Comment: I’ve been excited by LightSail 2 since before its launch. My interest in solar sails was sparked by Arther C. Clarke’s “The Sunjammer” when it appeared in Boys’ Life in 1964. The fact that LightSail 2 was the work of The Planetary Society done on the cheap with donations is just icing on the cake. For the last 30 months LightSail 2 has shown the world how to sail in space. NASA has learned a great deal in preparation for their future solar sail missions.
NEA Scout will be launched whenever the Artemis-1 Orion is launched to orbit the Moon. From near the Moon, NEA Scout will set sail towards a still to be determined near Earth asteroid. This uncertainty gives a “second star to the right and straight on ’til morning” air of adventure to the mission. This uncertainty is only tolerable because solar sails won’t run out of fuel. If NEA Scout ends up having to circle around to rendezvous with the asteroid, so be it. It might even take in more asteroids over its lifetime.
The Solar Cruiser will be the biggest sail yet, about a third the size of a football field. This craft should launch in 2025 or so and is destined to hover in an orbit staying on the Sun-Earth line just on the far side of the Sun-Earth Lagrange Point. A conventionally powered spacecraft would have to expend a lot of fuel to maintain that position. The Solar Cruiser can stay there indefinitely.
I finally found a simple description of how these solar sails are controlled on an episode of Planetary Radio where Les Johnson, NASA’s Principal Investigator for the NEA Scout and Solar Cruiser, explains how it’s done. Much like sailing a windsurfer, you change the center of effort of the sail in relation to the center of mass of the board. With a solar sail, you shift the mass of the craft in relation to the center of effort of the sail. One method is to physically adjust the location of the central part of the craft along the x and y axes. Another way, just developed by NASA, amounts to scandalizing a specific area of the sail with photovoltaic panels on the sail that can adjust the amount of light that is passed through without imparting momentum to that part of the sail. Japan came up with this concept for their pioneering IKAROS solar sail launched in 2010. Japan would not share the technology with NASA.
I think we’re going to see a lot more of these solar sails in space exploration, especially as we move towards more multi-year and multi-decade interstellar missions. It will be a new age of sail… and just as adventurous and exciting.
I’m sceptical that a sailing craft immersed in the solar wind can extract a force which will allow it to navigate in a direction other than that in which it is being ‘blown’. However, that prejudice aside, the exciting factor in this exercise is that Earthly water-borne sailing craft can exceed the speed of the wind which is driving them by a considerable margin; notably catamarans sailing a course perpendicular to the wind.
I’m not sure of the physics of this at all, but LightSail 2 has been in orbit for 30 months and has been able to move to higher orbits during that time. And that’s with reportedly crude attitude control. Sure it also takes advantage of gravitational and centrifugal/centripetal forces, but somehow it works.
A windsurfer has a lot of force of friction from water slowing it down when you make a direction change. You’re not going to have that in space.
True. The vacuum of space offers no resistance to a centerboard or fin. Nor does Bernoulli’s principle apply in space. My best guess is that a solar sail is angled to the direction of the solar wind and, as the photons are banked off the angled sail, momentum is imparted to sail in relation to that angle. The similarity between a solar sail powered craft and a windsurfer lies in changing the relationship between the sail’s center of effort and the center of mass for the solar craft and the center of resistance for a windsurfer.
Yes, but you’ll still be moving at the same velocity in the prior direction.
LightSail 2 has been in a changing elliptical orbit of the Earth for 30 months. That’s a lot of changing direction even though a lot of that can be attributed to the Earth’s gravity. Changing the angle the solar wind hits the sail changes the direction of the momentum imparted to the craft by the photons. Speed will also increase as long as the solar winds hit the sail. Solar sails are the fastest means of space propulsion we have over long distances and time.
If the spacecraft is in a gravity field it is going to be able to tack and gybe. If there is no gravity field then I think its only going to be able to gybe and travel “down wind”, but in a gravity field it should be able to beam reach. I don’t know if it can travel against the solar wind but it may be possible if it trades some velocity and slows down. I’ll see if I can find another scientific paper somewhere.
Thanks, Walrus. That’s pretty much what I was thinking. I knew they were more maneuverable than just running down the solar wind.
