There is a pretty cool gallery on Gizmodo of various pieces of spacecraft and rocket debris that have fallen to Earth, intact enough to have recognizable shapes.
A fallen rocket stage in Russia
This gallery puts in perspective what we mean when we say things “burn up” in the atmosphere. The friction of re-entry generates enormous heat, usually enough to break up rockets and satellites into tiny, unrecognizable pieces. But every now and then, conditions are just right for bits of spacecraft to make it all the way down to Earth. Some of the pieces are clearly spent rocket parts, which might not have been all the way up in orbit, but a few are most definitely chunks of previously orbiting spacecraft.
This sort of thing highlights one reason why launches all have range safety officers. If there is an unrecoverable problem during a launch, standard procedure is usually to blow up the rocket. The goal of such an action is to break the vehicle up into small pieces that will be much less likely to hit things on the ground.
As usual, the physics here is something that future spacecraft designers might be able to take advantage of: clearly, some spacecraft shapes other than the standard aeroshell capsules can make it to a planet surface. This idea has inspired certain spacecraft research groups to look at developing spacecraft that can get from orbit to the ground by fluttering down benignly.
Note: the problem of orbital debris, shown in the map in that gallery, is related but very different from the issues surrounding re-entering debris.
Jupiter is one of the most useful planets for planning spacecraft trajectories, and it’s home to some of the most interesting science targets in the Solar System. However, it also happens to be one of the most dangerous planets for spacecraft.
Jupiter is so dangerous because of two things: its magnetic field, and the moon Io. You see, Io is extraordinarily volcanically active – the only known extraterrestrial body with active volcanism, in fact – and constantly spurts all sorts of particles out from its interior. The energy of Io’s eruptions gives some of these particles escape velocity, and they end up orbiting Jupiter. Jupiter’s immense magnetic field or solar radiation can then strip electrons off these particles, creating a torus of raging ions around the planet. It’s the same phenomenon as Earth’s Van Allen radiation belts, but without the nuclear explosions and, well, Jovian in scale.
Jovian radiation belts. Sort of. My Jupiter model didn't work, so this picture looks terrible.
Jupiter’s radiation belt kills spacecraft. The Galileo probe, a Jupiter orbiter, eventually died when it crashed into the planet, a protective measure to prevent it from contaminating Europa with Earth life in the event its electronics became too fried to control the spacecraft. (Galileo contrasts quite a bit with the Cassini mission to Saturn, which has been considerably extended from its main science mission!) The next mission to orbit Jupiter, Juno, is encased in a titanium radiation shield. And you can pretty much forget human exploration of Europa.
But it’s not just tempting to send robotic probes to Jupiter – it’s practically a necessity! Any spacecraft headed to the outer Solar System, or trying to do wild orbit maneuvers, needs a lot of delta v (that is, the capability to change velocity – in magnitude, direction, or both). The Ulysses spacecraft needed a bunch of delta v to kick its orbitwaaaaaaay up over the sun. And New Horizons needed a ton of delta v to get out to Pluto and beyond. One efficient way to get delta v is to perform a planetary swingby, or flyby, maneuver – and you can get more delta v from a bigger planet. Jupiter is king of the planets and gives space vehicles a huge boost. So, every spacecraft that has ever gone into the outer Solar System has visited Jupiter.
That magnetic field, though. Jupiter has a magnetic moment 18,000 times as large as the Earth’s. That sort of magnetic field is useful, if a spacecraft has the hardware to take advantage of it. For instance, an electrodynamic tether: a long conductive filament stretching from the spacecraft, along which the spacecraft runs a current. (Currents through the rarefied plasma filling the Jovian orbit environment complete the circuit.) The spacecraft will then experience a force proportional to the cross product of the current along the tether and the local magnetic field. If you don’t like vector math, don’t worry – just remember that the force is perpendicular to both the tether and the planet’s magnetic field near the spacecraft (which, near the planet’s equator, will run approximately parallel to the planet’s spin axis). In Earth orbit, there have been several missions testing tether physics, with applications including both electrodynamic propulsion and harvesting power from the Earth’s magnetic field. Around Jupiter, these methods will be even more powerful.
