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Astronomers have searched over many decades for comets that have come from interstellar space, perhaps from a planetary system orbiting a nearby star in the Milky Way. A blank was drawn in this quest for a long, long time… and now, similarly to London buses, two have come along almost at once.
The diagram at the head of this post shows the trajectory of the recently-discovered comet C/2019 Q4 (Borisov). Later I will explain how that label/name comes about (and why it will likely soon be altered), but first let me say why it is newsworthy and noteworthy. It’s moving too fast. That is, it’s moving too fast to be a member of the solar system, and it seems to be a transient visitor from interstellar space, cast out from its parent stellar/planetary system aeons ago (by which I mean: millions or billions of years), since when it has been wandering the galaxy, and is now zooming through our own cosmic backyard. It’s only the second such vagrant to be spotted by astronomers, the first appearing less than two years ago. For context, let me first discuss that earlier interstellar object…
In 2017 the astronomical world was intrigued by the discovery of an object that was given the nick-name ‘Oumuamua (yes, there is an apostrophe at the start of that appelation), which was the first such large body to be found flying through the solar system but coming from interstellar space. That is, it is on a hyperbolic heliocentric orbit: it is not bound to the solar system by gravity, unlike all other comets, asteroids and planets (major, minor, or dwarf).
The name ‘Oumuamua derives from a Hawai’ian language word meaning ‘scout’, someone who reaches out into the distance and is a messenger. It was discovered by Canadian astronomer Robert Weryk, using a search telescope at the Haleakala Observatory in Hawai’i. I worked with Robert at the University of Western Ontario in 2014, and just this past April had a chance to discuss ‘Oumuamua with him whilst we were both attending the Planetary Defense Conference at the University of Maryland. Normally it would have been termed Comet Weryk but, hey, naming rules are there to be broken… and later observations led astronomers to question whether it should be classified as being a comet, or an asteroid.
The formal designation of ‘Oumuamua is 1I/2017 U1. The capital letter ‘I’ means Interstellar: this is a new class of object, hence the numeral ‘1’ at the beginning (and with apologies that the font here does a poor job of differentiating between 1/one and a capital letter I). The ‘2017’ shows that it was discovered in that year. The ‘U’ indicates the discovery half-month (the second half of October). The final ‘1’ means that it was the first comet (again noting the comet/asteroid confusion) discovered in that half-month. There is method to this coding madness, then.
Observations of ‘Oumuamua using large telescopes have shown that it must be highly elongated, because its brightness varies regularly in time and with a large amplitude, indicating that sometimes we are seeing it side-on, and at other times end-on, so that the amount of reflected sunlight it is sending our way alters substantially. This leads to an idea of what it might look like, as below.
A rapid word of caution, though: that really is an artist’s conception of what ‘Oumuamua could look like if we could see it up close. In reality we can’t. We know that it is very dark because of the ratio of scattered sunlight (not much) compared to the radiation emitted in the thermal infrared of the electromagnetic spectrum (i.e. the sunlight absorbed heats the object’s surface, that energy then being re-emitted as longer wavelengths than the incoming visible sunlight). Much more than that we do not know. We think it’s between 100 and 1,000 metres long, but really we are unsure. To show what I mean, here is one of the best images of ‘Oumuamua that were obtained whilst it was still close enough to the Sun and the Earth to be followed using large-aperture telescopes:
‘Oumuamua is regarded as an important discovery then, with discussions in both the mass media and the scientific literature continuing. Is it really a comet, or an asteroid, given that it showed no cometary activity (outgassing of vapour, tail or associated dust cloud)? If this were an object that had been thrown out of another planetary system (a complex of exoplanets orbiting another star), might it carry evidence of microbial life within it? Could such microbial life still be viable after a voyage taking millions of years?
How did we know that ‘Oumuamua is interstellar?
Fundamentally, the answer is its speed. It was measured to be going too fast to be a member of the solar system, bound to the Sun’s gravitational pull. Above I wrote that it has a “hyperbolic heliocentric orbit”, but that’s a bit technical. Let’s look at it another way, simply by saying that it had too much kinetic energy for the Sun to capture it. Kinetic energy? That’s 1/2 m v², where m is the mass, and v is the speed. ‘Oumuamua had a speed above the limit for its (changing) distance from the Sun.
