5 ms·

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 investi

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 8y ago
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 8y ago
  Or was it a lighter sail, still carrying a payload?!
  - jacobush 8y ago
    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 8y ago
      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 8y ago
        Interesting thought! And also, maybe some of the stored and emitted heat can be directed somehow.
- skykooler 8y ago
  Microscopic debris impacts could have degraded it over its lifetime to its current state.
- ansible 8y ago
  Yes. Something of that size (area) and weight doesn't seem like some kind of naturally occurring object though. Even if it is a piece of debris, that is super, super interesting. Eh, who am I kidding, even if it was produced through some natural process, that is still interesting.
- bottled_poe 8y ago
  Perhaps, but would the design you are proposing survive millions of years of space travel? This appears to have zero moving parts.
mannykannot 8y ago
Assuming a uniform thickness and density, is the acceleration of a light sail not independent of area? I get the impression that the radius given is an upper limit based on the apparent magnitude and assuming a high albedo. The light curve indicates that the object is tumbling, so a thin sheet would rarely (if ever) be oriented approximately optimally to act as a sail. I have not figured out how the authors have taken this into account in their calculation - could it be as simple as halving the area to give an effective value?
- deleted 8y ago
  [deleted]
- Qwertystop 8y ago
  Perhaps the acceleration is consistent with tumbling if it just-so-happened to tumble (slowly) into a sailable angle at around the time the acceleration was noted, or is tumbling fast enough for things to average out?
InclinedPlane 8y ago
> "Comet-like off-gassing has been mostly ruled out due to the lack of observed dust or gas around the object" This is a new one on me, I'm not aware of any rigorous lower limits on off gassing which have actually ruled it out as the source of the acceleration. Do you have any citations?
- Tossrock 8y ago
  Rafikov, 2018: https://arxiv.org/pdf/1809.06389.pdf https://arxiv.org/pdf/1809.06389.pdf

abecedarius 8y ago

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 8y ago
Or was it a lighter sail, still carrying a payload?!
- jacobush 8y ago
  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 8y ago
    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 8y ago
      Interesting thought! And also, maybe some of the stored and emitted heat can be directed somehow.
skykooler 8y ago
Microscopic debris impacts could have degraded it over its lifetime to its current state.
ansible 8y ago
Yes. Something of that size (area) and weight doesn't seem like some kind of naturally occurring object though. Even if it is a piece of debris, that is super, super interesting. Eh, who am I kidding, even if it was produced through some natural process, that is still interesting.
bottled_poe 8y ago
Perhaps, but would the design you are proposing survive millions of years of space travel? This appears to have zero moving parts.

mannykannot 8y ago

Assuming a uniform thickness and density, is the acceleration of a light sail not independent of area? I get the impression that the radius given is an upper limit based on the apparent magnitude and assuming a high albedo. The light curve indicates that the object is tumbling, so a thin sheet would rarely (if ever) be oriented approximately optimally to act as a sail. I have not figured out how the authors have taken this into account in their calculation - could it be as simple as halving the area to give an effective value?

deleted 8y ago
[deleted]
Qwertystop 8y ago
Perhaps the acceleration is consistent with tumbling if it just-so-happened to tumble (slowly) into a sailable angle at around the time the acceleration was noted, or is tumbling fast enough for things to average out?

InclinedPlane 8y ago

> "Comet-like off-gassing has been mostly ruled out due to the lack of observed dust or gas around the object" This is a new one on me, I'm not aware of any rigorous lower limits on off gassing which have actually ruled it out as the source of the acceleration. Do you have any citations?

Tossrock 8y ago
Rafikov, 2018: https://arxiv.org/pdf/1809.06389.pdf https://arxiv.org/pdf/1809.06389.pdf