Did We Miss an Interstellar Comet Four Years Ago? (arxiv.org)

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.

Comments

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:Interstellar travel

[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 Neptune at any one time. The estimate of the number of interstellar objects hasn't changed much - just your knowledge of that flight hazard.

Re:Interstellar travel

[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:Interstellar travel

[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.

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: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:Too many parabolic orbits

[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:Too many parabolic orbits

[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:Too many parabolic orbits

[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. This paper is a plea for more systematic observation procedures.

Re:Too many parabolic orbits

[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

[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 it was 200. In fact, most comets that are being discovered now hit the Sun. Partly this is because we have SOHO in space doing the observations, and lots of small comets suddenly brighten as they approach the Sun as they vaporize all at once under the super-spot-light that is the Sun, kind of like seeing lots of moths near a light. They do cluster there, and mostly can't see them at all unless they are near the light. Such observations were hard to do from Earth's surface due the scattered glare of the Sun.

The little ones that don't hit the Sun that are dim, we never see them at all.

The body of near-parabolic comets we know of are relatively large, but many are still dim enough that observations far from the Sun are difficult.

Re:Too many parabolic orbits

[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...) and there are commercial and freeware packages for doing the calculations for you (e.g. https://www.projectpluto.com/f... which includes other useful stuff too).

In practice, it is sufficiently automated that, for example, if you do Mike "@plutokiller" Brown's MOOC on the formation of the solar system (harder than your average MOOC ; strongly recommended) you'll be seeing and using plots of thousands of KBOs in "a:e" space. If you've got three observations, you've got an orbit. But if you've got four observations, you've got three orbits and you've got to be looking at the internal consistency of the data sets. Which is why TFP is complaining about the poor quality of the data in the sense of "high residual errors". The intrinsic problems of determining orbits in the Kuiper Belt are as they'll always be - faint objects (meaning noisy data) and slow movement (because the longer the arc between your first and last observations, the more accurate your orbit). But if you stick at it, and in particular, if your first orbit is good enough to collect the object a year later (when it's next in a good position in the night sky for observation - far from the Sun, far from your observatory's horizon at midnight), you'll get a reliable orbit. Unless of course, there is something like fragmentation, or gas ejection (propulsion) going on which you don't notice, and there is no single orbit for your object.

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:Too many parabolic orbits

[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/... 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 the orbit the object is, and what the heliocentric escape speed is at that distance, then calculate the object's speed at that distance ... several additional steps, increasing the error bars.

There were numerical techniques used back in the days of slide rules and log tables where you could shorten the calculation by initially assuming an eccentricity of 1 (if you thought your object was a comet) or 0 (if you thought it was in a circular orbit). But once you've got your initial orbit you need to do a couple of rounds of refinement of the orbit to improve the quality of fit - and you'd get to a true estimate of eccentricity then. If your orbit wasn't converging to a valid solution with each round ... you've got trouble. But that excuse was dieing by the end of the 1950s - you might have to wait a week or two to get your printout back from the computing centre after you set up the observations onto cards (hint : the standard format for reporting observations is still 80 columns wide!) and drop the cards into the internal mail, but you'd still get your computer-calculated orbit back while you're typing up your paper.

Re: Too many parabolic orbits

[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:Too many parabolic orbits

[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 discovered when it was somewhere out near Saturn's orbit. But that was 40-80km diameter, compared to Pluto's 2400km diameter.

The inner boundary of the Kuiper Belt is typically taken at Neptune's orbit. But it is rather fuzzy. Once a body evolves to high enough eccentricity(*) to start crossing Neptune, then it's increasingly likely to come close to Neptune, which is pretty unlikely to result in the eccentricity decreasing. Things start to happen rapidly then, and within mere tens of thousands of orbits it's likely to meet Jupiter. SL-9 did that in about 1970, did about 10 orbits of Jupiter, fragmented under the tidal stress in 1992, and impacted in late 1994. Comets evolve fast once they start to reach the inner Solar System.

(*) It's average kinetic energy, proportional to it's semi-major axis "a" changes much more slowly, because that energy has to go somewhere. Which is why a-e diagrams encapsulate so much about a population of bodies.

I'm no professional in these matters - just an interested amateur.

Re:Too many parabolic orbits

[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:Too many parabolic orbits

[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 orbits, the idea of so many comets having exactly the right velocity/speed for that seemed improbable. Now, of course, I know that TFS misquoted the paper, which explains the oddity.

Re:Too many parabolic orbits

[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: Too many parabolic orbits

[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 multiple interactions with Jupiter. It's not got a long life expectancy either.

Re:Too many parabolic orbits

[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 orbital solutions paid more attention to the V_inf (velocity at infinite range - velocity at apoapse for a hyperbolic orbit) as more indicative of an interstellar origin than the eccentricity. Several examples were discussed of records of objects of undisputed solar system origin being ejected on hyperbolic orbits (after interacting with Jupiter.

Given that TFS states (wrongly) that there were at least 100 comets in parabolic orbits

No it doesn't - and I wrote the damned summary, not just copy-pasting the article from some pre-digested pap website. I quote the paper about the assumption of parabolic orbits for hundreds of comets since the 1950s, and the context of that has the clear expectation (written by the paper's authors, not me) that at least some of these parabolic solutions would be found to be wrong, or at least, unjustifiable with today's better computational tools.

Now, of course, I know that TFS misquoted the paper,

Go to the paper - the published PDF, not just the abstract. Page 1, section 1, paragraph 1. Compare the paper with my alleged misquote. How have I misrepresented the authors?

Re:Too many parabolic orbits

[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 bragging about it.

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.

Miss it?

[CaptainDork]

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

Re:Why the hyperbole?

[RockDoctor]

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