|Number 22: 30/10/2003
A scientific publication by SGF and NEODyS
Hayabusa's Exploration Plans for Asteroid 25143 Itokawa
by Kelli B. Grant and Beth Ellen Clark - Ithaca College
One hundred and sixty-five million miles away from Earth, asteroid 25143 Itokawa is on an Earth-approaching orbit, moving between Mars and Jupiter after being pulled out of the asteroid belt by perturbations in Jupiter's gravitational force.
Japan's Hayabusa is on its way to intercept and collect samples from the asteroid, which has an orbit that brings it to within 1.8 million kilometers of Earth's orbit - far closer than most asteroids classified as near-Earth objects. "That's pretty close," said Don Yeomans, manager of NASA's Near-Earth Object Program Office and the U.S. project manager for Hayabusa. Any object within 9 million kilometers of Earth's orbit is deemed hazardous because the gravity of the sun and other planets can easily alter its course.
By retrieving samples of Itokawa, scientists will be able to increase knowledge and understanding of asteroid surfaces, directly improving our knowledge of the connections between asteroids and meteorites. As "leftovers" from the formation of the inner planets, asteroid samples may also give scientists clues to early solar system formation. "The holy grail of asteroid science is to find out the chemical composition of an asteroid and link it to the chemical composition of meteorites on Earth," says Yeomans. Likewise, knowing more about asteroid composition could help deflect an NEA on a collision course with Earth. "In terms of mitigating an Earth threatening object, it will make a big difference whether we are dealing with some wimpy ex-cometary fluff ball or a slab of solid iron."
Down the line, study of the asteroid samples could yield information useful for asteroid mining as man begins to build objects in space instead of hauling them up by rocket.
Launched May 8, 2003 from the Uchinoura Launch Center at Kagoshima on Kyushu Island, Hayabusa is scheduled to reach the asteroid in July 2005. The project, initially called Mu Space Engineering Spacecraft-C (MUSES-C), is a pioneering effort by Japan's Institute of Space and Astronautical Science (ISAS). Although there are several sample-return missions in progress in the world, Hayabusa will be the first two-way trip to an asteroid and the first to use a small launch vehicle instead of a large rocket.
Classified by the Japanese as a "technology test mission," Hayabusa is equipped with several high-tech components. According to the mission Web site, Hayabusa project scientists are "convinced that this mission will provide us with valuable technical data that will promote and enable ambitious sample-return missions in the future in the world." Hayabusa's optical navigation cameras will be used in combination with a laser altimeter device to maneuver the craft around Itokawa. Because the spacecraft will approach the target with several minutes necessary for light-time communication, it must use its high-tech instruments to autonomously "decide" how best to move around and land on the asteroid.
A second technological experiment is Hayabusa's four ion-drive electric propulsion engines. It is not the first craft to use such engines - NASA's Deep Space 1 had that distinction - but it is the first Japanese craft to employ the technology, and the first to use microwaves to ionize the xenon fuel. The engines propel the craft forward at 12 meters per second per day - a force 20,000 times less than traditional thrusters that operate once for only a few seconds. Yeomans said that in this case, "like the tortoise and the hare," slow and steady gets the craft to Itokawa in a much more energy efficient way.
The estimated $180 million mission was originally slated to be launched in late 2002. Its target: asteroid (10302) 1989 ML, known as Nereus. But after an M-5 failed to hoist Japan's Astro E X-ray observatory into orbit in February 2002, the project was stalled for a year while the space program's failures were investigated. During that time, MUSES-C was renamed Hayabusa, Japanese for Falcon. This particular bird of prey was chosen because of its ability to swoop down and snatch up its prey in its talons - a maneuver that some see as similar to the plans for Hayabusa, its prey being a sample of the asteroid surface.
|The orbit of the spacecraft|
Because the launch delay placed Nereus beyond reach, the new target 1998SF36 was selected. By July of 2003, the International Astronomical Union had obtained enough information about the 1998SF36's orbit to give it a name beyond a numerical designation. In honor of the father of Japanese rocketry, it is now called 25143 Itokawa - a namesake that sparked a yearlong debate, says Yeomans. Objects as close to Earth's orbit as Itokawa are called Apollo objects and are usually named for mythic figures or gods. "This was sort of a change in the guidelines."
