Is There Any Way To Slow Down A Solar Sail?

I recently read an article about a project called Breakthrough Starshot. It’s been proposed to send a tiny craft to Alpha Centauri at 20% the speed of light and the craft(s) would arrive in 20 years or so. Let’s say that there’s been a successful launch and the tiny ship is on it’s way. Would Alpha Centauri have enough gravitational pull to capture an object moving that fast or would it be a one time fly by like New Horizons and Pluto?
This image shows the closest stellar system to the Sun, the bright double star Alpha Centauri AB and its distant and faint companion Proxima Centauri. In late 2016 ESO signed an agreement with the Breakthrough Initiatives to adapt the VLT instrumentation to conduct a search for planets in the Alpha Centauri system. Such planets could be the targets for an eventual launch of miniature space probes by the Breakthrough Starshot Initiative. Image Credit: ESO/B. Tafreshi (twanight.org)/Digitized Sky Survey 2 Acknowledgement: Davide De Martin/Mahdi Zamani

This image shows the closest stellar system to the Sun, the bright double star Alpha Centauri AB and its distant and faint companion Proxima Centauri. In late 2016 ESO signed an agreement with the Breakthrough Initiatives to adapt the VLT instrumentation to conduct a search for planets in the Alpha Centauri system. Such planets could be the targets for an eventual launch of miniature space probes by the Breakthrough Starshot Initiative. Image Credit: ESO/B. Tafreshi (twanight.org)/Digitized Sky Survey 2 Acknowledgement: Davide De Martin/Mahdi Zamani

Originally posted on Forbes!

You’ve got a pretty good handle on Breakthrough Starshot - their goal is indeed to ship off a tiny little craft, attached to a huge solar sail, and use high powered lasers to accelerate the craft to a speed much faster than what the power of the sun could do alone.

The spacecraft itself will have no thrusters on it - the whole point is to keep the spacecraft as light as possible so it’s easier to accelerate to relativistic speeds. The more massive your object, the more energy it takes in order to increase its speed, and getting any object up to fractions of the speed of light is difficult under any circumstances. The strategy is to make a very light, thin (but large) sail, so that a very powerful laser can bounce off of it, gradually pushing the sail along faster and faster. The actual science instruments would be relatively tiny, suspended in the very middle of the sail, weighing as little as possible, while still being able to do the science required. (We are a significant technological distance from being able to do accomplish any kind of interstellar solar sail.)

The complete lack of thrusters, plus the single direction we can push from, does mean that once the spacecraft gets up to 20% of the speed of light, it’s not slowing down again unless it crashes into something. The gravitational pull of Alpha Centauri is certainly present, but as that star is only a little bit larger than our own Sun, it doesn’t have the kind of extreme gravitational pull you’d need to slow the craft down as it goes past. Not without stopping it violently, through a collision, anyhow. At best, you’d swing past the star, get a few good images, and then sling your spacecraft right on through the solar system and out the other side - just like New Horizons did for its flyby of Pluto.

Plutonian landscapes in twilight, under a hazy sky. Credit: NASA/JHU APL/SwRI

Plutonian landscapes in twilight, under a hazy sky. Credit: NASA/JHU APL/SwRI

However, New Horizons had a bit more control over itself coming into its Pluto encounter than any solar sail craft would. New Horizons does have thrusters- which meant it could make adjustments to its flight en route to Pluto, and that it could course correct into a good path to encounter another object, out beyond Pluto.

Using only gravitational arguments, any solar sail spacecraft is doomed to speed up and then pretty much continue to cruise at that speed. However - it’s possible that with a very careful calculation of the spacecraft’s trajectory, you might be able to slow down the spacecraft with another method. Stars very clearly have strong, non-gravitational influences over the region surrounding them, once you get up close. It’s not just our star that has a solar wind that could be used to propel a solar sail - every star has a solar wind.

