
‘Oumuamua, Thin Films and Lightsails - Osiris30
https://www.centauri-dreams.org/2018/10/29/on-oumuamua-thin-films-and-lightsails/
======
Tossrock
Given Oumuamua's well documented non-gravitational acceleration, what was the
cause? That's the key question this article – and the paper it cites – are
investigating.

Comet-like off-gassing has been mostly ruled out due to the lack of observed
dust or gas around the object, as well as lack of effect on spin. The paper
suggests that a highly reflective 0.3mm-thick film 20m in diameter could be
consistent with the observed acceleration. This also happens to be similar in
dimensions to lightsails that we've proposed (and in a few cases deployed)
here on/around Earth.

Of course, it's just one hypothesis. Could the acceleration be due to some
other effect, like the Pioneer anomaly? Sure. Unfortunately, there's no way to
ever confirm it given that the object is beyond our ability to observe or
catch. But it's still very interesting speculation.

~~~
abecedarius
It’s a fascinating mystery, but I gotta say that’d make a crappy lightsail,
considering that even at our current tech level we could make a much lighter
one if we could manufacture it in space. (The experiments that have flown were
all constrained to survive launch and unfolding.)

~~~
jacobush
Or was it a lighter sail, still carrying a payload?!

~~~
jacobush
Hm, say you are designing a light sail for interstellar travel. Instead of
having it be made of thin film and actuators capable of swinging these sails
like on a sail boat... because actuators are more likely to die after
millennia in space:

say you instead make the space craft tumble but make the sails able to change
their albedo (how reflective they are). If you wanted to make a course
correction, you'd alternate how reflective they were. If you wanted a push
away from a sun, you'd make the side facing a sun bright. If you wanted to
fall in more towards a sun, you'd make the side facing a sun dark. Would that
work?

How much do light sails depend on brightness (light pressure) vs just
absorbing molecules in the Solar wind?

Edit: Wikipedia says light pressure from photons is most of the effect, but
still I can't figure out the difference between a black body sail and a
reflective sail.

~~~
abecedarius
Wouldn't a perfect absorber facing the sun directly get half the light
pressure of a perfect reflector? I'm assuming after it heats up to equilibrium
it emits in all directions (no net momentum from emission, so you're left with
absorption only).

~~~
jacobush
Interesting thought!

And also, maybe some of the stored and emitted heat can be directed somehow.

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mrfusion
It makes sense you would jettison your light sail before starting deceleration
approaching earth. That means the things they jettison would pass by us first
while the actual spaceship is still slowing down.

Perhaps the main ship is on the way!

~~~
lmilcin
If you jettisoned lightsail while approaching solar system it would get SLOWED
down by Sol's light compared to the main mass of the craft. We would see the
ship first and only then the sail.

Solar sails to be effective would have to be much larger than the craft
itself, and also very reflective (by definition), though the reflection don't
have to omnidirectional (stealth aircraft/ship principle -- make it reflect a
lot but only in very specific direction so that it seems dark to most
observers).

Given this, the craft would most likely PRECEDE the sail but would also be
smaller and darker and we would most likely not be able to spot it.

~~~
lolc
Using a dinky star for acceleration is not going to get you real fast. There
could have been a targeted beam to accelerate the craft. And the craft carries
fuel for deceleration only.

~~~
Tuna-Fish
You can get right up to relativistic velocities if you first sundive as close
as you can while keeping your sail folded and protected by a heatshield, and
then deploy your sail right next to the sun.

But anyway, if you carry a solar sail to accelerate, why would you not use the
same sail to decelerate as much as possible too?

~~~
lmilcin
Especially if you are going to a star roughly same or higher luminosity, you
should have no problem braking with the sail. Also, you probably absolutely
need to break with sails since you don't want to carry much fuel to break. Any
reasonable amount of fuel required to break will be orders of magnitude more
than the payload -- reducing effectiveness of sails by same factor.

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jolesf
"Oumuamua appears to have originated at the Local Standard of Rest (LSR),
which is the galactic frame of reference." Is a very interesting side-note

~~~
coleifer
Wikipedia says:

> In fact, the strong correlation between ʻOumuamua's velocity and the local
> standard of rest might mean that it has circulated the Milky Way several
> times and thus may have originated from an entirely different part of the
> galaxy.

~~~
ianai
That really does just make it that much more interesting. Here’s hoping we get
a perfect sequence of coincidences to inspect a similar object someday.

