I’m conflicted as to what to write about today. Solar sails are in the news, but the floatability of brontosauruses was recently mentioned in my presence and trust me, there is a history (of love) there between me and the sauropod buoyancy. Astronomy… dinosaurs… astronomy… dinosaurs…
My very favorite episode of Bones* is on right now, so I’m in a good mood. A bright, sunny mood. The dinosaurs will wait for a rainy day.
I have a powerpoint on solar sails already prepared, but I can’t just post it right up and tell you all to go to town because it’s full of equations,** so I guess we need to talk.***
So, the thing here: the Japanese Aerospace Exploration Agency (JAXA) built a solar sail, shot that puppy up into space, and the sail has now successfully unfolded itself and is ready to be tested as a method of actually steering its spacecraft.
Solar sails work basically like boat sails, except instead of being pushed around by wind, they get pushed around by light. Yay for transparency in naming.
Wait, you might say, that is ridiculous. Light cannot push things around.
Oh, but it can, through the phenomenon named radiation pressure for reasons which are, again, hopefully obvious.
Light, you see, has momentum. Exciting! So do all things that are moving. Usually when we talk about momentum we talk about mass times velocity, but light is made of photons, which don’t have mass. But relativity provides for photons to have momentum anyway. The important thing to take away from this paragraph is that photons have momentum; why is not immediately relevant.
So. Photons whiz along at 186,000 mi/hr or 3×10^8 m/s, with momentum. And then they collide with things, things like you and me and everything we look at, and they impart some or all of their momentum to the things they collide with, exerting pressure on them. Radiation pressure.
Radiation pressure is exceedingly gentle; at 93 million miles from the sun (also called 1 AU, or astronomical unit; about the distance the Earth orbits at), the pressure of the sunlight, around 4.6×10^-6 N/m^2, is incomparably lighter than the brush of a moth’s wing.****
So on Earth, radiation pressure isn’t going to make much of a difference. But in space? It’s a vacuum, so there’s no air pressure, no wind, and once you get out from the influence zone of large objects, no gravity… radiation pressure, gentle though it is, isn’t facing any competitors. So theoretically we should be able to send up some kind of space robot, attach a giant sail to it, and use that sail to float that robot around space like an awesome space-sailing-boat-type-thing.
Sounds awesome, right? Clearly it is or I wouldn’t be writing about it. But because radiation pressure is so gentle–and the farther from the sun our space-boat-robot goes, the gentler the radiation pressure gets–to get up to speeds fast enough to go anywhere in a reasonable time, you need your sail to be both large (to catch as much light as possible) and lightweight (because the more payload the sail carries, the less effect a given amount of radiation pressure has). And it’s not like it’s easy to get large, delicate, lightweight objects into space.
So we can’t send a sail up ready-to-go, it would tear or run into stuff or burst into flame on the way. You need to roll it or fold it or something. We’ve attempted solar sails before, but they failed on launch or wouldn’t unfold and things of that nature.
But now Japan has launched IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun),***** and it unfolded, and now they’re going to test how the sail steers and moves its little robot payload around. Which is awesome: advent of new technology!
Additionally, artists’ renderings of solar sails look super-cool.
*The Witch in the Wardrobe. It makes my heart want to explode in a shower of confetti and stars and other tiny hearts full of sparkles.
**I have two powerpoints about the floating dinosaurs, also full of equations.
***Also, the slides are slightly nonsensical without accompaniment
… as it should be.
****Though, when panicked, luna moths can actually get some surprising force behind there. I know this because I have been repeatedly smacked in the face by one. But don’t let this anecdote distract you from the point: radiation pressure is minute.
*****No points for spotting the reference, sorry.
[note: this post originally appeared on July 20, 2010 at Original Blog (see here for details)]
…as the dinosaurs knew it, anyway.
Background: in 1980, Luis Alvarez, Walter Alvarez (his son), Frank Asaro, and Helen V Michel published a paper in Science claiming that the dinosaurs went extinct when an asteroid hit the Earth*. The impact (the crater was eventually found in Chixculub, Mexico) would throw up a huge dust cloud** that would literally darken the skies. Without sunlight, plants start to die off, so plant-eating dinosaurs die off, so the meat-eating dinosaurs that ate the plant-eating dinosaurs die off.
Part of their evidence was the worldwide layer of iridium*** laid down at about the same time the dinosaurs died off. Iridium is rare on Earth, but not in asteroids; if an asteroid hit, and iridium-laden asteroid dust was dispersed all over the world, that could explain the iridium layer.
Now Disney can at last add an asteroid hit at the appropriate part of Fantasia!****
*Walter also wrote a book about it called T. Rex and the Crater of Doom, which is an awesome title. The book’s not bad either.
**other exciting, but extinction-free, dust clouds: Krakatoa.
***the iridium satellite network is named because iridium (from iris, Latin for ‘rainbow’) is atomic number 77, and there were originally supposed to be 77 iridium satellites in orbit. As it turned out, there are only 66, but atomic number 66 is dysprosium (from dysprositos, Greek for ‘hard to get at’). You can see why they made that naming choice.
****right after they resolve their confusion about the appropriate number of claws for a T rex to have (2) and whether stegosaurus and T rex were contemporaries (no).
[note: this post originally appeared on March 9, 2010 at Original Blog (see here for details)]