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They're testing a solar sail concept with the idea of using it as an ultra-lightweight way to move around the solar system. It's been theorized for decades and

They're testing a solar sail concept with the idea of using it as an ultra-lightweight way to move around the solar system. It's been theorized for decades and this is the first attempt to actually put it up there in space. Lots more info here: http://sail.planetary.org/ http://sail.planetary.org/ edit: as others point out, this isn't quite the first time. but it is the first time in a long time!

mod 11y ago
Sweet! So this doesn't validate the theory, correct? (Is it actually validated yet?) Is this a viable method for long-distance traveling? Is it limited in direction (like the photons only push one way, and it's not like you can use a rudder in space, right?)
- ceejayoz 11y ago
  The theory is already validated (Mariner 10 used solar pressure for attitude control in the 1970s, and a Japanese mission successfully tested a solar sail in 2010 - http://en.wikipedia.org/wiki/IKAROS http://en.wikipedia.org/wiki/IKAROS - and it can be demonstrated in an Earthbound laboratory too). Yes, it's viable. http://en.wikipedia.org/wiki/Solar_sail http://en.wikipedia.org/wiki/Solar_sail
- jadell 11y ago
  It's not necessarily one way. Think of it like a sailboat: wind blows in one direction (at a time) but you can still steer, change direction, speed up, slow down, etc. just by changing the angle of your sail and how tense or slack it is. You can even travel against the direction of the wind. This is the same concept - a sail in wind - except the wind is solar wind: a stream of photons. The difference being that in sailing, the resistance of the water plays a part. For solar sails, you can't do as many complicated maneuvers as with sailing on water, but you can still make adjustments beyond juts being pushed in a single direction.
  - tedsanders 11y ago
    Just to clarify: Unlike a regular sailboat, you cannot sail against the wind with a solar sail. In space, there is no water equivalent to push off of.
    - SoftwareMaven 11y ago
      On the other hand, you have a constant accelleration towards the sun, so you can use parabolic orbits to move sunward (at least, until you reach Voyager I).
    - knodi123 11y ago
      acceleration can't move towards the sun, but you can build up some velocity with this, and then use a slingshot to head back to the sun. not very general-purpose, but sound in principal.
      - Tuna-Fish 11y ago
        To move towards the sun when orbiting it, you don't need to accelerate towards the sun, you need to accelerate towards your retrograde vector.
        TeMPOraL 11y ago
        Indeed. To explain the technical term, retrograde is the reverse of where you're going (where your velocity vector is pointing). Accelerating retrograde (against the direction of your movement) = dropping your current orbital velocity = when you "slingshot" to the other side, you'll be closer to the Sun. Orbital mechanics is fun :).
        knodi123 11y ago
        that's a good point, I forgot orbital mechanics. However, what's the ratio of the acceleration due to light pressure vs. due to gravity? If you are at rest 1 au from the sun, and you deploy your sail perpendicular to sunlight, which direction will you accelerate in?
- CydeWeys 11y ago
  To be clear, theory is being used here in the scientific sense, not in the colloquial sense of being a hypothesis. The science behind solar sails has been solid since Einstein's earlier discoveries (e.g. what he earned his first Nobel Prize for), when he discovered that light, despite having no mass, nevertheless does have momentum thanks to relativity. From there it's a straight shot towards realizing that you can make something move in space when it's hit by something carrying by momentum, exactly the same as a sail on Earth when it's hit by air particles carrying momentum. Basically it just became a problem of engineering at that point. Some important limitations to keep in mind with solar sails are that the momentum imparted on them tails off according to an inverse square law, and also that since they have no medium like water to push against (hence no rudder), you can't "tack into the wind" like a sailboat can. However, and this is a very important point, because of the way orbital mechanics works out, and assuming you can adjust your orientation of the sail on the fly, this matters less than you might think, e.g. imagine orienting your sail at apoapsis so that you create a net thrust in the retrograde direction, then you furl or orientate your sail edge-on to the Sun. You end up with an orbit that has a periapsis closer to the Sun (with an apoapsis further out), so you're still traveling inward in some sense, at least for part of your orbit.
  - hyperpallium 11y ago
    Niven and Pournelle's The Mote in God's Eye suggested something to push against: if you're in a magnetic field, you can turn by giving your craft an electric charge.
    - darkmighty 11y ago
      You can do better: you can separate an electric dipole across the direction you're travelling to create torque T=d v q B (since you can't create charge due to conservation laws).
david-given 11y ago
First attempt? http://www.isas.jaxa.jp/e/enterp/missions/ikaros/index.shtml http://www.isas.jaxa.jp/e/enterp/missions/ikaros/index.shtml IKAROS, launched by JAXA in 2010, passed Venus six months later in is still accelerating (slowly; it's getting about 1mN of thrust). It's even steerable, by using LCD panels in the sails to change the albedo. It was last contacted early this year. IKAROS' sail is 20m diagonal, which makes it considerably larger than LightSail1. There are even pictures of it --- it ejected a camera after deployment. http://www.space.com/25800-ikaros-solar-sail.html http://www.space.com/25800-ikaros-solar-sail.html
- devindotcom 11y ago
  totally forgot about ikaros. you're completely right, of course. Lightsail is a smaller, cheaper, privately (and crowd) funded solar sail project.
  - fapjacks 11y ago
    How long ago was Ikaros designed? I've been a member of the Planetary Society since I was a child and this has been talked about since at least the early 90s, iirc. This has been a major goal of the Planetary Society for years and years and years, finally overcoming the setbacks with a successful crowdfund. I don't doubt Ikaros may have been conceived much earlier, considering how spacecraft design takes time.

