There’s a good reason for the dearth of news from the Red Planet.

"We’ve been traveling," said Rongxing Li, an Ohio State University scientist.

The we he is referring to are the twin NASA rovers Spirit and Opportunity, which have been quietly scooting across the Mars surface for months.

In fact, with little fanfare, Spirit has logged more than 1.4 miles on its odometer, an achievement far exceeding what NASA expected of the sixwheel, golf-cart-size explorer that landed in January.

There’s no sign any wheels are ready to fall off one of the rovers, which are rolling into their fifth months on Mars. Their mission originally was slated to end after 90 days.

Li directs a team in California producing maps of Mars and the rovers’ paths. Ohio State scientists have been spinning 31,000 images and other data from the rovers into maps and 3-D images that show depths, diameters and other details for craters, rocks and other objects.

"We use the same techniques for mapping erosion on the Lake Erie shoreline," Li said.

Right now, Spirit is headed for the Columbia Hills, where scientists hope to find rocks older than those the rover has examined so far. Opportunity, which has traveled about seven-tenths of a mile, is now perched at the edge of a crater about the size of Ohio Stadium.

When Spirit landed, the lowlying Columbia Hills seemed too distant a target.

Instead, the rover was sent to examine soil, rocks and craters near its landing site.

Now, logging as many as 300 feet a day, Spirit is close to the hills.

Li said before Spirit tackles the hills, it might catch its breath for a day or two and do some touristy things, such as digging a few holes, boring into a few rocks and taking some pictures.

Then scientists might command Spirit to climb a low hill, where it would take a panoramic look at its surroundings before poking, probing and scraping away some more.

While every new turn of the rovers’ wheels is a milestone, just surviving the 305 millionmile journey to Mars is a feat.

Spirit and Opportunity were sent to opposite sides of Mars. Notably, Opportunity landed in a shallow crater close to exposed rock outcroppings that revealed that part of Mars once was covered with water.

With the twins working well, ecstatic NASA brass found another $15 million to extend the $820 million mission to September.

"They are pretty well-built rovers. It’s pretty reasonable to say they beat their warranties," said Jeff Landis, a rover scientist at NASA’s Glenn Research Center near Cleveland.

Landis helped design the solar panels that power the rovers. Dust now coats those panels, limiting the amount of energy they can produce.

"They’re getting a little bit slower," he said. "They can’t drive all day like they used to."

NASA engineer Naigin Cox said Opportunity soon will be put into a deep sleep in hopes of shutting off a heater that is draining power. Utilizing the rovers’ low-gain antennae more than its high-gain antennae also will save power.

Knowing the stress the rovers can handle will help Cox and other engineers who are planning the 2009 Mars mission. That mission involves landing a minivan-size rover capable of roaming Mars for a year.

If the rovers are still functioning in September, NASA might further extend the mission to enable scientists to watch them die.

"We’d like to go into the twilight slowly and gracefully," Cox said.

mlafferty@dispatch.com