Mars rover rolls in wheel-saving reverse toward objective

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One of the main objectives of the Curiosity mission to Mars is to study the lower reaches of Mount Sharp. The exposed geology here could add supporting evidence to the discoveries made at Yellowknife Bay last March.

Analysis of sediments there showed the presence of clays with many of the elements needed to support microbial life. Mars was indeed habitable billions of years ago.

Scientists believe that by studying the layers of rock on the lower slopes of Mount Sharp they can determine how the Martian environment evolved from a habitable one to the current dry, acidic desert.

But there are no safe landing sites at the base of Mount Sharp. The closest safe landing site is about five miles away, and that is where Curiosity landed in August of 2012. After working in this area for almost a year, Curiosity started its trek to Mount Sharp last July. It started out on a route that had been planned using data and images from Mars orbiters.

As it travels along the route, Curiosity routinely uses its Mars Hand Lens Imager on the end of its robotic arm to take images of the undercarriage and wheels. These images are used to check for damage and monitor normal wear.

The October images showed some small holes in the left front wheel. This is not a problem — holes in Curiosity’s wheels are expected as the one-ton robot travels over sharp, fixed rocks.

The wheels are made of aluminum, and it is easy for these sharp rocks to punch holes in the soft metal.

Follow-up images showed more holes at a rate that was higher than expected.

But even at the higher rate, this damage is not a problem. All the wheels need to do is rotate and grip the ground. Each wheel has a very high torque motor, and NASA says that even if the wheels were square the rover could roll across the surface just fine.

Curiosity’s top speed on hard, flat terrain is only 0.09 mph, and therefore the wheels never turn very fast.

Even though it is not a problem, NASA would like to limit the damage to the wheels and make them last as long as possible, so they are implementing two changes.

The first is to find a route to Mount Sharp with fewer sharp rocks.

It appears that valleys have fewer sharp rocks than ridges do, and the sand in the valleys means that the rocks are not fixed and will do less damage.

The next planned science location was a waypoint called Kimberley, so NASA plotted a new route to Kimberley through two valleys, the Moonlight Valley and then the Violet Valley.

To reach these valleys, Curiosity had to cross Dingo Gap with its 3-foot-high sand dune. After careful assessment of the possible dangers of crossing the dune, Curiosity entered Dingo Gap and crossed the dune with no problems on Feb. 6.

The rover has now driven through both valleys, and it does appear that wheel damage is significantly less.

The second change was to add backward driving to Curiosity’s proven abilities.

Driving backward is a known strategy for reducing wear on the metal wheels. Both of the Mars exploration rovers, Spirit and Opportunity, have used backward driving in their journeys. In fact, Opportunity is driving in reverse most of the time now.

On Feb. 18 Curiosity covered 329 feet in reverse, traveling on relatively smooth and benign ground. This drive was a test to validate the rover’s ability to drive in reverse.

NASA wants this option in the tool bag because parts of the future route could be more challenging.

The current plan has Curiosity arriving at the base of Mount Sharp sometime in June.

When Curiosity reaches it, it will climb the mountain’s foothills, discovering the history of the planet’s changing environmental conditions by reading the information left in the rocks.

Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at marty.scott@wallawalla.edu.

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