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.
- By Tony Reichhardt
- Air & Space magazine, November 1999
(Page 4 of 6)
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.
The can-do team at JPL didn't argue that it was impossible. "We wouldn't want to immediately take the easy way," says O'Neill. But bringing back more sample doesn't automatically increase the scientific productivity of the mission. The project had long ago rejected what was known as the "grab sample" scenario--dash to Mars, dig up a bunch of dirt in a hurry from right around the lander, then rush it back to Earth. To maximize their chance of finding evidence of water or some other geologic prize, the scientists much prefer collecting smaller samples from many diverse sites, with cameras and other instruments on the rover carefully documenting each setting. All they need is a few milligrams back in the lab--if it's the right rock.
This requires that the cache box be divided like a honeycomb into compartments, each containing a sealed sample from a different site. The greatest worry is time. It takes time for scientists on Earth to study the surface photos and determine which sites they want to explore, time for the rover to move on to the next location, time to photograph the site up close, time to inspect each rock in the vicinity, time to drill the inch-long cores, and time to seal them in the compartments. The rover is expected to last only 90 days on the surface before dust and general wear and tear reduce the effectiveness of its solar arrays and it runs out of electrical power.