Asteroids and comets: What’s the difference?

We watch comets with wonder when they fly by Earth. But when asteroids like 2024 MK fly across our orbit, we track them with fear. Here is why.

Halley’s comet: A once-in-76-year, spectacular sighting | Photo Credit: Deutsche Welle

Asteroids and comets were formed about 4.5 billion years ago, during the earliest moments of our solar system. They orbit the sun, just as planets do. But while it takes a terrestrial year for our planet to fly around the sun, an asteroid can take anywhere between three and six years—they are located in an “asteroid belt” between Mars and Jupiter.

Comets, on the other hand, can spend thousands, if not millions, of years way out in the solar system before coming back round the sun. Others do not even have fixed or “closed” orbits—so, their flight paths can be quite random.

How long it takes planets and comets to orbit the sun

The duration of planetary orbits ranges widely. Mercury—the closest planet to the sun—has an 88-day orbit, while Earth, the third from the sun, takes 365 days (1 year).

Mars needs almost twice that time to fly around the sun, while Saturn takes 10,759 days (29.4 years). Neptune, the furthest planet from the sun, has an orbit of 60,190 days (164.9 years).

Comet orbit periods have an even wider range. Here’s a select few as an example:

67P Churyumov-Gerasimenko: 6.6 years

1P Halley: 76.1 years

Hale-Bopp: 4,000 years

Hyakutake: 40-THOUSAND years!

How asteroids and comets orbit

Asteroids and comets have elliptical orbits—in fact, all orbits are elliptical—but there are differences in how elliptical they are. Planetary orbits, for instance, are almost circular, as are asteroid orbits, and quite stable, too—there’s not a lot of variation over time. But a comet’s orbit can be what’s called “highly eccentric” or even “squashed”.

So-called “long-period comets” have orbital periods of more than 200 years. Their orbits can be highly inclined to the ecliptic, and if that is the case, it suggests they are from the Oort Cloud.

The Oort Cloud is about 1,00,000 astronomical units (AU) from the sun—1 AU is the distance between the sun and Earth. So, 100,000 times that distance is a long, long way away. The North American space agency, NASA, estimates that these comets can have orbital periods of up to 30 million years.

Some comets are found in the Kuiper Belt, which is just beyond the orbit of Neptune—it is a little closer to Earth but still way out there in the solar system.

Have we ever landed on an asteroid or comet?

Yes! More often, we fly by other planets, moons, asteroids and comets. But we have landed on a few comets and asteroids and even retrieved bits of rock and dust and close-up images.

The Japanese Space Agency JAXA was the first to successfully land a spacecraft—Hayabusa—on an asteroid called 25143 Itokawa. It was also the first mission to have a spacecraft take off from an asteroid. In 2010, after seven years in space, Hayabusa completed its “sample return mission” by releasing a capsule, carrying primitive asteroid rock, down to Earth.

In 2022, JAXA repeated the feat with its Hayabusa 2 mission, which returned 5.4 grams of samples from asteroid Ryugu. Then, in 2023, NASA’s OSIRIS-REx returned a capsule of samples from the asteroid Bennu. So what if it took them three months to open the capsule? (It’s true: Two “stubborn” bolts refused to loosen at first.)

Space scientists have also landed probes on comets. NASA’s Deep Impact mission was the first, with an “impact” (or planned crash) landing on comet 9P/Temple 1 in 2005. The first “soft” landing was performed by the European Space Agency’s Rosetta mission and its Philae probe, which landed on comet 67P/Churyumov-Gerasimenko (aka “Chury”) in 2016.

What is the chance of an asteroid or comet hitting Earth?

Comets have crashed into Earth before, but asteroids never quite make it. They may be a greater threat to satellites orbiting Earth.

Asteroids enter Earth’s atmosphere about once a year. They can be about the size of a car. But they tend to burn up before they hit the surface. Meteoroids, meanwhile, do make it through the atmosphere. A meteoroid is a small particle from a comet or asteroid. It becomes a “meteorite” if it survives its journey through the atmosphere and lands on the surface.

It is generally thought that asteroids pose no immediate threat to Earth. But a meteoroid measuring about 25 metres could do significant damage on impact if it did hit the ground. NASA says that anything between one and two kilometres could have “worldwide effects”.

Comets have an average diameter of between 9 and 40 kilometres. Asteroids can be small rocks or more than 965 kilometres in diameter.

But they do not have to be that big to be a threat: Toutatis is one of the largest known “potentially hazardous” asteroids for life on Earth, and it’s only 5.4 kilometres across.

So, why do comets wow us and asteroids scare us?

In this writer’s opinion, it is all in the look. Because comets are made of ice, rock and hydrocarbons, they produce a spectacular tail when they get close to the sun. They become “active”, emitting gases or jets of dust, as their surface warms up and their materials melt and vaporise. It’s like Perseid meteor showers—everyone comes out for a glimpse of them.

Asteroids, on the other hand, are made of “inactive” rock and metal. They are solid and stable and a potential source of valuable minerals. But they don’t glow with life—they look heavy and brooding instead.