We Might Have Found Another Interstellar Comet, And This One's Coming Closer

Estimates of the path of gb00234 are subject to change as more data comes in, but the green hyperbola indicates the best estimate we have. Jupiter is in pink and Neptune in blue. Tony 8730004 via Wikimedia commons. CC-by-SA-4.0

On August 30 astronomers at the Crimean Astrophysical Observatory detected what looked like a perfectly ordinary comet. However, excitement is rising with each new observation of its movement through the skies. Astronomers now think the object currently known as gb00234 has a hyperbolic orbit, meaning it comes from outside the Solar System, only the second such object we have observed. Better yet, it is coming closer, and by the end of the year it could be so bright you can see it with a backyard telescope.

The discovery of Oumuamua in 2017 was a historic development, the first time humanity had knowingly observed a visitor to the Solar System. The fact that it was such an unusual object, with a length to width ratio greater than any local comet or asteroid, added to the excitement – even feeding claims this was an alien spacecraft, although few astronomers take this idea seriously after no radio signals were found.

There was, however, one disappointing aspect to the whole event. We only noticed how unusual Oumuamua's orbit was when it was already past the Sun and on the way out, limiting opportunities for study. If gb00234 (not to be confused with the women's running shoe of the same name) really is interstellar, this time things will be very different. Gb00234 is on its way in, and will make its closest approach to the Sun either late this year or early in 2020 – some predictions even have it occurring on New Year's Day.

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There are still question marks over gb00234's interstellar status because it is around 3.5 astronomical units (the radius of the Earth's orbit) from the Sun. Moreover, like all comets, it is a slightly fuzzy blob, rather than a sharp point, which makes precise measurements of its position hard. Nevertheless, the data we now have suggests this has a velocity in interstellar space slightly greater than Oumuamua's, and a much higher eccentricity – two measures used to identify objects from outside the Solar System.

Gb00234 moving against the background stars, photographed by its discoverer Gennady Borisov

 The brightness of comets is notoriously hard to predict – generations of amateur astronomers have been disappointed at those that failed to live up to expectations. That goes double when we still have enormous error bars on how close to the Earth gb00234 will get. However, there is a high chance at its peak it will be visible with small amateur telescopes. If so, the Southern Hemisphere is likely to get the best view.

Update: Gb00234 has now been renamed C/2019 Q4 (Borisov) with an official, although still preliminary, orbit plotted by the International Astronomical Union's Minor Planet Center. This proposed orbit would be very hard to explain for an object whose origins lie within the Solar System.

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