By Holly Spanner

Published: Saturday, 28 May 2022 at 12:00 am

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We’ve observed space rocks for thousands of years. Iron meteorites have been prized throughout history: who can forget King Tutankhamun’s meteoric iron dagger, or the Buddha carved from a meteorite that fell 15,000 years ago? Likewise, our history of observing comets is extensive, many of which have had a significant impact on human history and the development of legends (or should that be omens?). Halley’s Comet, of course, was immortalised in the famous Bayeux Tapestry, made sometime in the 11th Century.

But what’s the difference between an asteroid and a comet? Or a meteorite and a meteorid? Answers to these questions, as well as a round-up of the different types of space rock, are below.

If you’re looking for more stargazing tips, be sure to check out our astronomy for beginners guide and our UK full Moon calendar to make the most of the night sky. For a full roundup of this year’s meteor showers, we’ve got them all listed in our meteor shower calendar.

What’s that space rock?

Asteroids: Small, rocky objects, often irregularly shaped, leftover from the formation of the Solar System

Comets: Large, icy bodies of frozen gases, dust and rock, with a frozen nucleus

Meteoroid: Fragments and debris from asteroids and comets

Meteor shower: Multiple meteoroids burning up in the Earth’s atmosphere

Fireball: An exceptionally bright meteor that can be seen over a wide area

Meteorite: Meteoroids that survive the journey through the Earth’s atmosphere and fall to the surface

Dwarf planet: A small spherical (or near-spherical) celestial body in orbit around the Sun, that lacks sufficient mass to clear its neighbourhood of debris

Asteroid

Asteroids are small, rocky objects that orbit the Sun. Most asteroids are irregularly shaped, although there are a few that are nearly spherical. There are over a million known asteroids, and most can be found in the main asteroid belt between Mars and Jupiter. Asteroids are rocky remnants left over from the early formation of the Solar System.

When an asteroid shares its orbit with a planet, it’s known as a Trojan asteroid. There are two trojan asteroids for Earth, however, both are difficult to see as they are leading the Earth in our orbit around the Sun, and therefore appear on the horizon, close to the Sun at sunrise. Not ideal viewing conditions.

Asteroid Bennu (shown in the above video by NASA’s Scientific Visualization Studio) made the news in 2020 when NASA’s OSIRIS-REx mission successfully landed on the asteroid’s surface and collected over 400g worth of samples. It was an impressive achievement, and one which smashed the original target of 60g.

Bennu is estimated to be around 4.5 billion years old, forming just after the formation of our Solar System. It’s most likely a piece of a much larger carbon-rich asteroid, breaking off around 700 million to 2 billion years ago and making a close approach to Earth every six years.

But astronomers must be patient for a little while longer as the spacecraft is still on its way back home, due to swing by on 24 September 2023 to deliver the precious cargo. After that, the spacecraft will head out again, this time to study the near-Earth asteroid Apophis, under a new mission name OSIRIS-APEX.

Comet

Comets are the snowballs of the cosmos. These icy bodies of frozen gases, rocks and dust orbit the Sun on highly elliptical paths, often tumbling as they go. When their orbit brings them close to the Sun, they heat up. This causes the solid ice to turn into gas, which is swept into the distinctive comet tail. Arguably the most famous comet is Halley’s Comet, which is due to return to our skies in July 2061.

Comets are so varied in size, orbit and composition that this has resulted in numerous classifications over the years. However, for argument’s sake, they can be sorted into four broad categories:

• Non-periodic comets: comets that have only passed through the Solar System once
• Short-period comets: comets with an orbit of less than 200 years
• Long-period comets: comets with an orbit of more than 200 years
• Lost comets: comets that have ‘disappeared’ and were not seen at their most recent perihelion (closest point to the Sun)

There are three main parts to a comet:

• Nucleus: the solid core
• Coma: gases expelled by the nucleus
• Tail: the stream of gas and dust left in the comet’s wake

Meteor shower

As comets or asteroids travel around the Sun, they leave behind a trail of debris in their wake. When Earth’s orbit intersects with this debris, the result is hundreds (or thousands) of bright trails, appearing to radiate from one point in the night sky. This is why we often see meteor showers at the same time every year, and meteors are more numerous on certain nights.

