Did We Miss an Interstellar Comet Four Years Ago?

Long-time Slashdot reader RockDoctor writes: A paper published on Arxiv last week reports on a project to redetermine the "orbits of long period comets... We recently attempted to check, whether the assumption of a parabolic orbit for hundreds of comets discovered after 1950 is fully justified in all cases." The full work by Królikowska & Dybczynski remains in preparation (which is perfectly normal), but this intriguing result deserved early attention.

During this research we found an interesting case of the comet C/2014 W10 PANSTARRS.

(that's the 10th reported comet in fortnight W of year 2014, source : the PANSTARRS team)

After discovery on 2014-11-25, fourteen observations were made over three days, giving a first-estimate orbit with an eccentricity of 0.6039453. So far, so boring — as the temporary designation suggests, these get found on most days. But that orbit is subject to uncertainty so some more measurements were made on 2014-12-22 from a different observatory. When all of the data is considered, it becomes impossible to clearly assign an orbit to this object (this is possible if, for example, there is a fragmentation of the object between observations), but many of the solutions which can be obtained have a hyperbolic orbit — that is, the object is extra-solar.

If correct, this "post-covery" would double the size of the catalogue of interstellar objects known.

Unfortunately, the quality of the original data remains poor — estimates of the orbital eccentricity vary between 1.22 and 1.65 — which is in contrast to the prompt recognition and intense observation campaign for 'Oumuamua. The report's main conclusion is that

Our main purpose is to show that similar cases should be treated in future with greater care by more reliable preliminary orbit determination and alerting observers about the importance of the object to initiate more follow-up observations.

Which is exactly what happened with 'Oumuamua.

Re:weird measurements

[RockDoctor]

I take it that you didn't actually read the fucking paper? They're fully aware that the data set is poor. But that doesn't mean that the data is invalid. You might, for example, have done the two bouts of observation on either side of a large lump (but still too small to show up in the telescopes) falling off from a rotating and warming up body. There isn't enough data to decide, and there never will be. Which is the reason for stressing the need for prompt reporting of orbits and the prompt targeting of unusual orbits for additional follow-up.

No wonder you're an AC. I'd be ashamed to put my name to such an admission of incompetence.

Interstellar travel

[ShanghaiBill]

The may have serious implications for interstellar travel, if it turns out there is a lot more debris out there and space isn't as empty as we thought. A chunk of ice can do a lot of damage when you are going 0.2c.

Re:

[RockDoctor]

I think getting to 0.2c is the first problem to address. Then the problem of a 20-year voyage in a closed ecosystem with no resources you didn't accelerate to 0.2c (caveat : you might catch up with stores you dispatched 5 years ago at 0.18c - read any book on the logistics of "Expedition" style mountaineering for examples).

Actually, my first guesstimate is that if confirmed, this second pre-covery would generally agree with the estimate that there are of the order of 10000 such objects within the orbit of

Re:

[ShanghaiBill]

there are of the order of 10000 such objects within the orbit of Neptune at any one time.

This comet was about 800 meters in diameter. If there are 10000 that size, then there are likely millions or billions of smaller objects, the size of a refrigerator or a baseball. At 0.2c, even a pebble or grain of sand can cause enormous damage.

We may want to delay any interstellar colonization voyages for a few years, until we get a better understanding of this problem.

Re:

[RockDoctor]

I still think that developing an idea for a technology that would get a probe large enough to carry a vonNeumann machine and a fistful of gametes & bacteria at more that 0.1c is a more immediate concern. After all, if you've got a 0.2c propulsion technology, you can launch several robotic prototypes ahead of the actual probe with the expensive equipment (or even people) aboard.

Re:

[RockDoctor]

Have you ever worked in the discovery phase of an observational science? Obviously not.

Too many parabolic orbits

[techno-vampire]

It's been decades since I studied this, and most of the neurons I used to store the details have long since been recycled, but I do remember a little bit about orbital mechanics. If an object's velocity is lower than escape velocity, it's in an elliptical orbit, and if it's above, the orbit's hyperbolic. You only get a parabolic orbit if it's traveling exactly at escape velocity. OK, yes, I understand that there are limits to how precise our measurements are and that leads to a margin of error in the calculated orbit, but I can't help but think that there's something wrong when there are hundreds of comets discovered since 1950 with calculated orbits at exactly escape velocity, as close as we can calculate it. What I wonder is why it took astronomers this long to start checking their figures and finding all of these mistakes. Of course, they might just have been too busy to recheck all of those figures, but still, I'd like to find out.

