SAN FRANCISCO — A critically important element of NASA’s Mars Science Laboratory (MSL), the sophisticated radar needed to help guide the 1,000-kilogram Curiosity rover gently onto Martian soil, is set to begin an extensive series of tests. NASA engineers have attached the radar to a helicopter to evaluate the accuracy of its measurements over varied terrain throughout Southern California.
Those tests are scheduled to be performed in April and May, said Steven Lee, MSL guidance, navigation and control manager at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. In August, the Ka-band radar, formally known as the Terminal Descent Sensor, is scheduled to undergo another series of tests. It will be taken to NASA’s Dryden Flight Research Center in Edwards, Calif., and strapped to the bottom of a Boeing F/A-18 supersonic fighter, Lee said.
Unlike the Mars Exploration Rovers Spirit and Opportunity, which were protected by airbags as they bounced around the surface during landing, the Curiosity rover is designed to be lowered via winch and cable from the spacecraft’s carrier platform onto the planet’s surface. Curiosity will land somewhere in a 20-kilometer area, while Spirit and Opportunity aimed for a region 200 kilometers long, Lee said. That targeted landing opens up a range of exciting scientific prospects because the rover could touch down in a Martian crater or other area that holds the promise of water or environmental conditions favorable to microbial life, but it also presents significant challenges for the guidance team, Lee said.
The MSL radar, which includes six individual antennas, was designed specifically for the mission. Each antenna uses a narrow, pencil beam to make precise measurements of the spacecraft’s altitude and velocity. “It’s like looking through a straw,” Lee said. “The challenge is to use the measurements from the antennas, each at a different angle, to find out how far away the ground is and how fast we are going.”
During the first round of tests, which were scheduled to begin April 9, MSL radar was to be mounted on a helicopter that is usually employed by film studios. The cradle in front of the helicopter’s nose that is designed to hold a camera has been equipped with engineering models of the MSL radar and inertial measurement unit as well as GPS equipment and laser rangefinders to confirm the readings, Lee said.
The tests are designed to show the radar’s ability to provide accurate measurements over varied terrain, including lava fields, dry lake beds and sand dunes, and to ensure that it can function reliably when individual antenna beams are obstructed. That will be the case when the radar is used to help guide the Curiosity rover onto Mars from the spacecraft’s upper stage. To confirm that feature, a mock-up of the MSL rover will be lowered from the radar-equipped helicopter, Lee said.
When testing resumes in August, the MSL radar will be reconfigured so a single antenna and associated electronics can be mounted on an F/A-18. During that testing phase, which is scheduled to run through September, the guidance team will confirm the accuracy of the radar at high altitudes. “We want to prove it works at all altitudes and velocities,” Lee said.
That testing involves a complex series of maneuvers that require the pilot to carry the radar above 15,000 meters, slow the aircraft, roll upside down and then point the plane’s nose straight at the ground. “We are trying to mimic the flight profile of the parachute and descent,” Lee said.
During the MSL mission, scheduled for launch in October 2011, the spacecraft is designed to release a parachute once it enters Mars’ atmosphere and then turn on retrorockets located around the spacecraft’s upper stage to slow the vehicle for a powered descent. When the upper stage is 18 meters above the ground, the rover will be lowered to the surface on wires in the same way a Skycrane helicopter lowers a large beam onto a construction site, Lee said. Once the rover reaches the ground, the wires used to attach it to the spacecraft will be cut and the roving laboratory will be ready to begin using its 10 onboard instruments to search for signs of past or present life, such as organic carbon compounds and water.
As the MSL team prepares to flight test the new radar, NASA officials also are continuing to clear the technical hurdles that have plagued the $2.3 billion program. The latest issues concern the rover’s nuclear power source and metal used in the vehicle’s construction.
NASA officials have detected a faster-than-expected degradation rate in the multimission radioisotope thermoelectric generator that is designed to extend the rover’s range and lifespan. That degradation is not expected to have a serious impact on the program, but it may lead to a reduction in activity during the Martian winter because it means the rover’s batteries will take longer to charge, NASA officials said.
At the same time, MSL officials are addressing concerns that the titanium used to construct the rover is not strong enough to endure the harsh conditions the vehicle is destined to face. In 2008, NASA’s titanium supplier was charged with fraud for falsifying documents to show that the material being supplied to the space agency met military specifications. Through a painstaking review process, NASA officials are confirming that the metal used to build Curiosity is strong enough for the job.
That review is more than 90 percent complete, and no serious setbacks have been discovered. “None of the parts we have finished analyzing require replacement because they meet the strength requirements for the application,” said Doug McCuistion, Mars Exploration Program director at NASA headquarters in Washington.
