A beginner's guide to asteroids

What are asteroids, where do they come from and how can you see an asteroid in the night sky?

Image of an asteroid colliding and blocking out a star's light. Credit: NASA
Published: April 8, 2022 at 9:10 am
Try BBC Sky at Night Magazine today and save 30%!

For a long time, when we only had eyes to survey the heavens, the Solar System was a nice and simple place.

Advertisement

Back then just five big planets, the Sun and our Moon moved around the sky, along with the odd infrequent visiting comet.

Things changed with the invention of the telescope, because we could now peer into the dark with instruments that allowed us to see smaller, fainter objects.

An image of Eros's southern hemisphere, where the NEAR Shoemaker touched down. Credit: NASA
An image of asteroid 433 Eros's southern hemisphere, where the NEAR Shoemaker spacecraft touched down. Credit: NASA

Nevertheless, and quite incredibly, it was still approaching 200 hundred years after Galileo first pointed his scope skyward before the first asteroid was discovered.

The Near Earth Asteroid Rendezvous was the first spacecraft to orbit and later land on an asteroid.

After its launch in February 1996, the craft entered orbit around asteroid 433 Eros in February 2000 and landed one year later.

And now missions like the Japanese Hayabusa2 and NASA's OSIRIS-REx are returning pieces of asteroids to Earth for study.

A view of asteroid Bennu, its striking craters and surface covered in boulders, captured by OSIRIS-REx's OCAMS (MapCam) instrument on 28 April 2020 from a distance of 10km. Half of Bennu is bathed in sunlight, and half is in shadow. Credit: NASA/Goddard/University of Arizona
A view of asteroid Bennu, captured by OSIRIS-REx's OCAMS (MapCam) instrument on 28 April 2020 from a distance of 10km. Credit: NASA/Goddard/University of Arizona

The official International Astronomical Union, which is involved in the naming of asteroids (for example, there are asteroids called Flora, Hilda and Albert), also decides overall what they will be classed as.

The IAU has now officially designated asteroids as being in the Small Solar System Bodies category, along with everything else that isn’t a planet or a moon.

However, the use of the word ‘asteroid’, or alternatively ‘minor planet’, continues, not least because the official name is rather dull.

What is an asteroid?

Radar images of asteroid 2016 AJ193 GOLDSTONE OBSERVATORY, 3 SEPTEMBER 2021 CREDIT: NASA/JPL-Caltech
Radar images of asteroid 2016 AJ193, Goldstone Observatory, 3 September 2021. Credit: NASA/JPL-Caltech

So, what is an asteroid? Well, there are no hard and fast rules here.

Most are generally rocky, metallic or a mixture of both, but some appear to be icy.

Some are round, but many are odd-shaped rogues that look like they’ve been involved in some major celestial crashes.

Most live in the zone between the orbits of Mars and Jupiter in an area that is commonly referred to as the asteroid belt.

However, there are many that travel all over the place – occasionally hitting anything in the way.

An illustration showing a 'bird's eye view' of our asteroid belt, which lies between the orbits of Mars (red) and Jupiter (purple). Green dots represent asteroids. Earth's path round the Sun is in blue and red indications comets and asteroids with orbits that pass close to Earth. Credit: NASA/JPL-Caltech.
An illustration showing a 'bird's eye view' of our asteroid belt, which lies between the orbits of Mars (red) and Jupiter (purple). Green dots represent asteroids. Earth's path round the Sun is in blue and red indications comets and asteroids with orbits that pass close to Earth. Credit: NASA/JPL-Caltech.

While crashing into our planet and causing global devastation, an asteroid led to the extinction of the dinosaurs.

The largest asteroid in the asteroid belt is Ceres, which was the very first one that was discovered.

Italian astronomer Giuseppe Piazzi at his observatory in Palermo, Sicily, found an object that he named Ceres on 1 January 1801. This tiny world is a mere 975km (605 miles) across.

All the others in the belt are smaller than this.

Indeed, there are very few asteroids you would class as big: fewer than 20 are larger than 250km (155 miles) across, and once we get down to ‘shards’ of around 1km (0.6 miles) to 2km (1.2 miles) in diameter, the total could be well into the high hundreds of millions.

Where do asteroids come from?

