On August 30, Russian amateur astronomer Gennady Borisov captured a most unusual object in a photograph taken with his homebuilt 25.6-inch (0.65-meter) telescope. It didn’t look like much — just a fuzzy spot with a hint of fan-shaped tail. Yes, it was a comet, but not just any comet. This one had blown in from another star system!
Every comet discovered up till that moment until has orbited the sun. This one came from interstellar space, in other words, it once belonged to another star system who knows how many hundreds of light years away. It dropped into our neighborhood from the direction of Cassiopeia, the W-shaped constellation you can see right now in the northeastern sky at nightfall. And it’s heading toward the southern constellation of Telescopium. At the moment, the comet is a very faint object in Cancer invisible in amateur telescopes, but that will change.
We can tell the difference between comets from interstellar comets and ones that orbit the sun in at least two ways — orbit and speed. A sun-bound comet orbits our star in an ellipse, a sort of squished circle. Occasionally, a comet orbits along a really long ellipse that extends so far into the distance it looks open-ended. That’s called a parabola. Borisov’s comet tracks along a completely open-ended orbit called a hyperbola.
It’s been zipping through space for millions of years, perhaps passing other star systems along the way. At the moment, it can “feel” the sun’s gravity and will speed up a bit as it passes closest to our star on Dec. 10. at a distance of 180.4 million miles, about twice Earth’s separation from the sun. Then it will continue along its hyperbolic orbit and leave us behind, probably to never return again — a true interstellar vagabond cut loose from its origin-star either by a close encounter with one of its planets or possibly some sort of explosion that sent it reeling.
This is where speed is important. Stars in the sun’s neighborhood zip around the center of the galaxy at around 45,000 mph (20 km/s) relative to the sun. Comets within our solar system cycle around the sun zip along at anywhere from 10-70 km/s. Not only is Borisov on a hyperbolic orbit but its rather incredible speed of 92,000 mph (41 km/s) clearly tells us it’s not a member of the solar family but an object on its own path moving faster — relative to the sun — than many stars do. Even before it felt the sun’s touch the comet had been barreling along at 73,000 mph (32.6 km/s), the speed of the stars.
It won’t get very close to Earth unfortunately which means the comet will likely remain a faint object invisible in smaller telescopes. When closest to Earth on Dec. 28 it will still be 187 million miles away and glowing faintly at around magnitude 14, about the limit of a 14-inch telescope. I’m crossing my fingers I’ll pick it up in my 15-inch and can’t wait for the opportunity.
Despite how dim it is, amateur astronomers with 8-inch and larger telescopes and high-end camera have been busy as bees photographing the tiny smudge of light. Meanwhile, the comet offers an unprecedented opportunity for astronomers to study the comet up close with the biggest telescopes and spectrographs (instruments that determine a comet’s composition by the light it produces or reflects) on Earth and in space.
You might recall 1I ‘Oumuamua, the first interstellar object discovered back in Oct. 2017. We’re still not 100 percent sure if it was a comet or asteroid, but it was found after it’s closest approach to the sun when far off in space. Borisov is closing in as you read this and shows a nice tail and other cometary features, so the data collected will be much more complete. Already we know that color-wise, it resembles solar system comets.
The comet is a tiny blip in Cancer right now and moving to the southeast. In the next couple months it will cross into Leo and set up in the dim constellation Crater the Cup around the time of closest approach. I’ll share a map of its path closer to that date. If we get really, really lucky Comet Borisov may crumble a little as it’s heated by the sun. That could expose fresh ice and dust which could brighten the comet further. Who knows? Comets are always full of surprises.
For more details on the comet and speculation on its origin I encourage you to read this excellent FAQ written by Bill Gray, the creator of Guide astronomy software. By the way, the comet’s name is likely to be changed soon to 2I/Borisov, the second interstellar (I) comet-like object discovered.