What I mean is this… Let’s take Earth as an example. Our planet’s heliocentric distance varies over the year, between 0.9833 and 1.0167 astronomical units (perihelion and aphelion respectively), reaching the former in early January and the latter in early July, at present. In consequence Earth’s speed varies, between 30.3 km/sec at perihelion and 29.3 km/sec at aphelion. But let us imagine instead that the terrestrial orbit is circular, at precisely 1 AU. In that case our speed would be a constant 29.8 km/sec. Now, at 1 AU if any object is sent out into space (into a heliocentric orbit) with a speed greater than √2 times 29.8 km/sec (i.e. a bit over 42 km/sec) then it will escape the solar system: it has sufficient energy to escape the Sun’s gravitational pull.
Similarly, any object having a speed in excess of √2 times the circular orbit speed at any particular heliocentric distance will be on a hyperbolic orbit: a path that may pass through the planetary region, but will continue out and never return.
The shapes of orbits are defined by what we term the eccentricity (e). All orbits are conic sections. Orbits that are gravitationally bound to the Sun are elliptical, with eccentricities ranging from e = 0 (a circle) to just below 1: Earth has e = 0.0167 at present, Mars has e = 0.0933. When e = 1 the orbit is parabolic, and if e > 1 then that orbit is hyperbolic, and unbound. A parabolic orbit is right on the edge, as it were.
Most comets we observe are near-parabolic. In the past some were observed with eccentricities marginally above 1, but that was referenced to the centre of the Sun; for examples, see here and also here. When the reference frame was altered to be the solar system barycentre (the centre of mass of the Sun and all the planets, this barycentre shifting around as the planets move) almost all such comets with e only slightly above 1 in heliocentric terms were found to have e ≤ 1 when referred to the solar system barycentre. The only exceptions were those which had been observed to pass rather close to one of the planets (generally Jupiter), that planet’s gravity then causing a slingshot effect so as to hurl the comet out of the solar system onto an interstellar path. That is, those comets had not arrived from interstellar space, but they were shot out into interstellar trajectories.
It was for precisely this reason that the non-observation of any true interstellar object until 2017 was a bit of a mystery: if there are planetary systems around other stars, and there are also asteroids and comets there, then surely (we reasoned) those systems should be sending the occasional lump our way? ‘Oumuamua was the first definitely-interstellar large object to be detected, with an eccentricity near 1.2.
And so to the discovery of the apparently-interstellar comet now known as C/2019 Q4 (Borisov). The internet is already abuzz about this one, despite its nature only just having been recognised (the listing of its observations and strongly-hyperbolic orbit determination was published by the International Astronomical Union on the morning of Thursday 12th, NZ time). Here is a NASA webpage about it. Here is an article in Sky & Telescope magazine about it. This is an article on the Scientific American website. Here is an animation on YouTube showing its path through the solar system. Here is a GIF animation from NASA of that path.
Unlike ‘Oumuamua, this one surely is a comet, with a coma (cloud of vapour) and a small tail already being apparent, as seen in the image below and as reported by several observers soon after its discovery.
As can be seen, the comet will not come anywhere near the Earth: its perihelion will occur around December 7th this year, nearly three months from now, at 1.94 AU from the Sun. That distance places it passing through the inner parts of the main asteroid belt, and well outside the orbit of Mars (which has aphelion at about 1.67 AU). No reason for alarm, then.
The graphic at the head of this blog post also indicates this comet’s path: it is currently in the northern sky at a declination of about 33 degrees, which means that it is really too low in the sky to be seen from New Zealand, though it is moving south and observers here will have plenty of chances to monitor it as it remains detectable for about the next twelve months (assuming it does not behave anomalously, in that some comets sometimes become ‘dormant’ and stop producing the vapour clouds that make them more-easily seen because those clouds, and their tails, scatter a lot of sunlight).