If a spacecraft is in circular orbit around Jupiter and orients a tether parallel to its direction of travel, then the force will be either directly toward or directly away from the planet’s center. This means that the spacecraft can, by running a current through a tether, generate a force that has the effect of either enhancing or weakening gravity. So a robotic probe performing a Jupiter flyby could get a much bigger gravity-assist boost with a little help from a current-carrying tether. Or, if the current is high enough, the net force could repel the spacecraft from Jupiter, changing the direction of the delta v it picks up in the flyby. The capability for spacecraft to perform that kind of maneuver could open up more launch windows for outer Solar System missions.
Another idea is to use electrodynamic tether propulsion to keep a Jupiter orbiter out of the worst parts of the radiation belt, so that it can get lots of data on the Galilean moons. If the probe has a slightly inclined orbit, then it could vary the current in its tether over the course of each orbit so that the spacecraft pushes its semimajor axis in and out each period. With the right parameters, this non-Keplerian spacecraft trajectory would skirt the ring of hard radiation around Io’s orbital radius.
A trajectory that skates around the radiation belt
Perhaps the spacecraft has some leeway into how far into the radiation belt it can venture – or its orbit is just bigger than the “danger zone.” Then, it could follow a Keplerian orbit (affected only by gravity) some of the time and use the passage of the tether through Jupiter’s immense magnetic field to generate electricity. If engineers can balance the numbers, then such an exotic orbit might come out power-neutral over the course of each orbital period, giving spacecraft a free, and safe, way to explore Jupiter’s Galilean moons for a very long time.
Speaking of power harvesting: Jupiter is far enough from the Sun that spacecraft around there can’t really get all the power they need from solar panels. Dragging a conductive tether around and letting the planet’s magnetosphere drive charges along the length of the filament would be one way to overcome that challenge.
These are kinds of technologies that we can develop in Earth orbit and deploy in the outer Solar System, to take advantages of the resources out there and allow us to learn more about the things in our backyard. After all, the more we understand about the different regimes of our own Solar System, the more we understand about our origins – and about the possibilities that exist in planetary systems around other stars.
After much pressure from my girlfriend, I’ve signed up for a Twitter account. I’m going to use the account to echo posts here on Quantum Rocketry.
It’s not a communication medium I really see a lot of value in; if I want to convey information I would much, much, much, much rather do so here, where I have more than 140 characters to develop an idea, in context, without conforming to the soundbite-based, ultra-distilled, headline-only view of things that seems to be the current trend on the Internet. (There goes my liberal arts background, shouting opinions at anyone who will listen!) And if I feel like being silly and inane, I’d rather do that on a narrowcast medium like Facebook, where my silliness will go to my friends who know what to expect and how to interpret such activity, instead of a broadcast medium like Twitter.
But, though I shall continue my rebellion, I finally got an account anyway. (And I just have to continue my rebellion against all the people who instantly must check everything on their smartphones instead of interacting with me when I’m right in front of their faces!) It is jpshoer. This is 10% to allow further Twitter-based interaction with ‘netizens who read this blog, and 90% because a Twitter account is a requisite for NASA Tweet-ups.
Just remember, kids: one of the few things I think are more stupid than Twitter is the word “tweep,” so don’t call me that.
I have successfully defended my dissertation. I would appreciate it if you would address me by my correct title, now: Doctor of Rocket Science.
(I’m kidding.)
The funniest thing about this to me is that I know that the research I’ve been working on isn’t done. There are more investigations to pursue, more refinements to write into the code, more variations to try in simulation, and more experimental verification to perform. Research never stops. But at some point, we grad students have to decide, with our advisers, when we have made a sufficient contribution and should wrap up our work into a complete dissertation. Still, it doesn’t quite feel like I’m “done,” because I know that the research has much further to go! It’s kind of anticlimactic.
A rather nice capstone, though, was spending last week getting the lab ready for, and filming, a bit for the National Geographic show “Known Universe!”
Hello, Intertubes! I have been slacking off on the blog in favor of preparing my dissertation and the presentation for my defense. I know, excuses, excuses…
To keep all eighteen of my intrepid readers happy, here is a video that recently went up on my lab group’s YouTube channel:
That’s me demonstrating the physical principles that could be used to make a real-life tractor beam that can push, pull, and manipulate spacecraft. The device would work by pumping changing magnetic fields at a target spacecraft, exciting eddy currents in the spacecraft’s aluminum skin. These currents interact with the magnetic field from the tractor beam device, allowing it to push, pull, or rotate the target.
