Category Archives: Concepts

Jovian Electrodynamics

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 orbit waaaaaaay 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.

How to Build a Tractor Beam

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.

REAL Space Legos!

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?!

World-Building and the Real Universe

(Pardon me for the hiatus. Had to fly to Houston to do some flight testing at NASA.)

I spent a pretty good weekend doing some world-building. Since discovering the maps in the first pages of The Lord of the Rings, Redwall, and the like, I have really enjoyed sketching out maps of imaginary worlds and outlining details of the cultures and histories that play out over those maps. My maps started as knockoffs of Tolkien’s (with the bad guys sequestered in a nice, rectangular wall of mountains around some barren lands) or parallel-universe versions of the terrain around my house. Since then, though, I’ve started to inject a lot more realism into the worlds I create. Want to know where the tectonic plates and prevailing winds are on my map of Oghura? I could show you!

The Barovin Mountains are this world's ancient Himalayas. The desert is in the rain shadow of the Red Mountains - though it wasn't always, which explains some of the Oghuran-Kalatchali history!
Map of Oghura

Beyond the maps, some of my imagined cultures have fully fleshed-out languages, religions, and customs. Slowly, slowly, I’ve been compiling reference documentation on the Oghuran desert and people, the fantastical Cathedral Galaxy, and the future-universe of the Four Colonies. This weekend I was spending my time in the Cathedral Galaxy, putting together a master list of the major galactic regions and polities, along with distinguishing characteristics. Now I know a bit more about why the Imperium of the Triumvirate is split in three, how the far-from-galactic-center Traders’ Rim came to be populated by merchants and entrepreneurs, and the tumultuous history of conflict between Amseile and Shobah. I’ve also got the beginning of a couple more stories – one concerning an Imperium gladiator’s bid for freedom and another describing the Waygehn people, who evolved to sentience near the death of their star and outlived the event, leaving them homeless in the galaxy. That’s one of the most fun things about deciding to build a universe purely for short stories: I get to invent worlds, and then immediately show them off with snippets of detail!

Though the Cathedral Galaxy has some distinctly space-fantasy elements, I decided early on that it would be a universe based on hard science – though not necessarily our hard science. My short story “Conference” illustrates the point, as it shows that there are technical concepts built upon technical concepts – but at the level that Arthur C. Clarke would have described as “indistinguishable from magic.” I have no idea how the Channel Network could be set up, and building planet-size structures is clearly fantastical. (And none of you know yet what’s in The Cathedral!) But I made sure that the story was relevant to us Earthdwellers, and I lean strongly on plausible concepts to describe things like astronomical bodies or planetary orbits.

Great Galactic Map, showing major markers and the Channel Network

For example, take Heliast, the resort world on which much of “Conference” takes place. Here’s the description that conference-goers got of the world:

The tour guide explains how Heliast is an ancient world with a single moon nearly half its own size, and how that has dominated the history of the planet and made it ideal for resort paradises. A billion or so years ago, the planet spun many times under one orbit of the moon, and the energy input of ocean tides among all the planet’s archipelagoes – Heliast is over eighty percent water – gave rise to life. But nowadays, the moon orbits in tidal lockstep with one Heliast day, the prime factor contributing to the perpetual calm of its seas. The small radius of Heliast’s solar orbit leaves the planet with a reasonable day length, while the dimness of its sun places it in the liquid-water zone. Without tides, with a massive moon helping to protect the planet from asteroid impacts, and with barely any eccentricity in its orbit to create seasons, there have been few selective pressures on Heliast’s life forms. Life on the planet thus failed to diversify much, and after millions of years of evolution with few external stressors, there are now only a few ecological niches on the world. Three or four avian species, eight or ten surface-level swimmers, two or three land animals, and about six land plants are all most tourists have the chance to interact with. The rest of the planet is geological beauty for visitors to enjoy.

So, the planet’s “month” equals its “day,” but there are still many days per year and there is much liquid water on the surface. The dynamics shaped the world’s evolution. That was fun to think of! But, more and more, I am completely amazed by the strange worlds that actually exist in our own universe. Many Earth- and space-based observatories keep returning data on new exoplanet candidates, and in the last few years, the galaxy seems a lot more planet-populous than it has in the past.

This past Monday, I went to a fascinating astronomy seminar on the potential climates of Gliese 581g given by Dr. Raymond Pierrehumbert from the University of Chicago. (He’s preparing these climate models for an arXiv preprint.) Besides tying the Gleise 581 system with 55 Cancri for most number of known exoplanets around the same star (5), this planet is interesting because it falls right smack in the middle of the traditional “habitable zone,” the range of orbital radii necessary for planet surface temperatures that could support liquid surface water. Now, of course, the discovery of Gliese 581g has to be confirmed to become official – and there’s some doubt about that! – but it’s at least got scientists thinking about these dwarf-star systems in interesting ways. Continue reading World-Building and the Real Universe

Europa Mission Concept Followup

My Ice Fracture Explorer concept for getting a probe down into Europa’s subsurface ocean – one of the likely places in our Solar System to find extraterrestrial life – was just one way to dig beneath the ice crust. Other concepts often involve melting through the ice crust. However, I thought, what if we can take advantage of the places where Europa’s geologic dynamics allow access to the ocean without tunneling through the ice?

