Shoulder Motor Balks on Opportunity's Robotic Arm

A small motor in the robotic arm of NASA's Mars Exploration Rover Opportunity that began stalling occasionally more than two years ago has become more troublesome recently.

Rover engineers at NASA's Jet Propulsion Laboratory, Pasadena, Calif., are diagnosing why the motor, one of five in the robotic arm, stalled on April 14 after much less motion that day than in the case of several earlier stalls. They are also examining whether the motor can be used and assessing the impact on Opportunity's work if the motor were no longer usable.

The motor controls sideways motion at the shoulder joint of the rover robotic arm. Other motors provide up-and-down motion at the shoulder and maneuverability at the elbow and wrist. A turret at the end of the arm has four tools that the arm places in contact with rocks and soils to study their composition and texture.

"Even under the worst-case scenario for this motor, Opportunity still has the capability to do some contact science with the arm," said JPL's John Callas, project manager for the twin rovers Opportunity and Spirit. "The vehicle has quite a bit of versatility to continue the high-priority investigations in Victoria Crater and back out on the Meridiani plains after exiting the crater."

The performance of the motor in the past week is consistent with increased resistance in the electrical circuit, such as from degrading of wire in the winding, rather than a mechanical jam. Additional tests are planned for checking whether the apparent resistance is localized or intermittent.

Opportunity and Spirit landed on Mars in January 2004 to begin missions originally planned for three months. They have continued operating for more than four years, though each with some signs of aging.

Opportunity's balky shoulder motor began stalling occasionally in November 2005. The motor could still be operated by applying increased voltage. Engineers assessed it has an increased likelihood of becoming unusable, however, so the team changed its standard procedures for stowing and unstowing the arm.

Until then, on days when the arm would not be used, the team kept it stowed, resting on a hook under the front of the rover deck. Motion of the stall-prone shoulder motor is necessary to unstow the arm, so if the motor were to become unusable with the arm in the stowed position, the arm could not be deployed again. With diminished confidence in the balky motor, the team began unstowing the arm at the end of each day's drive rather than leaving it stowed overnight. This keeps the arm available for use even if the motor then stops working.

This spring, Opportunity is crossing an inner slope of Victoria Crater to reach the base of a cliff portion of the crater rim, a promontory called "Cape Verde." On April 14, Opportunity was backing out of a sandy patch encountered on the path toward Cape Verde from the area where the rover descended into the crater. As usual, the commands included unstowing the arm at the end of the day's short drive. The shoulder motor barely got the arm unstowed before stalling.

"We'll hold off backing out of the sand until after we've completed the diagnostic tests on the motor," Callas said. "The rover is stable and safe in its current situation, and not under any urgency. So we will take the time to act cautiously."

source : www.nasa.gov

NASA's moon rockets will sport shocks to smooth out bumpy ride

The nation's new moon rockets will be outfitted with shock absorbers to buffer astronauts from jackhammer-like vibrations during rocky rides into orbit.

A spring-and-damper ring will separate the first and second stages of Ares 1 rockets, which NASA is developing for missions to the International Space Station, the moon and later Mars.

Sixteen actuators that act like shock absorbers also will be added to the bottom of the rockets, significantly reducing the gravitational forces and vibrations astronauts will be exposed to in flight.

"The good news here is we've got a solution that will solve the thrust oscillation phenomenon," said Steve Cook, manager of Ares Projects at NASA's Marshall Space Flight Center in Huntsville, Ala.

NASA's Ares 1 rocket is being developed to launch Apollo-style Orion crew capsules.

The rocket's first stage will comprise a five-segment solid rocket booster derived from the four-segment boosters that help propel shuttles into orbit. The second stage will be powered by a liquid-fueled engine.

NASA design engineers found the Ares 1 — dubbed the "single stick" due to its slender shape — would shake violently near the end of a two-minute firing of the rocket's first stage.

This "thrust oscillation" is induced as solid fuel in the first stage depletes, leaving a long, empty metal case that takes on the characteristics of a pipe organ, resonating at frequencies between 12 and 14 hertz.

The resulting vibrations could shake the Orion crew capsule enough to make it difficult for astronauts to read cockpit displays. In a worst case, astronauts might be injured or critical components of the spacecraft could be damaged.