Craters on Eros, imaged by the NEAR Shoemaker spacecraft from a distance of 35 kilometres above the surface. Credit: NASA/JPL/JHUAPLNASA/JPL/JHUAPL
Craters on asteroid Eros, imaged by the NEAR Shoemaker spacecraft from a distance of 35 kilometres above the surface. Credit: NASA/JPL/JHUAPLNASA/JPL/JHUAPL

Some interesting ideas can be formed about how all these asteroids came into being.

The fact that their make-up varies from being all rock to all metal, and every stage in between, suggests that some used to be larger objects that went through a process of differentiation before being split apart.

Differentiation is when big objects with lots of gravity heat up, which causes molten material to move about.

The metals sink to the centre because they’re heavier and the rocks rise to the surface because they’re less dense.

When something comes along and smashes it up, you’re left with a variety of mixtures.

The reason they’re in a belt between Mars and Jupiter could be that there just wasn’t enough stuff to make a planet, or perhaps the massive gravitational tugs of nearby Jupiter just kept breaking apart anything that tried to form.

How to see an asteroid

The comet-like asteroid P/2010 A2. Credit: NASA, ESA and D. Jewitt (UCLA)
The comet-like asteroid P/2010 A2. Credit: NASA, ESA and D. Jewitt (UCLA)

If you’re looking for an asteroid, you ideally need two clear nights in a row, or at least within a few days of each other.

This is because these objects appear just like stars or planets – they don’t move as you watch them.

Therefore, you need a second observation to see their shift against the background of normal stars.

Finding them to start with is a matter of using a star chart, an astronomy smartphone app or software such as RedShift or Starry Night that shows you a location chart.

Or you can read our observing guide to find out what comets and asteroids are in the sky tonight.

5 of the brightest asteroids visible in the night sky

1

4 VESTA

Asteroid Vesta, as seen by NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech
Asteroid Vesta, as seen by NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech
  • Max. apparent magnitude: 5.3
  • Size: 578 x 560 x 460km (359 x 348 x 286 miles)
  • Discovered: 1807

The brightest of the asteroids, it’s visible to the unaided eye from areas where light pollution isn't an issue. It has a metallic iron-nickel core.

2

2 Pallas

An image of asteroid 2 Pallas captured by the Very Large telescope. Credit: ESO/Vernazza et al.
An image of asteroid 2 Pallas captured by the Very Large telescope. Credit: ESO/Vernazza et al.
  • Max. apparent magnitude: 7.0
  • Size: 570 x 532 x 500km (354 x 330 x 310 miles)
  • Discovered: 1802

An unusual main belt asteroid because it has a highly inclined orbit of nearly 35° as well as a high axial tilt of around 60°.

3

1 CERES

Dwarf planet Ceres. Credit: NASA/JPL-Caltech
Dwarf planet Ceres, as seen by NASA's Dawn mission. Credit: NASA/JPL-Caltech
  • Max. apparent magnitude: 6.7
  • Size: 975km-diameter sphere (605 x 565 miles)
  • Discovered: 1801

Ceres has been promoted to the title of dwarf planet. It’s believed to have a rocky core with an icy mantle and crust.

4

7 Iris

An image of 7 Iris captured by the Very Large Telescope. Credit: ESO/Vernazza et al.
An image of 7 Iris captured by the Very Large Telescope. Credit: ESO/Vernazza et al.
  • Max. apparent magnitude: 7.1
  • Size: 225 x 190 x 190km (139 x 118 x 118 miles)
  • Discovered: 1847

This is quite a bright asteroid for its size, probably indicating that its surface includes a large amount of iron and nickel metals.

5

433 Eros

433 Eros, as seen by the NEAR spacecraft. Credit: NASA/JPL
433 Eros, as seen by the NEAR spacecraft. Credit: NASA/JPL
  • Max. apparent magnitude: 8.5
  • Size: 13 x 13 x 33km (8 x 8 x 20 miles)
  • Discovered: 1898

One of the largest asteroids of the so-called Amor family. Asteroid 433 Eros approaches us after journeying from beyond Mars and was famously studied by the NEAR spacecraft.

Advertisement

This article originally appeared in the July 2007 BBC Sky at Night Magazine.

Authors

Astronomy writer Anton Vamplew
Anton VamplewAstronomy communicator

Anton Vamplew is an amateur astronomer, author and lecturer.

Sponsored content