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Comment by robotrout | Hacker News Reader

So, two masses were close enough in relative velocity and vector, that one of them didn't careen off the other one, and instead, allowed the weakest of all known forces to bond them together as they hurtled at high velocity through space.

This object(s) seem statistically unlikely to me. I'm not saying it's artificial. That would be even more unlikely, by orders of magnitude. Just saying "Wow!".

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Contact binaries are fairly common. Expected to make up about 10-15% of NEOs. My layman's understanding of how Kuiper object contact binaries develop is mutual capture during the early life of the solar system and angular momentum decay until they come into contact.

Can you say more about how angular momentum decays in orbital mechanics? I understand that, because the Earth rotates faster than the moon orbits, our tidal bulge will be ahead of the sublunar point, and will accelerate the moon in its orbit. Are there any other ways to get rid of angular momentum?

Tidal forces are one way - either between the two binary components, or from a close encounter with a third, larger body.

Another is the "YORP Effect". Sunlight falling on an asteroid produces a slight thermal radiation pressure (push). If, due to asymmetries in asteroid shape/albedo, the net radiation pressure force is not aligned with the asteroid's center of mass, it will produce a torquing force which will cause the asteroid to spin faster (or slower) over time. Applying the idea of YORP Effect to binary asteroids yields the "BYORP [Binary YORP] Effect", by which the orbital dynamics of the binary system are modified by this asymmetric radiation pressure over time, in a way that either pushes them together into a contact binary or apart into two unbound asteroids.

It's even hypothesized that some asteroids may be in a binary/contact-binary cycle on long timescales! There are solutions to the above in which the BYORP effect causes a loss of angular momentum in a binary pair, causing them to merge into a contact binary - but the contact binary may settle into a state where the YORP Effect actually causes the newly merged asteroid to spin faster, eventually flinging them apart due to centripetal forces... back into a binary state where the BYORP Effect may again cause them to merge someday.

Recommended reading: https://arxiv.org/abs/1010.2676

> This object(s) seem statistically unlikely to me.

At this mass these bodies are at, they aren't going to crush together from gravity to form a single round body.

However, if you've got two bodies in very similar orbits around the Sun, in close proximity, it doesn't seem incredible to me that they might eventually collide and stick.

When you think about the vastness of space (even in our own star system), as well as the incredible amount of time involved since the formation of our system, many things that seem statistically unlikely probably become very likely. How many objects are in orbit around the Sun in this system? We probably only know a small fraction of them, since we can't see ones this small very well from this distance, and there could be many more that are even more distant. So really unlikely objects like this could be more common than you think.

In the vastness of a universe as large as ours, polka dotted unicorns are probably very likely. But it's still unlikely to actually encounter one ... you know, because they're polka dotted unicorns and because the universe is so big...

What I mean is, your argument doesn't make any sense. Just because the universe is big, doesn't make unlikely events more likely to be stumbled upon.

>This object(s) seem statistically unlikely to me

Your post implied that the existence of such objects is unlikely, not the observation.

To which the other poster replied that (unlikely event) x (vastness of space) = likely occurance.

If a center of mass begins to aggregate in an area of space, smaller objects may get captured and will move in an elliptical orbit with the forming object at one of the foci of an ellipse. Because the other foci of the ellipse is also fairly stable, it is also a likely point for smaller objects to aggregate into another larger object. Objects forming in pairs can reinforce each other. Over time when the small objects around the two foci have all aggregated, the objects at the foci will fall together due to gravity.

It doesn’t seem unlikely to me. There are many many approach paths that would result in two similar sized objects ending up orbiting each other.

Once they are orbiting each other then it’s just a matter of time for the orbits to decay. In the final, they would be spinning very very fast but would be inching towards each other until they touch.

Given the slower speeds in the outer solar system, especially since they will be rotating in the same direction, so the relative speed will be less, I would imagine that contact binaries would be fairly common. And since they are primarily made of ices, the actual contact event should melt the ice, which will then re-solidify since the impact speed is so little.

