It is comet 2I/Borisov’s time to shine. The interstellar visitor has been steadily getting brighter since it was first spotted in September, and now it is reaching its brightest moments as it passes close to the sun and Earth.
We know that Borisov comes from another stellar system because of its trajectory: rather than orbiting the sun like comets from our own solar system, it is simply passing through. It came in from above the plane on which the planets orbit and is now passing between the orbits of Mars and Jupiter.
It makes its closest approach to the sun on 8 December, at just about 299 million kilometres away. Later in the month, it will pass slightly closer than that to Earth. That makes this month key in our quest to observe and understand the interstellar comet.
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“When it comes closer to the sun it gets more active because it gets heated more and releases more gas, so it gets brighter. And when it comes closer to Earth it gets brighter just because it’s closer,” says Anita Cochran at the University of Texas at Austin. “The brighter it is, the easier it is to observe it.”
Because of that, many of the world’s biggest telescopes will be trained on Borisov as it passes by. “We’re all just hammering on it because now it’s brightest,” says Michele Bannister at Queen’s University Belfast, UK.
For the most part, those telescopes aren’t taking pictures but rather looking at the chemical spectrum of the comet to try to figure out what it is made of. “Based on the molecules we see, we can put limits on the temperature and pressures under which the comet formed,” says Maria Womack at the University of Central Florida. We are already seeing signs that Borisov is similar to a relatively rare type of solar system comet that has fewer large carbon molecules than others, she says.
Read more: Alien water may have been found on interstellar comet Borisov
When regular comets get close to the sun, they sometimes send out huge flares of gas and dust that make them suddenly far easier to observe. Some telescopes are set up to start observing Borisov immediately if it suddenly brightens because that would reveal the most information about the comet’s interior. “If it’s going to cooperate, it will cooperate in the next few days, so it’s a bit of a nail-biting time,” says Gregory Laughlin at Yale University.
“The big hope is, or was, that there is something different, so that we learn something about other stellar systems that we could never learn from our own solar system,” says Pieter van Dokkum, also at Yale. “So far, all the observations seem to suggest that it’s very similar to comets in our own solar system, which is a little bit disappointing.” It tells us that our outer solar system is probably very similar to the outer reaches of other distant systems.
We should expect to see more comets like Borisov, Cochran says, so this isn’t quite a once-in-a-lifetime opportunity for observation. But within a year, we won’t be able to see it anymore even with the biggest Earth-based telescopes. “Most interstellar comets and asteroids never get anywhere near a star,” says Laughlin. “We’re excited about the visit, but for [the objects] it was everything – there’s never going to be another interesting encounter for them. They’ll be drifting through space long after the sun dies.”
