FAQ for gb00234 = C/2019 Q4 = 2I (Borisov)

You can click here for the "usual" pseudo-MPEC listing observations, orbital elements, and ephemerides. The following is aimed at a more general audience, to answer some of the questions I've had aimed at me regarding this object. I expect to add to it as more questions come in.

Please note that in writing this, I was able to recycle bits of my previous FAQ for 1I = ʻOumuamua. A lot of this will look very similar, except that numbers, dates, and various facts are different. (But the answer to why this object can't be a chunk from our own Oort cloud, for example, is pretty much the same as it was for ʻOumuamua.) I've also copied and pasted from some replies I've written to private inquiries... quite a bit of "self-plagiarism" going on here.

Where is this object going? Where did it come from?

With the current orbit solution, the object came from somewhere within about 2 arcminutes of the point RA=02h12m03s, dec=+59 27.5', in the constellation Cassiopeia. After going through the solar system, it'll be headed for somewhere within 6 arcminutes or so of RA=18h20m38s, dec=-52 05', in the constellation Telescopium. (Those positions will almost certainly be improved a lot as we get more data. They're already much more precise than they were when the object was first announced.)

As it went past us, its path was "bent" by about 34 degrees.

The uncertainties differ because we have a little bit of data, all of it before it reaches the closest point to the Sun on December 8. We've got a decent idea as to where it is "right now", but as you project that backward or forward in time, uncertainties accumulate. Project its trajectory past the sun, and small errors get magnified.

Also, be warned that these are "formal" uncertainties, based on some mathematical principles of error analysis that sometimes founder when faced with real-world errors. That is to say, these "formal" uncertainties usually turn out to be at least a little bit underestimated, and sometimes a lot. They also assume a very straightforward "non-gravitational" model, one that is reasonable but probably not exactly right. Comets often move a little "off course" as jets of gas boil on their surface.

Most likely, this object was created in the early stages of planet formation around another star. It's estimated that 99% of the material around the Sun when it formed got thrown out into interstellar space, and much of it is probably still roaming the galaxy. I would expect your average interstellar object to be billions of years old, which would mean it came from very far away.

Incidentally, it also almost certainly came from our galaxy, not another galaxy or the galactic halo. For the latter cases, it would have had to be moving really fast, hundreds of km/s. (Probably so fast that we would have missed it; our window of opportunity to observe it would have been tiny. I sometimes wonder if such "zingers" pass through the solar system, unobserved... probably not, though; it's a big universe, and it generally takes a lot of energy to eject an object from a galaxy. Unless it's small... but then you only get a small number of ejected objects.)

Also, the uncertainty is not really a circle; it's really a cigar-shaped area in space that gradually expands as you go into the past (i.e., go back 20,000 years, and it's about twice as big in every direction as it would be 10,000 years ago). Jon Giorgini at JPL posted a good description of how the uncertainty area for ʻOumuamua looks in the distant past; same general idea applies here.

Can we say anything about what star it came from?

I am reasonably sure that, as with the previous interstellar object ʻOumuamua, there will be some efforts to show that this object came from a nearby star in the direction of Cassiopeia. Those efforts will probably increase as we gather more data, and can say more precisely exactly what direction the object came from.

I can't say such efforts are pointless. But it's a real long shot, probably at odds of thousands to one. As described in the previous section, objects like this average billions of years old, and take a couple of million years to get here from a nearby star. That would mean 99.9% of them are older objects from far away. This object and its original star have had a lot of time to drift apart, get their orbits around the galaxy perturbed, and become completely unrecognizable as having started out from the same place.

So this object almost certainly got sent to us long, long ago, from a star far, far away. (But as stated above, not from a galaxy far, far away.)

How fast is it going?

As of mid-September, it's moving at about 41 kilometers a second (25 miles/second, or about 90000 mph) relative to the sun. (I'll get to speed relative to the earth in a bit.) It's still going toward the sun, picking up speed as the sun's energy yanks it in.

It'll reach its peak speed (still relative to the sun) at perihelion, its closest point to the sun, sometime around December 8. By then, it will have sped up to about 44 km/s. So it's already accumulated most of the speed it's going to have.