Physically, Yeomans described 25143 Itokawa as "potato-shaped." Current estimates measure its diameter at 1,650 feet. Earth-based observations obtained thus far suggest that Itokawa is probably made of Ordinary Chondrite meteoritic material, the type that currently comprises about 85 percent of all meteorites in our Earthly collections. Classified as a type-S body; its spectral analysis indicates an undifferentiated metamorphosed asteroid. And although its surface may be somewhat weathered, it is probably fresher and covered with fewer particulates than most asteroids of its type. Observations in the infrared indicate that there is little or no water in the form of hydrated minerals on the surface.
The only difference in Itokawa as compared to the meteorites is a slight reddening that could be caused by minor surface alteration processes occurring in interplanetary space, a theory consistent with the findings from the visible wavelength studies. Compared with other S-type asteroids, Itokawa is fairly bright. The average percentage of light reflected from main belt S-types is 20 percent, while Itokawa's is somewhere between 23 and 53 percent, depending on the measurement methods used and assumptions made.
Broadband visible wavelength-color comparisons of Itokawa and other asteroids always place it at the outer edges of the color-space occupied by S-type asteroids. Its color, which is both redder and brighter than its cousins, could indicate a surface that is fresher or more recently exposed to the space environment. Measurements of Itokawa in the thermal infrared reveal higher values of thermal inertia than expected for most S-type asteroids, indicating a very thin surface regolith and/or the presence of exposed bare rocks.
|The spacecraft and its' scientific instruments (click on the image for an enlargement)|
Hayabusa also has several devices onboard for complex analysis of Itokawa. A second LIDAR device will be constantly measuring the distance between the spacecraft and Itokawa to determine the asteroid's shape and volume. The resulting data can be used to create an accurate model of Itokawa - down to the smallest crater. A near-infrared spectrometer (NIRS) will be used to map the mineral composition of the asteroid surface. An asteroid multi-band imaging camera (AMICA), "will do a great deal," Yeomans says. Not only can the AMICA measure the asteroid's size, shape, volume and rotation characteristics, it can also map satellites around Itokawa and note slight differences in surface colors. Finally, an alpha proton x-ray spectrometer (APXS) will detect elements on the asteroid surface and provide abundance ratios, information that could forge a more detailed link between the asteroid and meteorites.
MINERVA, a small jump robot developed by ISAS, will image the asteroid's surface from very close range using its three onboard cameras. Mechanics inside the solar-powered cylindrical rover will allow it to autonomously explore Itokawa, relaying data to Hayabusa. In addition to providing photographs of Itokawa, MINERVA will also relay data about the surface temperature, gravity, and friction.
Data obtained in the first month will be used to select three sampling sites. Before executing a touchdown, the spacecraft will release a grapefruit-sized target marker made of highly reflective material, and use its positional accuracy on the surface as a homing beacon to safely navigate a landing.
|MUSES C and its' collecting horn. On the left, a representation of how the horn works, on the right, a picture of the horn|
Collecting the samples will be the most difficult part of the mission. A sample return from an asteroid has never before been attempted, and there are numerous obstacles to overcome - namely the asteroid's low surface gravity (less than 1/100,000 of Earth's), its 12-hour rotation period, and its unknown composition (density estimates average 2.7 g/cc). As a third technological experiment, Hayabusa will carry a horn that will be brought into contact with the asteroid's surface. A small pyrotechnic charge will fire a projectile into the surface, ideally smashing part of it. The resulting impact fragments will be captured by the funnel and sucked into a sample container. The spacecraft will only remain on the surface of the asteroid long enough to collect a single sample. Once it has done so, its engines will restart and it will return to a hovering position 100m away from the surface. Yeomans said the process will be repeated at least once for a total of two to three samples.
After a two-year flight back toward Earth, Hayabusa will eject the sample canisters, which after re-entry will parachute down near the town of Woomera in Southern Australia. Yeomans said there is an extremely low chance of the samples being hazardous, a fact that helped convince the Australians to reduce quarantine requirements. "They gave us the thumb up," he said. "This is one of the safer sample return missions." A sample return from any astral body with water presents a greater chance of microscopic life forms, and would have been a much tougher sell.
The total mass of the return samples is predicted to be .035 ounces - less than a teaspoon, but more than enough to revolutionize current asteroid knowledge. After it lands, scientists will transport the sample to Japan to be opened. Japan will study the samples for a year, then offer portions for international study. Yeomans said 10 percent of the sample will be given to NASA, and an additional 40 percent will be archived for future studies.