Two NASA spacecraft circling Mars have begun repositioning their orbits to provide engineering insight into the landing of the Curiosity rover on the red planet in August, supplying engineers on Earth with vital data during the robot's dramatic rocket-assisted touchdown.

Artist's concept of the Curiosity rover being lowered from the Mars Science Laboratory's rocket-powered descent pack in the final stages of entry, descent and landing. Credit: NASA/JPL-Caltech
 
Engineers are shifting the orbits of the Mars Reconnaissance Orbiter and Odyssey probes, ensuring they have line-of-sight communications with Curiosity as it lands at Gale crater, a 96-mile-wide impact site adorned with rugged rock formations and a colossal central peak.

Landing is scheduled for early Aug. 6, U.S. Eastern time.

"Odyssey and MRO have begun positioning their orbits so that they will be overflying [the Mars Science Laboratory] during entry, descent and landing," said Fuk Li, director of Mars exploration at the Jet Propulsion Laboratory in Pasadena, Calif.

Without MRO and Odyssey, controllers would lose crucial information on how the $2.5 billion mission performs as it plunges through the Martian atmosphere at hypersonic speeds, deploys parachutes, fires a rocket back and lowers the six-wheeled rover to the surface on a bridle.

NASA has never tried such a landing system before, and if it works, it will allow the space agency to place much larger, and heavier, payloads on Mars. Live insight into the landing will yield crucial information on how the system works, and would give NASA reams of data in the event of a mishap or failure.

Artist's concept of the Odyssey spacecraft. Credit: NASA/JPL-Caltech
 
The Odyssey spacecraft, which has orbited Mars since late 2001, will be the primary means of monitoring Curiosity's progress during landing. Odyssey will be in view of both Curiosity and Earth, so the orbiter will offer bent-pipe communications, receiving transmissions from the rover and relaying them directly to Earth.

Traveling at the speed of light, the signals will reach Earth several minutes later.

The rover carries its own X-band direct-to-Earth transmitter, but it only provides a communications rate of about 1 bit per second, just enough to send status tones back to mission control at JPL.

"It will only tell us roughly what it is doing," Li said.

With Odyssey, engineers will receive a stream of data at 8,000 bits per second.

After losing the Mars Polar Lander mission on descent to the red planet in 1999, NASA began requiring live communications with probes heading for the Martian surface, according to the space agency. NASA received no data from MPL during landing, challenging an engineering investigation tasked with finding a cause of the failure.

Artist's concept of the Mars Reconnaissance Orbiter. Credit: NASA/JPL-Caltech
 
Launched in 2005, MRO will collect data from Curiosity and play it back to Earth about an hour after landing.

The European Space Agency's Mars Express spacecraft will also listen to Curiosity during the landing, giving NASA four methods of obtaining information on how the lander performed.

"Spacecraft all around Mars will be listening as we enter that day," Li said.

Odyssey also provided real-time bent-pipe communications from NASA's Phoenix lander when it touched down on the northern polar plains of Mars in 2008.

The Spirit and Opportunity rovers, which arrived at Mars in 2004, only communicated with a direct-to-Earth transmitter.

The Mars orbiter fleet will also be essential during Curiosity's science operations. Odyssey and MRO will relay commands to the rover and research data, imagery and health information back to Earth.

"For MSL, they are instrumental to make it work," Li said. "Without them, MSL's operation would probably slow down by a factor of ten."