA member for the Orbital LIDAR
Q. What is your personal scientific and technical contribute in MUSES C mission?
A. There is a joint NASA-ISAS science team, and I am the NASA member for the Orbital LIDAR. I am helping with science planning, asteroid operations, and data analysis. I was the Project Scientist, or the lead scientist, for the NEAR mission which was developed and operated by the Applied Physics Laboratory where I have worked for the past 19 years. Since MUSES-C is a Japanese mission, with the spacecraft and launch vehicle built by ISAS, the Project Scientist for MUSES-C is Akira Fujiwara of ISAS.
The MUSES-C spacecraft has been renamed Hayabusa, which means 'falcon' in Japanese. The name is most appropriate, as it recalls not only the soaring flight of the bird but also the way it swoops down upon its prey. Hayabusa will fly to the asteroid 25143 Itokawa (also named just this year after a Japanese rocketry pioneer) and return samples to Earth. To acquire these samples, Hayabusa will make only momentary contact with its target. It descends to the surface of the asteroid, and immediately fires a small (5 gram) projectile into the surface, causing small fragments from the surface to be collected by a sample collection horn. This is a funnel which guides the fragments into a collection chamber. After less a second on the surface, Hayabusa fires its rocket engines to lift off again - very much like a falcon seizing its prey.
The flight of Hayabusa to the asteroid is also unusual, because it uses ion engines. These are unlike ordinary rocket engines, which burn chemical fuel. The ion engines use electricity to create and exhaust Xe ions at an extremely high velocity, ten times greater than that of ordinary chemical rockets. This means that the ion engine derives ten times as much thrust from the same rate of fuel expenditure. However, the rate of fuel expenditure for ion engines is very low compared to that for chemical rocket engines, so ion engines produce only low amounts of thrust, and the spacecraft must also carry a set of chemical rockets. The Japanese ion engines are flying for the first time on Hayabusa and are working well. NASA has used ion engines only once before on a planetary mission, the flight of Deep Space 1 by Comet Borrelly in 2001. ESA is also planning a mission to the Moon using ion engines (SMART-1).
Q. Why are missions to asteroids important?
A. Because asteroids are primitive objects, which preserve materials dating back to the time of planet formation in our solar system. If we wish to study such ancient materials we must return to asteroids
Because asteroids have impacted Earth and other terrestrial planets throughout geologic time, and these impacts have shaped the evolution of life on Earth - an asteroid impact around 65 million years ago ended the age of the dinosaurs
Q. What do we actually know about asteroid composition and evolution? What answers are you still trying to answer to?
A. NEAR discovered that Eros was primitive and not a differentiated body (in other words, it was never melted and separated into different constituents). Also Eros is a geologically active body, even though Eros has no atmosphere and no flowing water to cause erosion like on Earth, and Eros has no volcanism to build new terrains. Nevertheless, Eros has features called ponds, which do not contain water but which are filled with fine particulate matter and which are surrounded by raised linear features that look like beaches. We think that ponds on Eros formed from seismic shaking due to impacts on the asteroid.
Still, after NEAR we still do not understand fully what Eros is made of, and we do not know whether Eros material is actually the same as that of any meteorite that ever fell on Earth. This is because the average density of Eros is lower than that of most meteorites, and because the elemental and mineral abundances do not quite match. We are not sure to what extent these properties have been affected by geologic processes on Eros. To resolve these questions, we would require a sample return from Eros. Also, we wonder if the geologic processes we see on Eros, such as the formation of ponds and the filling or erasure of craters, will also be found on other asteroids like the MUSES-C target.
Q. Can you trace the history of the MUSES C mission ?
A. The origins of the MUSES-C mission can be traced to a 1986 ISAS study of missions to near-Earth asteroids, including a possible sample return mission to asteroid Anteros. This study was led by Jun Kawaguchi, who is now the Project Manager in charge of the MUSES-C mission at ISAS. It is interesting that Anteros was also the original target of the NEAR mission as proposed in 1991 to NASA by my institution, JHU/APL. The target of the NEAR mission was changed to Eros in 1993, and NEAR was launched in 1996. The trajectory flown by the NEAR mission to Eros was similar to that proposed in the 1986 Japanese study. This is only one of the ways in which the histories of the two missions are linked. Because NASA had chosen in 1993 to do a near-Earth asteroid rendezvous (an orbital mission) in the form of NEAR, the Japanese decided to start the MUSES-C project in 1996 as the first near-Earth asteroid sample return. The original target for MUSES-C was another asteroid called Nereus, but in 1999 the target was changed to asteroid 1989ML (which still does not have a official approved name) and the launch was postponed to mid-2002. In 2000 the target was changed again, to Itokawa (at the time, this asteroid had only the provisional name 1998 SF36) and the launch date was postponed to late 2002. Then in 2002 the launch date was postponed one more time, because of a problem with the new Japanese M5 launch vehicle that MUSES-C was scheduled to use. The problem was fixed, and MUSES-C was launched successfully to Itokawa on May 9, 2003, the first day of the launch window.
