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Comment by quotha | Hacker News Reader

It seems like we've been probed when you look at this hyperbolic trajectory:

https://photojournal.jpl.nasa.gov/archive/PIA22357_JPL-20180...

You'll need to give me something a bit more convincing than a relatively small deviation from expected trajectory for a small object we know next to nothing about if you want me to believe we've been visited by aliens. Extraordinary claims require extraordinary evidence, this is not it IMO.

I mean, that's what the author is arguing for. That we need more evidence.

> Whereas the next time we see an object like this one, we can contemplate taking a photograph. My motivation, in part, is to motivate the scientific community to collect more data on the next object rather than argue a priori that they know the answer.

Basically, what's being said is: "Hey, all these things point to this and are explained by this, we need more data!"

If you mean that the "expected" trajectory is different than the "observed" one, it's not that we don't have examples where the simpler calculations result in difference from the observations, but then the newer, better calculations find these older simply insufficient. See the famous "Pioneer anomaly":

https://en.wikipedia.org/wiki/Pioneer_anomaly

"The anomalous acceleration was first noticed as early as 1980 but not seriously investigated until 1994." "Various explanations, both of spacecraft behavior and of gravitation itself, were proposed to explain the anomaly."

"By 2012 several papers by different groups, all reanalyzing the thermal radiation pressure forces inherent in the spacecraft, showed that a careful accounting of this explains the entire anomaly; thus the cause is mundane and does not point to any new phenomenon or need for a different physical paradigm."

The explanation I've seen is that Oumuamua is made of a "frozen cloud" and has lower mass than that used in the expected trajectory model, and was possibly pushed by solar wind into that observed trajectory.

If you had read the article you'd know that's not possible, as solar wind would have had a negligible effect - unless the object was in practice a light sail. OTOH that hypothesis also doesn't sound correct IMHO - why would it appear to be tumbling with a fixed period?

The article does not talk about the possibility of an ultralight frozen cloud. It only mentions one kind of "solar sail" structure.

The tumbling was a reasonable interpretation of light curve data. But the data was limited, and something else may have been happening.

its time to stop writing clunky js and start preparing for the alien wars

we'll use new framework press releases as a distraction.

Is there a way to search for objects that would go in solar system plane --|-- <--. Like something that could hit Earth in a way that would accelerate it to collide with Mars for example?

Those things are caught by Jupiter probably most of the time but still I bet those are mostly recorded somewhere.

Best if those things are outside of solar system, but those are not that common I think...

How can they predict the trajectory when the shape and material of the object is unknown?

So we can determine the orbit using only line of sight measurements of the object relative to the Earth.

This is a problem originally solved by Gauss for Ceres.

https://en.wikipedia.org/wiki/Gauss%27s_method

More generally given sufficient observations one can use many similar methods of orbit determination to get a relatively accurate orbit of the body. All of this is standard stuff in astrodynamics and happens on a near constant basis for both man made and natural objects.

A good introduction is the classic BMW

https://www.amazon.com/Fundamentals-Astrodynamics-Dover-Aero...

None of this requires the shape or material of the object, just some fuzzy measurements of the position

But surely it can be affected if the body outgasses because of some solar effect? I don't believe that applies to Ceres, or to satelites around the Earth, etc.

Outgassing should be seen. If it is close enough to the sun to melt anything, it is close enough that we should see the resulting tail. Once it is far away from the sun again, outgassing should stop as everything freezes up.

That depends quite highly on the object's mass.

The trajectory of an object in space really only depends on the (external) gravitational field applied to it.

No, it also depends on effects like the pressure from the solar wind and outgassing. Outgassing is caused by water on the side facing the sun evaporating, emitting a gentle pressure away from the sun. If the object is rotating, the force pushes at an angle to the sun because the outgassing rate lags insolation due to the surface water's thermal mass (ie, each point on the surface continues to evaporate for a while after its apparent sunset.)

The effects aren't significant for planets, but they are for asteroids. It seemed more significant for Oumuamua than most, suggesting a high surface area to volume ratio.

I thought outgassing and solar wind was only really relevant when applied for a long time over many periods of an orbit? Would it have a significant impact on a one-time hyperbolic trajectory like this one?

In fact, one of the things proposed in the article is that the object is a light sail being accelerated by the sun.

And its speed!

Well, yes. I guess I was talking more on the lines of acceleration and deviations from a straight line trajectory :)

I think the projected trajectory is the path expected without some unknown force acting on it.

Yes, but here's my problem with expected trajectory being used as evidence of aliens, and 'unknown force' being used as some mysterious alien technology.

If I remember right, one of the voyager or pioneer probes was moving much faster than it should've been (relatively), and we had no idea why for years. Then we figured out it was the cumulative effect of heat radiation(?) from something on board the machine.

If we can't figure out what WE made, how can we expect to figure out a once-in-a-generation event like this on the first shot?

rate of acceleration was more in line with object offgassing due to irradiation from the sun, at least as what I got from scientific sources last year.

Wow, that's crazy... It sure seems so.

Not really, the deviation is not that large for the scales we're talking about and there are still plenty of potential explanations. Hell, for the longest time we couldn't explain why our own Pioneer spacecraft deviated from expected trajectories and we knew precisely what they were made of.

... with the most likely explanation being outgassing.

Seligman et al. (2019): https://iopscience.iop.org/article/10.3847/2041-8213/ab0bb5

If that's so, all comets could be probes.

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