Re:Too many parabolic orbits

[RockDoctor]

Apparitions of "first appearance" comets didn't all produce eccentricities of 1.0000. - I recall for example reading papers by Opik (grandfather of the MP who promoted the UK actually investing money in planetary protection. Before getting "involved" with a pop starlet and losing his seat.) from the late 40s or mid-50s when for plotting and comparison purposes he worked on the semi-major axis of the reduced orbits because it made the small differences more obvious. That's what gave evidence for the Oort cloud - which we're still at the very edge of being able to directly observe.

But these days we're spending a lot more time observing with a lot bigger "light buckets", and reducing the data astrometrically to orbits a lot faster - which makes the recent discoveries (putative) much less surprising. We can look forward, on this basis, to seeing yearly or more frequent discovery of interstellar objects - exactly as we did with pulsars when I was a school kid and we've done with gravity wave astronomy in the last couple of years.

Re:

[techno-vampire]

Either you didn't understand what I meant, or you explained the situation poorly. I've no problem whatsoever with there being large numbers of interstellar visitors, or with recalculations of the obits of old comets revealing more of them. What bothers me is the large number of observed comets with velocities close enough to escape velocity to have a parabolic orbit; I'd expect most, if not almost all of them to have hyperbolic orbits.

Re:

[RockDoctor]

I'm not sure that I understand what you're trying to say then. Why the interest in "escape speed" (nb: it's a speed, not a velocity - https://en.wikipedia.org/wiki/... [wikipedia.org] paragraph 2), which varies at different heliocentric distances, over eccentricity (which is a property of the entire orbit, and doesn't matter where in the orbit the object is when you measure it. If you're interested in the escape speed, then you first need to determine the orbit (boing! you've got the eccentricity!) then work out where on t

Re:

[techno-vampire]

My concern is (or was, by this point) that the eccentricity of the orbit depends, in part, on the object's velocity. Unless it's travelling at least at escape speed (to use your pedantry) the orbit must be elliptical, with an eccentricity of less than one; if it's greater than escape, the orbit will be hyperbolic, (greater than one) and if it's exactly at the escape speed/velocity, it will move in a parabola. (exactly one) Given that TFS states (wrongly) that there were at least 100 comets in parabolic or

Re:

[RockDoctor]

I'm still not getting your point.

the eccentricity of the orbit depends, in part, on the object's velocity.

That's .... well, a ... tautology would be a polite way of putting it. Since orbits are all about the interplay of velocity with position and gravity field. Of course, the velocity varies somewhat between periapse and apoapse - if the latter has any meaning. Actually, reading up on the messages announcing the realisation of the importance of 'Oumuamua, the humans who looked at the machine-generated orbi

Re:

[techno-vampire]

TFS says that over a hundred comets are traveling in parabolic orbits, implying that they're all moving at escape velocity, the most common term for the concept. Another poster in this thread explains that the paper says nearly parabolic, meaning that they're moving slightly faster than needed to continue back into interstellar space and never be seen again, which sounds much more reasonable because it suggests a range of speeds, not one specific one. If you wrote TFS and left that word out, I wouldn't be

Re:Too many parabolic orbits

[pz]

I'm just an astronomy fan boy, but if something's coming in from the Oort Cloud, isn't that far enough away that all orbits are going to look parabolic to the limits of measurable accuracy? I mean we can barely determine orbits in the Kupier belt, right?

Re:

[techno-vampire]

My understanding is that any comet that originates in the Kupier belt will be in a highly elliptical orbit, and that by the time we can get clear enough observations to calculate that orbit, the comet will be much closer, possibly even within Pluto's and/or Neptune's orbit, but like you, I'm no professional.