While this history may sound complicated, it is still only part of the story. An agreement was reached in 1999 between NASA and ISAS for MUSES-C to fly a miniature, 4-wheeled rover to be built by the Jet Propulsion Laboratory. This NASA rover project, which was called MUSES-CN, was to develop an asteroid surface rover even smaller than the Sojourner rover flown to Mars in 1997 on the Mars Pathfinder mission. MUSES-CN would have carried a new, miniaturized instrument complement. Unfortunately, MUSES-CN encountered development problems and cost growth to an extent that NASA cancelled the program in 2001. However, the Japanese were still interested in flying a mobile, landed science package on MUSES-C, so they decided to develop a Japanese rover instead. This became the MINERVA rover, which MUSES-C will drop onto the surface of Itokawa, where it will take close-up color pictures and hop from one place to another on the surface.
Q. Which scientific instruments will the mission carry and for what purposes? In particular, how will the laser be used, how will you be making a 3d detailed model of the surface of the asteroid? What will data in other frequencies tell us of the surface?
A. To study the asteroid Itokawa and support sample acquisition, the mission will carry a color camera called AMICA, an near-infrared spectrometer called NIRS, an x-ray spectrometer called XRS, an orbital LIDAR, and the MINERVA rover which carries its own complement of color cameras and temperature sensors. AMICA maps the surface and the geologic features of Itokawa. AMICA is also used for optical navigation, to help Hayabusa find the asteroid and fly safely to the surface. NIRS spectra will be used to study the mineral composition of the surface. XRS spectra will allow us to measure the abundances of several important elements, like Mg, Al, Si, Ca, and Fe. The Orbital LIDAR measures the range to the surface of the asteroid, which will help to determine the shape of the asteroid and the heights of topographic features. All of these mapping data will be used to help select landing sites. In addition, Hayabusa carries equipment to extract samples from the surface and carry them safely back to Earth. This equipment consists of three projectile launchers, the sample collection horn, and the sample return capsule that re-enters into Earth's atmosphere with the asteroid samples. Samples will be recovered in Australia.
We will combine data from the AMICA imager and the Orbital LIDAR to determine an accurate, detailed shape model.
Q. What do you already know and how are you expecting asteroid 1998 SF36 (also thanks to the ground-based observing campaigns)? How will it be to land and stay on its surface?
A. From ground-based optical and radar measurements, we already know the approximate size, shape and spin rate of Itokawa. It is an Earth-crossing asteroid with a 12.12 hr rotation period and approximate ellipsoidal diameters 600 x 300 x 200 m from optical, near-infrared and radar measurements. We also know that the spectral type of Itokawa is similar to that of Eros (S-type, the most common type of asteroid in the near-Earth population).
Q. MUSES C is a joint effort by NASA and the Japanese ISAS and had to face a number of technical problems: what can you say about this kind of international cooperation ?
A. MUSES-C provides excellent illustrations of both the benefits and pitfalls of such cooperation. One benefit is increased opportunities for scientists - because NASA has not yet decided to fly any asteroid sample return mission, US scientists like myself would not have any opportunity to participate in the first such mission, except that Japan and the US have agreed to cooperate on the Japanese mission MUSES-C. I have benefited from this cooperation and am most grateful. Of course, the cooperation was originally intended to be more equal, with NASA supplying the MUSES-CN rover. When NASA cancelled its part of the mission, the roles and contributions of the two partners became unbalanced and created some issues. I think these issues not yet resolved.
International cooperation for space exploration will become increasingly important in the future, as space missions become ever more ambitious. It is very common to say that space missions are always risky and difficult, but it is nonetheless true. It is also true that when teams of engineers and scientists from different countries have to work together, then space missions are even more complicated. Unlike the situation for ESA, which has an established structure within which European nations cooperate for space exploration, there is no established structure for US-Japanese cooperation. Moreover, in recent years a new difficulty has developed for international cooperation, resulting from controls on export and import of technology. These regulations were never intended to hinder international scientific cooperation, but that has happened.