~~~
ddalex
Ramans make everything in threes.

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stupidcar
Given how weird, and apparently unexplainable, various aspects of this object
are, shouldn't we be moving heaven and earth to investigate the possibility of
some kind of mission to it? From what I've read, there are ideas at least for
how we might launch a spacecraft capable of catching up with it. Given the
potential of what we could learn, wouldn't that be a better way to spend a few
billion dollars than a return to the moon? (of course, ideally we'd do both,
but...)

~~~
savrajsingh
We can’t catch it, but we’re on the lookout for the next one

~~~
yeswecould
We could catch it with a bigger, faster lightsail

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KnightOfWords
Please bear in mind, the spectra of Oumuamua is very similar to that of a
D-type asteroid (organics and carbon), which isn't consistent with it being
highly reflective.

~~~
gibsons77
Could just be interstellar dust coating the sail.

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Zelphyr
Although unlikely to be a lightsail I think, this reminds me of the Rama
series by Arthur C. Clark. I really wish someone like HBO, Netflix, or Starz
would pick that series up and make a show out of it.

~~~
BillSaysThis
Morgan Freeman owns (owned?) the rights and has been trying to make it since
the late '80s:
[https://www.syfy.com/syfywire/morgan_freeman_insists_re](https://www.syfy.com/syfywire/morgan_freeman_insists_re)

~~~
Zelphyr
He wants to make it into a movie though. I hope he is considering a high-level
TV series a la Game of Thrones. I feel like there are a lot of nuances that
would get lost if made as a movie or series of movies.

~~~
howard941
A series that lasted more than one season? Brilliant!

Devil's advocate would raise the example of Dune perhaps, with a fairly decent
IMO SyFy network TV miniseries, a lot more nuanced than the film and far more
from the novel but didn't have the staying power of either of the two film
versions. (Not that any of the film versions or the miniseries seems to be
broadly loved)

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motoboi
If that is a discarded lightsail, maybe it's payload is right now decelerating
towards Sol. THAT would be something (pun intended).

~~~
scj
What if it's payload contains an AI in a tiny container rather than a
biological being?

~~~
Ftuuky
Like in the book Accelerando? Advanced civilizations might want to send
digital copies of their astronauts, to keep the operation light and lean.

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StreamBright
If I were a funny alien civilization and had some time to kill I would totally
play skimming stones involving solar systems and Oumuamua like objects.

~~~
trhway
in a short time, may be even in our lifetime, we'll probably be able to set up
manufacturing and sending out of the light sails like this just as a task
performed 24x7 by several weaving/printing machines planted on some asteroid
having needed raw materials. Given even today's technology something like on
scale of hundreds of thousands of such sails can be produced yearly. We just
have to get up there.

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xrayzerone
Centauri Dreams is one of my favorite sites to grab a coffee and settle in for
a read / geek out on high-concept space articles.

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rossdavidh
While I find it highly unlikely, it does raise an interesting point about
Fermi's Paradox, which is that even if there were enough alien life out there
for its discarded light sails to go hurtling through our solar system every
once in a while, we might not get any more evidence of it than this article
demonstrates. So, it's almost certainly not a light sail, but even if it were,
we would never know.

~~~
gwern
Fermi's paradox demonstrates it's almost certainly not a light sail. For it to
be a light sail which was observed by chance by our totally unprepared
observatories after a window of less than a century but also consistent with
all our observations of the universe, we would need to hypothesize that we got
extraordinarily lucky to observe a super-rare artifact, or that countless (or
one vast) interstellar, pan-galactic civilization throwing out light sails in
every direction constantly which also collectively choosing for
millions/billions of years to avoid any kind of stellar engineering or direct
colonization or Dyson sphere-like artifacts or even atmosphere composition
tweaks visible on exoplanet spectrographs all carefully preserving the
illusion of a 100% dead cosmos in every inch of our Hubble volume, or that
alternately, interstellar civilizations are super common and in fact routinely
throw out solar sails all the time, but some sort of interstellar-capable
Great Filter kills them almost instantly before they can have any visible
effects, leaving just a bunch of spreading artifacts. (And this Great Filter
can't be any of the usual ones like asteroids or nukes or pandemic or
development of intelligence.)

~~~
skykooler
Or that this was a relatively nearby civilization, and the light sail was
intentionally headed toward our solar system (like a few of our proposed
interstellar missions to Alpha Centauri).