In the video, I generate these changing magnetic fields by moving a big rare-earth magnet around. On a spacecraft, a more likely tractor beam device would be a set of electromagnet coils. I calculated that, with reasonable power requirements, such a device could exert ion-engine-scale forces on a target several meters away. More powerful electromagnets would increase that range.
So, MAKE Magazine has this on their current cover:
That’s a Lego Mindstorms NXT computer and other Lego pieces on a spacecraft. “Cool!” my labmate and I thought upon seeing this. “Satellites made out of Legos!”
Well, it turns out that the article says this is a picture of a functional satellite prototype made out of Legos by a group at NASA’s Ames Research Center. (The same group that recently launched a spacecraft that used a cell phone for its computer system!) But, you know…why not? Why not make a satellite out of Legos? I think this would be a great idea!
What would it take?
The physical structure of a Lego-brick satellite would have to withstand the rigors of a launch into space. This involves accelerating the satellite and subjecting it to heating from friction as the rocket climbs, among other things. Space Mission Analysis and Design, Third Edition, gives the following “typical values” for acceleration and thermal requirements of satellites in a launch vehicle:
Acceleration: 5-7 g, but up to 4,000 g shocks during stage separation and other events.
Temperature: 10-35°C (but the inner wall of a Delta II fairing could get up to ~50°C).
The acceleration requirements, though that shock value sounds drastic, may not be too much of a problem. G-hardening is potentially easily accomplished by potting components in epoxy. Modern cell phones, for instance, are rated to several thousand g‘s so that they work even after you drop them. A good epoxy applied to all the joints in the Lego spacecraft structure, and probably around the whole structure after it’s completed for good measure, could go a long way toward preventing this from happening during launch!
I’m more worried about the thermal requirements. Lego bricks are made out of acrylonitrile butadiene styrene, which seems like it starts getting deformed due to heat at about 65°C. That 50°C Delta II fairing seems a bit close for comfort! Plus, the temperature of some Lego blocks sitting in direct sunlight in space could climb above this value very rapidly – and lots of transitions between daylight and shadow would cause the parts to expand and contract thermally, working the pieces apart if they aren’t well-secured with epoxy. However, the Lego satellite could be wrapped in something like aerogel or MLI blankets to mitigate the thermal challenges. Somewhat.
Another challenge is survivability of the computer system in the space radiation environment. With no atmosphere to absorb radiation, a cosmic ray could hit the spacecraft and trigger a single-event upset, or “bit flip,” that switches the value of a bit from 1 to 0 or vice-versa. This kind of thing happens to spacecraft computers all the time and corrupts data, so spacecraft computers engage in a lot of error-checking. But the same cosmic rays can also burn out a bit, so that the computer can never read its value again – or even burn out a trace in an integrated circuit so that the circuit fails! That sort of thing would definitely be a problem for a Lego spacecraft, and would shorten the life of the computer substantially unless we did some radiation hardening of the NXT. A simple way to harden it would be to encase it in some metal, but that adds mass, which is always at a premium on spacecraft. However, another strategy is to simply accept that the spacecraft will have a short life in orbit!
…Because, after all, what would be the purpose of launching a satellite made of Legos? It would be to show that commercially available materials are sufficient for at least some space applications, without the millions of dollars of investment in robustness and fault tolerance that the spacecraft industry generally demands. If the satellite’s mission can be accomplished in a few days and the lifetime of the craft is a week, then why should all of its components be certified for years of operation in orbit? Perhaps we could, instead, come up with much cheaper – or much riskier – satellite designs. We could try out new materials, new components, and new mechanisms without designing them never to fail. Instead, we accept a few failures as learning experiences, and move ahead with the designs that work.
Legos are, at least, a fun place to start. Perhaps most importantly, they are easy to get into the classroom, so that students can think about building the structure, thermal, power, electrical, and payload systems into a functional satellite – and can re-arrange or re-format those systems at will. But hey – when they’re done, why not launch?!
Scientific theories are cool and complex beasts: you observe some data or think about previous results, formulate an idea about how the world behaves, and then test out how well that idea holds up in the presence of more observation and further development.