However, it came in to the solar system already moving at about 32.34 +/- 0.07 km/s. So it had a lot of speed to start with. It will eventually emerge from the solar system with pretty much the same speed. (If it went past Jupiter, it could pick up or lose a little bit of energy, the way the Voyager and Pioneer probes did. But it's not getting all that close to any planet.)

Incidentally, that initial speed is why we knew it was interstellar. The eccentricity of the orbit was a tip, but it was Vinf, the "speed at infinity" (or close enough to it), that really said "interstellar, no doubt about it."

Relative to the earth, things get more complicated. We're orbiting the sun at about 30 km/s. For half the year, that would have been 'toward' this object, and the other half, it would be 'away'. In mid-September, we're moving a little bit away from the object, subtracting a few kilometers/second, so that (relative to the earth) it's moving at about 38 km/s.

I expect that you'll see various figures on-line for the speed, depending on whether they're reckoned relative to the earth or sun and what time they're for. In space, nothing is ever easy.

How big is it?

We don't really know all that exactly. We can see how bright it is. Also, we're seeing not only the object itself, but its tail, which is a big, fuzzy blob. Even with a huge telescope, there would be no real hope of seeing its nucleus as anything but a point of light.

We do know how bright it is : at present, you need a decent telescope to see it. About twenty observers have imaged the object and reported positions for it; looking at the information they gave about the equipment they used, I see that one observer used a 20-cm (8-inch) diameter telescope. (Many of the observers on that list are amateurs with small telescopes. The discoverer, Gennady Borisov, previously discovered a comet using an 8" telescope he designed.)

Why is this a Big Deal?

There has been speculation for decades that we'd someday see an interstellar object. Just based on what we know about planetary formation, there ought to be a lot of rocks and ice flying around between the stars. Various efforts have been made to quantify how much that would be. The fact that we'd not seen any before now sets something of an upper limit ("if this stuff were more common, we'd have seen loads of it by now"), and other estimates based on planetary formation that said we ought to see at least one every few decades, depending on how thorough our asteroids surveys were. (Note that they've only gotten really thorough in the last couple of decades. And when new surveys get going, we might indeed find a lot more of these.)

With the discovery of ʻOumuamua in October 2017, we finally got a chance to see and study one interstellar object. It was a very strange object, moving in a way that suggests it was an extremely low-density object that could be pushed outward from the sun by the pressure of sunlight. This appears to be a more "traditional" comet. At least from the images, you couldn't tell that it actually had been roaming the galaxy for billions of years and originated in another star system.

It's apt to be different from what we see here and similar in strange ways. The composition of the cloud from which its original star formed could be quite different. We may notice some unusual things in its spectrum (which, I'm sure, will get a lot of study over the next year or so.) ʻOumuamua resulted in a lot of scientific papers. I expect this will result in even more, just because we'll get a long look at it and will accumulate a lot of data.

Can we send a spacecraft to it?

I only wish. As described above, when this thing leaves the solar system, it'll be moving at about 32.6 km/s. The fastest of our interstellar probes, Voyager 1, is going at about 16 km/s. At least with what we've got now, we can't do it.

However, when ʻOumuamua went past us, it was suggested that we keep careful track of it while we could so we'd have as accurate a trajectory as possible, in case future generations come up with something faster than our chemical rockets and ion propulsion; if we do our jobs right, they'll know where to go find it. (Not to mention that all that careful tracking told us that ʻOumuamua was being unexpectedly influenced by the pressure of sunlight.)

Keep in mind that this is a for-real comet from another star system. There would be a lot to learn by going out to it and maybe even bringing a bit back for analysis... but barring incredible breakthroughs, it's not apt to happen in our lifetimes.

Since catching up to an object moving this fast seems unlikely, the idea has been floated that we could have a "comet interceptor" on standby. When a suitable object comes zipping through the solar system, we'd launch the interceptor near the path of the incoming object and get a good look at it as it zooms by. The European Space Agency has selected a 'comet interceptor' concept as a new project. But, of course, it hasn't been built and won't be ready until well after this object has left the solar system (they're talking about a 2028 launch time, by which time C/2019 Q4 will be well past the orbit of Pluto). The interceptor would be used for some future object. (If we don't have a convenient interstellar object going by, it could be sent to a "traditional" solar system comet. That would be something of a letdown, but still of great scientific interest.)

