QUESTION:
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?

ANSWER from Robin Vaughan:
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' orbits are more nearly
circular than Pathfinder's. That's why Earth's and Mars' orbits look like
circles in our trajectory plots while the spacecraft's trajectory looks like
part of an ellipse.

--Robin Vaughan, Mars Pathfinder Navigation Team