Cassini’s scheduled suicide at Saturn
The heroic Cassini mission to Saturn is set to reach its dramatic conclusion on 15 September. After a seven-year journey from Earth, the probe has been studying the planet, its glorious rings and its fascinating moons for the past thirteen years. Now, with its fuel running low, it is time for the NASA probe to plunge into the Saturnian atmosphere where, in the interest of so-called planetary protection, it will disintegrate and vaporise.
To leave it in orbit around the planet would run the risk of it colliding with the rings or one of the moons, with the outside possibility of contaminating them with microbes from the Earth. This was of little concern when Cassini’s mission was planned, and it carried and delivered the European-built Huygens probe which parachuted to the surface of Saturn’s largest moon, Titan. It touched down on a world in which rivers of liquid hydrocarbons, chiefly methane, flow into lakes in a landscape dominated by water-ice mountains.
Now, though, we realise that despite Saturn’s remoteness from the Sun, the possibility of alien life there cannot be discounted. Indeed, it seems clear that its small moon Enceladus has a subsurface watery ocean and there has been talk of sending a mission to search for organic compounds in the plumes of water erupting from geysers on its surface.
Recent orbits of Saturn have seen Cassini piercing the gap between Saturn and its rings, and even skimming the planet’s outer atmosphere. It will continue to collect data as it begins its final suicidal dive into Saturn’s atmosphere on the 15th, but its signal will be lost at around 13:00 BST as aerodynamic forces cause it to tumble and, eventually, break apart and burn up.
The Sun crosses southwards over the equator at 21:02 BST on the 22nd, the moment of our autumnal equinox. Sunrise/sunset times for Edinburgh change from 06:17/20:07 BST on the 1st to 07:14/18:50 on the 30th. The Moon is full on the 6th, at last quarter on the 13th, new on the 20th and at first quarter on the 28th.
Now that Scotland’s persistent summer twilight is behind us, our nights offer views of the Milky Way as it arches directly overhead from the south-west to the north-east at our chart times, carving through the Summer Triangle formed by Deneb, Altair and Vega which now lies just west of the high meridian.
To the east of the Triangle is the distinctive form of the celestial dolphin, Delphinus, where the celebrated English amateur astronomer George Alcock discovered a famous and unusual naked-eye nova fifty summers ago in 1967. I remember watching the stellar outburst as it took five months to reach its peak brightness at magnitude 3.5. Now assigned the variable-star tag HR Delphini, the star is still visible as a twelfth magnitude object through telescopes.
Another 13° east of Delphinus is the globular star cluster Messier 15, 4° north-west of Pegasus’s brightest star, Enif. A tightly packed globe of perhaps 100,000 stars, all very much older than our Sun, M15 lies around 34,000 light years away and looks like a fuzzy star through binoculars.
Saturn is the sole bright planet to appear on our star maps. Look for it as the brightest object low down in the south-south-west at nightfall and even lower in the south-west by our map times, only thirty minutes before it sets. Edging eastwards in Ophiuchus, it shines 4° below-left of the Moon on the 26th.
Jupiter is bright at magnitude -1.7 but hard to see very low in the west-south-west just after sunset. By mid-month it is likely to be lost in the twilight.
Our charts plot the two outer planets, the ice giant world Uranus in Pisces and its near-twin Neptune in Aquarius, though we probably need more detailed charts to identify them through binoculars or telescopes. At magnitude 5.7, Uranus is at the verge of naked-eye visibility, while Neptune reaches opposition on the 5th and is dimmer at magnitude 7.8.
The other planets are about to join Venus low down in our eastern sky at the end of the night. The brilliant morning star shines at magnitude -4.0 when it rises in the north-east at 03:04 for Edinburgh on 1 September, and climbs 25° high into the east by sunrise. Catch it through binoculars before the twilight intervenes on that day and look 1.2° to its left for the Praesepe or Beehive cluster of stars in Cancer. Leaving the cluster behind, Venus tracks east-south-eastwards into Leo to pass 0.5° (a Moon’s breadth) north of the star Regulus on the 20th.
Mercury emerges from the Sun’s glare to stand 18° west of the Sun and 11° below-left of Venus on the 12th. Between the 6th and 23rd it rises more than 80 minutes before sunrise and brightens eightfold from magnitude 1.1 to -1.1. On the 6th, in fact, Mercury lies 2.5° to the right of Regulus which, in turn, is 0.8° to the right of the fainter magnitude 1.8 planet Mars. As Regulus climbs above them, the two planets then converge to lie less than 0.5° apart on the 16th and 17th.
Early risers are in for a special treat when the waning earthlit Moon joins the party on the 17th. On that morning, Venus stands 10° below-left of the Moon and almost 4° above-right of Regulus, with the Mars-Mercury conjunction another 8° below and to the left. On the 18th, the line-up is even more compact as the Moon shifts to lie 0.7° below Regulus. By the 30th, Venus rises in the east-north-east at 04:41 and is 3° above-right of Mars.
