Here’s my space picture redrawn.
Shown again are the planets’ courses in October, November, and December, and Oumuamua’s path in magenta before its discovery, yellow from then to the end of 2017. Perihelion is marked with a tick; closest moment to Earth, with a green line; the vernal equinox direction, with a dashed blue line.
I’ve used slightly revised orbital elements, and a viewpoint rotated from longitude 350° to 160°; and added the “line of nodes.” This is the dashed line from the descending node through the Sun to the ascending node. It is the intersection of the body’s orbital plane with Earth’s (the ecliptic), and should help you to visualize that orbital plane.
Perihelion was closer to the descending node; if you look at the stalks connecting the orbit to the ecliptic plane, you can see that the orbital plane is tilted to the left; the arriving course is more northerly than the departing one.
Some people found the first version unclear. While devising it I must have tried at least a dozen viewpoints, each involving a re-drawing, before deciding on one that seemed clear and did not obscure certain details. Perhaps I’m too accustomed to my own idiom. Jan Kok suggested a view with the body’s orbital plane flat on – parallel with – the paper, or screen. That would be a view from the north pole of the body’s orbit. It might suit a picture for a single body, but the ecliptic plane would be strangely tilted; also, it could not serve for a picture in which another body is added for comparison, or a picture containing several comets. I’ve found that a view from latitude 15° north of the ecliptic works generally for these pictures, with just the longitude varied. It’s like walking – but not flying – around a model to peer at it from various directions.
One commenter mentioned the “MOID” given in various sources. That is the minimum orbital intersection distance: the smallest distance between the orbits – not between the bodies. When Oumuamua was closest south of a point in Earth’s orbit, Earth had not yet reached that point.
It seems hard to find any source that mentions the date of the closest approach; my calculation now spits out Oct. 14. I would be more confident, of course, if Jean Meeus or Aldo Vitagliano, or Gareth Williams of the Minor Planet Center, were doing the calculating.
Yes, I should write ‘Oumuamua. The initial mark, which you can call an apostrophe or inverted comma or quotation mark or, perhaps most handily, quotemark, will probably get dropped in circumstances such as computer listings. The quotemark is not a letter in English orthography – it doesn’t represent a segmental phoneme, like p or a – but it is used in transcribing certain unfamiliar consonants from other languages. For the language of Hawaii, which its natives would prefer us to spell as Hawai’i, it represents the “glottal stop” (plosive opening of the glottis, or slit between the vocal cords in the larynx, as in a cough). Hawai’ian has, at least by some linguists’ analysis, the record smallest number of segmental phonemes: 5 vowels and only 8 consonants (including ‘). For comparison, English has around forty, varying with method of analysis, and Khoisan languages of southern Africa have around a hundred, about half being consonants of the “click” or double-articulation type.
The single quotemark (which European scribes invented several centuries later than the double quotemark) is a bit awkward when used as a letter. If we type a “straight” quotemark, our smart software will turn it into the 6-shaped form if it’s at the beginning of a word (taking it for the beginning of a quotation), and into the 9-shaped form if it is between letters (taking it for an apostrophe). So the ‘ at the beginning of ‘Oumuamua may stay straight or become 6-shaped depending on what the software does. For transcribing Arabic or other Semitic languages, the convention is to use the 6-shaped quotemark for the pharyngeal consonant (at the beginning of ‘Abd or ‘Ali) and the 9-shape for the glottal stop, as in al-`Islam. So that conflicts with the usage for Hawaiian.
Not to mention that French, for instance, uses an entirely different form of quotemarks.
But enough of the pharynx and larynx and back to ‘Oumuamua.
It was discovered on October 19 by Robert Weryk, in an image of Oct. 18 from Pan-STARRS (the Panoramic Survey Telescope and Rapid Response System) on Mount Haleakala in Hawaii. Measured positions and the calculations from them showed that it was in a highly eccentric orbit, as are long-period comets, so it was announced, on Oct. 25, as C/2017 U1 (PANSTARRS). “U1” meant that it was the first comet discovery in the second half of October.
