The biggest comet ever seen shows off its hair for Hubble

Last year, a team of astronomer announced that they had found a comet that was so large it was estimated to be 150 years old.

It is on an incredibly long path that takes it a significant chunk of a light-year from the Sun, but once every three million years or so it drops down to a minimum distance of about 1.6 billion km.

The Oort Cloud comet is an object that comes from a swarm of icy bodies that can be trillions of kilometers from the Sun.

How big is this thing? The estimates were made based on how bright it is, its distance, and how reflective it is. New Hubble Space Telescope observations show that it's likely to be 137 kilometers long. Not as large as first thought, but still huge. It's bigger than any other comet.

The issue is the reflectivity. A big dark object can be the same brightness as a small shiny one, so getting the size depends on how much sunlight reflects it. One way around this is to look in the longer wavelength, where the object's brightness depends on its size and temperature. Since it is warm by the Sun, the size can be determined.

Hubble and ALMA observations give the size listed above. It is1-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-6556 That is still really, really big.

The smallest object in the solar system that is still big enough for its gravity to crush it into a sphere is the moon Mimas, which is 400 km across. It is likely irregular in shape.

It is interesting that it has a coma. There is a lot of ice in comets. The ices move away from the nucleus when they are near the Sun. The comet has a lot of rocks and dust in it so that it can be carried away by the expanding gas. The comet was still over 3.5 billion km from the Sun. It's cold that far out, so seeing a coma is amazing. It is not impossible that activity can start over 5 billion km out.

The Hubble image shows a coma, and the astronomer used a model of what that looks like to subtract it away from the image to reveal the nucleus, which is still far too small to see as anything other than a dot. They were able to calculate how much mass the comet is losing, and while their numbers are uncertain, they find it blowing out one ton of material per second, even at its enormous distance from the Sun! It's very active, but also because it's so big, there's a lot of surface area to warm up and emit gas and dust.

Even though it is enormous, it can impact Earth since it never gets close to the Sun. An impact from a large object would be bad. The asteroid was about 10 km wide. It has 1,000 times the volume and is likely to impact at higher speed than the one that ended the Cretaceous Period. I don't know if anything could survive an impact like that, so the good news is UN271 stays over a billion km away.

This comet is a treasure. It hasn't gotten very close to the Sun, so it's likely to be in a state of near-pristine condition, like it has been for the past 4.5 billion year history of the solar system. It's a time capsule to the past that can tell us a lot about what happened when the Sun, the planets, and the comets were formed.

It's about the same distance as Uranus is, roughly 3 billion km. It will get closer, brighter, and more active as it gets closer to perihelion, the closest point to the Sun in early 2031. There will be a lot more fun observations made of it, and a lot more to learn from it.