This is a slightly-revised version of Alan’s article published in The Scotsman on August 31st 2017, with thanks to the newspaper for permission to republish here.
Brilliant Venus plunges into the evening twilight
Stargazers will be hoping for better weather as Orion and the stars of winter depart westwards in our evening sky, Venus dives into the evening twilight and around the Sun’s near side, while all the other bright planets are on view too. Indeed, Venus has the rare privilege of appearing as both an evening star and a morning star over several days, provided our western and eastern horizons are clear.
Orion still dominates our southern sky at nightfall as Leo climbs in the east and the Plough balances on its handle in the north-east. The Sun’s northwards progress and our lengthening days mean that by nightfall at the month’s end Orion has drifted lower into the south-west, halfway to his setting-point in the west. He is even lower in the west-south-west by our star map times when it is the turn of Leo to reach the meridian and the Plough to be almost overhead.
Leo’s leading star, Regulus, sits at the base of the Sickle of Leo, the reversed question-mark of stars from which meteors of the Leonids shower stream every November. The star Algieba in the Sickle (see chart) appears as a glorious double star through a telescope. Its components are larger and much more luminous than our Sun and lie almost 5 arcseconds apart, taking some 510 years to orbit each other. The pair lie 130 light years away and are unrelated to the star less than a Moon’s breadth to the south which is only half as far from us.
The Sun travels northward across the equator at 10:28 GMT on the 20th, the moment of the vernal (spring) equinox in our northern hemisphere. On this date, nights and days are of roughly equal length around the globe. Sunrise/sunset times for Edinburgh change from 07:04/17:47 GMT on the 1st to 06:46/17:49 BST (05:46/18:49 GMT) on the 31st after we set our clocks forwards to BST on the morning of the 26th. The lunar phases change from first quarter on the 5th to full on the 12th, last quarter on the 20th and new on the 28th.
Look for the young earthlit Moon well to the left of the brilliant magnitude -4.6 Venus on the 1st when telescopes show the planet’s dazzling crescent to be 47 arcseconds in diameter and 16% sunlit. Venus’ altitude at sunset plummets from 29° in the west-south-west on that day to only 7° in the west on the 22nd as its diameter swells to 59 arcseconds and the phase shrinks to only 1% – indeed, a few keen-sighted people might be able to discern its crescent with the naked eye and this is certainly easy to spot through binoculars.
Venus dims to magnitude -4.0 by the time it sweeps 8° north of the Sun and only 42 million km from the Earth at its inferior conjunction on the 25th. This marks its formal transition from the evening to the morning sky, but because it passes so far north of the Sun as it does every eight years or so, Venus is already visible in the predawn before we lose it in the evening. In fact, it is 7° high in the east at sunrise on the 22nd, and it only gets better as the month draws to its close.
Before Venus exits our evening sky, it meets Mercury as the latter begins its best spell as an evening star this year. On the 20th, the small innermost planet lies 10° to the left of Venus, shines at magnitude -1.2 and sets at Edinburgh’s western horizon 78 minutes after the Sun. By the 29th, it is 10° high forty minutes after sunset and shines at magnitude -0.4, easily visible through binoculars and 8° to the right of the very young Moon.
Mars, near the Moon on the 1st and again on the 30th, dims from magnitude 1.3 to 1.5 this month as it tracks from Pisces into Aries. By the month’s end, it lies to the left of Aries’ main star Hamal and sets at our map times. It is now more than 300 million km away and its disk, less than 5 arcseconds across, is too small to be of interest telescopically.
The Moon has another encounter with the Hyades star cluster on the night of the 4th-5th, hiding several of its stars but setting for Scotland before it reaches Taurus’ main star Aldebaran. The latter, though, is occulted later as seen from most of the USA. The Moon passes just below Regulus on the night of the 10th-11th and meets the planet Jupiter on the 14th.
Jupiter, conspicuous at magnitude -2.3 to -2.5, rises in the east at 21:37 GMT on the 1st and only 31 minutes after Edinburgh’s sunset on the 31st. Now edging westwards above the star Spica in Virgo, it is unmistakable as it climbs through our south-eastern sky to cross the meridian in the small hours and lie in the south-west before dawn. Its disk, 43 arcseconds wide at mid-month, shows parallel cloud bands through almost any telescope, while its four moons may be glimpsed through binoculars as they orbit from one side to the other.
Saturn, the last of the night’s planets, rises in the south-east at 03:44 GMT on the 1st and almost two hours earlier by the 31st. Improving very slightly from magnitude 0.5 to 0.4 during March, it is the brightest object about 10° above the south-south-eastern horizon before dawn. Look for it 4° below-left of the Moon on the 20th.
This is a slightly-revised version of Alan’s article published in The Scotsman on February 28th 2017, with thanks to the newspaper for permission to republish here.
