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LightSail 2 Spacecraft Successfully Demonstrates Flight By Light (planetary.org) 87

According to The Planetary Society, LightSail 2 has successfully raised its orbit using solar sailing, making it the first small spacecraft to demonstrate the concept. From the report: Since unfurling the spacecraft's silver solar sail last week, mission managers have been optimizing the way the spacecraft orients itself during solar sailing. After a few tweaks, LightSail 2 began raising its orbit around the Earth. 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. The mission team has confirmed the apogee increase can only be attributed to solar sailing, meaning LightSail 2 has successfully completed its primary goal of demonstrating flight by light for CubeSats.

The milestone makes LightSail 2 the first spacecraft to use solar sailing for propulsion in Earth orbit, the first small spacecraft to demonstrate solar sailing, and just the second-ever solar sail spacecraft to successfully fly, following Japan's IKAROS, which launched in 2010. LightSail 2 is also the first crowdfunded spacecraft to successfully demonstrate a new form of propulsion. The mission team will continue raising LightSail 2's orbit for roughly a month, until the perigee decreases to the point where atmospheric drag overcomes the thrust from solar sailing. During the orbit-raising period, the team will continue optimizing the performance of the solar sail.

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LightSail 2 Spacecraft Successfully Demonstrates Flight By Light

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  • The mission team will continue raising LightSail 2's orbit for roughly a month, until the perigee decreases to the point where atmospheric drag overcomes the thrust from solar sailing.

    It shouldn't take too much atmosphere for the atmospheric drag to come into action if the sail remains deployed or, do they fold it in at every pass? Folding/de-folding at every pass seems unlikely to me.
    https://en.wikipedia.org/wiki/... [wikipedia.org]

    To achieve its final "solar sailing" configuration, LightSail extends four 4-meter cobalt alloy booms that slowly spread open the mylar sail material.

    • by Anonymous Coward

      Couldn't you rotate the sail so it acts as a wing and boost at apogee to keep the orbit from decaying? Eventually you could achieve escape velocity and go into a heliocentric orbit. This could be useful for interplanetary repeater satellites to get around obstructions.

      • by EricTheRed ( 5613 ) on Thursday August 01, 2019 @04:54AM (#59021956) Homepage

        That's exactly what they do, using the momentum wheel to change it's attitude so the sail is only used when it can get the benefit of those photons.

        The downside is that although the apogee is risen from 725km to 727km, the perigee has been lowered from 710km to 707km.

        This is the reason that the mission duration is just about 1 month. This is the first week & it's proved it, the rest of August will be to see how far they can go. After that it will then begin to deorbit which will last about a year.

      • Couldn't you rotate the sail so it acts as a wing and boost at apogee to keep the orbit from decaying?

        How is a wing going to help when you have nothing to generate lift from?

    • "It shouldn't take too much atmosphere for the atmospheric drag to come into action if the sail remains deployed or, do they fold it in at every pass? Folding/de-folding at every pass seems unlikely to me."

      Sailors have been tacking for centuries.

  • or maybe straight to another solar system where them aliens might think that the US is now starting to dump their trash in other galaxies because they can't ship it to China anymore.
    • by Antique Geekmeister ( 740220 ) on Thursday August 01, 2019 @05:44AM (#59022046)

      I've long been an advocate of using solar mirrors to collect and focus solar radiation to Earth for power, and of solar sails to collect icy asteroids or icy ring material from Jupiter to harvest for water and metals in Earth orbit. It should be much safer and is potentially more economical than Lunar harvesting or shipping water and material to orbit for space stations and spacecraft.

      • by Anonymous Coward

        That sounds like a great way to prolong global warming. The problem with global warming isn't the greenhouse gasses per se, it's that the carbon is causing Earth's atmosphere to retain more heat. The problem with space-based solar power is that it increases the amount of energy the Earth is receiving from the sun. Eventually, that extra energy *will* find its way into the environment, because of thermodynamics. You might end up with less greenhouse gasses emitted with SBSP, but now you're warming up the pla

      • I've long been an advocate of using solar mirrors to collect and focus solar radiation to Earth for power, and of solar sails to collect icy asteroids or icy ring material from Jupiter to harvest for water and metals in Earth orbit.

        How about warp drives powered by unicorn farts? You are talking science fiction. How about we worry about figuring out what solar sails can actually do in reality before we go running off to Jupiter for some ill defined scheme to mine who knows what for undetermined purposes requiring technology we don't have for a space economy that doesn't exist.

        • It's hardly "warp drive". It's a matter of scaling up tested engineering, not inventing new physics for utter science fiction. It's also likely to be a far smaller investment, with well defined benefits, than a Lunar base that has to produce water, and aluminum.

  • Now let's make those solar sails reflective and able to block some sunlight from entering the atmosphere. We could actually provide a lot of cooling if solar mirrors are put in place.
    • It's theoretically possible, though expensive. Simply replacing fossil fuel sources with solar power beamed from solar mirrors could be very helpful.

