NASA's Spirit Rover Descends From Martian Hilltop
NASA's Mars Exploration Rover Spirit completed a difficult, rocky ascent en route to reaching a captivating rock outcrop nicknamed 'Hillary' at the summit of 'Husband Hill.' At the end of the climb the robotic geologist was tilted almost 30 degrees and had to wiggle its wheels to gain a more solid footing.
Credit: NASA/JPL-Caltech.

After nearly one year of rolling and scrambling up Husband Hill, NASA's Mars rover Spirit is headed back down towards new and rocky pastures.

The rover is making its way down Husband's slopes toward a basin to the south, where a science target known as "Home Plate" awaits.

"It is a big step," said Steve Squyres, principal investigator of the rover's science mission at Cornell University in Ithaca, New York. "We've really switched gears."

Spirit's descent marks a new phase of the rover's Mars exploration, which has carried it across the plains and up one hill of its Gusev Crater landing site, Squyres told SPACE.com.

Meanwhile, Opportunity - Spirit's robotic partner in Mars exploration - has overcome a series of glitches as it roves around a wide crater dubbed Erebus on the other side of the red planet.

Hilltop departure

Spirit reached the top of Husband Hill, part of the seven-rise Columbia Hills chain, in late August after steadily scaling 270 feet (82 meters) of rocky terrain.

After comparing rocks at its summit - the last target, an outcrop dubbed "Hillary," required a bit of rover wiggling to observe - with those at lower elevations, Husband Hill's geology appears to be very similar from the top down, mission scientists said.

"It's very similar in composition, with very similar textures when you look at it with a microscope, but it's dipping in all different directions," Squyres said of the hill's geology. "What it says is that the geology around here is pretty complicated."

Husband Hill's similar composition seems to speak toward a violent beginning, researchers said, adding that the rise may represent part of an ancient crater rim or have been formed through some other uplift process.

"My first personal reaction is that it's maybe not surprising since Mars gets beaten up a lot by impacts," said Albert Haldemann, rover deputy project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, during an interview, adding that researchers have not settled on an impact as the hill's genesis.

Spirit is currently making its may along an area dubbed "Haskin Ridge" and will head toward "Home Plate," though the science target is not an ultimate goal.

"There's a lot of good stuff beyond it," Squyres said.

Haldemann added that it could take two months for Spirit to completely descend Husband Hill, depending on the number of interesting spots it turns up on the way down.

Opportunity recovers

On the plains of Meridiani Planum on the other side of Mars, Spirit's robotic partner Opportunity is again rolling westward around Erebus Crater.

Unlike Spirit, Opportunity experienced a series of glitches this month that stalled science operations for several days.

"We had a little string of bad luck and some hiccups here and there," Haldemann said.

On Oct. 4 - its 603rd day, or sol, on Mars - the rover halted a planned 148-foot (45-meter) drive after just 16 feet (five meters) when its wheels began slipping in Martian sand. The incident triggered Opportunity's maximum slip limits, and canceled the drive as a precaution against bogging down, mission managers said.

But the unplanned stop was a success of sorts for Opportunity, proving to rover controllers and mission scientists that the measures taken to prevent the robot from trapping itself in the Martian sand again work well. The rover bogged down in a dune for nearly five weeks early this year.

"A sol like that gives you confidence," Squyres said. "I'm really convinced now that we're able to keep the vehicle safe."

Opportunity also experienced two separate computer reboots - one well understood, the other not so much - that also prevented the robot from carrying out its science duties. The first reboot was due to a documented flight software glitch, while the second one also appears familiar to mission scientists.

"The good news is that it's happened again in the same fashion as it did some 200 sols ago," Haldemann said of the second reboot glitch. "And that may help us determine its cause."

But despite the glitches, Opportunity is again making steady progress around Erebus, a shallow depression that stretches about 984 feet (300 meters) in diameter.

"We're not going to take any risks," Squyres said, adding that it's unlikely Opportunity will explore Erebus as in depth as it did Endurance Crater. "It's a big crater in terms of diameter, but really shallow. It's nothing like Endurance."

A Martian anniversary

While Spirit and Opportunity have each performed well beyond their initial, 90-day mission - they've kept scientists on Earth busy for more than one and a half years - the two rovers still have one distinctively Martian milestone ahead of them.

But both robots have to keep roving at just over one more month to make it. Unlike Earth, which takes about 365 days to circle the Sun, Mars takes about 687 days - or sols - to make the trip.

On Nov. 21, Spirit will have spent an entire Martian year trekking across the red planet's surface, with Opportunity following in turn on Dec. 11.

"I actually see the Mars year anniversary as being more significant," Squyres said, "We will have seen Mars over an entire seasonal cycle."