This story was updated at 3:55 EDT.
PASADENA, Calif. ? The big day has finally arrived: After 10 months of spaceflight, NASA?s Phoenix Mars Lander is headed for its long-awaited attempt to touch down in the arctic region of the red planet later today.
?We?ve bet the whole farm on this safe landing,? said Phoenix principal investigator Peter Smith of the University of Arizona in a Saturday briefing here at NASA?s Jet Propulsion Laboratory (JPL). ?We can?t do our science without the safe landing.?
The $420-million Phoenix mission, which launched in August, is expected to dig down into the rock-hard layers of water ice thought to lie under the Martian soil in the planet?s northern polar region. It carries a robotic arm, ovens and wet chemistry lab to test the soil and ice to see if the region could have once been a habitable zone for microbial life.
The craft has performed beautifully so far, with all systems checking out throughout the 422 million-mile (679 million-km) trip to Mars, mission scientists have said. ?Now what we need is a successful entry, descent and landing,? said Doug McCuistion, director of the Mars Exploration program for NASA.
Phoenix is slated to land on the Vastitas Borealis plains of Mars later today, with mission scientists expecting to receive the first signal that Phoenix has landed at 7:53 p.m. EDT (2353 GMT). (The signal should arrive at Earth about 15 minutes after leaving Mars due to the 171 million miles (275 million km) between the red planet and Earth.) The spacecraft currently has about 30,000 miles (50,000 km) left to travel, Smith said, adding that from the spacecraft, Mars would look about 10 times the size of a full moon.
If all goes well, Phoenix will touchdown under its own rockets to mark NASA?s first powered landing on Mars since the agency?s two Viking probes landed in 1976. The last spacecraft to attempt a powered landing - the ill-fated Mars Polar Lander (MPL) - crashed before reaching the surface of the planet?s southern polar region in 1999.
Phoenix mission engineers believe they have worked out all the problems that plagued MPL and are hoping the system is all set to guide the craft safely to the surface. If successful, the spacecraft would join NASA?s Spirit and Opportunity rovers, which used airbags to land in 2004 and are currently roaming the planet?s equatorial regions.
NASA will broadcast Phoenix?s approach and landing attempt live on NASA TV from the JPL control room here, with commentary set to begin at 6:30 p.m. EDT (2200 GMT).
Fiery descent ahead
By the time Phoenix arrived at Mars this morning, it was travelling at 6,100 mph (9,800 kph), the Mars Reconnaissance Orbiter, Mars Odyssey Orbiter and the ESA?s Mars Express will be in position to observe its landing attempt and relay information back to Earth.
To make a successful landing, the craft must execute a complicated series of actions in a very short amount of time; it will take just seven terrifying minutes for the spacecraft to plummet through the Martian atmosphere, mission managers have said.
?It?s not going to be an easy one,? said Phoenix mission manager Joe Guinn of JPL. But he added, ?I think we?re actually in fairly good shape.?
Mission scientists decided to forgo an opportunity to adjust the craft?s trajectory late Saturday and again this morning because Phoenix remains on course toward its target landing ellipse: a 50-mile (80-km) long drop zone that sits in a broad, shallow valley. Controllers have sent Phoenix its last instructions before landing, so ?the rest of the day is just watching and waiting,? Smith said.
All that watching and waiting, ?means I?m a heck of a lot more tense today,? said Barry Goldstein, Phoenix project manager of JPL.
Mars? gravity has started to accelerate the craft, which will eventually enter the Martian atmosphere at 12,600 mph (20,300 kph) as it approaches the planet. ?Today our spacecraft is starting to feel the pull of Martian gravity,? Smith said at press briefing today at JPL. ?I understand this well; I?ve been feeling the pull of Martian gravity for 15 years?
Phoenix should separate from its cruise stage at about 7:39 p.m. EDT (2339 GMT) tonight, after which there will be a three-second communication blackout before the craft?s UHF radio antenna is supposed to kick in. Goldstein has said that milestone is the one that worries him most. If communications aren?t restored between Phoenix and Earth and the lander fails, scientists won?t be able to glean any data to tell what went wrong, he added.
Smith agreed: ?If you see me freak out, it?s because we?ve lost the signal and we don?t know what that means.?
The spacecraft is then supposed to turn and enter the atmosphere, its heat shield guarding it from the superhot plasma created by friction as the craft falls. (This plasma could cause another blackout period by interfering with the UHF antenna.) This stage will reduce Phoenix?s speed by 90 percent until it?s falling at 1.5 times the speed of sound.
Phoenix is then supposed to deploy its orange and white parachute, which will slow the craft to about 1,100 mph (TK kph), jettison its heat shield and stick out its landing legs. Shortly after, its radar system should activate, providing the craft with its first measurement of where the ground is.
Once that occurs, Phoenix should separate from its back shell at 7:50:15 p.m. EDT (2350:15 GMT), ?and then shortly after that, the fireworks, literally, will begin,? Goldstein said, as the craft fires up its thrusters to further slow its descent and guide it to a three-point landing.
Smith and Goldstein will be with other mission scientists in mission control, monitoring the radio signal sent from Phoenix to Mars Odyssey Orbiter that could give them and idea of how things are going.
The first images from the spacecraft on Mars could be received as soon as a few hours after landing, but only after Phoenix?s vital solar arrays deploy to begin generating power for its planned six-month mission. Without the solar panels, Phoenix only has about 31 hours of battery power on which to live.
?This is truly the defining moment of this mission,? McCuistion said of today's landing attempt.
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