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Comets: Solar system’s tasty leftovers

Greetings, stargazers.

Just as when cooking a large meal you often end up with some delicious leftovers, the leftovers from the formation of our solar system are some of the most interesting objects to look at.

In the solar system, the main dish is, of course, the sun. It contains 99.9% of all the mass in the solar system, and almost all of it is hydrogen and helium. Of the last tenth of a percent, almost three fourths of that mass is in Jupiter. Except for a tiny fraction that are the leftovers of the leftovers, everything else ended up in one of the other planets or moons.

Gravity from the sun and planets is doing a great job of clearing out the rest of this debris. You can see this process at work every night whenever you see a shooting star. Each meteor is one fewer grain of dust roaming between the planets. After 4½ billion years, most of it has been removed. If all the asteroids and comets and dust were gathered into a new body, it would be much smaller than any of the planets.

Debris that is as close to the sun as the inner planets is dense and rocky because heat from the sun has evaporated all the lighter and more volatile components, such as water and methane. We call these denser leftovers asteroids. Debris that is far from the sun, still has that lighter material. The term “dirty snowball” is often used to describe what we call comets. If some gravitational nudge sends one of these more distant objects toward the sun, heat will start evaporating the volatile components and the resulting cloud of gas can be seen when sunlight hits it.

There are two sources of comets in our solar system – the Kuiper belt and the Oort cloud – and each place provides about half of the comets we see.

The Kuiper belt is the source of what are called short-period comets. These comets are mostly just beyond the orbit of Neptune in a wide disc that aligns with the plane of the solar system. Halley’s comet is the most well-known of these short-period comets. Pluto is also a Kuiper belt object, and it does have a very faint tail like a comet. If Pluto was somehow deflected into the inner solar system, we would likely be able to see its tail. The elliptical orbits of Kuiper belt comets are generally well known, and their trajectories are easy to predict.

The Oort cloud is a much more distant repository of comets. Instead of being a disc aligned with the rest of the solar system, the Oort cloud is a giant sphere that extends perhaps a light year from the sun. Instead of heading toward the inner solar system from near the plane of the ecliptic, these long period comets come from all directions equally. The orbits of objects from the Oort cloud are so stretched out that they appear to be almost parabolic rather than elliptical. Without being able to tell precisely the size of the orbit, it is impossible to tell the period, other than to put an estimated lower limit on a “really long time.”

This month

The long-period Comet C/2022 E3 (ZTF) was discovered in March 2022 using instruments at the Zwicky Transit Facility. Its closest approach to the sun was on Jan. 11, and its closest approach to Earth will be on Feb. 1. Between those dates, its path in the sky will be between the Big Dipper and the Little Dipper. Whether it will be visible to the naked eye remains to be seen.

On Jan. 22, Venus and Saturn will be a third of a degree apart. You should be able to cover both of them with your pinkie finger held at arm’s length. Venus will be by far the brightest thing in the southwestern sky at sunset, so the conjunction should be easy to see.

On the evening of Jan. 30, the moon will almost pass in front of Mars. We were clouded out during the Mars occultation last month, and this time Durango is not quite in the path where Mars will be blocked. However, Farmington is within the path, so if you want to see the occultation, you won’t have far to travel.

Useful links

Comet C/2022 E3 (ZTF)

https://en.wikipedia.org/wiki/C/2022_E3_ (ZTF)

Astronomy picture of the day


An Astronomer’s forecast for Durango


Old Fort Lewis Observatory



Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory.