Pasadena, Calif. ? After a journey of seven months and 303 million miles, a six-wheeled NASA rover will speed like a bullet tonight toward the surface of Mars and, if all goes as planned, stop with a bounce.

The plunge through the Martian atmosphere at 12,300 mph will mark the start of the riskiest portion of the voyage thus far.

As the unmanned spacecraft Spirit plummets to the rocky surface 80 miles below, it will rely on the precisely choreographed use of heatshields, parachutes and rockets to slow its descent. Just eight seconds before hitting the ground, the golf cart-size Spirit should inflate a set of airbags to cushion its impact.

The entire trip down should take just six minutes. A gust of wind or a single sharp boulder could doom the entire enterprise.

“It’s going to be six minutes from hell. It’s going to be high anxiety,” Ed Weiler, NASA’s associate administrator for space science, said Friday at NASA’s Jet Propulsion Laboratory.

Early Friday, Spirit was 231,000 miles from Mars, or about the distance separating the Earth and its moon.

The $820 million project also includes a twin rover, Opportunity, which is set to arrive Jan. 24 on Mars.

The camera- and instrument-laden rovers are designed to spend 90 days analyzing Martian rocks and soil for clues that could reveal whether the Red Planet was ever a warmer, wetter place capable of sustaining life.

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If successful, the 384-pound Spirit and its twin would become the fourth and fifth U.S. spacecraft to survive landing on Mars. If neither survives, they will join the ranks of some 20 other spacecraft from various nations that failed to successfully reach the planet.

“Some, including myself, call it the ‘death planet,'” Weiler said.

The latest to fail, apparently, were Japan’s Nozomi satellite and Britain’s Beagle 2 lander. Nozomi was unable to enter Mars orbit last month; Beagle 2 has been silent since it was to have landed on Christmas Day.

NASA hopes to learn whether Spirit has landed safely within 10 minutes of its expected 6:35 p.m. CST arrival.

If NASA scientists have not heard from Spirit within 22 hours of landing, the likelihood that it survived is very low, project manager Pete Theisinger said.