An internal NASA task force determined the second stage of the vehicle and the Orion crew capsule would naturally dampen resulting pressure pulses.

The astronauts, however, would be subjected to forces five or six times that of normal gravity — or about double the three G's shuttle crews are exposed to during nine-minute climbs into orbit.

The task force recommended NASA add the shock absorber system, which will reduce gravitational forces to 0.25 G, or about the same level that Mercury and Gemini astronauts were exposed to.

The system will reduce the lift capacity of the Ares 1 rockets by 1,200 to 1,400 pounds. But Cook said the rockets still will be powerful enough to fly missions to the space station and moon.

NASA aims to debut the Ares 1 rocket and Orion spacecraft by March 2015 — five years after the agency's shuttle fleet is retired. The target for the first moon mission is 2020.

source : www.usatoday.com

Archaeologists get a glimpse of life in a Sahara Eden

The tiny skeletal hand jutted from the sand as if beckoning the living to the long dead.

For thousands of years, it had lain unheeded in the most desolate section of the Sahara, surrounded by the bones of hippos, giraffes and other creatures typically found in the jungle.

A chance discovery by a team of American scientists has led to the unearthing of a Stone Age cemetery that is providing the first glimpses of what life was like during the still-mysterious period when monsoons brought rain to the desert and created the "green Sahara."

The more than 200 graves that have been explored so far indicate that, beginning 10,000 years ago, two populations lived on the shores of a massive lake, separated by a 1,000-year period during which the lake dried up.

Among the scientists' most surprising discoveries has been a poignant burial tableau of a woman and two children with fingers intertwined, a find that is putting a surprisingly human face on the little-known people who enjoyed a brief visit to Eden in what is normally one of the most forbidding places on Earth.

The first to settle the area was a group of tall, powerfully built hunters, gatherers and fishermen called the Kiffian, University of Chicago paleontologist Paul Sereno said at a news conference Thursday.

The group that followed the Kiffian was a physically smaller band of pastoralists called the Tenerian, who relied on fishing and hunting but also herded cattle, he said.

"They've managed to find these people," said archaeologist Anne Haour of the University of East Anglia in Britain, who was not involved in the research. "We've always suspected something was going on, but this is the first time it has been properly documented."

In addition to the graves, researchers found a massive collection of the remains of meals, tools, pots and other artifacts -- the detritus of everyday life.

"This is a real find . . . for a time period that is not very well documented in that part of the world," said archaeologist Kathy Schick of Indiana University's Stone Age Institute. "It's just a gold mine of information."

The new findings were published Thursday in the online journal PLoS One and in the September issue of National Geographic magazine.

The Sahara has been a desert for untold millenniums. But about 12,000 years ago, a faint wobble in Earth's orbit and some other factors caused Africa's seasonal monsoons to shift slightly north, bringing rains to the Sahara and greening it from Egypt in the east to Mauritania in the west.

About 8,000 years ago, the rains retreated, leaving the region arid once more and causing it to be abandoned. A thousand years later, the rains returned for two more millenniums, before again retreating.

The newly discovered site, called Gobero after the Tuareg nomads' name for the area, lies deep in Niger's Tenere Desert, a large region in the still larger Sahara. The site lay unobserved and untouched because it was literally "in the middle of nowhere," Sereno said. "There is absolutely no reason for anyone to go there."

Sereno had a reason: a nearby table of 110-million-year-old sandstone "that has more dinosaurs in it of high quality than any other rock in the continent of Africa."

In 2000, Sereno and a small group of colleagues were on one of their periodic forays in which they would load up a Land Rover and travel as far from their main site as they could in one day, looking for dinosaur bones.

"We were at the end of our rope," Sereno said, nearly out of water and ready to turn around, when he spotted a stone formation sticking up in the distance and decided to go a little farther.

When they got there, they found animal bones scattered on the surface, exposed by the weather. Photographer Mike Hettwer wandered off to a trio of small dunes, then rushed excitedly back to the group.

"I found some bones," he told them. "But they're not dinosaurs. They're human."

The hand, probably belonging to a child, stood out amid the flat landscape, its finger bones blackened but intact. The researchers saw parts of dozens of human skeletons, including jawbones with nearly full sets of teeth and skullcaps like serving dishes filled with sand.


source : www.latimes.com