The 420-million-U.S.-dollar Phoenix Mars Lander—seen here in this artist's conception—is expected to reach Mars in the spring of 2008, touching down near the red planet's north pole. Its mission: to determine whether the icy soil on far northern Mars has conditions that were once suitable for life.

Phoenix currently sits at the launch pad on top of a 13-story-tall stack of rocket engines. If anything delays tomorrow's scheduled 5:26 a.m. launch, NASA officials will have until the end of August to get the craft space-borne.

"We have a three-week launch window, which is quite an extensive period," Deborah Bass, NASA's deputy project scientist for the Phoenix mission, said during an August 1 press briefing. "I feel confident that we'll get off Earth and on toward Mars within that time."

Get up to speed on the search for water on Mars with our full news coverage.

• More Photos in the News
• Today's 15 Most Read Stories
• Free Email Newsletter: Focus on Photography