Opportunity knocks on Mars' atmosphere
Spirit 'upgraded from critical to serious,' NASA manager says
This white oval shows Opportunity's expected landing site.
PASADENA, California (CNN) -- The Mars Opportunity rover is hurtling toward the surface of the red planet for a touchdown planned Sunday at 12:06 a.m. ET (0506 GMT).
If successful, it would be the second U.S. craft to land on Mars this month. Opportunity's twin rover, Spirit, landed January 3 but on the opposite side of the planet.
Mission manager Jim Erickson said Opportunity was in "great shape" for the landing but the planned site would likely make its landing more difficult than Spirit's.
Opportunity is set to touch down on the Meridiani Planum, a smooth plain near Mars' equator, believed to be full of iron-bearing hematite. The semi-precious mineral usually forms on Earth in the presence of liquid water.
In addition, Mars' increased distance from Earth would lengthen the time needed to communicate with the spacecraft.
"Last night we performed the last engineering activity," he said, which prepared the rover's battery to control it once it lands.
Entry, descent and landing development manager Rob Manning said the rover was angled for entry differently from how Spirit was, meaning it could lose contact with Earth sporadically -- and possibly for long stretches of time -- on the way down or once it lands.
Although Spirit had a near-perfect landing, the rover mission has had complications during the past few days.
NASA engineers found a work-around Saturday for its problems, and established communication and regained the ability to control it.
"This is very good news," project manager Pete Theisinger told reporters.
Spirit's condition, Theisinger said, "has been upgraded from critical to serious."
The rover is probably "three weeks away from driving," he said, as engineers study the problems and try to correct them with additional work-arounds in the meantime.
Spirit uses Flash memory to communicate with the flight software to establish a file structure and will shut itself down if the process is interrupted, Theisinger said. Engineers guessed that Spirit's troubles were in its Flash memory and set about sending the rover a complex series of instructions to see if they could get it to bypass the corrupted memory.
Theisinger said engineers sent Spirit a command just before its daily "waking up," telling it to shut down and restart in what is known as "cripple mode," using RAM instead of Flash for its start-up instructions.
"That is precisely what happened," Theisinger said, and Spirit then sent an hour's worth of data back to Pasadena.
"Something in the flight software talking to the Flash memory is causing us difficulty," Theisinger said.
He said engineers did not know caused the problem, but that if it is purely a software problem, it is likely fixable. If, however, if a problem in the hardware is affecting the software, repair may not be possible.
But "we have a vehicle that is stable now," he said.