Active asteroid

Active asteroids are small objects in space that move like asteroids but sometimes look like comets. They have features such as a fuzzy area around them or a tail, which are signs that material is leaving the object. However, they travel in paths similar to asteroids, usually staying within the orbit of the planet Jupiter.

These objects were first called main-belt comets in 2006 by astronomers David C. Jewitt and Henry Hsieh. This name suggested they were made of ice, like comets, and only found in the main group of asteroids. But as more of these objects were found, it became clear that not all of them fit this description.

The first active asteroid discovered is called 7968 Elst–Pizarro. It was first seen in 1979 as an asteroid. Later, in 1996, astronomers Eric Elst and Guido Pizarro noticed it had a tail and gave it the comet name 133P/Elst–Pizarro.

Orbits

Active asteroids move around the Sun in paths that stay closer than the planet Jupiter. Unlike comets, which travel far from the Sun, active asteroids have orbits that look much like regular asteroids.

Scientists describe these orbits using special measurements. One important measurement is called the semi-major axis, which must be within Jupiter’s orbit. Another is the Tisserand parameter, which helps tell asteroids apart from comets. The first active asteroids found all move in the outer part of the asteroid belt.

Activity

Some active asteroids show a dusty tail only when they are close to the Sun. This suggests that materials on their surface are turning into gas, pushing dust away. The activity of one active asteroid, called 133P/Elst–Pizarro, happens every time it gets close to the Sun. This activity lasts for a month or more and is thought to be caused by ice exposed by small crashes in the past few hundred years.

In 2010, an object called P/2010 A2 (LINEAR) was thought to be showing comet-like behavior, but it is now believed to be the remains of a crash between asteroids. Another asteroid, 596 Scheila, was seen covered in dust after being hit by another asteroid.

P/2013 R3

Main article: P/2013 R3 (Catalina–PanSTARRS)

P/2013 R3 was discovered by two observers and was found to be breaking apart. Follow-up observations showed that this asteroid had split into pieces, with one main piece and three smaller pieces moving away from each other.

Dimorphos

Main article: Dimorphos

When a spacecraft called the Double Asteroid Redirection Test hit the asteroid moon Dimorphos, it caused the asteroid to lose a lot of material and create a long dust tail. This was the first time scientists were able to watch an asteroid become active from a crash. The impact changed Dimorphos's shape and rotation, and it is expected to settle back into a steady spin over time.

Composition

Some active asteroids have icy parts like traditional comets, while others are rocky like typical asteroids. Scientists think that these icy active asteroids might have brought water to Earth a long time ago. This is because the water in Earth's oceans has a special mix that doesn’t match what we usually find in comets. Some European scientists have suggested a mission to collect samples from an active asteroid named Caroline to learn more about its icy parts and dust.

List

Identified members of this group (T_Jup>3.08) include:^: 17 

Exploration

JAXA’s DESTINY⁺ is a planned mission to visit 3200 Phaethon and collect dust. It is expected to launch in fiscal year 2028.

Castalia is an idea for a robot spacecraft to study 133P/Elst–Pizarro. This could help us learn about where Earth’s water came from. The idea was suggested in 2015 and 2016 to the European Space Agency, but it was not chosen at that time. The team is still working on the plan, and a possible launch date is October 2028.

In 2019, the OSIRIS-REx mission saw small pieces of rock and dust leaving 101955 Bennu when it flew close to this near-Earth asteroid. This was the first time scientists saw an asteroid acting like this up close. Since then, they have seen this happen at least 10 more times, though these events are smaller than ones seen from far away.