I was looking over the daily distance and relative speed information and noticed that the spacecraft is losing “significant” speed every day. Why is Pathfinder slowing down?
Pathfinder is now in an elliptic orbit around the Sun. The basic laws of gravitational attraction lead to varying speeds for a body as it moves along an elliptical orbit. The body moves fastest when it is closest to its center of attraction and continually slows down as it moves away from this center. It moves most slowly when it is farthest away from the center and begins to speed up again as it moves back in towards the center to complete its orbit. For Pathfinder, the center is the Sun, the closest point in its heliocentric orbit is nearer to Earth and the farthest point would be somewhere just beyond Mars. Pathfinder is constantly slowing down on the part of its orbit that leads from Earth to Mars. If the spacecraft were allowed to continue without landing on Mars, it would begin to speed up again as it came back in towards the Earth’s orbit and the Sun. This also happens to the planets as they move in their orbits about the Sun. However it’s a much smaller effect since the planets
