The Mars Global Surveyor mission is the oldest of the three, having been in orbit around the Red Planet since 1997. Mars Odyssey, meanwhile, has been circling the planet since 2001 whereas Mars Express is the relative newcomer in the group, after entering its orbit near the end of 2003. All three spacecraft are traveling at speeds of nearly 7,000 miles an hour. The Odyssey and Global Surveyor crafts both travel in nearly circular orbits around the poles of Mars, with Odyssey maintaining a slightly higher altitude, mainly to avoid a collision. The closest the two come to each other is 15 kilometers, and the new pictures show Odyssey from distances between 90 and 135 kilometers.
In comparison, Mars Express, which travels in an elliptical orbit, keeps its distance from Global Surveyor. As a result, the shot of the European spacecraft--taken from more than 250 kilometers away--is less defined. The Mars Orbiter Camera took all the pictures on April 20 and April 21. When trained on the planet, the instrument can resolve features as small as a few meters across.