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LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

planetary.org

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Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#91

Post by Retric »

mabbo wrote: post #69

My understanding of what's going on here is that the light sail isn't aiming or changing at all. It's just a big sail, in orbit. When it's moving away from the sun, the sail is being pushed "forward", which adds to it's velocity- this increases the apogee on the opposite side of the orbit. When it's moving towards the sun, the sail is being pushed "backwards", lowering it's current velocity and reducing the perigee.…

Changing orientation is likely easier than retracting the sail. Even just rotation per orbit could bias the acceleration gain.

However, I suspect the goal was kept as modest as possible to increase odds of success and lower costs.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#92

Post by Gustomaximus »

alpha_squared wrote: post #46
b_tterc_p wrote: post #39

Not really relevant but, if we we had an arbitrarily super fast spacecraft, how would we prevent it from hitting a small rock and exploding?

Ultimately, you can't. Space is really, really, really big and empty, though. Unimaginably so, which would make an incident be some really bad luck. In something like the lightsails, you'd just launch multiple simultaneously. Space is so big and empty that even for something like the asteroid belt you could pass through any point randomly and on average be thousands of kilometers from the nearest rock. Perhaps even t…

Is it though? To try and quantify risk I looked up how many metoeites hit the earth each day and its 18,000 to 84,000 that are over 10 grams.

Whiile the earth is obviously much bigger than a space craft, you would think should there be a bunch of craft flying around in the future, theres a fair odds of it happening every few years.

Also for example, the ISS has already been hit a couple of times: https://www.iflscience.com/space/an-astronaut-used-his-finge...

If we have thousands or 10's of thousands of craft in space in the future I would suspect this will be a very real issue on a semi regular basis.

Take what I say with a healthy dose of skeptism as not an expert or even well researched, but logically on above seems quite a reasonable risk.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#93

Post by jessriedel »

sdenton4 wrote: post #90
jessriedel wrote: post #75

Earlier quoted context omitted.

Unlike Lightsail 1, Lightsail 2 operates at an altitude where the atmosphere is negligible.

Hmm.... But the site says this: "In the past 4 days, the spacecraft has raised its orbital high point, or apogee, by about 2 kilometers. The perigee, or low point of its orbit, has dropped by a similar amount, which is consistent with pre-flight expectations for the effects of atmospheric drag on the spacecraft." They also say this on the mission page: "LightSail 2's attitude control system does not have the precisio…

Based on my previous reading, I was under the impression that the atmosphere would be negligible until the perigee dipped much lower than the initial orbit. (Obviously, if you keep lowering the perigee then eventually it becomes important.) That's consistent with your second quote, but your first quote suggests I was wrong and the atmospheric effects are detectable even now. Thanks for the correction.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#94

Post by azernik »

adrianpike wrote: post #27

Can someone more familiar with orbital mechanics help me understand why it looks like the perigree is coming down in opposite to the apogee? I'm but a mere KSP player, but I've always thought the perigee comes down in a fairly linear fashion based on drag, which is mostly a function of altitude - I would assume they'd be using the sail to just "burn prograde" whenever the solar angle is possible, raising both the apo…

Second paragraph:

"The perigee, or low point of its orbit, has dropped by a similar amount, which is consistent with pre-flight expectations for the effects of atmospheric drag on the spacecraft."

Basically, they're using the sail to boost their apogee while at the same time drag is bringing the orbit as a whole down.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#95

Post by tfolbrecht »

umvi wrote: post #2

Very cool! I know nothing about solar sails, but I wonder if the craft could have LEDs mounted to it so it could bombard different regions of the sail with photons to make it turn appropriately.

I'm curious if you could selectively turn off the reflective effect in different regions of the material and steer it that way.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#96

Post by azernik »

kmm wrote: post #63
hairytrog wrote: post #47

If a photon hits the sail and is reflected, it must transfer two photon momentums to the sail. The energy should be less than before it the sail. Is it redshifted?

Correct. Consider an initially stationary sail. Even though this isn't physically what is happening, you can imagine the reflection from the point of view of the sail as a photon getting absorbed and a photon of exactly the same frequency being emitted immediately after in the opposite direction. Now, to a stationary observer, the sail will have started moving after the absorption, so the emmitted photon will be reds…

Reflection can indeed be used! However, it's only useful for pushing apart two objects that you've built - e.g. a large station with a mirror and a laser, and a small probe with just a mirror.

This is called a photonic laser thruster (https://en.wikipedia.org/wiki/Photonic_laser_thruster) and is a variant of laser propulsion (https://en.wikipedia.org/wiki/Laser_propulsion). Like a mass driver, it gets around the rocket equation by separating the propulsion system from the spacecraft.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#97

Post by moron4hire »

Cogito wrote: post #88
moron4hire wrote: post #44

Earlier quoted context omitted.

Which can be done, because the sail is not just being pushed by the air, it's also redirecting the flow of air. The momentum of the fan pulling the air in does cancel any momentum imparted to the sail as the air pushes it, but there is also momentum gained by the air molecules rebounding back, away from the sail. It's (partially) the same reason you can sail perpendicular to the wind (the shape of the keel is also im…

> Sailing is not air pushing on a sail, it's air rebounding away from the sail. To add just a tiny amount of nuance - sailing is redirecting airflow with a sail. Surface and flow effects mean that very little air should be bouncing off the sail. If there is you probably have a lot of turbulence, and a 'bubble' will form around the sail around which most of the wind will simply divert, rather than redirect. Or to put…

Right, a sail has more in common with a wing than a parachute. The force that is propelling the sailboat is basically lift—turned sideways—not collision.

Which, to bring it back to the origin of this thread of armchair physicists incorrecting each other, means that an LED mounted on a solar sail would not work, because the only force it imparts is collision. Which makes the sail analogy kind of a bad one for the use of solar pressure as a driving force.

Edit: Hmmm, but what if the surface material of the sail were phosphorescent and could gain energy state from the solar pressure, then kick it a photon when dropping energy state? And then what if there were a fly wheel (magnetic axel, in the vacuum of space, extremely low friction) that could act as the keel... Hmmmmmmm

Edit 2: I wonder if a lens that refracted light similarly to how air flows over an airfoil would create a "solar lift" effect? Unfortunately, lenses are heavy.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#98

Post by foota »

yarg wrote: post #82
c3534l wrote: post #76

Earlier quoted context omitted.

Gravity is exponential, which makes lifting fuel off of Earth a losing strategy for long-distance travel. Anything that is fueled by something already in space seems incredibly promising.

Newton's approximation of gravitational force is: G = gMm/d^2 The variable here is d (the distance between the centres of mass) - gravity is inverse quadratic ( https://en.wikipedia.org/wiki/Inverse-square_law ), not exponential.

I suspect they were referring to the tyranny of the rocket equation.

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#99

Post by JshWright »

umvi wrote: post #2

Very cool! I know nothing about solar sails, but I wonder if the craft could have LEDs mounted to it so it could bombard different regions of the sail with photons to make it turn appropriately.

Changing orientation is easily done using spinning masses (this is how most satelites maintain their orientation)

Re: LightSail 2 Spacecraft Successfully Demonstrates Flight by Light

#100

Post by Skunkleton »

f5thesystem wrote: post #66
Skunkleton wrote: post #53

Earlier quoted context omitted.

Of course it would, but what wouldn't work is bouncing that CO2 off of some sail.

I would think a sail, or rudder, could redirect the force from the CO2 canister.

It could, but it would be significantly more efficient to just point the CO2 canister in a different direction.
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