Stargazing in May: An interstellar journey

“Comets are like cats: they have tails, and they do precisely what they want.” So says Canadian astronomer David Levy, who knows more about comets than most. He’s found 22 of these cosmic beasts, including the co-discovery of Comet Shoemaker-Levy 9 which spectacularly smashed into Jupiter in 1994.

His words came to mind when I recalled my signoff from this column last month: “According to the most optimistic forecasts, Comet Atlas will be brilliant in May … watch this space!

Well, Comet Atlas decided it would do something totally different. Overheated by the warmth of the Sun as it approached, this cosmic lump of ice and rock has disintegrated. The Hubble Space Telescope saw it break up into at least 30 fragments, which are now rapidly fading from sight.

But, as Atlas began to break up, an Australian amateur astronomer discovered a faint blur on images sent back to Earth by the Solar and Heliospheric Observatory. Since 2004, Michael Mattiazzo has been regularly checking images from this spacecraft, and has previously picked out eight new comets. His new discovery has been named Comet Swan, after the Solar Wind Anisotropies camera that captured the image.

And it’s coming our way. Currently, Comet Swan is only visible from the southern hemisphere, but it’s heading rapidly northwards in the sky. It passes closest to the Earth on 12 May, at 83 million kilometres, and – as it travels towards its rendezvous with the Sun on 27 May –the comet appears low down in our evening sky, in the constellations Perseus and Auriga (see the starchart).

The key question, of course, is how bright it will be – assuming that this comet holds itself together. No one is suggesting it will be “brilliant”, certainly nothing like the wonderful Comet Hale-Bopp which many of us remember from 1997. The best guess is that it will be a bit fainter than the stars making up the well-known Plough. You may not be able to see it well with the naked eye, but binoculars – or a small telescope – should reveal a distinct glowing ball of gas with a comet’s trademark elongated tail.

Professional telescopes will be trained on the comet to study the gases it’s emitting. This visit is probably the first that Comet Swan has made to the Sun from the frozen depths of the Solar System, and researchers want to find what molecules it’s bearing. Many scientists think that the raw materials of life may have been brought to the Earth by comets.

Ideally, we should despatch a spacecraft to investigate the comet at close quarters, as the European Space Agency (ESA) did with its Rosetta mission to Comet 67P/Churyumov–Gerasimenko in 2014. But it takes a long time to design, build and launch a spacecraft like Rosetta – by which time a newly discovered comet will have disappeared again. And, although the mission returned a lot of interesting results, Churyumov–Gerasimenko is a comet that’s been baked by the Sun many times, and its material is longer pristine.

That’s why the ESA has designed a mission to catch a virgin comet. After launch in 2028, the Comet Interceptor will wait in orbit around the Sun until astronomers find an exciting new comet for the spacecraft to intercept. The target could be a first-time visitor from the outer parts of our Solar System, like Comet Swan, packed with still-frozen ices preserved from the birth of the planets. Or it could be a comet from another star, which has escaped to stray near our Sun and can provide us with a unique insight into the material that makes up other planetary systems.

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