Variations in its faint point of light allowed deduction that it is tumbling, rather than spinning, and is a pencil of reddish rock, 250 yards long and 40 wide. This reminded science fiction fans of Arthur C. Clarke’s Rendezvous with Rama, and there was a wish to dub the object “Rama.” This, I think, would have led to chronic confusion between the real object, the fictional one, and the Hindu god. Radio telescopes have searched for artificial signals but found none, so far.
A better idea was the Hawaiian word, said to mean “scout” (‘ou, “reach out”; mua, “forward,” with reduplication for emphasis, a regular feature of Hawaiian and some other languages and an occasional one in English – “bye-bye”).
Deep images did not show any haze like that given off by comets. And the eccentricity of the orbit proved to be significantly higher than any ever known: nearly 1.2.
What does this mean? Eccentricity is the number that expresses the shape of the curve. A perfect circle has eccentricity 0. Bodies that stay around the Sun travel ellipses, with eccentricity between 0 and 1: the planets have moderate eccentricities, up to 0.2 (Mercury); Comet Halley is in a long ellipse with eccentricity almost 0.97. Eccentricity of 1 means a parabola, which like the circle is a limiting case and no body could actually stay in it: it can’t decide between closing on itself or continuing to widen. Beyond the parabola, with eccentricities above 1, are the hyperbolas, which open out infinitely.
What we call the long-period or non-periodic comets are generally treated as having parabolic orbits: they are so long that it’s difficult to tell the difference, but they are thought to be in enormous ellipses with outer ends still in the solar system. A few comets have been found to be in slightly hyperbolic paths, but that is because they have been perturbed into them by passing among the planets. If a comet stays on a hyperbola, it is on its way out of the solar system.
Oumuamua is the first observed body that must have arrived along a hyperbola. It couldn’t have been perturbed into it by any planet, even a remote unknown one; it was traveling far too fast. It could only have arrived from outside the solar system. The Sun became the focus of the hyperbola, and after whipping around it Oumeamea is on its way along the other arm of the hyperbola, back out to the stars in a different direction.
When discovered it had already passed that perihelion and passed its nearest to us, was hurtling away, and was of such obvious interest that the Minor Planet Circular sent to the world’s observatories asked for observations urgently.
Since it is not a comet of the solar system, the designation became A/2017 U1. The “A” prefix is for objects that are mistakenly taken to be comets but are really solar-system minor bodies of other kinds, usually, asteroids. This prefix had been set up for this kind of situation, but had never yet been used. Plenty of “apparently asteroidal” discoveries have shown cometary “activity” and been reclassified as comets, but no comets had had to be reclassified as asteroids.
Oumuamua may be an asteroid, but it is not a member of the solar system’s fleet of asteroids. An email conversation “between the IAU [International Astronomical Union] General Secretary, the IAU Division F President, the co-chairs of the IAU Working Group on Small Body Nomenclature and the Minor Planet Center” led on November 6 to the creation of a new class, “I,” for interstellar objects. This first of the kind becomes 1I/ U1 ?Oumuamua.
You will notice two other awkwardnesses resulting from our alphanumeric system, which is handier than Egyptian hieroglyphs or Roman numerals but not a creation of fully intelligently design. Capital I and lowercase l and numeral 1 (and capital O and numeral 0, and numerals 5 and 6) are similar enough to cause millions of hours per year of hesitation and confusion. As Anthony Barreiro suggested, it would have been better to use E/, for “extra-solar.” It isn’t reserved; the prefixes in use for comets are C/, P/, D/, A/, and X/ (for non-periodic or not yet known, periodic, disappeared, really an asteroid, and insufficiently known).
And: at least some keyboards have not just one key for the single quotemark, but a second, which gives a backward-sloping mark like a grave accent, so that you can force the 6-shape where the 9-shape would otherwise appear. But not all fonts or software understand this, so they find it to be an unknown character and turn it into a question-mark. Not the worst thing to turn it into: the International Phonetic Alphabet character for the glottal stop looks like the question mark without a dot.
The Minor Planet Electronic Circular announcing the decision included the sentence “A solution has been proposed that solves the problem.” I caught this for my collection of examples of redundancy. But is that unfair? I suppose there are solutions that do not solve. Like, we may unkindly say, this one.
It may not matter that we’re stuck with 1I/ U1 ?Oumeamea, since in ordinary discourse we may drop the first six characters, and since we may not live to see 2I.
It is amazing that a rock wandering interstellar space aimed so close to our Sun. This is where the rareness lies. Stars are around 8 light-years apart in our region of the galaxy (it’s not a number that can be strictly given; see the Astronomical Companion section on “Nearest Stars”). That’s around 40,000,000,000,000 times the length of Oumuamua. Comets are known to have been diverted into hyperbolic orbits, and other fragments have probably been ejected during the evolution of the solar system, so it is likely that Oumuamua was ejected from another star system and that there are trillions more like it. They have a fair chance of passing through a star’s outer Oort Cloud, which may reach out to around 3 light-years and is sparsely inhabited by proto-comets; if this has happened for the Sun’s Oort Cloud, it would have been unobservable by us and would have negligible effect on the rock’s path. Oumuamua is just one that happened to aim almost directly at the Sun. Not quite directly, nor as nearly as at Mercury’s orbit, but near enough to be pulled by the Sun’s gravity inward into what became the hyperbola with the Sun at the focus, and thus sent out in an entirely new direction.
By mid December, Oumuamua was 2.5 astronomical units from the Sun (as far as the main belt of asteroids), beyond the reach of the largest telescopes. Some elaborate ways of getting a space probe to catch up with it, before it gets utterly too far away, have been proposed, making use of light-sails or gravitational wells.
Here is a chart of how Oumeamea’s path on the sky as it arrived would have appeared, if we could have seen it.
As with any comet approaching from far out, the path makes apparent loops, gradually widening, because viewed from Earth circling the Sun.
And here is the rapid path across the sky in 2017 – red from discovery onward – and the departure.
The receding path, like the approaching, goes into annual loops, but otherwise they are not symmetrical. This November, Oumuamua crossed the asterism known as the Circlet, otherwise the western of the two “fishes” of Pisces, and then seemed to make a sharp northeastward turn. This was because it was moving directly away while Earth was curving away around its own orbit.
Plainly, Oumuamua came from a direction in Lyra, near to Vega and even nearer to the three fainter stars called the Kids which are just south of Vega. And the direction to which it is now heading is in Pegasus. I calculate that the spherical angle between these points in Lyra and Pegasus is about 68°. And that does appear to be the angle between the two arms of the hyperbola in the space picture.
What is interesting is the direction from which Oumuamua came. On average the stars must be moving in circular orbits around the galaxy’s center, and for us that means a direction in the Milky Way at galactic latitude 0°, longitude 180°, in the constellation Cygnus, not far from the supergiant star Deneb. But individual stars have their individual orbits, slowly changing under the influence of each other, and the Sun’s is carrying it slightly aside from the general direction, toward what is called the apex of the Sun’s way. This is near Vega, though actually just across the border from Lyra into Hercules. (See the Astronomical Companion, beginning of the “Outrush” section.)
So does this mean that Oumuamua was coming to meet us, sent from somewhere in almost exactly the opposite direction relative to the galaxy? No, I think its speed from the apex direction toward us means that, relative to the galaxy, it was moving forward in the same direction as us and some of the stars around us but with slightly slower speed. Imagine flipping a ball off the back of a moving truck. It would keep rolling forward along the highway, but would be run over by another truck. This may be a helpful though not precise analogy.
There is fascinating speculation about which star system Oumuamua may have been ejected from – maybe not Vega, which a million years ago was in a different position relative to us – how long ago this might have been, whether it could have been billions or years ago – many revolutions around the entire galaxy – because of the small chance of centering another star system. Watch this space, so to speak. Well, not this space, but the space of professional astrodynamics. Having at last almost hit but not been captured by a star, Oumuamua is out along a different path. Will this too be just a variation on the general orbit around the galaxy, or will it eject Oumuamua from the galaxy?
In the new year, Oumuamua will reach the Great Square of Pegasus, the winged horse, on which it is destined to ride away into, as far as we are concerned, infinity. If those who believe it is a cylindrical starship like Rama are right, it is wiring back to you a Vegan wish for a happy Earthyear.