"Pseudo-MPEC" for A/2017 U1

Updated 2017 Nov 10 21:30 UT

Those looking for the "usual" pseudo-MPEC listing observations, orbital elements, and ephemerides should click here. The following is aimed at a more general audience, to answer some of the questions I've had aimed at me regarding this object.

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

With the solution below, the object came from RA=18h 39m 14s, dec=+33 59' 50", with an uncertainty of about 2'. This is about five degrees away from the bright star Vega.

After it leaves the solar system, it'll be headed toward RA=23 51 29.5, dec=+24 44.4', with an uncertainty of about 15 arcseconds. This is in the direction of the constellation Pegasus.

(The uncertainties differ so much because all our data comes from after it passed the sun. Try to project the trajectory before that, and any small errors get magnified; a small tweak toward the sun means the object's path gets "bent" at a sharper angle. 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.)

Most likely, A/2017 U1 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.)

There's been a lot of speculation that this might have actually come from Vega, or at least some other nearby star in that direction. I can't totally rubbish this, and I know efforts are already underfoot to figure out a possible nearby origin. But I figure the odds of this having come from a nearby, traceable star are thousands to one. That's based on these objects averaging a couple of billion years old, and that they'd 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.

So this got sent to us long, long ago, from a star far, far away. (But not from another galaxy far, far away.)

Note that while the above incoming point is somewhat uncertain, we'll doubtless get more data and reduce that uncertainty (it's already come down a bit from where it was). In fact, we may get more data and a lower uncertainty than I would have thought, if this proposal to observe A/2017 U1 with the Hubble telescope is approved.

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 this object looks in the distant past that expands on this and may be a little clearer than the above.

How fast is it going?

Its incoming and outgoing speeds as it went through interstellar space were 26.33 +/- 0.01 km/s. (Which is why we knew pretty quickly that 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.")

As it approached the sun, it picked up speed (the sun pulled it inward). At its closest point to the sun (almost a quarter of our distance from the sun, on 2017 Sep 9), it was moving at 87.71 km/s relative to the sun. After that, it started climbing its way away from the sun and began to lose the speed it gained while "falling in".

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.

Incidentally, I said the eccentricity suggested that it was interstellar, and that's true. But it's really Vinf that matters here. In fact, the only reason the eccentricity is as low as it is is because the object came so close to the sun. If it had only come as close to the sun as we are (one astronomical unit), its orbit would have had an eccentricity of 1.77. Put it further out (and most interstellar objects won't pass this close to the sun) and its eccentricity will be correspondingly higher. (To be precise: an object with Vinf=26.12 km/s will have a semimajor axis of -1.30 AU. With a perihelion distance q, that will lead to an eccentricity of 1+q/1.30. This object actually came within a quarter of an AU of the sun, leading to an eccentricity of 1+0.25/1.3 = 1.19.)

How big is it?

We don't really know all that exactly. We can see how bright it is. A lot of rocks in the solar system reflect about 10% of the light that hits them (their "albedo"). If A/2017 U1 does that, we're looking at something about 160 meters across.

If it reflects just about all the light that hits it (Saturn's moon Enceladus does this), it would be about 50 meters across. That's a solid lower limit on size, though; it can't reflect more than 100% of the light that hits it.

There is no similarly firm upper limit on size. If it absorbs most of the light that hits it (some asteroids in the solar system are just about totally black), it might be a fair bit larger than 160 meters across.

There's a proposal to get observations to determine the albedo and diameter of A/2017 U1, using the Spitzer Space Telescope (which "sees" in infrared, and is therefore well-suited to this sort of purpose.) Because Spitzer orbits the sun a good distance from us, its observations would also help nail down A/2017 U1's orbit more precisely (something that surprisingly doesn't show up in the proposal). The proposal notes that the observations would be made in late November, "the last telescopic observations that will ever be made of this object." Which may not be true; there's also a proposal to observe A/2017 U1 with the Hubble telescope that would conceivably run until 2018 January 1.

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.)

However, knowing that it can happen and actually finding one are two different things. This gives us a chance to study the composition of an object from another star. Any chance to do that is a Big Deal.

Why was it first designated as a comet, then as a rock?

I think the Minor Planet Center folks looked at the orbit and assumed anything moving this way must be a comet. That wasn't an entirely unreasonable decision, and they did have to give it a designation, so they went with C/2017 U1 (which follows the usual pattern for comet designations).

However, comets have gas and dust and (sometimes) tails. Everybody looking at this was seeing a starlike object. The gas and dust is sometimes rather faint, but even a very long-exposure image from the Very Large Telescope didn't show cometary activity. So it got re-designated as an asteroid, the C turned into an A, and it's now A/2017 U1.

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 26 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, it's been suggested that we keep careful track of it while we can and come up with 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. Again, since this is a for-real rock 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.

What's this I hear about it not really being interstellar?

Unfortunately, there was a posting on Twitter to the effect that a much more ordinary elliptical orbit had been found for this object. The author is not a crank, so this got some traction. But I don't think he knew much about asteroid orbit determination. I posted an explanation of why we're quite confident that this is interstellar. It essentially boiled down to not understanding that the data are of greater precision than he thought (the elliptical orbit would have required huge errors in almost all of the data) and that professional astronomers are not (in this instance) all that much more accurate than amateurs doing professional-level work.

Maybe this is a solar system object after all?

I should first note that I am utterly confident that the object is moving as fast as I've said it was. When I first posted a comment about this object, I determined its incoming speed ("Vinf", its speed when far away from us) as 26.0 +/- 0.5 km/s. Further data has only served to confirm and refine that point.

That definitely makes A/2017 U1 interstellar... at this point. But five suggestions have been made as to how a mundane, slow object within the solar system might have been sped up to be the oddball we're seeing now. Four of them are quite definitely wrong. The fifth is very improbable, and became even less likely when we learned that this isn't a comet, but it can't be totally rubbished.

The first idea suggested to me after I posted an interstellar orbit for this object was that 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, A/2017 U1 is most likely something that got kicked out of another star system when it formed; that's when most of the kicking-out takes place.

However, by the time I posted that interstellar orbit, we had enough data (about six days' worth) to figure out how close it could have come to the gas giants in the solar system. The closest approach to Jupiter as it came in was at 4.9 +/- 0.1 AU. For Saturn, 8.3 +/- 0.1 AU. Uranus and Neptune weren't anywhere in the way. (The other planets are too small to kick anything up to this kind of speed.) Basically, A/2017 U1 didn't come close to anything on the way in.

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 26 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. 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 came about as close to the sun as Mercury does; if it were made of some sort of icy material (water ice or something else), it would be entirely reasonable to expect it to get pushed around by such forces.

The problem is the amount of change of speed required. The best-case scenario (the one where you'd need the least amount of speed added to get what we're looking at here) would be that as it came closest to the sun, it got a kick of 3.59 km/s, all of it making it go faster (nothing wasted at an angle). That's not the sort of gentle kick comets get from boiling patches on them. So we had to drop that idea.

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. Aside from the sheer low probability, though, there's another problem with this scheme: the cloud of comets around the solar system's edges are... comets. And we now know this object isn't one (the gent who proposed this idea did so before the MPC announced that it wasn't actually a comet.)

Some links about this object