In less than four hours, Phoenix will begin its final descent. And, at this point, there’s nothing that mission scientists and engineers can do about it. “The rest of the day is watching and waiting,” said Phoenix PI Peter Smith at the final pre-landing press briefing from Jet Propulsion Laboratory in California.

Mission engineers decided to forgo a final course correction. A lumpy mound of rocks sits a few kilometers to the northwest of the expected centerpoint of landing. But the chance of landing in the rocks is only 1%, and within the mound, the chance of settling on a rock is only 10%. So engineers didn’t think the 1 in a 1000 risk of a rock was worth the risk of altering the trajectory.

The spacecraft has sidled up to Mars, accelerating from 6,300 mph this morning to 8,500 mph by the time of the press conference. By the start of entry this afternoon, just before 5 pm Pacific time, it will be moving at 12,700 mph.

“Today our spacecraft is starting to feel the pull of Martian gravity,” said Smith. “But I’ve been feeling the pull of Martian gravity for 15 years.”

After separation from its cruise stage, Phoenix will have to decelerate from 12,700 mph to zero – within seven minutes – using a combination of atmospheric friction, parachute, and thrusters. Phoenix will also lose its high bandpass signal to Deep Space Network antenna on Earth. Relying only on its battery powered radio, Phoenix will pump out a weak UHF signal that is likely to be picked up by both the Greenbank radio telescope in West Virginia and the Odyssey orbiter.

But it won’t be until a second overhead pass from Odyssey, at 6:45 pm Pacific time, that Smith is expecting to get the first pictures that will provide proof of a safe landing and solar array deployment.

Smith said that from the spacecraft’s perspective, Mars appears to be 10 times the size of the Moon from Earth. “It’s growing rapidly in the sky, and my anticipation is growing just as rapidly.”

Image: Phoenix Mission, University of Arizona