As I expected, there wasn't a lot of news from today's Phoenix press briefing, which I watched from home on NASA TV as my daughter industriously colored at the coffee table. One item is that their landing ellipse has shrunk slightly. The three-sigma ellipse (see Rob Manning's message to me for background on what that means) used to be about 20 kilometers across and 100 kilometers long. The length of the ellipse results mostly from uncertainty in the atmospheric conditions that will prevail during the landing. Just as weather predictions on Earth are more reliable for tomorrow's than they are for next week's weather, the Phoenix team's weather predictions are getting more certain as the day draws closer, so that uncertainty is decreasing, and the ellipse is now only 80 kilometers long. (That's what they said, but if I've interpreted their map correctly, the ellipse is actually roughly 23 by 70 kilometers.)
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However, as the length of the ellipse has shrunk, its center has moved slightly off target, a bit to the northwest of the desired target point. They do have one more chance to change course, but they explained at today's press conference that it is not so off-target that the course correction would be necessary if they had to make the decision right now. They'll make the final decision tonight.
Phoenix landing site map as of May 24, 2008 Phoenix is scheduled to land May 25, 2008 in a region above Mars' Arctic Circle. Its 3-sigma landing ellipse (largest yellow oval, the region in which there is a 99% certainty that the lander will come to rest) is about 70-80 kilometers long and 20 kilometers wide. This map is current as of May 24, 2008, a day before the landing, but would change if they choose to do a final trajectory correction maneuver Saturday evening. Credit: NASA / JPL-Caltech / Washington Univ. St. Louis / JHU APL / Univ. of Arizona / Emily Lakdawalla
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