Love this. I'm happy to say I just talked to Bill Nye on Friday (LS was still AWOL) about this and a recent science competition for kids and I love his enthusiasm for everything. He thinks of it as carrying on work that Carl Sagan started. Just gonna hijack this thread to share a couple quotes from my notes that didn't make it into my article[1], since you guys might appreciate them:
"for me it's [lightsail is] really personal. i had carl sagan for one class, and it changed my life. and so i'm really excited to get this thing working. if not this mission, the next mission. oh god, the concept is just cool, to just be sailing around the solar system essentially for free. It's elegant."
"there are two questions we all ask, and if you meet somebody who claims they've never asked these questions they're lying and that's all there is to it: where did we come from and are we alone in the universe? if you want to answer those questions, you have to look at the past and the future. You have to look at paleontological evidence, and you have to explore space."
"talk to younger people. they expect great things from technology. that expectation i believe will carry over to every aspect of life - that we can do technological things to prevent climate change. that for any problem there's a technological solution."
"this is what we talk about all the time in science ed. we want science every day in every grade. this is obvious to me and my colleagues in science ed. what's amazing is it's not amazing to everybody. science is as important a topic as writing and reading and arithmetic. if you want to have innovation, if you want to have the engineers of tomorrow, you have to have science."
"kids have no trouble understanding climate change - it's grown-ups that are confused. climate change denial is almost entirely a generational issue."
"climate change denial is almost entirely a generational issue" ...
I do wonder if this is a US thing. I've not seen a denier over here in the UK. Sure, on the TV they have debates about climate change with scientists. They argue and present data and talk about change. They may argue about the rate of change and the potential effects. But everyone agrees that change is happening.
Just take a look at the G7 fossil fuel pledge by the Germans. That's pressure from everyday people over here in Europe, that's millions of us who understand.
I wonder if it is more a phrasing issue in American polls. I live in a small, rural, Southern town. I've had a few conversations, as well as watching American media. I think the term climate change or global warming means man-made warming in most people's head. Many people I've seen pressed on the issue, even by gentle conversation show this conflation. The deniers tend to be people saying, even through a veil, "I don't think man has everything to do with it".
Oddly, in the back of many people's head that I've spoken with is the Cambrian period. They have a vague remembrance that Earth was hotter and more swap like. Since there were no men then, it is possible for the planet to heat up on its own. They latch onto this thought and back their view that modern peoples can't be causing this much damage.
James Delingpole in the Telegraph is probably the leading UK denier. But there are quite a lot of "silent" denialists who won't make a big noise about climate itself but will attack environmental, energy or "green" projects on spurious grounds.
Global warming wouldn't really be bad for Poland, and CO2 tax is very bad for Polish economy, so there's more people choosing to belive it's all a scam.
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:
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.
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.
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?)
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 - and it can be demonstrated in an Earthbound laboratory too).
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.
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.
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).
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.
If you angle the sail to send light in front of you, you will speed up and fall towards the Sun. If you angle the sail to send light behind you, you will slow down and move away from the Sun. Aim to either side and you will move away from the light and also move up slightly. Between these effects you can manage to get from any orbit you might be in to any other possible orbit. It will just take a while.
This is all assuming that there is not enough force from reflecting the light back into the Sun to beat the Sun's gravity.
This is a kind of thing that is totally counterintuitive until you spend a few (dozen) hours in Kerbal Space Program. Basically keep sails angled at 45 degrees so that the light reflects in the direction you're moving, and you'll be getting closer and closer.
It wouldn't be called "the satellite paradox" if it did what you would naively expect. :-)
If you have 2 objects in the same orbit and one accelerates backwards, that one winds up below and in front of the first. This happens because accelerating backwards causes it to lose energy, so it falls, and in falling picks up more kinetic energy than it had before.
It depends on the frame of reference. Obviously we're talking about a solar reference frame here with respect to linear velocity (it's the only other object), but it will also be rotating.
If the reference frame is initially rotating at with your orbit or a closer one, "speed up" is accurate. If it's rotating with some more distant orbit, "slow down" is accurate.
This, by the way, is one reason why physicists tend to just say "accelerate" in both cases: then, it's technically true for any external reference frame.
There's gravity from the sun but not the planet, and yes, that ought to be worth adding. I'm not sure what'd help the most to turn this into a fun game, though, so I'm working on other things. Thanks for the suggestion; a time control's probably a good idea too.
Ah yes some kind of normalization for the sail's velocity would be cool. The whole systems dynamics gets a lot slower as you get away from the sun, so it gets frustrating to see what happens with those; and it is hard to execute manuevers near the sun. (Maybe normalize for constant angular speed and display e.g. 'Time at 3.5x'?)
You can angle the sail to be close to your retrograde vector, which will allow you to decrease your orbital altitude around the sun. You could use this to move closer to the sun or to the inner planets (or to return from a distant mission).
