<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Jim, June 27: So, a comet if it comes from some place outside the solar system it is coming into the gravity system of the Sun at a speed above the excape velocity of the sun's gravity and will slow down relative to the excape velocity as it nears the Sun. <hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Less confusion would result if you say what you mean: i.e.: A fast incoming comet loses <u>energy</u>, while a slower one gains energy. Obviously, either one will gain speed until it reaches perihelion and lose speed until it reaches aphelion.
It is true that most comets are negligibly influenced by the planets, and their trajectories are therefore 2-body problems. Capture requires planetary influence.
I don't claim to be an authority on this subject, but as I understand it: The "slighshot" effect only boosts a comets energy if the comet passes close to a planet on the planet's right side (with north up). If it passes on the left, the comet will lose energy. In either case, the comet's new perihelion may be below the Sun's surface. A subsequent planetary encounter is then needed to prevent collision with the Sun.
The same is true if a comet or asteroid passes the Earth-moon system: Depending on size and incoming velocity, it could be slowed by passage thru the upper atmosphere to a collision trajectory, and on passing the right side of the moon, it could be slingshotted into a more stable orbit. Subsequent passes by the moon would tend to circularize its orbit.
Schumacher-Levy 9, 1993, is a made-to-order example of cometary capture. The commet's initial energy loss was due to extreme tidal forces tearing it apart, though Jupiter's moons may have also played a role. While most of the comet impacted on Jupiter, it apparently left a [url]
www.atsnn.com/story/36924.html
[/url] new ring of cometary fragments. Had the initial encounter been very slightly different, Jupiter might have acquired several additional moons.