My favorite theory is, hands-down, Theory of Special Relativity. It’s not my favorite because of its far-outside-common-experience implications, or because of mathematical obscurities, or because of its attachment to the great celebrity-scientist Albert Einstein, or even because of its amazing practical applications (like nuclear power and lasers and GPS) – cool though all those things may be. It’s my favorite theory because it springs from just a couple simple ideas, and I can derive its wild and wonky implications straight from those ideas using nothing more than basic geometry. It’s a testament to the power of the “thought experiment,” and a wonderful demonstration of how a few brilliant ideas can lead to extraordinary outcomes!
The Theory of Special Relativity basically boils down to just two postulates:
The laws of physics are the same in all non-accelerating reference frames.
The speed of light is the same when measured in any reference frame.
That’s it! Now, check this out: I’m going to derive in a few lines the famous relativistic effect known as time dilation.
Suppose I go screaming by you in a cartoon rocket ship while you stand bewildered on the ground. My rocket’s velocity v is a substantial fraction of the speed of light c. Because it’s just that awesome.
Whoosh!
Inside my rocket ship I have a special type of clock. It’s like a pendulum clock, but it works with light. In a pendulum clock, the pendulum swings through a certain arc in a certain time. In my clock, a laser bounces a pulse of light off a mirror a certain well-measured distance away, and a detector right next to the laser picks up the light pulse. (The laser and detector are so close together that the light basically retraces its steps back from the mirror.) A timer hooked up to the whole thing tracks the amount of time between the laser firing and the detector registering the light.
My light clock
Here’s a closer look at the clock and how it works:
I know that the distance from the laser to the mirror is d, so the beam has to travel a distance 2d every time it fires. I also know that the speed of light is c, so the total time the light beam takes to travel this distance is t’ = 2d/c. That’s one tick of the clock, as I measure it.
Now suppose the clock is right near the porthole on my rocket ship, so that you can see it as I whiz past. You see the entire rocket traveling with speed v to the right, so in a time t the rocket moves a distance vt. And you see the light beam travel along a slightly different path than I do:
Your view of the clock
Why do you see the light travel along this angled path? Why, the first postulate of Special Relativity is the reason! The laws of physics have to be the same for both of us. I look at the laser and see that it has zero horizontal velocity (because we’re both standing on the cartoon rocket deck), so the beam just goes straight up and down. But you look at the laser and see it zooming along with horizontal velocity v, so the light the laser shoots out picks up that additional velocity.
Let’s look at that beam path carefully for a minute, and add some math – don’t worry, nothing too scary! Just the Pythagorean Theorem, to figure out the distance the light beam had to travel.
Okie-dokie. Sounds great, but here’s the thing: Special Relativity Postulate #2 says that the speed of light in vacuum is constant as measured by all observers. So how long do you measure it takes the laser beam to travel this path?
Now, hold on here – I measured one tick of the clock as t’ = 2d/c. You measure it as t’ = 2d/sqrt(c2 – v2). But because of postulate #1, we know that we are describing the same physics! Let me write t in terms of t’ for comparison.
This gamma quantity is kind of a funny thing, and it shows up all over relativity. Since v always has to be smaller than c, then (1 – v2/c2) is always less than one and gamma = 1/sqrt(1 – v2/c2) is always greater than one. That little fact means that t will always be greater than t’. That’s relativistic time dilation! Put in simple terms, if I am traveling very fast with respect to you, then the time of one tick of my clock seems longer for you than it does for me. In fact, since gamma depends on my rocket ship’s velocity, the effect gets more and more pronounced the faster I go, getting towards infinity as I get closer to the speed of light:
Gamma as a function of speed
It’s not just my light clock that gets stretched out in this way. All clocks, and all processes that involve time are subject to time dilation! (You could figure that out by the same method that I just did, if you carefully track the paths of light beams.)
Eventually, what I think is one second on my clock will be a year according to you. If I go faster still, I could get one second on my clock to be a century or a millennium to you! This phenomenon is one reason why we know that nothing can travel faster than light: because if my rocket ship could go at light speed, then time dilation would stretch things out such that (according to you) an infinite amount of time would elapse if I go anywhere!
The coolest thing about all this, to me, is that Einstein came up with these ideas through careful consideration of “thought experiments:” what if we could ride along with light beams? what if I zoom by you on a relativistic rocket? He formulated his postulates carefully, and he fleshed out their implications carefully – but the derivations themselves are wonderfully simple and easy to follow. The physics that result, though…crazy!