This image is of a four-quadrant solar sail system, measuring 66 feet on each side that was tested in 2005 in the world's largest vacuum chamber at NASA's Glenn Research Center at Plum Brook Station in Sandusky, Ohio. Image Credit: NASA

This image is of a four-quadrant solar sail system, measuring 66 feet on each side that was tested in 2005 in the world's largest vacuum chamber at NASA's Glenn Research Center at Plum Brook Station in Sandusky, Ohio. Image Credit: NASA

A recent paper worked out how the solar wind of the star you’re heading towards could help slow down a solar sail, perhaps enough to send it into orbit around its destination star. There are a bunch of limitations to this approach - the biggest one being that solar sail has to be able to endure quite a rapid deceleration - if the sail shreds, you’re not stopping the craft. The sail also has to be relatively gigantic - their calculations rely on something about 315 meters to a side, which is much larger than anything we’ve currently built. (The most massive one currently underway is 50 meters to a side.)

The orbit also has to be pretty precisely known - according to this paper, if you come too close to the star, you destroy the solar sail and crash into the star. However, you’d still have to sail into the solar system veryvery, close to the star for the pressure from the star to slow down your spacecraft. It’s an extremely narrow range of allowable arrival positions, which the authors measure in solar radii. On an astronomical scale, this is tiny. You’re aiming to sail in to a solar system, so close to the star that you'd be about 20 times close to the Sun than Mercury, if you were arriving in our solar system. If your solar sail were a comet, this would put you in sungrazer territory.

The difficulty of managing this needle-threading exercise aside, there are still a number of technological problems - the calculation assumes that the solar sail is made of graphene, for its lightweight nature and relative strength. Graphene is not very reflective, though, which means that we'd have to get really good at coating extraordinarily thin layers of graphene with something reflective without making it brittle and prone to tearing. In principle, it's physically possible. It's just technologically improbable, for the moment.

Have your own question? Feel free to ask! Or submit your questions via the sidebarFacebook, or twitter.

Sign up for the mailing list for updates & news straight to your inbox!

Are There Any Planned Missions Using Solar Sails?

Are there any planned planetary missions using solar sails for propulsion?
This is a picture of the Sunjammer solar sail being tested, before the project was canceled. Image Credit: NASA/L'Garde

This is a picture of the Sunjammer solar sail being tested, before the project was canceled. Image Credit: NASA/L'Garde

Originally posted at Forbes!

There aren’t very many, but there are a few! I’ve poked around and found three major projects which are actively being worked on, though there have been more than that in the past, or which were tested on the ground, and never made it to space.  If you’re curious to learn about how solar sails work, exactly, check out this post which covered just that!

The solar sail project that might be most familiar to you is the LightSail, brought to you by the people at the Planetary Society. Because the Planetary Society is not a government agency, they’re free to fund their missions however they please, and the LightSail wound up being significantly funded by a Kickstarter campaign. The LightSail Kickstarter in 2015 coincided with their proof-of-concept solar sail deployment test; this was a quick orbit around the planet, just long enough to deploy it from its launch rocket, check that the sail could unfurl as expected, and grab a quick image of itself. The Kickstarter was a tremendous success, raising a million more than their goal of $200,000, meant to help pay for the last bit of unfunded construction and launch costs. Since the proof-of-concept craft worked almost perfectly (there were a few communication glitches), the Planetary Society is going forward with their first full-scale solar sail mission, which should leave the Earth’s atmosphere and hitch a ride on our Sun’s solar wind.

Artist's impression of a solar sail beginning its journey, accelerated (slowly) by the solar winds. Image credit: Kevin Gill, CC A-SA 2.0

Artist's impression of a solar sail beginning its journey, accelerated (slowly) by the solar winds. Image credit: Kevin Gill, CC A-SA 2.0

The LightSail 2, as the next one will be called, is scheduled to launch this year, on a SpaceX Falcon Heavy rocket. The LightSail is still in pretty early days, in terms of what kind of science it’s doing - the next launch is primarily to check that the light sail of its size works the way we think it should, and can successfully accelerate a small craft away from the Earth. As each incarnation of the LightSail succeeds, the scientific scope of the missions will increase- which makes sense, you don’t want to put an expensive scientific instrument on a craft if you’re still worried about the craft being able to spread its wings.