Mars meets Regulus as Jupiter dominates before dawn
Our main interests this month lie in our predawn sky where Jupiter shines brighter than any star and where Mars is in conjunction with the star Regulus in Leo. Sweeping close to Mars is Comet ISON which remains dim at present but may yet brighten to become an impressive sight later in the year.The Summer Triangle of Vega, Altair and Deneb still looms high on the meridian at dusk but has tumbled westwards by our star map times. Trailing behind it is Delphinus where a nova flared in August. That stellar outburst was still just visible through binoculars in late September but was perhaps on the verge of plunging much fainter as is becomes shrouded in dust forming in the wake of the explosion.
Our charts show the Square of Pegasus approaching the meridian as the Plough rotates counterclockwise below Polaris in the north and the “W” of Cassiopeia nears the zenith. Taurus and the Pleiades are climbing in the east while Gemini is rising in the north-east with the Twins, Castor and Pollux, one above the other.
Jupiter stands 8° below and to the right of Pollux and rises less than 30 minutes after our map times, climbing to prominence high in the south-south-east to south before dawn. The planet creeps 2° eastwards during October, passes only 7 arcminutes north of the magnitude 3.5 star Delta Geminorum on the 4th and is above the Moon on the 26th. Jupiter improves from magnitude -2.2 to -2.4 this month and swells from 38 to 41 arcseconds wide if viewed telescopically.
Our other pre-dawn planet, Mars, stands above-left of the Moon on the 1st and again on the 30th. Rising for Edinburgh in the east-north-east at 02:27 BST on the 1st and by 01:18 GMT on the 31st, it is climbing well up into the south-east before dawn. From 9° above-right of Leo’s leading star Regulus as the month begins, it speeds eastwards to pass 1.0° north of Regulus late on the 14th and lie 9° below-left of the star at month’s end. It improves slightly from magnitude 1.6 to 1.5 but remains just fainter than the magnitude 1.4 of Regulus. Note, though, the contrast in appearance with Mars being steady and reddish while Regulus is bluish-white and twinkling. Through a telescope, the planet’s tiny ochre disk is 4 to 5 arcseconds wide.
Comet ISON remains something of an enigma. Discovered just over a year ago when it lay beyond the orbit of Jupiter, it has been diving towards the Sun on a path that carries it 1,100,000 km above the Sun’s surface on 28 November. Some predictions have it as bright as the full moon at that point, albeit swamped in the Sun’s glare. In fact, its brightening has been slower than most were expecting and it was still a dim 12th magnitude object in late September. Hopes remain that it will be a naked-eye object in mid-November and again during December when it may well sport an impressive tail.
Its orbit takes it within 10.5 million km of Mars on the 1st and it is already being observed by spacecraft at the planet. It stands 2.0° above-left of Mars in our morning sky on the 2nd, moving to lie within 0.9° of Mars on the 18th and 6° below-left of the planet by the 31st. We can only hope that it might be visible through binoculars by the month’s end.
An unfortunate fact concerning our autumn sky is that planets that lie to the east of the Sun are hugging our horizon at sunset. Take Venus, for example. As seen from Edinburgh at sunset on the 1st, the brilliant magnitude -4.2 evening star stands 135 million km away and 45° away from the Sun but is only 5° high in the south-west. It sets 53 minutes later and should be obvious as the twilight fades, but only if our horizon is clear. Contrast this with Australia where Venus is a stunning object 43° high in the W at sunset and remains visible for almost four hours.
The reason, of course, is that the planets never stray far from the ecliptic, the Sun’s apparent annual path against the stars. At present the ecliptic slants low across Scotland’s south-western sky at nightfall as it traces the Sun’s southerly motion until midwinter.
By the 31st, Venus stands 5° above Edinburgh’s south-south-western horizon at sunset, shines a little brighter at magnitude -4.4 and is 6° below-right of the Sun’s midwinter position against the stars. As Venus approaches to 101 million km this month, its telescopic diameter swells from 19 to 25 arcseconds while the dazzling sunlit part of its disk changes from 63% to 50%. Look for the planet 4° below the young crescent Moon on the 8th and 1.5° above the red supergiant Antares in Scorpius on the 16th.
We have no chance of seeing Saturn or Mercury which are both fainter and closer to the Sun in the evening sky than Venus.
This month the Sun tracks 11° southwards as sunrise/sunset times for Edinburgh change from 07:16/18:48 BST (06:16/17:48 GMT) on the 1st to 07:18/16:34 GMT on the 31st. Nautical twilight persists for about 82 minutes at dawn and dusk. The Moon is new on the 4th, at first quarter on the 12th, full on the 19th and at last quarter on the 27th.
The southern 76% of the Moon’s disk slips through the fringe of the Earth’s shadow on the night of the 18th/19th to cause a penumbral lunar eclipse. Although only a slight dimming may be noticed, at least the Moon is well placed being in Pisces in the middle of our southern sky. The eclipse lasts from 22:51 to 02:50 BST.