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Asteroids and comets: How to tell them apart

September 27, 2018

Landing a probe on an asteroid? It's been done before, hasn't it? Nope. The last time space scientists landed a probe on a smaller celestial body, it was a comet. So what's the difference?

Comet Lovejoy with its blue-green tail visible in the night sky
Image: Alan Dyer/amazingsky.com/dpa

Asteroids are from the future and the past. It all depends on whom you ask.

Space scientists say asteroids, like comets and other celestial bodies, were formed around 4.5 billion years ago, during the earliest moments of our solar system. 

They orbit around the Sun, just like planets, although their orbits are particular. Comets, for instance, have an elliptical orbit — so they can spend thousands of years way out in the solar system before coming back round to the sun.

Some asteroids and comets come incredibly close to Earth. And when space scientists get a chance to put a lander on one, they hope to learn about those dark beginnings of life, and how it came to Earth.

Read more: ESA's Rosetta space probe ends its life

That's happening right now. The Japanese Space Agency (JAXA) has just jumped an asteroid called Ryugu.

JAXA's Hayabusa 2 mission aims to bring back samples from the near-Earth asteroid. There are three landers and rovers on the mission. One of the rovers, MASCOT, was built jointly with the German Aerospace Center (DLR). And it's worth singling out, because it holds the instruments that will study Ryugu's surface. 

Japan's asteroid explorer, Hayabusa2, hovers over Ryugu
Japan's asteroid explorer, Hayabusa2, hovers over Ryugu — a strange-looking near-Earth asteroidImage: picture alliance/AP Photo

It's the first time space scientists have landed on an asteroid. As a result, this moment will be watched with some wild intrigue in the New Space community, especially among commercial asteroid prospectors and miners. For them, asteroids represent our future too. 

But it is the second time space scientists have landed on a moving rocky body in space. The last time was a European mission called Rosetta, which landed on Comet 67P/Churyumov-Gerasimenko - also called "Chury".

Both missions are landmarks in space exploration.

But does it really make a difference whether it's an asteroid or a comet? Kinda.

Location, location, location

The first difference between asteroids and comets is their location. Asteroids are found within the Asteroid Belt, which is between Mars and Jupiter.

Comets, on the other hand, are found a little farther out in the Kuiper Belt, which is beyond the orbit of Neptune, or in the Oort Cloud in the outer solar system. So, yeah, they're a lot farther out — about 186 billion miles (300 billion kilometers) from the Sun.

Collisions with Earth

Comets have crashed into Earth before, but asteroids never quite make it. Asteroids enter the Earth's atmosphere about once a year. They can be about the size of a car. But they burn up before they can hit the surface.

Image of Comet 67P/Churyumov-Gerasimenko captured by Europe's Rosetta probe
Think asteroids look strange? Comet 67P/Churyumov-Gerasimenko looked like a duck.Image: picture-alliance/AP Photo/ESA/Rosetta/Navcam

Meteoroids, meanwhile, do make it through the atmosphere. A meteoroid is a small particle from a comet or asteroid orbiting the Sun. It becomes a meteorite if it survives its journey through the atmosphere and lands on the surface.

It's generally thought that asteroids pose no immediate threat to Earth. But a meteoroid measuring about 25 meters could do significant damage on impact. NASA says that anything between one and two kilometers could have "worldwide effects." 

Size matters

It's estimated that the asteroid Ryugu has a diameter of about 900 meters. The comet Chury is 4.1 kilometers along its lengths.

Comets have an average diameter of between 9 and 40 kilometers (6 to 25 miles). Asteroids can be small rocks or more than 965 kilometers (600 miles) in diameter.  

But they don't have to be that big to be a threat: Toutatis is one of the largest known "potentially hazardous" asteroids to Earth. It's only 5.4 kilometers in diameter.

Constituent parts

Comets are made of ice, rock and hydrocarbons.

Artist impression of predatory dinosaurs hunting smaller prey
Did an asteroid really cause the mass extinction of the dinosaurs?Image: Reuters/Courtesy Jorge Gonzalez and Pablo Lara/Handout

Asteroids are made of rock and metals — hence the interest from future miners, and the new race for resources.

Read more: Who owns space?

Dead or alive?

NASA describes asteroids and comets as being "relative small." But the American space agency distinguishes between the two by saying that asteroids are inactive, while comets are "at times active."

They are both neither really dead nor alive — rocky, dusty, imperfectly shaped objects, and potentially home to living, organic stuff. Scientists just don't know for sure.

But comets become "active" when they emit gases or jets of dust, as was observed by the Rosetta team investigating comet Chury. And that tends to happen the closer a comet gets to the Sun. Its surface warms up, and materials on the dirty snowball melt and vaporize to produce a comet's tail. When Halley's comet flies by Earth every 70 or so years, it's the spectacular tail that people tend to watch.

This also means that a comet's surface is described as very unstable. The surface of an asteroid is solid and stable, with visible craters and other evidence of objects having crashed into it. Asteroids do not produce a coma or a tail.

How the two orbit

Both comets and asteroids have elliptical orbits — in fact, all orbits are elliptical. But there are different degrees. A planet's orbit is almost circular. A comet's orbit is what is called highly eccentric or even "squashed," whereas an asteroid's orbit is relatively circular and stable.

Which would you rather have?

Well, scientists think it's possible that comets may have contributed to our having water on Earth. And asteroids are thought to have caused mass extinctions on Earth — like what happened to the dinosaurs. Both remain propositions at best. But we'd go with the one that brings life, rather than takes it away.


DW Zulfikar Abbany
Zulfikar Abbany Senior editor fascinated by space, AI, the mind, how science touches people, European perspectives