Comets

FAS Astronomers Blog, Volume 32, Number 9.

The Solar System is full of interesting objects – planets, dwarf planets, moons, and asteroids. These objects are very predictable and show up month after month and year after year. On the other hand, comets are different. Although, some return on a regular basis, many comets surprise us and unexpectedly appear, swing around the Sun, and then head off into deep space – many never to be seen again.

Comets are often described as “dirty snowballs.”, a concept introduced by Fred Whipple back in the early 1950s. They are objects left over from the formation of the Solar System and are composed of frozen gas, dust, rock, and ice. They are typically small objects with a nucleus only a mile to several miles in diameter, although some can be much larger. Most travel a very elliptical path taking them from the outer Solar System, around the Sun, and then back out again.

There are generally two types of comets: 

• Short-period comets, which take a hundred years or so to orbit the Sun and come from around the Kuiper Belt out beyond the orbit of Neptune. 
• Long-period comets, which can take millions of years to orbit the Sun and come from a spherical region called the Oort cloud. The Oort cloud is thousands of times farther from the Sun than the Earth – around ¼ of the way to Proxima Centauri, the nearest star to the Sun.

As a comet approaches the inner solar system, solar radiation causes some of the comet’s material to vaporize. This creates a coma that could be a thousand times the size of the comet’s nucleus. Material is also pushed away from the Sun by the solar wind. This creates a tail millions of miles long. 

Interestingly, comets often have two tails. 

• A blue ion tail composed of gases is created by the solar wind, and points away from the Sun. 
• A dust tail, composed of dust, is illuminated by the Sun’s light and curves back toward the path of the comet’s orbit.

Comets have always been a big deal for astronomers. Back in the day (ok, the 17^th century), everyone was looking to discover new ones. To make it easier, Charles Messier published a list of objects that were not comets to help those who might mis-identify one of them. We know these objects today as Messier objects with their familiar M numbers such as Andromeda (M31), the Orion Nebula (M42), the Pleiades (M45), and so on. 

The most famous comet is Halley’s Comet (officially 1P/Halley) which returns every 74-79 years, although it was a bit disappointing for most viewers back in 1986. Astronomers did get a close-up view as several spacecraft, including ESA’s Giotto, Japan’s Suisei and Sakigake, and Russia’s Vega-1 and Vega-2, rendezvoused with Halley as it passed close to the Earth.

1P/Halley wasn’t discovered by Edmond Halley. Comets had been spotted in 240 B.C.E., 1066, 1531, 1607, and 1682. However, it was Halley who first recognized that these were a periodic comet that returned on a regular basis. He predicted that it would return in 1758, and it did. Halley will return in 2061, so be sure to mark the date on your calendar.

Another is Hale-Bopp, which was one of the brightest comets of the 20^th century. It was visible to the naked eye for a record 18 months back in 1996-97. I remember looking up at it in the night sky while I was standing on the shore of Cumberland Island off the coast of Georgia. No, unfortunately, I didn’t take a picture. 

Some of us remember comet C/2020 F3 (NEOWISE), which was almost visible to the naked eye. It was, however, close enough for several FAS members to capture some good images. 

In October 2024, the comet C/2023 A3 (Tsuchinshan-ATLAS) paid a close visit and was ever so slightly visible to the naked eye. For more on this comet, see our previous post, Comet C/2023 A3. 

Back in 2014, The European Space Agency (ESA) sent the Rosetta spacecraft to orbit Comet 67P/Churyumov-Gerasimenko. The comet itself has an interesting shape and was often referred to as a “rubber ducky.”

The small probe Philae was sent down to land on the surface. Because of the comet’s light gravity, the probe was supposed to hook on to the surface. However, the hooks failed, and Philae bounced a few times before coming to a stop. Because of this, it took a month for scientist to locate the lander on the surface. The folks from ESA commented, after watching the lander bounce across the surface, that they achieved the first, second, third, … landing on a comet. 

In July 2005, NASA released a probe from its Deep Impact spacecraft that smashed into the surface of comet Tempel 1. Just for clarification, this is a different spacecraft than NASA’s DART, which deliberately rammed into the moon Dimorphos of the asteroid Didymos in September 2022. 

Then there was P/Shoemaker-Levy 9. In July 1994, it got a little too close to the planet Jupiter. Jupiter’s gravity ripped it apart. Astronomers watched as the chunks crashed down into the Jovian atmosphere and created huge black marks that were visible from Earth bound telescopes. According to NASA, the impacts had a force of 300 million atomic bombs and created plumes 1,200 to 1,900 miles high.