The most successful solar sail to date was launched in 2010, by the Japanese space agency JAXA. IKAROS, which stood for Interplanetary Kite-craft Accelerated by Radiation Of the Sun, was launched along with Akatsuki, their craft currently orbiting Venus. IKAROS successfully opened up, and formally operated as a light sail, gaining speed from the solar wind. IKAROS had a few science instruments aboard - a gamma ray burst detector and a dust particle counter. IKAROS was cleverly also equipped with small solar panels embedded in the sail, which provided power to the satellite. IKAROS surpassed its initial mission timeline, which was 6 months, and continued to communicate with Earth (with exceptions for its hibernation periods when the sunlight was too weak to power its instruments) until 2015.

The Japan Aerospace Exploration Agency (JAXA) successfully took images of the whole solar sail of the Small Solar Power Sail Demonstrator "IKAROS" after its deployment of a separation camera* on June 15 (Japan Standard Time, JST.) The IKAROS was launched on May 21, 2010 (JST) from the Tanegashima Space Center. Image credit: JAXA

The Japan Aerospace Exploration Agency (JAXA) successfully took images of the whole solar sail of the Small Solar Power Sail Demonstrator "IKAROS" after its deployment of a separation camera* on June 15 (Japan Standard Time, JST.) The IKAROS was launched on May 21, 2010 (JST) from the Tanegashima Space Center. Image credit: JAXA

JAXA is planning to launch a much bigger solar sail around 2020. This craft is scheduled to head off to investigate a set of asteroids which share an orbit with Jupiter, making a return trip with a small chunk of an asteroid sometime in 2050. Where IKAROS was 14 meters to a side (by no means small), this new solar sailboat will be 50 meters per side - more than three times the collecting area. JAXA recently showed off a full-sized model of one of its four wings, which took their volunteers a careful 10 minutes to unfold.

The last major ongoing solar sail project is Breakthrough Starshot, but it’s both the one which is aiming the highest, and the least far along in its progress to launch. They’re hoping to construct a solar sail which could be accelerated not just by the solar wind, but by high powered lasers, with the aim of getting a craft near Alpha Centauri in ~20 years. This is ambitious, to put it kindly, and the folks at Breakthrough Starshot are well aware. They have an entire page dedicated to major challenges that we don’t yet know how to solve.

As the technology develops for solar sails, they will become cheaper and easier to produce. I suspect we will start seeing more people attaching them to nanosats and cubesats which are relatively cheap to create and launch!

Have your own question? Feel free to ask! Or submit your questions via the sidebarFacebook, or twitter.

Sign up for the mailing list for updates & news straight to your inbox!

How Do Solar Sails Work?

I understand that a photon has no mass... so how can it be used to push a solar sail ? How can a “massless” thing transfer momentum?
A 20-meter solar sail and boom system, developed by ATK Space Systems of Goleta, Calif., is fully deployed during testing at NASA Glenn Research Center's Plum Brook facility in Sandusky, Ohio. Blue lights positioned beneath the system help illuminate the four triangular sail quadrants as they lie outstretched in Plum Brook's Space Power Facility -- the world's largest space environment simulation chamber. The sail material is supported by a series of coilable booms, which are extended via remote control from a central stowage container about the size of a suitcase, and is made of an aluminized, plastic-membrane material called CP-1. The material is produced under license by SRS Technologies of Huntsville, Ala. The deployment, part of a series of tests in April, is a critical milestone in the development of solar sail propulsion technology that could lead to more ambitious inner Solar System robotic exploration. Image credit: NASA

A 20-meter solar sail and boom system, developed by ATK Space Systems of Goleta, Calif., is fully deployed during testing at NASA Glenn Research Center's Plum Brook facility in Sandusky, Ohio. Blue lights positioned beneath the system help illuminate the four triangular sail quadrants as they lie outstretched in Plum Brook's Space Power Facility -- the world's largest space environment simulation chamber. The sail material is supported by a series of coilable booms, which are extended via remote control from a central stowage container about the size of a suitcase, and is made of an aluminized, plastic-membrane material called CP-1. The material is produced under license by SRS Technologies of Huntsville, Ala. The deployment, part of a series of tests in April, is a critical milestone in the development of solar sail propulsion technology that could lead to more ambitious inner Solar System robotic exploration. Image credit: NASA

Originally posted at Forbes!