I love winter. Maybe it’s just my New England heritage, but you know, I really enjoy it when the temperature drops below freezing outside and there are eight or ten inches of snow on the ground.
Suited up to ski!
I love when it’s cold enough outside that I can taste the air as I breathe in, and I have to bundle up in layers of sweatshirts, snow pants, parkas, boots, hats, scarves, and gloves to go outside. It adds a sense of adventure to the mundane: the need for extra preparations makes even familiar locales more exotic! It’s like outfitting for a hike in the desert, or (for me, growing up) preparing to explore a strange alien planet.
Wachusett Reservoir
In fact, snow and ice does turn the world into something new – something unpredictable, something different every time, with snowfall determined chaotically, water iced over, and winds sculpting the shape of these temporary surfaces. There can be places to explore that seem like exciting new discoveries which, in summer, might be only the usual woods or paths or buildings. Everyday objects in the snow take on a new aspect. All my senses are affected: the chill of wind on my cheeks, the soft white snow, the utter stillness of places deep in the woods, the muffled crunch of snow under my boots all make an impression.
Birdhouse
Extraordinary locations, too, take on a completely new character in winter. That which I enjoy in summer or fall becomes something else, entirely – adding to the value I place on it during the warmer months of the year. When the snow is gone, the rain and mud has washed away, and green things begin to come out again, I can appreciate springtime even more.
Ithaca Falls as an Ice Fortress
Winter brings with it a whole new gamut of things to do: from skiing and skating to just running out to the backyard and shoving, building, or sculpting with snow. These things cannot be done any other time of year, and during winter, they come right to my backyard!
Egyptian Snow-Ruins
Even the chores – the shoveling, the ice-scraping – are cause for me to take a moment outside and enjoy the sights and sounds of winter, savor the process of going out the door and coming back in, or just focus on physical exertion for a time.
And when the outdoor activities are finished, what could be better than to come inside, strip off all the layers, and sit someplace with some hot chocolate or tea? It’s a perfect time for board games or movies or idle chats. Winter drives people together.
Cornell arch and clocktower at night
I love all the seasons – maybe with the sole exception of that time between when the snow starts melting and the flowers come out, when the world consists of rain and mud – but winter always carries a little bit of extra excitement for me. I suspect I may be destined to start my career in California, and a New England winter is one thing I will certainly miss!
After buying my third computer (I have a work desktop in my office, a personal laptop, and a personal tablet), I became a big fan of Dropbox. The service is a paradigm of cloud computing: I get a folder on all my computers that acts like a normal Windows folder, but syncs up with a remote server every time a file changes. I immediately started using the service for, say, my dissertation-related files – which are now accessible from all three computers. As a plus, Dropbox downloads and keeps a local copy of all files in the folder, so my dissertation exists in four identical copies (all my computers plus the Dropbox server – which gets backed up on its own!) so I don’t ever have to worry about that work disappearing into some black hole if my hard drive crashes. And since I got a Droid Incredible, I can even access files in my Dropbox from there. Yippee!
I just came up with a devious new use of the software to add to all that. I do a lot of Matlab simulations these days, and they run fastest on my work desktop. However, these simulations take a long time, so I’d like to be able to set them up and get their results in short, intermittent checks while I’m traveling for the holidays. (Hey, I’m trying to move my research along efficiently and finish up my degree! Really!) But I haven’t been able to get Windows Remote Desktop to work – it seems that my department at Cornell keeps those ports closed and I haven’t been able to find a way around it.
So here’s what I did: I wrote a Matlab script that checks for the presence of other Matlab scripts in an input folder in my Dropbox. It then runs any scripts it finds, captures their output, and deposits that into another folder in my Dropbox. (I encapsulated the run command inside a try/catch block which also plops any errors into the output folder.) The script then deletes the file from the input folder and loops. If I put a file named “stop” in the input folder, the script cuts itself off. I think next I will add some code looking for a file named “clean” and responding to that by clearing all variables except those used in the wrapper loop.
From any of my computers, I can now write a Matlab script to do some simulations and copy it into the “input” folder. When my work desktop syncs up with Dropbox, the Matlab loop catches the script and runs it. I can check the Dropbox output folder later, again on any of my computers, to see what happened!
Maybe this little trick will be useful to someone else out there, so I decided to share it. Happy Hanukkah, grad students of the world!
