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How do planets capture satellites?

Mariner 9 went into orbit around Mars because it was slowed by rockets from its interplanetary speed. The question occurred to me that if Mariner 9 had to loose energy in order to be captured by Mars, how do the planets capture satellites as has been surmised by all astronomy books?

Yes, you are right. Planets can capture satellites only if the satellites have low enough energy as to be in a bound orbit. If an object has too much energy and it comes close to a massive planet like Jupiter, then it will be gravitationally scattered into a different orbit, but will not be captured.

All the planets with satellites have had the experts theorize that certain satellites are captured,I would like to know how this could be. While the characteristics of the satellite is somewhat different,I don't know of any means to slow a passing body down in order for the planet to capture it as a satellite. Could you please enlighten me as to this problem?

Only few satellites are captured for all planets. The theory of planet formation is that they formed from a protostellar disk. Similarly, when massive planets form, they have protoplanetary disks from which most of the satellites form. As a result, most satellites orbit in the same direction as the the spin of the planet and lie close to the equatorial plane of the planet.

Some satellites around planets have wierd inclinations and exhibit retrograde motion. These are the satellites that are postulated to be captured. One of the ways in which a satellite can be captured is if an object coming towards a planet undergoes a collision with another body which causes it to lose energy. Another way is through three body interactions where one body is captured and the other escapes with the excess energy.

However, the problem of how exactly a planet captures a satellite is not a trivial one and is the topic of current research. This is so because the energy of the satellite has to be within a tight range to be in orbit. Otherwise, it will escape or crash into the planet.

June 2002, Jagadheep D. Pandian (more by Jagadheep D. Pandian) (Like this Answer)

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