This is not an easy thing to wrap one’s head around, and part of the reason it’s tricky is because light is a little special. Light manages to behave both like a particle and like a wave. In either case, you’re absolutely right that the particle of light has no mass.

To start to get a handle on how this works, let’s think about light as a wave, and ignore the photon-particle behavior. The way we break up the whole electromagnetic spectrum is by the amount of energy carried by that wave. The more energetic the wave is, the higher its frequency, and so dividing by frequency is just another way of slicing by energy levels. When these waves are absorbed by a surface, they will deposit the energy they carry in that surface. This is why we burn in sunlight; the ultraviolet, which we can’t see, carries a lot of energy with it, and that energy is deposited in our skin. Our skin doesn’t handle this energy dose very well, and so we wind up with a burn, as though we’d touched something hot. (Technically, we did! The sun.)

A partial reflective surface, reflecting light but at a weaker intensity than the incoming beam. Image credit: wikimedia user Zátonyi Sándor, CC BY A-SA 3.0

A partial reflective surface, reflecting light but at a weaker intensity than the incoming beam. Image credit: wikimedia user Zátonyi Sándor, CC BY A-SA 3.0

But what about if the surface doesn’t absorb the light, but reflects it, as a mirror does for visible light? In a perfect world, the wave is totally reflected off of the surface, and none of this transfer of energy to the surface happens. In practice, however, most materials are not perfectly reflective, and so the reflected wave has lost some of its energy to the mirror. In most cases, this energy loss is pretty small (otherwise it’s a terrible mirror), but if you’re in the business of trying to make a solar sail, this energy donation pushes the sail along a little bit.

If we go back to thinking about light as a particle, the light-particle must still carry energy. The light particle is a little weird, because it does manage to carry momentum, even without a rest mass. But there is an energy-momentum translation, even without mass, which is in play for photons. Now, if you think about a string of particles bouncing off of a surface, bouncy-ball style, those collisions also transfer a bit of energy into the surface. This energy transfer is giving a little bit of momentum to the surface, so if that surface is floating freely, as a solar sail does, you’ll slowly add speed to the sail.

It’s important to note that both the wave method and the particle method of thinking about light are totally equivalent, but in some cases it is simpler mentally or mathematically to think about light more as a particle or more as a wave. In this case, either method is a decent description of how light can propel a solar sail along.

Model of the Japanese interplanetary unmanned spacecraft IKAROS at the 61st International Astronautical Congress in Prague, Czech Republic. Image credit: Pavel Hrdlička, Wikipedia. CC A-SA 3.0

Model of the Japanese interplanetary unmanned spacecraft IKAROS at the 61st International Astronautical Congress in Prague, Czech Republic. Image credit: Pavel Hrdlička, Wikipedia. CC A-SA 3.0

The technical term for this gradual, tiny momentum-energy transfer is called radiation pressure, and this gradual pressure across the solar sail is what can propel it through the solar system. In any patch of space where there is strong radiation — a.k.a. a huge source of light, of any frequency — you can wind up with radiation pressure shaping the behavior of things around it.

Our sun isn’t even that extreme of an environment for radiation pressure! If it were more extreme, our solar sails might not need to be quite so large, as the pressure would be stronger and we wouldn’t need such a large surface area. The current solar sails have to be tens of meters on a side to be practical – the Japanese IKAROS light sail is a little over 150 feet along one edge, with an even larger one planned now that we know that this sort of technology is feasible!

Have your own question? Feel free to ask! Or submit your questions via the sidebar, Facebook, twitter, or Google+.

Sign up for the mailing list for updates & news straight to your inbox!