Do planets and comets travel at a constant speed

[ErnieM]

As planets and comets revolves around the Sun in their respective orbits, do they respectively travel at a constant speed? As they pass in proximity to each other, does gravity have any sling shot effect?
By extension, do stars and galaxies (and clusters) behave the same way?

[owlice]

This link might help: http://www.freemars.org/jeff/speed/index.htm

(Yes, speed varies.)

[Chris Peterson]

As planets and comets revolves around the Sun in their respective orbits, do they respectively travel at a constant speed? As they pass in proximity to each other, does gravity have any sling shot effect?
By extension, do stars and galaxies (and clusters) behave the same way?

For bodies orbiting another body which has much greater mass, the orbital velocity is approximately described by Kepler's Laws. If the body is in a perfectly circular orbit (which never happens, although it may be very close) its orbital velocity is constant, and determined by its distance from the central body and the mass of that central body. In elliptical orbits with eccentricity greater than zero, the orbital velocity varies throughout the orbit (which should be intuitive, given that such a body has a varying distance from the central body).

The situation becomes much more complex when you have multiple bodies, as their mutual gravitational effects tug on one another. The effect is usually a small one (which is a good thing, or we would not be here discussing this), but can be large when bodies pass close together. This probably happened a lot in the early Solar System, and partly explains the current position of the planets, and the reason that we don't have more planets. It continues to happen with comets and asteroids, which are often affected hugely by interactions with other planets- especially Jupiter.

And yes, these effects all happen to stars, clusters, galaxies, even clusters of galaxies. Anything that has mass.