Smashing comets and spectacular scorpions: Heather Couper and Nigel Henbest scan the July skies to preview a lunar anniversary, a collision on Jupiter and a constellation with a sting in its tail
Monday 27 June 1994
The biggest collision, with a fragment estimated to be 4km across, will coincidentally take place on the 25th anniversary of the other space event that will be celebrated this month - the first manned Moon landing. Among all the nostalgia and reminiscing, there will doubtless be a look to the future - and a call for us to return to the Moon.
But we have already begun our return. Earlier this year, on 19 February, an American spacecraft called Clementine slipped into lunar orbit. For more than two months it surveyed the whole Moon, including some regions near the poles that have never been mapped before (where it may have discovered ice pockets deposited by comets). The 1.6 million images it obtained at different wavelengths reveal the Moon's geology in unprecedented detail, with its various soil and rock types picked out in 11 colours. These images, which are widely available in the US on the Internet, will help in planning future lunar exploration, and, eventually, in exploiting the Moon's mineral resources.
Clementine is a new breed of spacecraft. A collaboration between the National Aeronautics and Space Administration (Nasa) and the US Department of Defense, it was designed and built in under two years with the cheap price-tag (for a space probe) of dollars 55m ( pounds 36m). It is a test of new technologies - in particular, lightweight sensors and electronics - that could be used for both military and civil applications. Ironically, it was old technology - in the shape of the computer - that prematurely ended Clementine's mission.
After leaving lunar orbit, the probe had been scheduled to rendezvous in late August with the near-Earth asteroid 1620 Geographos. But a computer malfunction triggered the 12 attitude-control thrusters to vent all their hydrazine propellant, leaving the spacecraft unsteerable. Despite this, the Clementine mission has been judged an outstanding success, and Clementine 2 is now on the drawing board.
Venus is still a brilliant object in the north-western sky after sunset, although by the end of the month it sets only an hour and a half after the Sun. On 12 July, it forms a pretty grouping with the crescent Moon. The other planet prominent in evening skies is Jupiter, although by the end of the month it will set at about midnight. Readers with access to moderate or large telescopes (with apertures of at least 15cm) might like to monitor the planet between 16-22 July to see if the Comet Shoemaker-Levy's impacts have any effect on Jupiter's vast bulk.
Saturn is now starting to put on an appearance in evening skies, rising at about 10.30pm mid-month. The remaining planets visible to the unaided eye - Mars and Mercury - are both morning objects. Mars is rising at about 2am, and is currently tracking past the stars of Taurus. Mid-month it passes close to the bull's 'eye', Aldebaran (which is slightly brighter than Mars and almost as red), and the Pleiades star cluster. On the morning of 5 July, it will make a striking grouping with the Moon, with the latter less than a moon's width to the south.
Mercury is a morning star this month. It rises with the Sun at the beginning of July, but gradually draws away until mid-month, when it rises an hour before the Sun.
This month affords the best view of one of the most spectacular constellations in the sky: Scorpius. Seen from more southerly latitudes, it is an unforgettable sight: a huge Scorpion-shaped curve of stars, with a vicious-looking sting in its tail. In Britain we are too far north ever to see the tail, so much of the effect is lost. But we can see the heart - the star Antares - and the 'pincers', low in the south.
In mythology, Scorpius was the scorpion whose sting killed Orion. The two constellations are on opposite sides of the sky, apparently a deliberate ploy by the gods to keep them apart. It is one of the most ancient of all constellations, dating back to 5,000 BC when it was one of the six original zodiacal signs of the Euphratean astronomers. The constellation's brightest star is Antares, whose name stems from the Greek 'Anti-Ares' - rival of, or similar to, Mars. It is a red giant star close to the end of its life, and changes slightly in brightness as its loosely held atmosphere pulsates in and out. Recent estimates of its size - almost one billion kilometres across - make Antares nearly as large as Betelgeuse, its counterpart in Orion.
Scorpius harbours an object that gave rise to a whole new field of astronomy. Close to the borders with Ophiuchus is Scorpius X-1, a faint double star with strange characteristics. As well as giving out light, it emits powerful X-rays, which were detected serendipitously on a routine flight by an Aerobee rocket in 1962. The X-rays come from fiercely hot gas dragged from one star of the pair on to the other - probably a neutron star with an immense gravitational pull.
Since the discovery of Scorpius X-1 - still the brightest X-ray source known - thousands of other X-ray sources have been pinpointed by rockets and satellites (fortunately, X-radiation does not penetrate Earth's atmosphere). Most, like Scorpius X-1, are regions of extreme cosmic violence, and some may even be the environs of black holes.
Diary (all times BST)
5 July Earth farthest from Sun (152m km)
8 10.38pm, New Moon
16 2.12am, Moon at First Quarter
17 Mercury at greatest W elongation
22 9.16pm Full Moon
30 1.40pm Moon at Last Quarter
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