Re:

[RockDoctor]

It is possible to get a fair-quality orbit from one night's observations - at least, good enough to know where to point the CCD for follow-up observations the next night. And the industrial-scale data pipelines will be back to photograph (well, "CCD") the same regions of sky repeatedly over the next few nights. So if the body is large enough (or bright enough at some points in it's light curve - which you're already starting to record), you'll know about it a long long way out. IIRC, comet Hale-Bopp was dis

Re:

[techno-vampire]

IIRC, comet Hale-Bopp was discovered when it was somewhere out near Saturn's orbit.

I'm not sure, but that sounds about right. In any event, though it wasn't discovered while it was still in the Kuiper Belt, when it would have been too faint to get accurate enough data to calculate the orbit very well.

Re:

[careysub]

I'm just an astronomy fan boy, but if something's coming in from the Oort Cloud, isn't that far enough away that all orbits are going to look parabolic to the limits of measurable accuracy? I mean we can barely determine orbits in the Kupier belt, right?

Bingo.

Not really to the "limits of measurable accuracy" but to the "limits of accuracy measured", sure. Accurate orbital determination requires lots of observations of very dim objects, and telescope time is limited. Not enough observations along a long arc, and it becomes insufficiently determined.

But we do know which are comets are closest to being true parabolas. It is easy to determine. They hit the Sun.

This is not rare, in fact about 100 comets a year do this. In 2010, a good year for sun-diving comets

Re:

[RockDoctor]

But we do know which are comets are closest to being true parabolas. It is easy to determine. They hit the Sun.

"A chance no astrophysicist could pass up."

"I am sorry, Louis Wu. I do not understand."

"The opportunity to study the underside of sunspots."

Re:

[RockDoctor]

Nope. Ever since Gauss's work on the determination of the orbit of Ceres (the discoverer Piazzi only got 41 a day long arc of observations before Ceres got too close to the Sun - turning those data into an orbit was suddenly urgently necessary to "recover" the object when it came back out of the Sun's glare. As a part of that calculation, the eccentricity of the (calculated) orbit pops out. The maths goes over my head, but there is no shortage of descriptions of it (e.g. http://sce.uhcl.edu/helm/Space... [uhcl.edu]) a

Re:

[careysub]

You only get a parabolic orbit if it's traveling exactly at escape velocity. OK, yes, I understand that there are limits to how precise our measurements are and that leads to a margin of error in the calculated orbit, but I can't help but think that there's something wrong when there are hundreds of comets discovered since 1950 with calculated orbits at exactly escape velocity, as close as we can calculate it.

Hang on. In other papers these same researchers use the term "near-parabolic" for this same class of comets, this short paper needed an editor. In some cases the observational errors (particularly in the earlier comets) are large that we cannot distinguish them from parabolic orbits. Nothing mysteriously wrong.

FWIW there is no great discovery revealed by this paper. It shows that a recent comet has a sufficiently poorly known orbit that it could be significantly hyperbolic. Or not. The data isn't available.

Re:

[techno-vampire]

In other papers these same researchers use the term "near-parabolic" for this same class of comets...

OK, thanx. That clears everything up for me. It was just a case of sloppy reporting by somebody who didn't quite understand the article enough to see how much of a difference that one word makes.

Re: Too many parabolic orbits

[Xova]

This is far from my area of expertise but is it not just because objects with orbital eccentricities above one donâ(TM)t stick around?

Re:

[techno-vampire]

Yes, because orbits with eccentricities greater than one have hyperbolic orbits. However, orbits with an eccentricity of exactly one have parabolic orbits and also don't stick around. My concern had to do with the fact that there looked like there were an unreasonable number of comets with parabolic orbits, but another poster explained that the paper's authors used the words "nearly parabolic," which is far more believable for me.

Re:

[RockDoctor]

Look at it from the other point of view. If something has an orbital eccentricity of more than 1, then we get one chance to see it (one "apparition"). But if something is perturbed into an orbit with an eccentricity of 0.96 (eg Halley's Comet), we'll get multiple chances to see it (30, so far, for Halley).

To have lower eccentricity and be close enough to the Sun to be seen through much of it's orbit ( e.g 67P/Churyumovâ"Gerasimenko, eccentricity 0.64, perihelion 5.7AU, aphelion 1.2AU) required multip

Miss it?

[CaptainDork]

Yes, I do recall a sort of emptiness and longing as it swooshed by.