Does a comet’s period change because it loses mass every time it nears the Sun?

Do the periods of comets decrease because they lose mass every time they get near the Sun?

Robert Bailey
Conover, Wisconsin

Great question! Assuming you’re referring specifically to the orbital period — in other words, how long comets take to make one revolution around the Sun, or a cometary “year” — according to basic physics, the period of an object does not depend on the mass of that orbiting object, and instead only on the size of the orbit and the mass of the central body being orbited (in this case, the Sun). So, if some of a comet’s mass suddenly disappeared one day, its orbital period should remain unchanged.

In the real world, however, mass does not simply disappear — it has to go somewhere. In a comet’s case, ice turns into gas when heated by the Sun, and that gas escapes into space (a process called outgassing), usually also carrying away some amount of dust along with it.

Cometary outgassing carries mass and momentum away differently on different comets, depending on their circumstances. For some comets, gas and dust might escape more or less evenly over the comet’s entire surface, like a melting ice cube. In other cases, gas and dust may escape from a specific location in a preferred direction, like a balloon that’s been blown up, but not tied off, and then released. The escaping material can cause significant changes in a comet’s orbit, acting like natural rocket engines, with the exact changes depending on the direction, speed, and amount of the escaping gas and dust.

For this reason, astronomers regularly look for orbital changes each time comets return near the Sun and update those orbits as needed, leading to reference catalogs listing multiple orbits for some comets, corresponding to each return if the changes are large enough. Besides having the practical benefit of keeping comet orbits updated so that we can continue to accurately predict when and where they will appear in the sky, tracking orbital changes during close approaches to the Sun is also of scientific interest to comet researchers because it provides a means for indirectly tracking the evolution of the strength and other details of a comet’s outgassing over time.

With regard to your specific question, the orbital period does not always decrease each time a comet approaches the Sun, but can increase, decrease, or stay the same depending on the details of the comet’s outgassing during that particular close approach to the Sun. In addition to the period, other aspects of a comet’s orbit that might change due to outgassing include the size of the orbit, how elongated the orbit is, and the tilt of the orbit relative to the rest of the solar system. So, these are all changes that astronomers will look for each time comets come close to the Sun. And in some cases, orbital changes in a small solar system object can be used by astronomers to infer the presence of outgassing when it may not be directly observable for some reason, such as in the case of the interstellar object 1I/2017 U1 ‘Oumuamua.

Aside from the effects of outgassing, comet orbits are also constantly changing simply due to the competing gravitational pulls of the major planets in the solar system. While the Sun’s gravitational pull is by far the largest force affecting a comet’s orbit, the comparatively weaker gravity of planets can still influence comet orbits in significant ways. As the largest planet in the solar system, Jupiter has the strongest influence after the Sun, but even smaller planets like Saturn, Mars, or Earth can produce small but noticeable changes in comet orbits, especially for comets that pass close to these worlds. As with outgassing, changes in a comet’s orbit due to the combined gravitational influence of the major planets will depend on the exact details of those interactions, and orbital parameters can increase, decrease, or stay the same as a result. For most comets that don’t pass extremely close to a planet, these changes are typically more subtle compared to those caused by outgassing, but can add up over time and so cannot be ignored by astronomers interested in tracking the orbits of comets over long periods of time.

Just like Earth, comets also spin on an axis (or sometimes tumble on multiple axes, like a wobbly, spinning top, but that’s a topic for another day!) as they orbit the Sun. The time it takes to spin once — one “day” on a comet — is also called a period, but in this case, it is specifically called the rotational period. Just in case you’re referring to this kind of period instead to the orbital period, the rotational period can also change due to cometary outgassing and for the same reasons. Again, as with orbital period changes, the rotational period can increase or decrease depending on the details of a particular comet’s outgassing — mainly the positions, directions, and intensities of any cometary jets.

Henry Hsieh
Senior Scientist, Planetary Science Institute, Honolulu, Hawaii