Maybe this is a solar system object after all?

(Much of this is recycled from what I wrote about ʻOumuamua, since many of the same alternative ideas have been raised or could be raised.)

The speed of this object definitely makes C/2019 Q4 interstellar... at this point. But six suggestions have been made as to how this might be something else. Four of them are quite definitely wrong. The other two would be really unlikely.

Maybe this was something that had gone past a planet in our solar system (most likely Jupiter) and had picked up some speed. This is how the Voyager spacecraft became interstellar objects: after going past Jupiter, they had enough energy to depart the solar system at high speed. In 1770, Anders Lexell computed that a comet had gone past Jupiter and gotten ejected; comet C/1980 E1 Bowell came in on an elliptical orbit, passed Jupiter, and is now wandering between the stars. (I've seen people pointing out that C/1980 E1 had an eccentricity of about 1.057. What they fail to mention is that on its way in, its barycentric eccentricity at great distance -- which is what matters -- was 0.999910 +/- 0.000009, perfectly consistent with a plain old comet. Then it passed Jupiter at a mere quarter of an AU.) And when the solar system was young, stuff got thrown out at interstellar speeds all the time... in fact, both ʻOumuamua and C/2019 Q4 are most likely objects that got kicked out of another star system when it formed; that's when most of the kicking-out takes place.

However, we have more than enough data to say that C/2019 Q4 never came at all close to a gas giant.

• The second suggestion was that maybe there's an unseen planet way beyond Neptune and Pluto, and it took an object orbiting the sun and threw it toward us as a fastball. The problem with this is that distant objects orbiting the sun are moving pretty slowly, a few kilometers a second at most. You can't go past something at a few kilometers a second and emerge at a blistering 32.6 km/s. This would be similar to tossing a ball gently against a wall and having it rocket back at you like a bullet.

• The third suggestion was that non-gravitational forces were at work here. (OK, that was my suggestion, which turned out to be stupid.)

Comets do this sort of thing all the time: as they get close to the sun, bits of them boil off and spray material, giving them a little boost in one direction or another. This object shows a tail, so it's clearly outgassing.

A couple of days before the discovery of this object was announced, I was still flailing about looking for an alternative to the object being interstellar, so I computed an orbit that assumed the object was orbiting the sun and had non-gravs working on it. And I got a decent fit to the data (with some ugly data points). However, the non-gravs involved were ludicrously strong. I really shouldn't have posted that orbit solution, except to say "we considered non-gravs and decided they wouldn't explain the motion."

• The fourth suggestion was that two objects collided, and a bit flew off at really high speed, and that's what we're seeing. Aside from the problems in getting two objects to hit that hard, these are not rubber balls; when rocks or ice chunks hit, you get rubble not moving much faster or slower than the incoming objects. You don't have some bits coming out at interstellar speeds.

• The fifth suggestion, as mentioned above, is just about impossible to rubbish, but also seems very unlikely: that a rogue planet, floating between the stars, passed through the cloud of comets at the edge of the solar system. As it did so, solar system objects passed by it and got kicked around by its gravity. Most just went into interstellar space. One got kicked toward the sun, passed it, and was spotted shortly afterward and is the object we're talking about.

This is not a likely event, but we can't totally dismiss it; it could happen. But it would take a gas-giant sized passing object to do it, and it would have to be a very near miss, with sheer luck bringing the object into the inner solar system, from a direction within the galactic plane, at roughly the speed we'd expect for an interstellar visitor.

Thinking that it's an interstellar comet seems more likely.

• The sixth suggestion is that it's an alien spacecraft. Exactly why the aliens would make their spacecraft look like and behave like a comet is left unexplained. We do expect that when planetary systems form, a lot of comets get thrown out into interstellar trajectories; it seems reasonable to assume that there would be billions and billions of comets for every spacecraft.

Some links about this object

More to come...

Contact info

If you remove the diacritical marks added to (I hope) confuse spammers, you can e-mail me at p‮ôç.ötulpťcéjôřp@otúl‬m.


All text on this page was written by me, Bill Gray, and is in the public domain. (A reference to this URL is welcomed, though, and probably a good idea. We're still collecting data on this object, and I expect to provide updates.)