f you’ve always harboured a yen to spot the innermost planet of the Solar System, then this is the month to tick it off your bucket list. Shy Mercury is putting on its best evening display of the year, and we even have a signpost to help you find it, in the shape of Venus, the brilliant Evening Star.
Look west after sunset and you can’t miss glorious Venus. Wait until the sky gets almost dark, around 9pm, and you’ll spot another “star” above it, about ten times fainter. That’s your quarry. And you can rightly feel smug. It’s rumoured that the sixteenth-century Polish astronomer Nicolaus Copernicus, who said that the Sun rather than the Earth is the centre of the Solar System, wasn’t able to observe Mercury because of the mists rising from the nearby River Vistula.
Even a powerful telescope isn’t going to reveal much on this tiny planet, hardly larger than our Moon. Fortunately, its surface has been scanned in close-up by two visiting Nasa spacecraft, Mariner 10 and Messenger. They have revealed a barren surface, almost completely disfigured by craters of all sizes.
Mercury has scarcely any atmosphere. Its surface is blasted by solar radiation and by the Sun’s heat – during the daytime, at least. The nightside is exposed to the total chill of space, so temperatures at the equator swing from 430C during the day – 170C at the dead of night. Mercury rotates so slowly that its “day” – from noon to noon – is actually twice as long as its year.
Another oddity is Mercury’s extreme density. Unlike the Earth, it can’t be made mainly of rock. Instead, almost the whole planet consists of a huge iron core, with just a veneer of rocky crust over its surface. Astronomers don’t why. According to one theory, most of the crust was stripped away by a giant cosmic impact early in the planet’s history. Or it may just have been born so close to the young Sun’s intense heat that rocks remained as a gas while iron condensed into a giant metallic blob.
Scientists hope to learn the answer when the European BepiColombo arrives in 2025 and slips into orbit around Mercury to investigate the planet’s remaining mysteries. Even more intriguing than its birth is that fact that, at its poles, this scorched planet has craters harbouring vast amounts of ice.
Yes, frozen water on the closest planet to the Sun! It’s hidden in the depths of craters at Mercury’s poles, where the Sun never rises and conditions are perpetually dark and cold. Astronomers first detected the ice in 1991, using powerful radar based on the Earth, and the Messenger spacecraft has revealed there’s up to a trillion tonnes of ice, in places possibly 20m deep.
This discovery opens up the possibility of astronauts voyaging to the innermost planet, and living on Mercury. There’s water aplenty for drinking, and for growing crops in pressurised domes. More than that, colonists could use the power of the intense sunlight to split the water into oxygen for breathing, and to burn with hydrogen as a rocket fuel.
With Venus far too hot for habitation, and the outer planets so remote and cold, some space scientists are saying that – after Mars – Mercury is mankind’s next natural home in space.
What’s up
The brilliant Evening Star, Venus, hangs low in the northwest as the sky grows dark, outshining all the stars.
As I’ve mentioned above, near Venus you can locate Mercury. The innermost planet never strays far from the Sun in the sky and this month provides our best opportunity for seeing this elusive world in 2021: after Venus has set, we have the unusual treat of seeing Mercury so long after dusk that the sky is dark.
The night sky at around 11pm this month
(Nigel Henbest)
At the beginning of May, Mercury lies to the left of the famous Seven Sisters star cluster, the Pleiades. By 9 May, this little world has moved upwards and Venus is close to the Pleiades. Mars, shining a distinctive ochre hue, lies to the upper left of Mercury, and considerably fainter.
We’re in for a lovely sight on 13-15 May, as the crescent Moon moves in turn past Venus, Mercury and Mars.
If you’re up in the wee small hours, look out for the bright giant Jupiter and its fainter sibling Saturn, rising in the southeast around 3 am.
We’ll all certainly be looking towards the sky on the evening of 26 May, for the biggest and brightest full moon of the year. It’s just 357,314km away from the Earth, and appears 30 per cent more brilliant than the faintest full moon. People around the Pacific Ocean will experience a total lunar eclipse.
Diary
9 May: Venus near the Pleiades
11 May, 7.59pm: new moon
13 May: Moon near Mercury and Venus
14 May: Moon between Mars and Mercury
15 May: Moon near Mars
16 May: Moon near Castor and Pollux
17 May: Mercury at greatest elongation east
19 May, 8.12pm: first quarter moon near Regulus
23 May: Moon near Spica
26 May, 12.14pm: full moon near Antares; supermoon; lunar eclipse
28 May: Mercury near Venus
Philip’s 2021 Stargazing (Philip’s £6.99) by Heather Couper and Nigel Henbest reveals everything that’s going on in the sky this year
