Because of their orbital patterns, Mars is getting very close to Earth: about 55 million-plus kilometers. In Washington, that will happen just a few minutes before six on the morning of August 27. Just six months ago, the planets were separated five times that distance. To take advantage of this relatively close encounter, the closest in about 60,000 years, the American space agency, NASA, has sent up two spacecraft as part of its Mars Exploration Rover Mission.

One of the mission's key scientists, Jim Bell, discussed the Mars Rovers, as part of the Smithsonian Institution's recent Mars Day program.

A video of past Mars exploration rocket launches repeats over and over at the National Air and Space Museum display where the scientist is standing. But Mr. Bell doesn't mind watching it again; in fact, he says it was such news footage that sparked his interest in planetary exploration as a child.

"Watching the Viking [spacecraft] missions in the 1970s when I was in elementary school, and seeing those pictures come back ? [I] realized that somewhere else out there is like the Earth in a lot of ways. But it's unlike the Earth in so many other ways," he said. "It's really such a fascinating place. I went on to study geology as well. Mars is another spectacular geological place in the solar system."

Mr. Bell teaches astronomy at Cornell University where the late, renowned scholar Carl Sagan also taught. He is also the lead scientist for the NASA Rover cameras, and says the Mars-Earth close encounter will be a once-in-a-lifetime opportunity for amateur and professional astronomers alike.

"Since it's close, it means we can see more detail on the planet with telescopes on the ground, the Space Telescope," he explained. "You can study the planet in a lot of detail you don't get when Earth and Mars are far [apart]. When Earth and Mars are far [apart], [Mars] is a fuzzy little blob in the sky. When it's close, you can see [its] polar caps. Some of the volcanoes can be seen from Earth, even clouds and other geologic features."

For the American space agency, the unique opportunity provides the best "window" for a spacecraft journey to Mars.

"Because the planets are closer, the travel time is shorter," said Mr. Bell. "The energy to get a rocket from Earth to Mars is less. That means you can send smaller rockets, like the Delta rocket we're using for the Mars Rover; [it's] a relatively small rocket."

And that, according astronomer Bell, means another opportunity for the researchers. "Because we got to use a smaller rocket and the costs were less, we could afford to send two," he said. You reduce the risk. If one mission fails, you still have the other one. If they both succeed, you increase the scientific potential because we're picking two different landing sites. You study one place that's mostly about geology. Another place is mostly about composition and mineralogy. If both of them succeed, we get spectacular science. We have an insurance policy, the way NASA used to do it: NASA used to send two of everything."

The Rover Spirit probe was launched June 10. Rover Opportunity followed on July 7. They are expected to land on Mars next January. So what do scientists hope to find on Mars with the two Rover spacecraft?

"I think anything they find is going to be new and interesting," stresses Mr. Bell. "It's a big planet, it's complex, and we don't know much about its surface in detail. It would be kind of arrogant to say, 'I know exactly what we're going to find. It's going to be this, that and this.' There's a reason that 'exploration' is the middle name of the mission. We don't know what we'll find." Mr. Bell notes that astronomers do have some clues of what's on Mars, so they've chosen two areas for landing the spacecraft.

"You try to pick places that could potentially be interesting," he said. "The two landing sites we picked have a lot of potential. One of them looks as if it used to be a lake, a long time ago. It's a big crater and looks like it has sediment on the bottom of it. It might have been a long-standing body of water at a place that could have very much like the Earth.

"The other landing site has certain kinds of minerals that have been found in orbit that on the Earth- have been associated with long-term standing bodies of water, with certain kinds of iron oxides and minerals like that. There's evidence in the kinds of chemistry there that Mars is very different and much more like the Earth."

For Jim Bell, that may be most fascinating aspect of Mars exploration: to see if the planet can support life as we know it, or whether it did in the past.

"One of our goals is to test this hypothesis," he said. "Scientists like to propose a hypothesis and test it try to beat it down and throw it away if it's no good. The hypothesis is that Mars was like the Earth a long time ago with warmer temperatures, a wetter environment, and maybe a place where life could have existed! We want to find clues that support that or don't support that."

Life on Mars? For astronomer Jim Bell and colleagues, it's one of science's most intriguing questions. And what if the Rovers' data still don't solve this puzzle? Well, the next close encounter between Earth and Mars will come much sooner than 60,000 years: in a mere 284 years and the planets will be even closer together then.