Opportunity Gained:

Rover Escapes Sand Dune

* After five weeks of being the Martian equivalent of a minivan stuck in the snow, rover Opportunity is on the move again.

On April 26, the rover got stuck in a foot-deep sand dune on Mars. Over the weekend, NASA scientists giving the 380-pound vehicle commands from Earth finally got it free.

Opportunity's problem was a lot bigger than getting a van stuck in a snow bank -- so the space agency couldn't use some of the tricks your parents might try.

Gunning Opportunity's engine to power it out of the dune wasn't an option. The solar-powered rover has a top speed of one-third of a mile per hour.

Rocking the rover out of the dune by shifting gears forward and back repeatedly wouldn't work either. It takes 10 minutes to send a command from Earth to Mars and another 10 to get back video of the command's effects.

Opportunity's cameras gave scientists a good idea of what the problem was: The dust-like substance in the dune had clogged up the blades on the rover's wheels, making them spin like bald tires.

So scientists at the Jet Propulsion Laboratory in California where Opportunity and its twin rover, Spirit, were built, headed to an enclosed sandbox the size of a big indoor swimming pool. They created soil similar to that in the dune and used models of Opportunity to come up with a plan.

After weeks of watching and calculating, the scientists and mathematicians had a strategy. They had the rover reposition its wheels, then go through a series of spins that pushed the craft forward an inch at a time.

In testing in the huge sandbox, "the [model] rover eventually got out every time," said astronomer Steve Squyres.

On Saturday, Opportunity got out for real, leaving tread marks in the Martian soil as it continued its exploration. The rovers landed in January 2004.

Opportunity makes tracks after

escaping a Martian sand trap.