You wait forever for an interstellar comet to arrive, and then three appear at once. Or at least for a decade. The latest alien from another star system is Comet 3I/Atlas. which was first discovered in July. As space agencies track the accelerating object, here's what we know so far.
What are comets?
Comets are made up of material left over from the formation of star systems. In the solar system, this means dust and ice that are 4.6 billion years old. At the heart of the comet is a central solid core, or “dirty snowball”, consisting of frozen water, dust and volatile substances such as carbon dioxide, carbon monoxide, methane and ammonia.
As the comet approaches the Sun, heat sublimates the surface ice into gas, creating a temporary atmosphere, or coma, around the nucleus. Emissions of dust and gas give the comet a blurred appearance and form tails that can stretch for millions of kilometers. Comets have two tails. One is white and formed by dust rising from the comet, the other is bluish and made of electrically charged molecules or ions. The ion tail points directly away from the Sun, regardless of the comet's position.
What about interstellar comets?
As the name suggests, they came even further back, formed from the remains of other star systems. A chance collision with the gravitational field of a passing star or other massive body can knock these comets off course and send them in our direction. Until now, astronomers have only seen three interstellar comets streaking through the solar system. The first, 1I/Oumuamua, was discovered in 2017. The second, 2I/Borisov, was spotted in 2019, and the last, 3I/Atlas, was reported in July of this year.
What can they tell us?
Interstellar comets are the only material from other star systems that astronomers can observe relatively closely. When comets pass close to the Sun, the dust and gases they emit reveal the chemical ingredients of the nascent star system. “These objects are the first building blocks that we can observe from these systems,” said Michael Kuppers, scientist on the European Space Agency's Comet Interceptor and Hera missions. “They tell us about the conditions in the star system where they formed.”
The 3I/Atlas studies may be more revealing than observations of previous interstellar comets. Astronomers haven't been able to detect much gas and dust around 'Oumuamua, and 3I/Atlas will come much closer to the Sun than Borisov, causing it to spew out more gas and dust for analysis.
Is 3I/Atlas special?
Astronomers have seen too few interstellar comets to know what is normal; their rarity makes them all fascinating. But some features are especially interesting. Early measurements show 3I/Atlas to be between 440 meters and 5.6 kilometers in diameter, making it potentially larger than the cigar-shaped 'Oumuamua, which is up to 400 meters long, and Borisov, which is about 1 kilometer wide.
An interesting feature found in a Hubble Space Telescope image shows that 3I/Atlas has a coma that is swelling towards the Sun. This “anti-tail” is believed to be caused by uneven sublimation of the ices on the comet.
The comet also differs from the more famous solar system comets in the amount of certain elements ejected from the surface. Astronomers using the European Southern Observatory's Very Large Telescope in Chile have detected a high ratio of nickel to iron in the comet's plume, which can be explained by the sublimation of nickel tetracarbonyl compounds and iron pentacarbonyl compounds in the comet. Other preliminary observations have shown that the light scattered by the comet is unusually polarized, but this can be explained water ice and silicates rich in magnesium.
Where do they come from?
Comets orbiting Earth often originate from the Kuiper Belt, a group of cool bodies lurking beyond Neptune. It takes less than 200 years to complete each revolution around the Sun. Others come from a region called the Oort cloud, which starts further out and extends halfway to our nearest neighboring star. Oort cloud comets can take 30 million years to complete one orbit around the Sun. There are believed to be billions of comets in the Kuiper belt, and even more in the Oort cloud.
What route does the comet take?
The comet entered the solar system close to the ecliptic – the imaginary plane along which the Earth's orbit around the Sun passes. Other planets orbit close to the same plane. The comet will not collide with Earth or any other planet in the solar system. The maximum distance it will approach the Earth will be 240 million kilometers, which is more than 1.5 times the distance between the Earth and the Sun.
American astronomer Avi Loeb suggested that “'Oumuamua was alien technologyalso suggested that comet 3I/Atlas maybe alien technology. He believes that the comet's trajectory, passing near Jupiter, Mars and Venus, appears to be planned. Moreover, he says that the comet came within nine degrees of Earth! Signal, radio waves discovered in 1977, which some consider potential alien transfer. But Loeb admits that “by far the most likely outcome is that 3I/Atlas will become a completely natural interstellar object.”
Küppers agrees. “It looks like a comet and behaves like a comet. There's no reason to think it's anything else,” he said. “If you start with a trajectory and then look at all sorts of distances and angles and so on, you'll always find something relatively unlikely.”
What happens next?
Astronomers hope to get more images of Comet 3I/Atlas using ground- and space-based telescopes, orbiters and rovers, and other probes such as the European Space Agency's Juice mission. This week, ESA released images taken by two Mars missions, Trace Gas Orbiter and Mars Express, as the comet streaked past the red planet. The comet was 30 meters away from us, so it looks like a small dot, but the coma is visible, showing that the sun's heat and radiation are bringing the comet to life. As the comet approaches Earth, it will be behind the Sun but will reappear in late November, giving astronomers another chance to observe it.
