Comets lose their mass through water evaporation due to close encounter with sun, so my question is does the loss of mass create an observable change in a comet's orbit? What kind of change that may be shrinking orbit or expanding orbit?


1 Answer 1


This problem was studied in Yu & Zheng (1995), who evaluated the effects of the change in the Sun's mass over time and the change in a comet's mass over time, for the case of Shoemaker-Levy 9, which had recently crashed into Jupiter. Given their mass model for the comet (Equation 6), they found that the Sun's mass loss created an increase in semi-major axis of about 8.5 centimeters per year, whereas the comet's mass loss created an increase in semi-major axis of about 10,000 kilometers per year.

Several things to note:

  • The comet orbited Jupiter prior to breaking up, so it did not come as close to the Sun as most short-period comets.
  • Mass loss rates can change over time, depending on the distance from the Sun.
  • Shoemaker-Levy 9 should not be considered a normal comet, in any sense, given its orbit and eventual destruction.

10,000 kilometers a year, though, is nothing to sniff at. Over the course of an orbit, that can be quite a lot - although keep in mind that longer orbits involve much larger semi-major axes - and I'd argue that it should be observable, given the correct calculations of how the orbit should evolve over time.

  • $\begingroup$ Your answer is good in its own but not as specific as i asked. What argument is right but you don't specifically mention that what kind of that change may be? My field is not astronomy so please bear with me. $\endgroup$
    – Shuji
    Oct 1, 2016 at 7:44
  • $\begingroup$ Why does comet orbit change? Because the mass loss is asymmetric? $\endgroup$
    – ProfRob
    Oct 1, 2016 at 15:56
  • $\begingroup$ @ShujaatAliKhan The orbital parameter we're concerned with is the semi-major axis, one half the distance between the two furthest points in the elliptical orbit of the comet. It increases as the comet looses mass. $\endgroup$
    – HDE 226868
    Oct 1, 2016 at 22:32
  • $\begingroup$ @RobJeffries It turns out that the mass loss of the comet causes a tangential acceleration perturbation, coming from matter being blown off the comet at a constant velocity. $\endgroup$
    – HDE 226868
    Oct 1, 2016 at 22:37
  • $\begingroup$ So with every revolution a comet actually becomes more eccentric? That is interesting. If you wanted to cause the opposite and reduce the semi-major axis, I suppose you could 'block' the mass losses natural direction and only allow the opposite direction to be exposed to mass loss. $\endgroup$ Sep 19, 2017 at 18:30

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