      • Eew, no! You put the panels in orbit and beam microwaves (or whatever) down to a receiver. Mirrors would just act like furnaces.
        • Most designs reflect the power to a microwave transmitter, which can then beam microwaves to a large scale ground array. The approach is described in numerous NASA papers, and in the "Fallen Angels" storay by Larry Niven and Jerry Pournelle. I admit that I left out mentioning that part as adding complexity to the explanation.

    • Now let's make those solar sails reflective and able to block some sunlight from entering the atmosphere. We could actually provide a lot of cooling if solar mirrors are put in place.

      How many of these things do you want up there? To block/reflect any appreciable amount you are going to need A LOT of area.

      • Here is an article by an astronomer who proposed something like this - using a swarm of satellites using solar-sails to move the the L1 point and perform station keeping; he estimates it would need about 20 million tons worth of satellites. If Space X's Starship works as planned this ends up being within the realm of possibility. You would need 200,000 Starship launches, or 10k a year for 20 years. Huge by today's space flight standards, though trivial next to the number of commercial aircraft (there are
    • Now let's make those solar sails reflective and able to block some sunlight from entering the atmosphere. We could actually provide a lot of cooling if solar mirrors are put in place.

      Oh yes let's put a bunch of space junk into orbit to block sunlight without any clear idea what the actual results would be or what specific problem it would solve. That sounds like a great idea. #sarcasm

      You engineer the world climate at our collective peril. We only have one Earth and if we get it wrong we are fucked. Let's not get too excited about geo-engineering our entire planet before we really know WTF we are doing. We're already doing enough damage as it is thanks to our technology exceeding o

      • Oh yes let's put a bunch of space junk into orbit to block sunlight without any clear idea what the actual results would be or what specific problem it would solve. That sounds like a great idea. #sarcasm

        True, but if things actually do get undeniably bad and look to get worse real quick, launching solar shades would be a rather straight forward engineering solution that wouldn't require the fight to get people/countries to change their life styles (get off oil and coal). Even then, the point which things could changed by lifestyle changes would probably be gone since the lag between entering CO2 and max heat retention due to it is many years*. Many thousands of square kilometers of shade could be placed in

        • Doing some quick math. Putting enough mylar in LEO to block out 1% of the sun would take an estimated 44,000 FBR/Falcon Super Heavy launches which would be about $4 trillion in expected launch costs alone.
  • by mpercy ( 1085347 ) on Thursday August 01, 2019 @06:10AM (#59022118)

    And Murcheson's Eye?

    • 2450 Jasper Murcheson explores the region beyond the Coal Sack
      2862 Coherent light from the Mote reaches New Scotland

  • then how is the perigee lowering until it encounters atmospheric drag? When my Kerbals raise their orbit, they get further and further away from the atmosphere.
    • by lgw ( 121541 )

      Unlike Kerbin, Earth's atmosphere doesn't have a sharp cutoff where drag drops to 0. At 50 miles/80 km perigee, you're "in space" in the sense that atmospheric drag is low enough that lightweight station keeping thrusters can keep your satellite up, but there's still non-trivial drag. Even at 200 km a satellite won't stay up very long without a little boost now and then, but you're starting to get to where thrusters are more for dodging debris than boosting against drag.

      Unless this solar sail can increase

      • by vrt3 ( 62368 )

        The perigee initially was 710 km, and has decreased to 707 km. I'm surprised the atmosphere has enough density at that height to accomplish that.

        • by lgw ( 121541 )

          Looking at their graph of their orbit over time, it seems like they're just getting most of their thrust far away from perigee, and so they're lowering perigee by about the same amount they're raising apogee.

          Practically, that doesn't accomplish much. It does demonstrate that they're actually getting thrust from the solar sail, no doubt about that, but thus far not enough to be very useful. You need enough thrust in the relatively small window near perigee to accomplish something.

          Still, it's a step. Hopefu

          • > Practically, that doesn't accomplish much. It does demonstrate that they're actually getting thrust from the solar sail

            It seems valuable, to me, to work out solar sail mechanics in orbit, where tracking and telemetry are rather easy.

            Perfect it here, then send one proven-reliable craft out to the planets or the stars with confidence.

            • by lgw ( 121541 )

              Yes, you've just described the difference between "research" and "practical". Hopefully the one leads to the other.

        • by Anonymous Coward on Thursday August 01, 2019 @09:16AM (#59022750)

          The perigee initially was 710 km, and has decreased to 707 km. I'm surprised the atmosphere has enough density at that height to accomplish that.

          The thing about orbits is in where you input the energy. I suspect the perigree is along the side of the planet that they are boosting the energy with the sun behind the vector the craft is following. This will raise the orbit on the apogee side (in this case the side where the craft is heading toward the sun). But the rub is that when heading toward the sun, they are turning the sail to the side to reduce the effect of the sun yet not eliminating it. This means some thrust is being applied against the vector and is assisting the drag to lower the perigree. I think *both* effects are showing that the lightsail is effective.