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).
Are you sure about this? If you have a sail in a heliocentric orbit, I would think you could increase or decrease your orbital speed based on the direction you deflect the solar wind, thus slowly increasing/decreasing altitude. I would assume it's possible to modify orbits around other bodies in a similar fashion.
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.
Of course, no spacecraft will ever be at rest with regards to the Sun -- it would take 30km/s of delta-v to get there, and there's rather little point. They will be orbiting, starting from leo, which means that the meagre acceleration from a sail is more than enough to get basically anywhere.
Long story short, photons (and solar wind) impart a small amount of momentum on the sail. This can be used instead of rockets - as long as you don't mind it taking an absurd amount of time to get you anywhere, that is. And its a whole lot lighter.
An absurd amount of time can be a workable tradeoff depending on the purpose of the mission.
Historically, both thrust and delta-V have been sorely constrained on spacecraft. This is trading a lot of thrust for an infinite* amount of dV, which is a really valuable trade.
(*Practically infinite. It runs out when the sun stops giving solar pressure ;) ).
I am well aware of the potential advantages of solar sails for many things.
I'm just saying they aren't the be-all and end-all. There are many things where they aren't particularly useful (for instance: anything time-constrained. Or anything that requires largeish maneuvers to be done in the outer solar system. And there are certain maneuvers that are... difficult... with a solar sail.)
Also, it's not infinite, nowhere near. Among other things, solar radiation and solar wind will degrade the sail over time. Especially with a lot of the materials they suggest using (mylar, etc.)
Technically, it's as long as you don't mind taking an absurdly long time accelerating. If you're traveling an absurdly long distance, then absurdly slow, but constant, acceleration can actually get you there quicker than chemical rockets.
It's mainly targeted at cube sats, which are very small satellites with usually one specific task or purpose. This could open up space even more to smaller teams of scientists.
I don't know where you got the LEO restriction from. All of the Planetary Society pages are clear that this LEO test is a test, and the real deal is above LEO.
It's not a restriction for solar sails. I know that.
But the grandparent comment said "It's mainly targeted at cube sats".
And cubesats tend to be LEO-based. (They tend to be the cheapest thing possible - and its harder to get into a higher orbit. And they tend to be launched parasitically to boot.)
They're hoping to enable cubesats (or at least small sats) to do deep space or planetary missions by proving a low-mass and low-complexity propulsion system.
This is absolutely amazing!! Pardon my ignorance, but how does this compare to ion thrust? Unless that's still theoretical, that is -- this seems like an even more elegant solution that ion thrusters.
Ion propulsion is not purely theoretical - the Dawn mission currently entering orbit around Ceres got there under ion propulsion, having previously rendezvoused with Vesta.
Ion thrust of course has the advantage that it keeps working if you get farther away from the sun. But for the inner solar system, light sails are very promising.
So this comment is not a substantive engagement with the topic of the article; instead, a reference to a familiar (to a nerdy in-group) pop culture touchpoint.[1] As such, it is more of an /r/ comment than an HN comment.
Nevertheless, it made me happy to read it, and I welcome it :-)
(One could note the pleasing irony of the comment itself being an instance of the type of communication-by-reference-to-shared-myth that occurs in the episode. However, since its contribution to this conversation is not about that mode of communication, it's ultimately a distraction .)
> As such, it is more of an /r/ comment than an HN comment.
The river Temarc in winter. Kiteo, his eyes closed.
;).
Honestly, this "familiar pop culture touchpoint" inspired a big part of a generation of scientists and engineers. As someone who thinks it has a significant impact on his life, I too appreciated the comment :).
"for me it's [lightsail is] really personal. i had carl sagan for one class, and it changed my life. and so i'm really excited to get this thing working. if not this mission, the next mission. oh god, the concept is just cool, to just be sailing around the solar system essentially for free. It's elegant."
"there are two questions we all ask, and if you meet somebody who claims they've never asked these questions they're lying and that's all there is to it: where did we come from and are we alone in the universe? if you want to answer those questions, you have to look at the past and the future. You have to look at paleontological evidence, and you have to explore space."
"talk to younger people. they expect great things from technology. that expectation i believe will carry over to every aspect of life - that we can do technological things to prevent climate change. that for any problem there's a technological solution."
"this is what we talk about all the time in science ed. we want science every day in every grade. this is obvious to me and my colleagues in science ed. what's amazing is it's not amazing to everybody. science is as important a topic as writing and reading and arithmetic. if you want to have innovation, if you want to have the engineers of tomorrow, you have to have science."
"kids have no trouble understanding climate change - it's grown-ups that are confused. climate change denial is almost entirely a generational issue."
[1] http://www.nbcnews.com/science/science-news/bill-nye-boosts-...