~~~
gwern
That falls under the 'we got ludicrously lucky' category: in the entire galaxy
of billions of stars over billions of years we just happened to arise
_exactly_ simultaneous with the other civilization in time to see it? (Also,
if it was coming from a nearby solar system, why did it pass right through, do
nothing, and is at rest with respect to the galaxy? I also think if its
trajectory led straight to a nearby star system with any potential for life,
someone would've noted that.)

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angel_j
If it was a large hollow body, then it would have particular resonant
frequencies. Is there such a thing as a resonance sail? Total speculation, but
I think such a thing would work the opposite of a lightsail, in that it would
tend to go toward bodies than emanated it's resonant frequency, rather than be
pushed away by radiation.

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RootKitBeerCat
This is quite possibly one of the most incredible hypotheses I’ve read, and I
love that it is so well supported!

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stephengillie
If we send a Voyager-sized probe to Alpha/Beta Centauri, could we use a
similar gravity assist to slingshot around one of the stars?

How good is our aim across 4.4 light years?

~~~
Chris_Jay
Wouldn't we have a robot steer the light sail as it got closer?

Genuine question, this is well outside my wheelhouse.

~~~
undersuit
I think that's the right answer, as it got closer the exact parameters for the
slingshot could be refined with images of the system taken a few lightdays out
with a strong camera. So we just need that robot really.

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otras
As another comment pointed out, the actual title of this article is
_‘Oumuamua, Thin Films and Lightsails_. While the question " _Could ‘Oumuamua
be debris from a technological civilization, a discarded lightsail?_ " occurs
in the article, this title does the article a clickbaiting disservice.

(" _Is the interstellar object Oumuamua a lightsail from an alien
civilization?_ " at the time of this comment)

~~~
dang
Yes, that title broke the site guidelines, which ask: "Please use the original
title, unless it is misleading or linkbait; don't editorialize." We've since
reverted it.

All: Accounts that do that eventually lose submission privileges on HN, so
please don't!

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howard941
But if it was, is it friendly, or is this place a Dark Forest?

[https://en.wikipedia.org/wiki/The_Dark_Forest](https://en.wikipedia.org/wiki/The_Dark_Forest)

~~~
lallysingh
Dark forests wouldn't waste a round-trip. They'd shoot first. This probe isn't
aimed at Earth directly.

~~~
flukus
They have to know where to shoot, the dark forest theory doesn't seem to
account for the ability to send probes.

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sigi45
No

~~~
gota
This is a law, I think, maybe with the capital L and maybe with very few
exceptions: every headline that states a yes or no question can be truthfully
answered with 'no'.

~~~
mikek
Betteridge's law of headlines

[https://en.wikipedia.org/wiki/Betteridge%27s_law_of_headline...](https://en.wikipedia.org/wiki/Betteridge%27s_law_of_headlines)

~~~
tome
tome's law

[https://news.ycombinator.com/item?id=17978737](https://news.ycombinator.com/item?id=17978737)

~~~
credit_guy
In any HN thread where Betteridge’s law is invoked, tome’s law follows with
probability 0.85. Anonymous law.

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vosper
The title of the post is "‘Oumuamua, Thin Films and Lightsails"

Why was this changed in the HN submission? It sounds much more click-baity
than it is.

~~~
dang
Because the submitter broke the HN guidelines. See
[https://news.ycombinator.com/item?id=18341433](https://news.ycombinator.com/item?id=18341433)
and
[https://news.ycombinator.com/item?id=18341439](https://news.ycombinator.com/item?id=18341439).

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DrBazza
Betteridge's law of headlines. No.

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d_burfoot
Paging Elon Musk, Jeff Bezos, et al: the next time one of these objects
appears, we should try to capture it somehow, or at least divert its
trajectory enough that it stays in the solar system and we can study it
further. Nabbing one of these things would be a _spectacular_ achievement,
comparable to the moon landings, and would likely be of significant scientific
value.

~~~
stephengillie
Musk missed, and his car is headed for the asteroid belt instead of Mars.

~~~
dang
Please do not take HN threads on off-topic flamey tangents. They lead to off-
topic flamewars, the last thing we want here.

~~~
stephengillie
Sorry