Having written that, please don’t expect to see it with a backyard telescope: all detections so far have been by proficient, experienced observers using telescopes with apertures of at least 30 centimetres, using tracking on the comet’s calculated motion along with CCD detectors. That is, this is not expected to be a naked-eye or small-telescope comet: its significance derives from its peculiar orbit. The image below – showing a characteristic fuzzy cometary appearance (the word ‘comet’ is derived from a Greek word meaning ‘hairy star’) – makes the comet look obvious, but take into account that this covers a tiny part of the sky, and the picture was obtained using a telescope with a main mirror 3.6 metres in diameter, collecting a lot of light!
What of the comet’s name? Whilst it is currently designated as C/2019 Q4 (the fourth comet discovered in the second half of August 2019), one would anticipate that once its hyperbolic nature is confirmed it will be re-designated as 2I/2019 Q4. An estimation of its eccentricity indicated e ≈ 3.1, substantially higher than ‘Oumuamua; a more-recent estimate (i.e. using more positional observations from the past day or so) has rendered e ≈ 3.6. To look at it another way, the comet seems to be moving more than 30 km/sec faster than the parabolic limit (the maximum speed at which it could be bound to the solar system, as discussed earlier).
Why Borisov? The comet was discovered just two weeks ago, on August 30th, by Gennady Borisov (a keen amateur comet hunter) using a self-constructed 65-cm aperture telescope at the Crimean Astrophysical Observatory. Since then, as one might anticipate, there has been a scramble to get observations of it, particularly by well-equipped amateur astronomers in the northern hemisphere.
Whilst I wrote above that this particular comet will not come close to the Earth, and so poses no danger to us, the size of it should give us pause. ‘Oumuamua was at most 1 km long, and slender, and so had a relatively low mass. The present subject (C/2019 Q4) is far bigger, though as of yet we know little about its size. Estimates for its diameter (assuming it to be spherical) are between 2 and 16 km, but that is based mainly on how much water vapour it seems to be spawning. We cannot see the solid nucleus directly, and so it is difficult (even impossible) to gauge its actual size and mass.
Another point to ponder is the fact that this comet was discovered only three-and-a-bit months before its closest approach to the Sun. That is, it was not spotted when far out in the region of the giant planets. Similar comets (or asteroids) on paths bringing them closer to the Sun, possibly crossing Earth’s orbit, must also exist. We must hope that one does not arrive soon (the next century, say), and take us unawares. The problem of detecting and tracking comets arriving from deep space is one that I have investigated in the past, jointly with my late colleague Brian Marsden.
Earlier I mentioned that the discovery of interstellar comets had been long-anticipated, and the lack of any such detection was a real puzzle for theorists. Now, though, the theorists have a potential bonanza. Would you believe that the first discussion paper regarding this newly-discovered object has already been submitted to the arXiv.org website?
To conclude, I must mention a New Zealand connection; and a strong one. Whilst ‘Oumuamua was the first large body from interstellar space to be detected in the solar system, in the past far smaller interstellar objects have been detected above the skies of NZ.
For my PhD project at the University of Canterbury I began the construction of a large meteor orbit radar system at Birdlings Flat, just south of Christchurch. This was under the supervision of Professor Jack Baggaley, who recently retired after a full half-century on the faculty at that university. The radar was ably completed by the next PhD student, Andrew Taylor, who now teaches physics at Christ’s College in Christchurch.
Using that radar we identified an influx of small high-speed meteoroids (or interstellar dust – pieces rather smaller than a millimetre in size) into Earth’s atmosphere, demonstrably on hyperbolic heliocentric orbits due to their tremendously high-speeds, far too fast to be members of the solar system. This result we published as a Letter in the world’s premiere science journal, Nature. That discovery seemed to pass by most of the astronomical community working on comets and the like, but since the discovery of ‘Oumuamua (and now C/2019 Q4 Borisov) I have found that the paper seems to have been re-discovered, with it now being cited by many researchers internationally.
Update Saturday September 14th, 10:46: With more than 200 positional measurements in hand, it seems secure that 2019 Q4 is indeed an interstellar visitor. The latest orbit published by the IAU Minor Planet Center appeared just under eight hours ago.
I give some useful additional information in my reply below to a query from Derek Syms.
Update Monday September 16th: I have updated and expanded various pieces of the original post above.