Researchers have calculated that carbon dioxide snow particles on Mars are roughly the size of a human red blood cell. Martian snow is depicted in this artist's rendering as a mist or fog that eventually settles to the surface.
PASADENA, Calif. (AP) — NASA's most high-tech Mars rover on Sunday zeroed in on the red planet where it will attempt a tricky celestial gymnastics routine during a "seven minutes of terror" plummet through the atmosphere.
The Curiosity rover was poised to hit the top of the Martian atmosphere at 13,000 mph. If all goes according to script, it will be slowly lowered by cables inside a massive crater in the final few seconds.
NASA was ready for the "Super Bowl of planetary exploration," said Doug McCuistion, head of the Mars exploration program at NASA headquarters.
"We score and win or we don't score and we don't win," said McCuistion.
If all goes well, mission control at the NASA Jet Propulsion Laboratory should hear a signal at 10:31 p.m. Pacific. The space agency warned that confirmation could take longer if an orbiting spacecraft that's supposed to listen for Curiosity during the descent is not in the right place.
Curiosity's trajectory was so accurate that engineers decided to wave off a last chance to tweak its position before atmosphere entry.
"We're ready to head in," said mission manager Brian Portock.
Not ones to tempt fate, flight controllers planned to break out the "good luck" peanuts before Curiosity takes the plunge as part of a long-running tradition.
One scientist who can relate to the building anxiety is Cornell University planetary scientist Steve Squyres, who headed NASA's last successful rover mission in 2004.
This time around, Squyres has a supporting role and planned to view the landing with other researchers in the "science bullpen."
"Landing on Mars is always a nerve-racking thing. You're never going to get relaxed about something like landing a spacecraft on Mars," said Squyres.
Sunday's touchdown attempt was especially intense because NASA is testing a brand new landing technique. Due to the communication delay between Mars and Earth, Curiosity will be on autopilot. There's also extra pressure because budget woes have forced NASA to rejigger its Mars exploration roadmap.
"There's nothing in the pipeline" beyond the planned launch of a Mars orbiter in 2013, said former NASA Mars czar Scott Hubbard, who teaches at Stanford University.
Curiosity was launched to study whether the Martian environment ever had conditions suitable for microbial life.
The voyage to Mars took over eight months and spanned 352 million miles. The trickiest part of the journey? The landing. Because Curiosity weighs nearly a ton, engineers drummed up a new and more controlled way to set the rover down.
The last Mars rovers, twins Spirit and Opportunity, were cocooned in air bags and bounced to a stop in 2004.
The plans for Curiosity called for a series of braking tricks, similar to those used by the space shuttle, and a supersonic parachute to slow it down. Next: Ditch the heat shield used for the fiery descent.
And in a new twist, engineers came up with a way to lower the rover by cable from a hovering rocket-powered backpack. At touchdown, the cords cut and the rocket stage crashes a distance away.
The nuclear-powered Curiosity, the size of a small car, is packed with scientific tools, cameras and a weather station. It sports a robotic arm with a power drill, a laser that can zap distant rocks, a chemistry lab to sniff for the chemical building blocks of life and a detector to measure dangerous radiation on the surface.
It also tracked radiation levels during the journey to help NASA better understand the risks astronauts could face on a future manned trip.
After several weeks of health checkups, the six-wheeled rover could take its first short drive and flex its robotic arm.
The landing site near Mars' equator was picked because there are signs of water everywhere, meeting one of the requirements for life as we know it. Inside Gale Crater is a 3-mile-high mountain, and images from space show the base appears rich in minerals that formed in the presence of water.
Previous trips to Mars have uncovered ice near the Martian north pole and evidence that water once flowed when the planet was wetter and toastier unlike today's harsh, frigid desert environment.
Curiosity's goal: To scour for basic ingredients essential for life, including carbon, nitrogen, phosphorous, sulfur and oxygen. It's not equipped to search for living or fossil microorganisms. To get a definitive answer, a future mission needs to fly Martian rocks and soil back to Earth to be examined by powerful laboratories.
The mission comes as NASA retools its Mars exploration strategy. Faced with tough economic times, the space agency pulled out of partnership with the European Space Agency to land a rock-collecting rover in 2018. The Europeans have since teamed with the Russians as NASA decides on a new roadmap.
Despite Mars' reputation as a spacecraft graveyard, humans continue their love affair with the planet, lobbing spacecraft in search of clues about its early history. Out of more than three dozen attempts — flybys, orbiters and landings — by the U.S., Soviet Union, Europe and Japan since the 1960s, more than half have ended disastrously.
One NASA rover that defied expectations is Opportunity, which is still busy wheeling around the rim of a crater in the Martian southern hemisphere eight years later.
Mars mission: http://www.nasa.gov/msl
Follow Alicia Chang's Mars coverage at: http://www.twitter.com/SciWriAlicia