Thanasis “Tom” Economou holds the instrument he designed for the MUSES-C mission scheduled for the year 2002.

From Mars to asteroids

The first object that humans will land on the surface of an asteroid will contain a miniature version of the University instrument that helped make aerospace history during the Mars Pathfinder mission in 1997.

Chicago’s alpha proton X-ray spectrometer, or APXS, which was carried aboard the Sojourner rover, provided the first-ever chemical analysis of native Martian rock during the Pathfinder mission. Now a much smaller, Chicago-built, alpha X-ray spectrometer, AXS, will provide similar data during the joint U.S.-Japanese MUSES-C mission to asteroid Nereus in 2002. The AXS may even play a role in selecting a sample of the asteroid for return to Earth during the same mission three years later.

“We are now in the final stages of design,” said Thanasis “Tom” Economou, Senior Scientist at Chicago’s Enrico Fermi Institute. The prototype is being built, and in a few months, it will be integrated with the nanorover “to make sure everything works in coordination,” he said.

The AXS instrument is key to accomplishing mission objectives, said Donald Yeomans, a senior research scientist at NASA’s Jet Propulsion Laboratory and U.S. science team leader for the asteroid lander mission.

“As the rover wanders around the surface of the asteroid, we’re counting on Tom’s instrument to tell us what the various soils and rocks are made of,” Yeomans said. “It makes a big difference as to how the object formed. Is it a chip off a bigger object or is it an accumulation of various bits and pieces of asteroids, sort of a rubble pile?”

The chemical composition of the asteroid can also be compared to that of meteorites found on Earth. “Once we make the link between a certain type of asteroid and a certain type of meteorite, we can remotely observe asteroids and infer what they’re made of,” Yeomans said.

This is possible because asteroids, planets and stars have specific spectral characteristics ––they give off certain types of electromagnetic radiation that depend on the object’s chemical composition. “Presumably, every other asteroid having the same spectral characteristics will be made of the same stuff,” Yeomans said.

MUSES-C is a cooperative effort between the National Aeronautics and Space Administration and Japan’s Institute of Space and Astronautical Science. NASA will supply tracking and navigational support for the spacecraft, is building the nanorover and will recover the sample-return capsule. Japan’s ISAS will act as overall mission manager.

The mission is scheduled for launch from Japan in January 2002. The spacecraft will arrive at Nereus in early April 2003 and will land later that month. Nereus, discovered in 1982, measures less than a mile in diameter.

“When the spacecraft is 50 feet above the surface, it will gently drop the nanorover and then go land, grab some samples, then take off and stay nearby for a couple months while we’re doing the surface analysis and investigations,” Economou said. “There will be an attempt to do two landings, so it is conceivable that we will tell mission controllers where to land for the second time to grab a sample of particular interest.”

The solar-powered nanorover, mounted on four wheels, will measure only 6 inches square. It will be equipped with a camera and a near-infrared spectrometer as well as the AXS.

“The nanorover is a nice little toy,” Economou said. “It can go over much larger rocks than itself.”

The APXS that Economou built for Pathfinder’s rover weighed 570 grams (1.2 pounds). But for MUSES-C, Economou has to think on the Lilliputian scale of Gulliver’s Travels. His weight limit for the nanorover’s chemical analyzer is no more than 100 grams (3.5 ounces), packed into an area measuring less than 3 cubic inches.

Together the AXS’s alpha and X-ray detectors can detect any chemical element except hydrogen at concentrations as low as a fraction of 1 percent. The instrument’s design saves weight by dispensing with the proton detector and by sharing capabilities with the nanorover.

“The proton mode was very important when we did not have the X-ray mode available in our instrument,” Economou said. “It helped to determine sodium, magnesium and some other light elements. Now, we can get all these elements from the X-ray mode. But it is always nice if you can compare the data from all the modes and get the same results.”

In November, Economou was one of 11 Mars Pathfinder team members and instrument developers to receive special recognition from the National Air and Space Museum of the Smithsonian Institution. The Pathfinder team has been awarded the 1998 National Air and Space Museum Trophy for Current Achievement for its demonstration of several technologies and concepts for use in future Mars missions.

Economou still is analyzing data collected during the Pathfinder mission, which officially ended in 1997. He also is designing and building two more APXS instruments for the rovers of the Mars Surveyor 2001 and 2003 missions. The rovers of these two missions will visit more Martian sites to collect samples, some of which will be returned to Earth during a mission set for 2005.

The rover for Mars 2001, called Marie Curie, is similar to Sojourner.

“For 2003, there will be the Athena rover, a new type of rover, capable of traveling farther distances, carrying more instruments and carrying a drill bit to collect samples,” Economou said.

The Athena rover’s robotic arm will pick up a sample and bring it to the APXS and other instruments. After examination, the robot arm will either drop the sample on the surface or place it in a special container for return to Earth.

“It will be exciting times,” Economou said.