After submitting my comment at November 21, 2021 at 12:44 am, I searched the www for information on “sailing faster than the wind” seeking information on how much faster than the wind it’s possible to sail, which produced this link:
It includes contraptions which sail on land and ice and can exceed the speed of the wind by a factor of 4+. However, all Earthly contraptions rely on contact with land or water to resist the tendency for the wind to push them off course.
For this reason, and the fact that Gravity is a very weak force compared to physical contact, I speculate that, in the absence of a stabilising medium, a solar sail will make very slow progress through Space with very limited ability to sail into the wind. I further speculate that had the sponsors of LightSail launched it into the null gravity region between Earth and the Sun they would already have access to its true Deep Space performance envelope.
But I hope I’m mistaken.
LightSail 2 was done on the cheap with private donations. For what it is, it’s been wildly successful. NASA’s NEA Scout will be a real demonstration of the technology. Although acceleration of a solar sail is slow, it is constant. They are supposed to reach 10% the speed of light, certainly not faster than the solar winds, but faster than what conventionally fueled spacecraft are capable of achieving.
For wind sailing here on Earth, I think iceboats are the thing to beat. Although not near as fast, I’ve had my 7.2 meter windsurfing sail, almost a rigid airfoil, planing into the wind and it sure seemed like I was approaching the speed of light. Exhilarating.
Thank you for the helpful and optimistic feedback.
Given NASA’s established expertise in planning and executing multi-destination space missions, and the Wiki page on their NEA Scout, I’m preparing to abandon my LightSail scepticism. I’m confident that NASA’s boffins are more than capable of accurately assessing its potential and limitations.
I note that Wiki’s NASA NEA page mentions several aids to optimising and fine-tuning a LightSail’s configuration/orientation including a cold gas thruster and some rotating weights which, I imagine, would help to correct minor negative action/reaction outcomes during reconfiguration events.
Your rigid airfoil windsurfer sounds more like a hotrod than an aid to relaxation 🙂
The key to sailing faster than the wind is Bernoulli’s effect. Drag from the ground is not the key. The best example I know of is the Blackbird project, in which a couple of engineers sought to settle a bet that it was possible, using a propeller as a sail, to exceed the speed of the wind while going dead downwind.
Most unfortunate that Bernoulli is irrelevant in a vacuum, but the elimination of drag makes him unnecessary.
LightSail is feasible proven technology since JPL experiments in the 70’s and since then has involved into many practical devices as you describe. Check out https://en.wikipedia.org/wiki/Solar_sail for very useful information including design calculations, materials, maneuverability (including tacking) and operating, completed, and proposed projects. This stuff is real.
On the other hand, please check out this startup company from Long Beach CA. https://www.spinlaunch.com/. There are plenty of 3d visualizations and even a real demo to persuade investors. It all looks very promising…
Until you examine some finer details of this scheme. For starters, ignore the problems with large vacuum tanks, accurate release mechanism, high centripetal g forces, unbalanced rotors etc. as these are possibly solvable. What is not solvable is the density of air at low altitudes and its associated drag on the projectile. For physics enthusiasts drag is proportional to velocity squared times cross section area (A) and a drag coefficient (Cd -that’s related to streamlined shapes).
The Spin Launch device works similarly to large artillery piece e.g., a M777 with its high muzzle velocity and aerodynamic shell. I’m sure there are some ‘arties’ reading this blog and If so, they may be able to drag out their calculus textbooks very quickly prove that scaling up a BFG will not make the projectile go and higher and further. In other words, drag will always get the better of you with its v^2 relationship. I look forward to being proven wrong here but technically I think this is an investor scam although maybe not criminally illegal like Theranos.
Sorry for butting in on your LightSail article TGG, but my scam-buster seemed more suited to your realistic space vehicle project article than the recent open thread.
p.s. Spin Launch work perfectly on the moon.
That Spin Launch reminds me of Jules Verne’s “From the Earth to the Moon.” It does seem limited to small craft, but Cubesat style craft may be perfectly suited for this. Maybe it’ll get something to orbit or maybe it won’t, but it strikes me more as a risky, wild eyed dream rather than a scam. Spin Launch on the Moon sounds like the launching of Moon ore to Earth in Heinlein’s “The Moon is a Harsh Mistress.” These Spin Launch folks must be avid readers.