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PASADENA, Calif. - NASA's Mars Science Laboratory is performing perfectly, and the weather on Mars is cooperating, for a landing in the planet's Gale Crater Sunday night at 1:31 a.m. EDT Aug. 6.

Controllers at the Jet Propulsion Laboratory cancelled a scheduled trajectory-correction manuever Saturday night when Deep Space Network tracking indicated the MLS spacecraft will hit its target atop the  planet's atmosphere.

"Our trajectory inbound to Mars has been right down the pipe," Arthur Amador, the MSL mission manager, told reporters here today.

If anything changes, there will be three more chances to tweak MSL's course before entry -- one today and two more tomorrow. Aside from that, the $2.5 billion spacecraft has been operating  autonomously since Monday, Amador said, and the only command definitely remaining to be uploaded is software for the  backup flight computer in case the prime fails before the long-awaited entry, descent and landing (EDL).

If the nuclear-powered Curiosity rover tucked in behind a 4.5-meter heat shield survives its "seven minutes of terror" -- slowing from 13,000 mph to 1.7 mph with atmospheric friction, parachutes, eight retrorockets and the untested "sky crane" descent to touchdown - the control room here may know as soon as radio signals make the 13.8-minute trip from Mars to Earth. "You'll see us celebrate," said Richard Cook, the MLS deputy project manger.

But that will depend on the Mars Odyssey orbiter, which will be in position to relay UHF signals from the rover to Earth in a "bent-pipe" setup. If there is a problem, and the signals aren't received, the seven minutes of terror could stretch into a day or more. Odyssey, the Mars Reconnaissance Orbiter (MRO), and Europe's Mars Express orbiter all will be receiving UHF signals from MSL as it lands. But only Odyssey will be able to reroute the signals immediately.

The MRO and Mars Express will record them for rebroadcast to Earth, and controllers will have several different chances to verify their spacecraft has landed safely in the hours after touchdown. If the silence stretches to 24 hours, Cook said, hope will begin to fade.

"There are certainly very credibly scenarios  by which those Odyssey and MRO passes might not happen in the firist 18 hours," Cook said. "Because of that, we might identify
ways in which we would have to wait untilo the next morning, for example, to hear the MRO  pass."

In addition to UHF broadcasts, the rover is set up to broadcast X-band "beeps" containing minimal  data. That would reassure controllers that the spacecraft is in one piece, even if the primary UHF radio fails for some reason. It would take several days for the backup UHF radio to activate itself, so if the rover isn't beeping when Mars swings it back into line of sight with Earth, it probably will mean trouble.

"Once we get past that first beep from the X-band system, if we haven't heard from it in any of those communications paths, including that one, then I think ... more likely than not that we would have a problem," Cook said. "It would take 24 hours."

                            

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