Amateur astronomer in Crimea discovers first interstellar comet
It is two years since astronomers in Hawaii discovered the first object known to have approached the Sun from beyond our solar system. Given the Hawaiian name “Oumuamua”, it appeared to be a reddish and elongated slabshaped body of about the size of a skyscraper that passed 38,000,000km from the Sun before sweeping within 24,000,000km of the Earth. It came from roughly the current direction of the star Vega and headed away towards the Square of Pegasus, though it may take 20,000 years to leave the solar system completely.
Its small size meant that it was followed only faintly, and for barely a month. Astronomers were surprised to notice no sign of cometary activity – no surrounding fuzzy coma and no tail – while suggestions that it was an alien probe prompted unsuccessful scans for any artificial radio emissions.
Now the second-known interstellar intruder has been sighted, and this one appears larger, brighter and is surely a comet. It was discovered photographically on 29 August from an observatory in Crimea by the amateur astronomer Gennady Borisov using a telescope he built himself. Initially called C/2019 Q4 (Borisov), or Comet Borisov for short, it was clearly speeding along a strongly hyperbolic path past the Sun, very unlike the elliptical or nearly parabolic orbits followed by all previous comets. Now it has been awarded the official interstellar designation of 2I/borisov.
The comet was travelling at about 33km per second as it entered the solar system from the direction of the constellation Cassiopeia, fast enough to cover the four-light-years distance of the nearest star in under 40,000 years. Perihelion, its closest point to the Sun, occurs at 303,000,000km on 8 December, putting it still beyond the orbit of Mars, and it reaches its closest to the Earth at 293,000,000km 20 days later.
It is still faint, no better than magnitude 17, but may attain magnitude 14 near perihelion and, while it will never reach naked-eye or binocular visibility, is likely to be within telescopic range until at least the middle of next year. This gives plenty of time for astronomers to study a comet that probably formed elsewhere in the Milky Way galaxy at a different time and with possibly a different composition than those that formed alongside the Sun and Earth. October has Comet Borisov travelling south-eastwards to the west of the Sickle of Leo and passing within a Moon’s-breadth east of the star Regulus on the 24th.
Leo’s Sickle rises in the northeast in the early morning and stands some 30° high in the east before dawn as our southern sky is dominated by the glorious constellation of Orion. The pre-dawn also gives us a chance to spot Mars as it emerges from the Sun’s far side. The planet rises in the east one hour before the Sun on the 1st and two hours before sunrise on the 31st. Moving east-south-eastwards in Virgo, it shines only at magnitude 1.8 and lies 8° below the slender earthlit Moon on the 26th.
As the Sun tracks southwards by 11° during October, the sky at nightfall is changing only slowly. The Summer Triangle is still high in the south as darkness falls, although its three stars – Vega in Lyra, Deneb in Cygnus and Altair in Aquila – have shifted into the west by our star map times. By then, Pegasus, the upsidedown flying horse with his nose near Delphinus the Dolphin, stands high in the south.
The sunrise/sunset times for Edinburgh change this month from 07:15/18:49 BST (06:15/17:49 GMT) on the 1st to 07:17/16:35 GMT on the 31st, following Summer Time’s end on the 27th. The Moon reaches first quarter on the 5th, full phase on the 13th, last quarter on the 21st and new on the 28th.
Like mars, venus is also coming into view from beyond the Sun, but this time into our evening twilight in the westsouth-west. Although brilliant at magnitude -3.9, it stands a mere 3° high at sunset for Edinburgh and sets at present only 30 minutes later, so we need good weather and a clear horizon to catch it. On the 29th, look for it 2.8° below the sliver of the earthlit young Moon, only 3° illuminated. Mercury is fainter and even lower at sunset and not visible from Scotland.
Jupiter is well past its best as an evening object, although it remains obvious low in the south-west at nightfall, sinking to set at Edinburgh’s southwestern horizon at 21:14 BST on the 1st and as early as 18:34 GMT by the 31st. At magnitude -2.0 to -1.9 and 36 to 33 arcseconds in diameter, it lies close to the Moon on the 3rd and 31st.
Saturn, one tenth as bright at magnitude 0.5 to 0.6, lies some 25° to the left of Jupiter. When it is close to the first quarter Moon on the 5th, its disk and rings span 17 and 38 arc seconds respectively. It is in Sagittariuslow in the south at night fall and sets in the south-west soon after our map times.
Neptune and Uranus are binocular brightness object of magnitudes 7.8 and 5.7 in Aquarius and Pisces respectively. There is little hope of locating them using our chart, but a web search, such as “Where is Uranus?”, should bring up information and a finder chart.
Uranus, in fact, reaches opposition at a distance of 2,817,000,000km on the 28th when it stands directly opposite the Sun and appears as a tiny 3.7 arcseconds blue-green disk through a telescope.
STELLAR FACT
Comet Borisov was clearly speeding along a strongly hyperbolic path past the Sun, very unlike the elliptical or nearly parabolic orbits followed by all previous comets