mod 11y ago

Sweet! So this doesn't validate the theory, correct? (Is it actually validated yet?) Is this a viable method for long-distance traveling? Is it limited in direction (like the photons only push one way, and it's not like you can use a rudder in space, right?)

ceejayoz 11y ago
The theory is already validated (Mariner 10 used solar pressure for attitude control in the 1970s, and a Japanese mission successfully tested a solar sail in 2010 - http://en.wikipedia.org/wiki/IKAROS http://en.wikipedia.org/wiki/IKAROS - and it can be demonstrated in an Earthbound laboratory too). Yes, it's viable. http://en.wikipedia.org/wiki/Solar_sail http://en.wikipedia.org/wiki/Solar_sail
jadell 11y ago
It's not necessarily one way. Think of it like a sailboat: wind blows in one direction (at a time) but you can still steer, change direction, speed up, slow down, etc. just by changing the angle of your sail and how tense or slack it is. You can even travel against the direction of the wind. This is the same concept - a sail in wind - except the wind is solar wind: a stream of photons. The difference being that in sailing, the resistance of the water plays a part. For solar sails, you can't do as many complicated maneuvers as with sailing on water, but you can still make adjustments beyond juts being pushed in a single direction.
- tedsanders 11y ago
  Just to clarify: Unlike a regular sailboat, you cannot sail against the wind with a solar sail. In space, there is no water equivalent to push off of.
  - SoftwareMaven 11y ago
    On the other hand, you have a constant accelleration towards the sun, so you can use parabolic orbits to move sunward (at least, until you reach Voyager I).
  - knodi123 11y ago
    acceleration can't move towards the sun, but you can build up some velocity with this, and then use a slingshot to head back to the sun. not very general-purpose, but sound in principal.
    - Tuna-Fish 11y ago
      To move towards the sun when orbiting it, you don't need to accelerate towards the sun, you need to accelerate towards your retrograde vector.
      - TeMPOraL 11y ago
        Indeed. To explain the technical term, retrograde is the reverse of where you're going (where your velocity vector is pointing). Accelerating retrograde (against the direction of your movement) = dropping your current orbital velocity = when you "slingshot" to the other side, you'll be closer to the Sun. Orbital mechanics is fun :).
      - knodi123 11y ago
        that's a good point, I forgot orbital mechanics. However, what's the ratio of the acceleration due to light pressure vs. due to gravity? If you are at rest 1 au from the sun, and you deploy your sail perpendicular to sunlight, which direction will you accelerate in?
CydeWeys 11y ago
To be clear, theory is being used here in the scientific sense, not in the colloquial sense of being a hypothesis. The science behind solar sails has been solid since Einstein's earlier discoveries (e.g. what he earned his first Nobel Prize for), when he discovered that light, despite having no mass, nevertheless does have momentum thanks to relativity. From there it's a straight shot towards realizing that you can make something move in space when it's hit by something carrying by momentum, exactly the same as a sail on Earth when it's hit by air particles carrying momentum. Basically it just became a problem of engineering at that point. Some important limitations to keep in mind with solar sails are that the momentum imparted on them tails off according to an inverse square law, and also that since they have no medium like water to push against (hence no rudder), you can't "tack into the wind" like a sailboat can. However, and this is a very important point, because of the way orbital mechanics works out, and assuming you can adjust your orientation of the sail on the fly, this matters less than you might think, e.g. imagine orienting your sail at apoapsis so that you create a net thrust in the retrograde direction, then you furl or orientate your sail edge-on to the Sun. You end up with an orbit that has a periapsis closer to the Sun (with an apoapsis further out), so you're still traveling inward in some sense, at least for part of your orbit.
- hyperpallium 11y ago
  Niven and Pournelle's The Mote in God's Eye suggested something to push against: if you're in a magnetic field, you can turn by giving your craft an electric charge.
  - darkmighty 11y ago
    You can do better: you can separate an electric dipole across the direction you're travelling to create torque T=d v q B (since you can't create charge due to conservation laws).

david-given 11y ago

First attempt? http://www.isas.jaxa.jp/e/enterp/missions/ikaros/index.shtml http://www.isas.jaxa.jp/e/enterp/missions/ikaros/index.shtml IKAROS, launched by JAXA in 2010, passed Venus six months later in is still accelerating (slowly; it's getting about 1mN of thrust). It's even steerable, by using LCD panels in the sails to change the albedo. It was last contacted early this year. IKAROS' sail is 20m diagonal, which makes it considerably larger than LightSail1. There are even pictures of it --- it ejected a camera after deployment. http://www.space.com/25800-ikaros-solar-sail.html http://www.space.com/25800-ikaros-solar-sail.html

devindotcom 11y ago
totally forgot about ikaros. you're completely right, of course. Lightsail is a smaller, cheaper, privately (and crowd) funded solar sail project.
- fapjacks 11y ago
  How long ago was Ikaros designed? I've been a member of the Planetary Society since I was a child and this has been talked about since at least the early 90s, iirc. This has been a major goal of the Planetary Society for years and years and years, finally overcoming the setbacks with a successful crowdfund. I don't doubt Ikaros may have been conceived much earlier, considering how spacecraft design takes time.