          The next challenge is how to use a craft with a lightsail to boost the whole orbit. Charged ionic particles are sticky. Turning the sail to sort of tack doesn't work like we want. The particles hit, transfer momentum in a vector directly away from the sun, bounce a small bit as they are drastically slowed from the collision and apply a tiny amount of force against the vector they were directed to(this part is actually similar in many ways to wind). On a sailing vessel we use a neat trick called a keel to apply a lifting body under a dense fluid and convert the force the wind applied into a drag against the dense fluid and that is what lets us vector the force in the direction we want.

          Lightsails are waiting for space keels to be invented.

      • So they are actually just making the orbit more elliptical. I don't see how that is considered raising it, when the other end is lowering. It's like taking two inches off of a ruler to add to the other end and claiming you just made the ruler longer.
  • It seems they plan to deorbit the spacecraft after their tests are complete.
    Does anyone know why they don't just try to sail out of earth's gravity well? It would surely take a very very long time, but since the sail doesn't use fuel...
    • It doesn’t have enough power. The cool thing they demonstrated was that you could use a sail near a planet and maneuver it... but it’s not adding energy fast enough to head out.
      • But why would it have to add energy 'fast enough'? They could just veeeeeery gradually raise their orbit arbitrary high, no?
        • No, you can't just keep raising orbit to escape. I probably won't do a great job explaining this but I will give it a try. If you don't "get it" after this the problem is probably my explanation.

          When Lightsail raises its orbit it actually slows down. Orbital mechanics aren't the most intuitive thing in the world. However as the orbit gets higher and the orbital velocity lowers the escape velocity lowers. Remember, it's still 1.4x your current velocity, so if you orbital speed drops from 7402m/s to 7400

    • Getting out of Earth's gravity well is going to require hitting escape velocity which is always 1.4x (square root of 2 to be exact) your current orbital velocity. For this cubesat the orbital velocity is probably around 7500m/s (figure 7200-7600 is the range, not sure what it's altitude is) so you need to kick your velocity up to around 10,500m/s to get out.

      That, uh... not happening with solar power.

      To deorbit it you just wait for the little bit of atmosphere out there to slow it down enough to bring it ba

  • by n2hightech ( 1170183 ) on Thursday August 01, 2019 @09:03AM (#59022684)
    Orbital energy is proportional to (perigee + apogee)/2. Apogee increased from 725 to 727 a gain of 2. Perigee however decreased from 710 to 707 a loss of 3. So the total energy of the satellite has decreased. For solar sails to be a benefit they must be able to increase the orbital energy of a spacecraft. This is not happening. The question then is the solar sail decreasing the decay rate due to atmospheric drag? No information was shared about what the atmospheric drag was doing to the orbit before solar sail deployment and after deployment. It kind of looks like the atmospheric drag on a deployed sail is larger than the thrust the sail produces. Again not what I would call a successful demonstration of the technology.
    • by pi_rules ( 123171 ) on Thursday August 01, 2019 @09:29AM (#59022808)

      I'd call it a successful demonstration. They raised apogee just with solar pressure; that's all they really wanted to show. If they were able to kick the thing out into an orbit far enough away from Earth that the atmospheric drag wasn't overcoming the total energy produced by the solar sale then they could stay up indefinitely, or at least as long as the cubesat still functioned.

      But getting that far into orbit is expensive, and they had to piggy back on other experiments just to get where they ended up.

      I can't help but wonder if this would be useful for cubesats in lunar orbit. There's no atmospheric drag there (or at least much much less than near Earth) and the sucker is "lumpy" mass-wise so things don't like to stay in orbit there. A little solar boost might solve that problem. Maybe. I'm sure people smarter than me have tried working through it.

      • Full disclosure/virtue signalling. I donate a little money (employer match!) to the Planetary Society in support of projects like this. I like the fact that there is an org that will pursue some basic, applied, science that hasn't been fully fleshed out by the national space administrations. Japan has done little solar sail work, the US has, and the Planetary Society is adding to this knowledge and experience.

        I would love to see them try and pursue some of the laser propulsion type experiments as wel

    • by ath1901 ( 1570281 ) on Thursday August 01, 2019 @09:56AM (#59022954)

      Drag alone can not raise the apogee, only lower it since the force is always in the retrograde direction. The raised apogee is proof that the solar sail has produced thrust even if the total orbital energy has decreased. It means the sail works even if it is not enough to make the craft stay afloat. The test will probably give them lots of data which they can use to verify the sail works according to calculations or perhaps find unexpected side effects. Since reality doesn't have a quick-save/load button it is common practice to build small scale test vehicles before building the real thing.

      As long as the sail works, atmospheric drag can be reduced by just putting the craft on a bigger rocket and sending it into a higher starting orbit.

    • by Toad-san ( 64810 )

      Thanks for the math: that makes a lot of sense. But I think they didn't prove much if they have a net energy loss. Why didn't they position the thing to increase orbital speed, thus moving the entire orbit further out? THAT would be the real proof.

  • (get it?) ...but seriously, I'm curious how well its performed according to theory and where they have explainable variations or unexplainable variations.

  • So we've established that photons pushing against (or bouncing off?) another object produce a force. Could it be that all the photons emitted from all of the stars in the universe hitting other space bodies is, at least partly, responsible for the expansion of the universe?

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