So, a meteor could more accurately be described as an event, rather than an object: when that tiny particle (called a meteoroid) enters the upper parts of the atmosphere and heats the air around it to incandescence. This is the glow we see as a meteor.

The Eta Aquariid and the Orionid meteor shower are created by the Earth’s orbit passing through the trail left by Halley’s comet, while the impressive Leonids are remnants of the comet Tempel-Tuttle.

Meteoroid

Meteoroids are your classic space rocks. As fragments and debris from asteroids and comets, they are some of the smallest bodies in the Solar System. These particles continue orbiting the Sun in approximately the same orbit as the parent body from which they came, and over time, they get further away from the parent as the orbit becomes littered with these particles.

When a meteoroid enters the Earth’s atmosphere and burns up, it leaves a glowing trail in the sky and is known as a meteor – or more popularly, a shooting star. Meteoroids can range in size from as small as a grain of dust to small asteroids.

If a meteoroid’s collision with the atmosphere is head-on, it can reach a velocity of up to 45 miles per second, whereas if it’s striking the atmosphere from directly behind the Earth, it might come in at around 7.5 miles per second. Usually, the velocity is somewhere between the two.

Fireball

If a meteor entering the atmosphere exceeds Venus in terms of brightness, this is a fireball. A large fireball will be visible for 5-10 seconds, and if you see a fireball that looks like it was close, be sure to listen out for a rumbling, or ‘detonation’ type sound! A fireball that lasts more than 10 seconds, is, however, more likely to be a satellite or some form of aircraft falling back to Earth.

Sometimes a fireball can explode, such was the case with the Tagish Lake meteorite that fell in 2000. It exploded in the sky with the force of around a quarter that of the Hiroshima bomb, breaking into around 500 pieces that rained down around Tagish Lake in British Columbia, Canada.

Meteorite

These are the meteoroids that survive the journey to Earth’s surface. So far, more than 69,000 meteorites have been found (and named) on Earth.

If you see a meteor streak across the sky with a brightness similar to that of a quarter Moon, there’s a good chance that it might survive the journey and land on Earth. Meteorites fall every day, however finding one is incredibly rare.

There are different types of meteorites, and most can be classified as either an iron meteorite, a stony meteorite or a stony-iron meteorite. These categories are broadly defined by the amount of iron-nickel metal the meteorite contains:

• Iron meteorite: those which are almost entirely made of metal
• Stony meteorite: those which are almost entirely made of silicate crystals
• Stony-iron meteorite: those which have similar amounts of both metal and silicate crystals

These broad categories are further subdivided, depending on the structure, chemistry and minerals that the meteorite contains. For example, pallasites are a beautiful type of stony-iron meteorite with large, green crystals of translucent olivine, embedded entirely in metal.

Pallasites are rare, and only 61 are known to date. Their exact origin is still hotly debated, but some scientists believe they originate from the core-mantle boundary of ancient worlds, while other research supports their origin as being higher up in the mantle. But however they’re formed – pallasites are awesome.

Dwarf Planet

Dwarf planets are massive enough to be affected by gravitation and can pull themselves into a round, or nearly round shape. However, unlike planets, they are unable to clear their orbital path. Each of the known dwarf planets in the Solar System is smaller than our Moon. There are five officially recognised dwarf planets in our Solar System: Pluto, Ceres, Makemake, Haumea and Eris.

Although we all know Pluto should be recognised as a planet…

As set out by the International Astronomical Union, the criteria for a dwarf planet are:

• It’s in orbit around the Sun
• Round (or nearly round) in shape, having been pulled into that shape by its own gravity
• Not a satellite of another planet
• Is not massive enough to clear its neighbourhood of debris

Another potential dwarf planet, 2015 RR_245, was discovered by the Outer Solar System Origins Survey at Mauna Kea Observatories on the Big Island of Hawaii in 2015, and was imaged by Hubble in 2020. It measures approximately 600km in diameter and is currently being investigated to determine whether it has a satellite (moon) and what the orbit of that satellite could be.

Read more:

• How do we know when a rock is from space?
• If all the asteroids in the asteroid belt had coalesced to form a planet, what size would it have been?
• What does a comet smell like?
• How can we tell that a meteorite has come from a particular planet?

To submit your questions email us at questions@sciencefocus.com (don’t forget to include your name and location)

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