- The Washington Times - Wednesday, March 10, 2004

Now that NASA and the Europeans have successfully put mechanical crawly things on Mars, the question arises: How to make rovers explore larger regions of the planet?

Rovers are slow. They cover distances measured in meters instead of miles. This is fine if you want to examine a small area in detail. Suppose you want a grander tour? Today’s rovers are also mechanical, which means they tend to fail. So how do you make a rover that will go long distances but doesn’t have moving parts?

The folks at the National Aeronautics and Space Administration have come up with an idea I wish I’d thought of. Tumbleweeds, sort of. There’s wind on Mars. Why not use it? The idea is to build a really tough balloon, with a diameter of about 6 feet, put instruments inside, and put it on Mars. The instruments would be held in the center by tension cords. The balloon would then blow wherever the wind took it and radio information back to, say, an orbiter. NASA estimates that such a globular rover could reach speeds of 20 miles per hour.

Astrobiology magazine explains how NASA came up with the idea by accident. The agency was experimenting with a rover having huge spherical wheels. One of them got loose by mistake and set out across the terrain at a good clip, propelled by the wind. The NASA-ites, no dummies, said, “Aha.” So they built a trial tumbleweed rover and set it loose in Antarctica. Eight days later, despite low winds, it had gone 40 miles. Given that the Mars rovers Spirit and Opportunity average something like .03 miles an hour, this was spectacular.

Says Astrobiology, “Along the way, the beach-ball-shaped device, roughly [6 feet] in diameter, used the … Iridium satellite network to send information about its position, the surrounding air temperature, pressure, humidity and light intensity to a JPL [Jet Propulsion Lab] ground station.”

Tumbleweeds ought to be fairly cheap. A mechanical, wheeled rover to go hundreds of miles would cost more than three dozen aircraft carriers.

But tumbleweed rovers have their limitations. You would have little control over where they went. On the other hand, you would learn where the winds were coming from. Since we don’t know much of anything about most of Mars, there is nowhere it could go that wouldn’t be of interest.

Engineers talk of perhaps designing the critter so that it could partly deflate itself on command, causing it to stop rolling. There is further talk of including a system to pump a liquid from one side to another to give a degree of control.

The trick, it turns out, is that the balloon has to be big enough. In earlier experiments elsewhere, NASA tried globes the size of beach balls. They got wedged between things and never got loose. But if the ball is large compared to the size of obstacles, it just rolls over them.

Mars, if one may judge by the photos returned from the various landers, is short on trees, barbed-wire fences, automotive graveyards, and other things that might be bad for balloons. In fact much of it is flattish and looks to be splendid tumbleweed territory. And of course the “balloons” would in fact be made of formidably tough materials. NASA also found that being inside a balloon helped keep the instruments warm, which is important.

Now, if it blew into a crater and fell into wind shadow, or dropped into a canyon, it would just sit there and not be real useful. But that is true of anything you send that far to do very difficult things.

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