Thisstory was updated at 3:55 EDT.
PASADENA, Calif. ? The big day has finallyarrived: After 10 months of spaceflight, NASA?s Phoenix Mars Lander is headedfor its long-awaited attempt to touch down in the arctic region of the redplanet later today.
?We?ve bet the whole farm on this safelanding,? said Phoenix principal investigator Peter Smith of the University ofArizona 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, whichlaunched in August, is expectedto dig down into the rock-hard layers of water ice thought to lie under theMartian 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 couldhave 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 successfulentry, 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, withmission scientists expecting to receive the first signal that Phoenix haslanded at 7:53 p.m. EDT (2353 GMT). (The signal should arrive at Earth about 15minutes after leaving Mars due to the 171 million miles (275 million km)between the red planet and Earth.) The spacecraft currently has about 30,000miles (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 touchdownunder its own rockets to mark NASA?s first powered landing on Mars since theagency?s two Viking probes landed in 1976. The last spacecraft to attempt apowered landing - the ill-fated Mars Polar Lander (MPL) - crashed beforereaching the surface of the planet?s southern polar region in 1999.
Phoenix mission engineers believe they haveworked out all the problems that plagued MPL and are hoping the system is allset to guide the craft safely to the surface. If successful, the spacecraftwould join NASA?s Spirit and Opportunity rovers, which used airbags to land in2004 and are currently roaming the planet?s equatorial regions.
NASA will broadcast Phoenix?s approach andlanding attempt live on NASA TV from the JPL control room here, withcommentary set to begin at 6:30 p.m. EDT (2200 GMT).
Fiery descent ahead
By the time Phoenix arrived at Mars thismorning, it was travelling at 6,100 mph (9,800 kph),the Mars Reconnaissance Orbiter, Mars Odyssey Orbiter and the ESA?s MarsExpress will be inposition to observe its landing attempt and relay information back toEarth.
To make a successful landing, the craft mustexecute a complicated series of actions in a very short amount of time; it willtake just seventerrifying minutes for the spacecraft to plummet through the Martianatmosphere, mission managers have said.
?It?s not going to be an easy one,? saidPhoenix mission manager Joe Guinn of JPL. But he added, ?I think we?re actuallyin fairly good shape.?
Mission scientists decided to forgo anopportunity to adjust the craft?s trajectory late Saturday and again thismorning because Phoenix remains on course toward its target landing ellipse: a50-mile (80-km) long drop zone that sits in a broad, shallow valley. Controllershave sent Phoenix its last instructions before landing, so ?the rest of the dayis just watching and waiting,? Smith said.
All that watching and waiting, ?means I?m aheck of a lot more tense today,? said Barry Goldstein, Phoenix project managerof JPL.
Mars? gravity has started to accelerate thecraft, which will eventually enter the Martian atmosphere at 12,600 mph (20,300kph) as it approaches the planet. ?Today ourspacecraft is starting to feel the pull of Martian gravity,? Smith said atpress briefing today at JPL. ?I understand this well; I?ve been feeling thepull of Martian gravity for 15 years?
Phoenix should separate from its cruise stageat about 7:39 p.m. EDT (2339 GMT) tonight, after which there will be athree-second communication blackout before the craft?s UHF radio antenna issupposed to kick in. Goldstein has said that milestone is the one that worrieshim most. If communications aren?t restored between Phoenix and Earth and the lander fails, scientists won?t be able to glean any data totell what went wrong, he added.
Smith agreed: ?If you see mefreak out, it?s because we?ve lost the signal and we don?t know what that means.?
The spacecraft is then supposed to turn andenter the atmosphere, its heat shield guarding it from the superhot plasmacreated by friction as the craft falls. (This plasma could cause anotherblackout period by interfering with the UHF antenna.) This stage will reducePhoenix?s speed by 90 percent until it?s falling at 1.5 times the speed of sound.
Phoenix is then supposed to deploy its orangeand white parachute, which will slow the craft to about 1,100 mph (TK kph), jettison its heat shield and stick out its landinglegs. Shortly after, its radar system should activate, providing the craft withits first measurement of where the ground is.
Once that occurs, Phoenix shouldseparate from its back shell at 7:50:15 p.m. EDT (2350:15 GMT), ?and thenshortly after that, the fireworks, literally, will begin,? Goldstein said, asthe craft fires up its thrusters to further slow its descent and guide it to athree-point landing.
Smith and Goldstein will be with othermission scientists in mission control, monitoring the radio signal sent fromPhoenix to Mars Odyssey Orbiter that could give them and idea of how things aregoing.
The first images from the spacecraft on Marscould be received as soon as a few hours after landing, but only afterPhoenix?s vital solar arrays deploy to begin generating power for its plannedsix-month mission. Without the solar panels, Phoenix only has about 31 hours ofbattery power on which to live.
?This is truly the defining moment of thismission,? McCuistion said of today's landing attempt.
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