Classification, i.e. putting things into groups based on their common characteristics, is a basic way of studying objects in many branches of science. Astronomy is, of course, no exception. That is why the question of Pluto being or not being a planet steered such a debate among planetary scientists.

1. Asteroids that grow tails (or everything we know about
active asteroids)

2. Classification in Astronomy
Classification, i.e. putting things into groups based on their
common characteristics, is a basic way of studying objects
in many branches of science. Astronomy is, of course, no
exception. That is why the question of Pluto being or not
being a planet steered such a debate among planetary
scientists. Examples of categories of the astronomical
objects include stars on various stages of evolution,
planets, moons of planets, brown dwarfs and so on.
But there are spacey things that demonstrate fundamental
features of more than one class of objects. Take, for
example, active asteroids.

3. Asteroids, comets and everything in between
What is an asteroid and what is a comet?
You may say that an asteroid is a small space rock (high
five to those who remember that asteroids reside in the
Asteroid Belt between Mars and Jupiter) and that a comet
is a “dirty ice ball” that grows tails when it gets close to the
Sun.
Well, it turns out that the distinction between those two
types of small Solar System bodies is not as clear as we
though.
Recently, astronomers started discovering very peculiar
objects: asteroids that grow comet-like tails, the so-called
active asteroids. In some literature they might be
called Main-belt comets, but the correct way is to say that
main-belts comets are a subclass of active asteroids. To
date, only twenty such objects have been found. Not
much, we know, but they do exist nevertheless. So the
question arises: what are they, asteroids with tails or
comets that pretend to be asteroids? Or maybe something
else altogether?

4. Active asteroids
Active asteroids have comet-like appearance and
asteroid-like orbits. They sport short-lived comas and tails,
but at the same time their orbital parameters tell us that
they are asteroids and nothing else! Although all active
asteroids have these features, the actual mechanism that
drives such behavior is different in different active
asteroids.
First active asteroids
It all started with an object called 4015 Wilson-Harrington.
Discovered in 1949 as a comet, it was soon lost and
rediscovered a few decades later as an asteroid. It took
until 1992 for the observers to figure out that the two were
in fact the same thing! The next bizarre celestial object
with a similar behaviour was identified in 1996. That
was 7968 Elst-Pizzaro. Known since 1979, this Asteroid
Belt resident with a period of 5.62 years went bananas in
1996 and grew a tail that was visible for a few months.
Actually, it also showed a tail in the later observations in
2001 and 2007, that is once every orbit near the point
closest to the Sun (i.e. at perihelion).

5. Why asteroids grow tails
Many other active asteroids we know behave the same
way, i.e. their activity is recurring and happens near
perihelion. This is a typical comet behaviour and the
reason for it is well understood: sublimation of the surface
volatile materials. Or, in volatile-poor asteroids, thermal
fracture (similar to mudcracks on Earth). That’s what’s
happening to the famous “comet rock” 3200 Phaeton, the
parental body of the December Geminid meteor shower.
On the other hand, a few active asteroids show repeated
activity at a great distance from the Sun that therefore has
nothing to do with heating. A more exotic mechanisms,
such as rotational breakup, cryovolcanism or even
component rubbing in a binary asteroid might be
responsible for the tail-making here.
And, finally, some active asteroids have only been seen
“active” once. In that case scientist suspect that a collision
with another body caused an asteroid to loose material
and exhibit tail. This is likely the case with asteroids 354P/
LINEAR and 596 Scheila.

6. Recent discoveries
•(6478)Gault
Main Belt asteroid Gault was first spotted in 1988. In early 2019 two
sky surveys, Zwisky Transient Facility (Palomar Observatory,
California) and Asteroid Terrestrial-impact last alert system (Hawaii)
reported the discovery of Gault’s comet-like tail.
The news encouraged observers to point their telescopes to Gault to
see what was going on there. Xingming Observatory in China followed
the asteroid from January to April 2019 (read the full paper here). At
the same time scientists started to comb through the old image
database in hope of spotting the signs of Gault’s past “comet activity”
(read the full paper here). They found that indeed Gault has been
demonstrating the outbursts of come-like behaviour since 2013.
Interestingly, there is no correlation between the activity of the asteroid
and the distance to the Sun. Astronomers suspect that the material
loss in Gault can be explained by its too-fast rotation (which in turn is
a result of a Yarkovsky-Keefe-Radzievskii-Paddack effect).
With only a handful of active asteroids known to date, this is a very
exciting discovery!

7. •101955 Bennu
Another object that surprised scientists this January is
Near-Earth asteroid Bennu. Bennu is currently being
explored by NASA asteroid sample-return
mission OSIRIS-REx. At the start of the year the
spacecraft watched Bennu eject material on more that 11
occasions over the course of one month. The ejected
particles ranged in size from centimeters to tens of
centimeters. They moved slowly and therefore ended up
either orbiting around Bennu or raining back on its
surface. At the moment scientist have no clue what is
going there. Luckily, OSIRIS-REx has until the summer of
2023 (that’s when the spacecraft is expected to start its
journey home) to find out!

8. Still curious?
Visit our portable planetarium, send your questions and
comments to our inflatable space dome team or check out
our other resources for more information about asteroids
and comets!
•Blog post Asteroids, friends or foes? Part one: foes! (all
about asteroid hazard, Near-Earth asteroids and impact
avoidance strategies)
•Blog post Asteroids, friends or foes? Part 2: friends! (all
about why we study asteroids and how we plan to mine
asteroids in the future)
•Lesson Plan (with additional resources) Small bodies of
the Solar System: asteroids, comets and meteoroids