Let’s imagine that the approaching comet is not just a lump of ice and dirt that received a slight gravitational nudge from a passing star. It is a being that planned, four or five million years ago, to do what it is now about to do.
This part of a diagram from Astronomical Calendar 2014 shows the paths in space of Mars and Comet C/2013 A1 Siding Spring, intersecting on October 19. Blue lines on the ecliptic plane are 1 astronomical unit (Sun-Earth distance) apart; stalks connect the bodies to the ecliptic plane at one-month intervals. The inward tick on the comet’s path indicates the point where it will be at perihelion, closest to the Sun, 6 days after its Mars encounter.
From out there in the Oort Cloud of trillions of other slow-moving lumps, 50,000 times as far away from the Sun as we are – about 5 million million miles – it measured the angle between the Sun and tiny Mars buried in the Sun’s light – about 0.002 of a degree, though the comet would have to know this much more exactly. It must have had a powerful telescope and an advanced astronomical brain! It would have had to take into account the elliptical shape of Mars’s orbit, and perturbations to that orbit caused by the other planets over those next millions of years, and Mars’s exact position around its orbit on an October day millions of years ahead. It calculated how to nudge itself so that it would fall almost exactly toward the Sun but just slightly enough off so that on that far-off instant – 2014 October 19 at 14 hours by the Universal Time of another planet near Mars, or 10 a.m. by the Daylight-Shifting clock time of the eastern part of a continent on that planet – it, the comet, would cut northward through the orbit of Mars on the far side of the Sun, just as Mars arrived at that point in its orbit!
Quite a feat. And this is what Comet C/2013 A1 Siding Spring is about to do.
The “A1” part of its designation means that it was the first comet discovered in the first half of January 2013, actually Jan. 3. It was found by Robert McNaught with a 20-inch-aperture telescope (called the Uppsala Southern Schmidt Telescope) at the Siding Spring observatory in Australia, where the Scottish-born McNaught works. He is the discoverer of (so far) 475 asteroids and 70 comets. By the current system, his 26 short-period comets bear his name but the long-period ones, such as this one, do not. One of his discoveries, C/2006 P1, was the Great Comet of 2007, brightest in forty years, easily seen by naked eyes in the southern hemisphere in early 2007.
(McNaught’s 70 comet discoveries may be the world record, though this is complicated by co-discoveries and naming conventions. The record for visual discoveries – with eyes and telescope, as opposed to photographic plates and CCD images – is still held by Jean-Louis Pons, the Marseilles observatory janitor who started observing with telescopes of his own around 1800. He found 37 – probably. He was a naive fellow who didn’t keep methodical notes and was mocked by the professionals. But there the comets were, where he had been the first to see them.)
Comet C/2013 A1 Siding Spring when discovered on CCD (charge-coupled device) images was a very dim spot (about 18,5 in the magnitude system of brightness), still 7.2 astronomical units (Sun-Earth distances) out, in the small constellation Lepus, the “hare,” south of the feet of Orion. By last month it was getting significantly closer to us; one result – only because of our visual angle from the moving Earth – was that it appeared to dive farther south, and on September 2 it was briefly at 75 degrees south in the constellation Octans that surrounds the south pole of the sky. In space it is all the while coming north; climbing rapidly through Scorpius, it has come within reach of north-hemisphere observers. They, or at least great comet observer (and discoverer) Alan Hale, report that instead of becoming as bright as magnitude 8 as had been predicted, it is between 10.5 and 11 – which means 10 or 15 times fainter. The magnitude limit for the naked eye is 5 or 6.
That’s comets for you; predicting their brightness is notoriously guesswork, because of their powdery nature and spinning irregular shapes and how much gas and dust they will give off as the Sun de-freezes parts of them. In this sense Comet C/2013 Siding Spring is a mild disappointment. But the planet it intended, for all those millions of years, to visit and dazzle was not the third one from the Sun but the fourth, Mars. How will it fare?
An early estimate was that it would miss Mars by 0.001 of an astronomical unit – 150,000 kilometers – but with uncertainty larger than this, so that it might hit. The nucleus of this comet (the solid body, as distinct from the vast cloudy coma and tail which are what we see as they shine in sunlight) was at one time estimated to be 4 kilometers wide, or anywhere between 1 and 50 kilometers; this has been reduced to only 700 meters. Still, it is a flying mountain. Striking at a Mars-relative speed of 56 kilometers per second, it would knock out a crater more than ten times its own width. If this happened on Earth, it would, at the least, speed up the Sixth Global Extinction, the one now in full progress. Or stop it, by putting an end to the species that is causing it, Homo sapiens.
Observations by NASA continue to refine the mathematics of the comet’s orbit, and for a while the miss distance was even smaller. The most recent estimate is about the same as before but more exact, 0.000932 a.u. (139,000 kilometers), with smaller uncertainty, so that there is no chance of impact. This is about six times the distance from Mars to Deimos, the outer of its two satellites. From Mars, this will certainly be a Great Comet: about magnitude minus 6 – far brighter than Venus is from Earth, almost like a quarter-Moon – and with a huge tail. The comet all along planned not to bomb Mars but to give a fine display for its inhabitants. Alas, there are inhabitants on Mars no more.
As witnessed from Earth, the performance is for the telescopes and for the mind but not for the naked eye. The comet will come up out of Scorpius and on October 19 cross Mars’s path in the southern corner of the constellation Ophiuchus, about a degree south of the ecliptic and 25 degrees south of the celestial equator. It is 59 degrees east of the Sun, that is, in the evening sky, at a rather southward angle, which will make it better seen from the southern hemisphere.
The disappointed but not disappointing comet will arrive, only six days later, at its perihelion or closest point to the Sun, just inside the orbit of Mars; and then begin its curve back out. It will climb northward in early 2015 through Ophiuchus and Hercules, in May reach 57 degrees north in Ursa Major, curve southward past Mizar, the star at the bend in the handle of the Big Dipper, and on south into Canes Venatici as it recedes into the distance. It won’t go all the way back to the edge of the solar system: its passage among the planets will have deflected it into a shorter orbit of only about one million years – a captured comet that will one day come again.
Comets do strike planets; the famous first-known instance was D/1993 F2 Shoemaker-Levy 9, which in 1994 broke into a long train of pieces before punching a line of dark holes in the cloud-tops of Jupiter. But it, rather than aiming from afar, had been an ordinary short-period comet buzzing around in the inner solar system before being captured by the massive planet and falling into a gradually shortening orbit around it. The “D” designation is given to periodic comets, about 36 of them known, that are “disappeared” – or “departed” or “destroyed” or “defunct” or “dead.” Or the grade that’s just above an F? If C/2013 A1 had hit Mars it would have found itself thus “downgraded.”
Is it really so amazing that this comet, dropping from so far away, almost manages to hit a planet in a relatively microscopic orbit around a distant star? For this comet the chances of its happening were close to zero, and no comet or astrophysicist however brainy could have programmed itself to do it, but for it to happen sometime or sometimes is inevitable – a case of, if something can happen it eventually will.