[X] My Assistant

[nnyspace]

So has a landing site been chosen yet, anyone know? and what is the specific (or general) location... and don't just say "up north", latitude and longitude would be better.

[dmuller]

When Phoenix meets Mars, the spacecraft will have a lower orbital speed around the Sun than Mars. Why? because the Phoenix orbit would take it all the way back to where it came from ... Earth (though Earth wouldnt be there at that particular time). So, in theory, Phoenix would have to speed up to match Mars' orbital speed and "stay" with it.

BUT

Phoenix (or any other craft going there) will get very close to Mars, and you want to stay there (land or orbit). So you cant fly "along" Mars because it's gravity would pull you in if you just match its orbital speed at close distance. The issue now is that you are too fast for Mars to capture you in its orbit, or land at a decent speed, so you have to slow down your orbital speed around MARS (not the Sun). Close to Mars, the gravitational force of Mars exceeds that of the Sun anyway, so you dont really care whether your orbital speed around the Sun goes up and down as you enter an orbit around Mars or land on it (it's a whole different story though if you use Mars for a swing-by).

Phoenix will reach the point from where the gravitational force to Mars exceeds the one to the Sun on 25-May-2008 11:25am SCET UTC, 12 hours before landing, at a distance from Mars of about 139,000 km

Phoenix will slow down its orbital speed around Mars as follows:
1. friction (glowing as it streaks through the atmosphere)
2. parachute
3. retro rockets for the last 500m or so only
4. ground impact

The remaining engine firings, TCMs, are not meant to slow the craft down to Mars, only to make sure it hits the atmosphere where it is supposed to.

Phoenix landing times are as follows:
Entry interface expected on 25 May 2008 23:31:12 UTC, landing expected on 25 May 2008 23:38:32 UTC Spacecraft event time
Entry interface expected on 25 May 2008 23:46:32 UTC, landing expected on 25 May 2008 23:53:52 UTC Earth received time