NASA's Mars rover may not phone home right away

PASADENA, Calif., Aug 5 (Reuters) - NASA cheekily calls the Mars Science Lab’s risky approach and landing attempt the “seven minutes of terror.” In reality, the anxiety at mission control could last much longer.

The robotic lander dubbed Curiosity, a $2.5 billion mission to search for life-friendly habitats on the Red Planet, was on autopilot for touchdown at 10:31 p.m. PDT Sunday (1:31 a.m. EDT Monday/0531 GMT).

If all goes as planned, NASA will heave a big sigh of relief immediately. But word of whether Curiosity survives may not come for hours.

Because Earth sets over the Martian horizon minutes before Curiosity is due to land, direct communication with the lander will be cut off. Scientists tracking the craft from the Jet Propulsion Laboratory in California will depend on three other spacecraft in orbit around Mars to transmit information about the rover’s fate.

One of those orbiters, nicknamed Odyssey, can potentially relay Curiosity’s descent and landing signals in real time. But NASA won’t know if this satellite can be properly positioned for live coverage until about 15 minutes before Curiosity hits the Martian atmosphere.

The Odyssey orbiter, flying about 250 miles (400 km) over Mars, lost one of its steering wheels in June, and flight controllers have had to learn how to maneuver it with a spare.

All communications between Curiosity and Earth, whether directly with the probe or relayed via the Mars orbiters, are sent and received by NASA through its Deep Space Network, a collection of satellite dishes arrayed in California, Spain and Australia.

Once direct communication with Curiosity is cut off, it will be another day before Earth and Mars are properly aligned again for a “line-of-sight” linkup.

If proper alignment of Odyssey fails, that leaves NASA to seek data from two other orbiters to confirm whether Curiosity safely touched down or crashed.

Like Odyssey, both the Mars Reconnaissance Orbiter and Europe’s Mars Express will be flying overhead during Curiosity’s descent and landing and should be recording the event. But getting those signals back to Earth and deciphered will take eight hours or more.

Moreover, there is no guarantee that the Reconnaissance Orbiter or Mars Express would succeed in their first attempts at relaying landing data to NASA.

“If we get past 24 hours, it’s more likely than not that we had a problem,” deputy project manager Richard Cook said. (Editing by Steve Gorman and Stacey Joyce)