MARIE - which is on board the Mars Odyssey orbiter -- is designed to collect and characterize aspects of space radiation both on the way to the Red Planet and in the Martian orbit. Her goal is to predict the radiation doses that would be encountered by future astronauts. "What MARIE allows us is the ability to see any source of radiation - background, solar - outside the vicinity of Earth's magnetic and atmospheric system," elaborates Roger G. Gibbs, Odyssey's project manager at NASA's Jet Propulsion Laboratory (JPL).

Space radiation - which is caused by cosmic rays emanating from the Sun and from stars beyond our solar system -- is one of the most deadly hazards human crews of interplanetary missions will confront. MARIE's findings, therefore, are vital to preparations for future human missions to Mars and other planets.

This intense form of radiation - which mostly falls in the energy range of 15 Me V to 500 Me V per nucleon - expels the kind of energy that can damage human DNA, catalyze cancer, and cause serious damage to the central nervous system. Not surprisingly, the team is, in Gibbs' word "ecstatic" that she is back in operation.

MARIE had done a fine job during the first part of Odyssey's journey to Mars, but the two suffered a communications breakdown last August, about four months into the flight. Although NASA and JPL scientists and engineers had seen the same sort of problem on other similar instruments before, it was, for the team, "a terrible shock when it went down," recalls Timothy F. Cleghorn, one of the MARIE team members at NASA's Johnson Space Center in Houston. "It went down the day before Gautam Badhwar, {MARIE's principal investigator} had his heart attack." Badhwar passed away shortly thereafter.

Late last week, however, MARIE responded to a series of calls from NASA and JPL engineers. Then last weekend, controllers sent a series of new commands and returned the radiation monitor to science collection mode.

It took until now to get her back online, Gibbs says, because the majority of the team had to focus on Odyssey's orbit insertion and aerobraking. "When communication with MARIE ceased, we made a determination that we would take about two weeks to evaluate what was going on. If we could recover - fine. If not, we decided we would stop recovery activities to focus on Mars orbit insertion and to do the MARIE troubleshooting offline," he explains.

While most people think the process is as simple as sending a command to flip a switch, troubleshooting activities actually involve several, very deliberate procedures. "Because the assets are out of our hands and a loss would be terrible, we are very, very careful to first do no damage, do no harm," Gibbs expounds. "One of my biggest fears was that we would do something that would hinder, hurt, lose the mission by a command that was unnecessary."

In fact, sending a command to the instrument on the spacecraft is, Gibbs says, "the tail end" of the intellectual process. First the team must consider the possible cause of the problem and what the potential recovery actions might be.

Then they evaluate a selected set of test commands on a spacecraft simulator and an instrument simulator. Finally, they review that process. So while Odyssey was undergoing orbit insertion and aerobraking, specific MARIE team members continued their troubleshooting efforts offline.

"The space radiation environment is extremely harsh," reminds Cleghorn. "You can have what is called a single event upset -- where a single particle hits the wrong bit on your CPU, and that can flip the bits," he says. The results of tests seem to indicate that the problem may have been related to a memory error in MARIE's onboard software.

Now, as Odyssey orbits Mars, MARIE's spectrometer - which has a 68-degree field of view - is sweeping through the sky and measuring the radiation field.

This, says Cleghorn, is what MARIE's first data sets have revealed:

o The radiation exposure in the transit period was "approximately double" that which the astronauts are receiving on the International Space Station (ISS).

o The exposure to the heavy nuclei, up to and including iron nuclei, is "approximately three times" what the astronauts are receiving on the ISS. (The heavy nuclei are more dangerous and of considerable concern, in terms of manned space flight.)

In addition, scientists also infer from MARIE data that on the surface of Mars, the radiation is comparable to the exposure the astronauts are receiving in the space station, Cleghorn adds, "although the charge composition - the elemental composition -- is different."

"These problems, of course, must be addressed," Cleghorn says. "They are not insurmountable, but it's going to take a lot of clever study and clever engineering to make space flight safe given the ground rules we have now."

MARIE measures an energy spectrum that reveals how many particles are detected with each energy.

Although high energy radiation exists in Earth's upper atmosphere - and humans are exposed to small amounts of it while in flight on commercial airliners -- the Earth's atmosphere and protective magnetosphere prevent most of the harmful radiation from reaching the surface of our planet. Mars, on the other hand, has an atmosphere that is less than one percent as thick as Earth's and the Red Planet has no global magnetic field to shield it from solar flares and cosmic rays.

While similar instruments fly now on the space shuttles and on the international space station, MARIE is the first radiation sleuth to have ventured out from Earth's protective magnetosphere. With her glitch now corrected, mission scientists are optimistic that she will reward them with more data.

"We have accomplished a lot of good things," offers Gibbs. "We have traveled 470 million kilometers to Mars and we hit our target within 750 meters - it was a bulls eye. We did aerobraking and without going into detail there was a lot we did and did well. I was afraid we were going to be a mission with an asterisk, but now we've removed the asterisk and I am ecstatic."

Cleghorn cautiously agrees. "Anything, of course, can happen," he says. "But we are going to get data."

Now that MARIE is once again at work, Cleghorn says, "I am overjoyed. I only wish that Gautam could have seen it."

The Mars Odyssey orbiter was launched April 7, 2001 from Cape Canaveral.

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