I don't know if this is how it actually happens, but it gives a mechanism for this being fairly common. The only information I could find was a short wikipedia article:

https://en.wikipedia.org/wiki/Contact_binary_(small_Solar_Sy...

the comet rosetta visited was quite similar. it's either humanity's dumb luck or those things are more common than previously thought.

As this is the first Kuiper Belt Object we've visited, it's hard to draw much about the statistics of the thing.

What's exciting to me is, both the first interstellar object we've observed visiting our solar system (Oumuamua) and the first Kuiper Belt Object we've visited have been "unusual". That strongly suggests to me that the unusual is more usual than we would have suspected, which means there's probably a great deal of opportunity to increase our understanding of the universe!

They deliberately picked targets that seemed unusual from initial readings.

Oumuamua was literally the only extra-solar object known. There was nothing to be picked.

Similarly for Ultima Thule. They may have been able to find a different target, but it was very difficult to find any target at all in the first place (lots of Kuiper belt objects are known; the difficulty was finding one that could be reached by New Horizons). See this Twitter thread that was linked elsewhere: https://twitter.com/Alex_Parker/status/1077986070128668674

It's easier to observe bigger objects than smaller objects.

Bigger objects have greater gravity than smaller objects, therefore are more spheroid.

As we get better at observing, we'll see more objects that are less spheroid.

At least in the case of Oumuamua though, being non-spheroid wasn't really the weird thing about it. It was that it exhibited comet-like acceleration, without any visible off-gassing, and without breaking up as it passed the sun, as a comet would be expected to do. Also the fact that from our current understanding, it is much more likely for a comet to be ejected from a solar system than an asteroid, so it's surprising (not impossible, certainly, but unlikely) that the first interstellar object was more like an asteroid.

Fair enough!

Re: first Kuiper Belt Object we've visited

Pluto may be a KBO also. Further, it's possible some moons of the gas-giant planets are captured KBO's.

The unusual thing about Ultima Thule is that it's in a nearly circular and "flat" orbit, which many experts interpret to mean it's mostly undisturbed from its point of origin. It's a prime "fossil".

Pluto has a "disturbed" orbit such that its origin is currently unknown. Same for gas-giant moons. Pluto has also been turned inside-out, perhaps multiple times, by a still unidentified force. Ultima Thule is probably mostly as-is since formation.

> Pluto has also been turned inside-out, perhaps multiple times, by a still unidentified force. Whoa, come again? How do we know this? What does it mean for a planet-like object to be turned inside out?

Inside out just means that the local equivalent of Earth's geologic plates seem to be subducting below the surface and fresh geologic plate equivalents are abducting back up. You can estimate how long a planetary surface has been exposed to the elements by the statistical distribution of number and size of craters. We have a reasonable idea of the likelihood of objects impacting Pluto, and it should have a lot of craters. Pluto's moon Charon has many more visible craters but Pluto has large areas with no large craters, so some sort of geologic process must be going on that's recycling areas of the landscape and covering or subducting the craters.

Given lack of fast erosion from liquid water or significant wind (in a very thin atmosphere), whatever geologic process is smoothing out Pluto is happening quickly compared to other points of reference we have.

I use the example of Earth's geologic plates, but that's only one possible explanation. There's also speculation about freeze/thaw cycles of the planetary material and atmosphere (since it has an irregular orbit), movement of water-ice mountains, cryovolcanos, and other theories.

Some examples: https://www.nasa.gov/sites/default/files/thumbnails/image/du... https://www.nasa.gov/sites/default/files/thumbnails/image/nh... https://www.nasa.gov/sites/default/files/thumbnails/image/co... https://www.nasa.gov/sites/default/files/thumbnails/image/nh...

The second, actually - Pluto is generally recgnised a Kuiper Belt object.

Or the seventh, since New Horizons imaged several satellites in the Pluto system, some at similar resolutions to the latest pictures.

If they're all like this one, that would be puzzling indeed! Well done Mr. Spock!

Remember, the Moon is covered with the results of astronomical odds.

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