The One-Pound Problem- page 3 | Space | Air & Space Magazine

The One-Pound Problem

All the Mars Ascent Vehicle has to do is deliver 16 ounces of rocks in a container the size of a grapefruit to Martian orbit. If only it were as easy as it sounds.

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(Continued from page 2)

"God created guidance systems for a purpose," says Caldwell. So the engineers admitted defeat and added active control--thrusters, inertial position sensors, computers, and power--to the first stage, which pushed the weight up from 80 pounds to almost 300.

Another issue the Mini-MAV design hadn't addressed was how the sample canister would keep the Martian dirt pristine. This was important not only to scientists, who wanted uncontaminated samples, but to NASA's "planetary protection" watchdogs, who had to guarantee to the public that no dangerous Martian bug--a remote possibility, but not entirely out of the question--would be returned to Earth. The sample-return project hadn't yet come to grips with the planetary protection dilemma when Brian Wilcox proposed his Mini-MAV. By the time it did, the sphere that could hold the Martian dirt and keep it sterile weighed ten times more than the sample itself.

This creeping weight gain was fairly typical for the early design phase of a space engineering project, says Caldwell. "Push the system here and it bulges out there with some kind of problem--too much heat, or cost, or weight." It was mainly that damn eight-to-one mass penalty. When anyone suggested adding another pound or two to the sample canister, he says, "I wanted them to know how painful it is."

By the spring of this year, the Mini-MAV had ballooned up to a 375-pound, plain old MAV, which stood about as tall as a person. It was still a lot better than the old liquid-fueled MAV, which had trimmed down to 600 pounds but appeared to be stuck there. Even so, with all the commotion that had been made over the Mini-MAV, Caldwell couldn't help feeling like the guy who's been given a perfect handoff, only to fumble it inches from the goal line.

Asked what's the hardest part of bringing back a sample from Mars, Bill O'Neill thinks for a moment, looks down at his desk, and takes in a deep breath. Then he lets it out with a laugh. "It's all hard!"

He considers each task of the three missions in order (see diagram, opposite page). Getting to Mars is straightforward enough. But two of the flight plans call for "direct insertion" by aerocapture. In other words, the spacecraft will come screaming into the Martian atmosphere, where drag, not retrorockets, will slow it down. It's never been tried.

Once safely on the ground, the 2003 and 2005 landers will deploy rovers about the size of a child's wagon, which will roam the Martian surface, collecting rocks and drilling cores from some 20 sites over the course of three months. The most interesting samples will go into a cache box, which will be transferred to the MAV when the rover returns to the lander. A mechanical arm then hoists the MAV to vertical launch position, and off it goes.

Still pondering the relative risks, O'Neill says the landing on Mars isn't too bad, since it's been done before. The rover can be tested on Earth. The transfer from rover to MAV is tricky, but also can be practiced beforehand. "I suppose if I had to pick something, I would say the ascent vehicle, because it's the least demonstratable," says O'Neill. "There's no environment you can find on Earth that can completely mimic what we're going to do at Mars."

The sample-return strategists will therefore rely mostly on computer simulation, just like the Mars Pathfinder team did, testing only those few key elements that are particularly worrisome and are possible to try out on Earth. For example, Doug Caldwell wants to run what he calls a "burp test" in a 100-foot-high chamber filled with simulated Martian atmosphere. A MAV would be loaded with just enough solid propellant to burn for a second or so--long enough for the rocket to clear the launch deck so the engineers can see if its exhaust plume has some funny interaction with the lander.

The sample return mission becomes much more difficult the more Martian dirt the project is asked to bring back, and last spring this was a topic of vigorous discussion between JPL and NASA Headquarters. By then Caldwell and company knew how to build a MAV that could lift about 12 ounces--350 grams. But NASA was "adamant," says O'Neill, that it wanted the full pound--a little more than a pound, in fact, 500 grams, about 17 ounces--and was pushing for a "stretch goal" of 1,000 grams.

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