Stargazing in May: An interstellar journey

Comet Swan is due to make an appearance over the northern hemisphere as it travels towards the sun

The floating rock has emerged from the icy depths of the solar system (AFP)

“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.

To the ancient Greeks these stars, plus many fainter ones around, made up the outline of a bear. The four stars in a rectangle made its body, while the outlying three stars marked its tail

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.

What’s Up

Brilliant Venus starts the month as the glorious Evening Star, gracing the dusk sky as it has since the beginning of the year. But all is about to change, as Venus – on its smaller and faster orbit around the Sun – overtakes the Earth on the inside track. With a small telescope, you can see the planet growing larger and its crescent shape shrinking inexorably throughout the month as it sinks lower into the twilight. And, almost unbelievably, Venus has gone by the end of May.

Before that, use the Evening Star as a guide to its smaller and fainter sibling, Mercury. You’ll find the innermost planet just to the left of Venus on 21 and 22 May, and 30 times dimmer. There’s a lovely sight on 24 May, when the crescent Moon joins in: Mercury is lying between the Moon (to the left) and Venus (to the right).

The familiar shape of the Plough lies almost overhead this month, its seven main stars making up the shape of an old horse-drawn plough. To modern eyes, it’s perhaps better described as a saucepan, and the Americans know this shape as the Big Dipper.

The night sky at around 11pm this month (Nigel Henbest)

To the ancient Greeks these stars, plus many fainter ones around, made up the outline of a bear. The four stars in a rectangle made its body, while the outlying three stars marked its tail – rather oddly, as real bears don’t have such magnificent appendages.

Follow the curve of the tail, and you come to the bright star Arcturus, its name meaning “the bear driver”. Extend this line to the lower left, and you come to Spica, the brightest star in Virgo (the Virgin). And under the body of the bear crouches the celestial lion, Leo, with bright Regulus marking its heart.

If you’re up before dawn, look to the southeast for the giant planet Jupiter, outshining all the stars. To its left is fainter Saturn, and further to the left again, the reddish planet Mars.

Diary

7 May, 11.45am: Full Moon

12 May (am): Moon near Jupiter and Saturn

13 May (am): Moon near Jupiter and Saturn

14 May, 3.02pm: Last Quarter Moon

15 May (am): Moon near Mars

21 May: Mercury near Venus

22 May, 6.39pm: New Moon; Mercury near Venus

24 May: Moon near Venus and Mercury

26 May: Moon near Castor and Pollux

27 May: Moon near Praesepe

29 May: Moon near Regulus

30 May, 4.30am: First Quarter Moon

Philip’s 2020 Stargazing (Philip’s £6.99) by Heather Couper and Nigel Henbest reveals everything that’s going on in the sky this year

Fully illustrated, Heather and Nigel’s The Universe Explained (Firefly, £16.99) is packed with 185 of the questions that people ask about the Cosmos