I can think of two surface features on Europa that mark potential exposure of the ocean to space. One is the “chaos,” which may be formed when ocean-floor volcanoes or rising blobs of warm water melt through the ice crust all the way to space. However, we don’t yet have a good way to predict when chaos features would form – unless the impact theory of chaos formation, my personal favorite, is correct, and we can track a large meteoroid on its way to hit Europa. The second, the double-ridge features marking cracks in the ice crust, are potentially more predictable so it makes better sense to plan a mission around penetrating the crust through these fractures.

My IFE concept involved a disposable probe landing on a double-ridge, rolling to the center, and hanging over the crack as it opens up under Jupiter’s tides. The hanging probe could then drop a penetrator into the fracture, to punch through the thin layer of ice below and dive into the ocean water.

Hanging drop concept

A number of readers left me comments here and on io9 pointing out various challenges with this design. Getting the lander to hang, suspended, in the middle of the crack might stretch our space-tether technologies a little too far. Timing transmissions to an orbiter before the closing crack crushes the lander is a problem. Communications from the penetrator are also an issue: since those have to cross a water/space boundary, I wanted to just reel out a long data line from the hanging lander to the penetrator – but the length of this cable could be an issue  if the ice crust ends up being 100 km thick. And, since the probe would probably have to be powered by an RTG, when the fracture closes and squishes the probe, we’d be dropping radioactive gunk on the Europan natives. While I don’t think any of the stages of the IFE concept stretch our technologies much more than, say, the Mars Science Lab’s Sky Crane, it certainly wouldn’t hurt to make things easier on ourselves!

One such idea might be to drop the miniature penetrators directly from orbit. There wouldn’t be any suspended platforms, data cables, or rolling around on an unpredictable surface. There also wouldn’t be as much of a challenge in lining up the orbiter for receiving data, since the penetrator came from the orbiter in the first place. However, determining which ice fractures are opening and closing, and timing the drop from orbit to coincide with those events, might be tricky. Landing on the surface first adds an additional safe-hold point to the mission: controllers can wait to establish good telemetry from the lander after it’s on the surface and before ordering it to commence penetrator deployments.

Another suggestion might be to keep the lander on one side of the double-ridge interior. It could shoot a cable across to the other side, and reel the penetrators out to the middle before dropping them:

Penetrator drop concept

This concept buys the IFE a number of things: first, it can drop more than one penetrator. My original concept called for several IFE’s to be dropped in tandem to several ice fractures to increase the chance of success. However, if each IFE can deploy more than one probe into the ocean, then the mission managers can get several chances to successfully drop the mini-probes as the crack opens and closes and opens again. Second, the lander won’t be crushed, meaning that we won’t have to worry about radioactive contamination of the ocean (as long as the penetrators run on batteries) and we’ll get the chance to have the landers keep performing science operations after all the penetrators are expended. Third, the lander can buffer data from the penetrators and uplink the information to an orbiter at leisure – no rushing to time the drop for an orbiter pass!

One thing scientists don’t really know yet about Europa is how wide these cracks open up. The tether-based ideas I’ve outlined work as long as the crack is big enough to admit the penetrators – but they have the advantage of working if the fractures end up being many meters wide. However, that might not be an advantage the spacecraft needs if the cracks are very narrow. In that case, why not just have the lander come down with footpads on either side of the fracture?

Straddler concept

As the crack opens and closes, damped mechanical joints in the legs could take up the motion and keep the lander centered over the crack. This lander would also be able to buffer data, survive for many tidal cycles, and be able to drop as many penetrators as it has packed into its body.

I think the biggest issue with my designs is that data line: the images and biochemical experiment results from the penetrator have to get transmitted to the lander somehow. (From there, they can get to orbit and then to Earth.) Direct transmission via radio or optical signal could be very difficult from beneath the alien waves, and speed is a factor, so I opted for a hardline. But how long does the cable need to be? At least tens of meters. Probably around ten of kilometers. But maybe as long as 100 kilometers, which could be prohibitively long! One possible solution might be to drop a two-segment penetrator into the crack: the upper segment would have floats – probably some sort of inflated bags – and a radio transmitter. The lower segment would contain the ice-shattering hard shell and all the science instruments. The two halves of each probe would be connected by an unreeling data cable. So, the probe would drop from the lander, smash through the ice, and then split into halves – with one half floating on (and them freezing on to) the ocean surface while the other half continues its plunge into the depths. The probe would collect its data, then zap that data up the cable to the surface unit. From there, the data would travel via radio to the lander, which would relay it to the orbiter and then the Deep Space Network.

Certainly, any concept for a Europa mission strains our ingenuity. But that is one reason why it’s so fun!

The Ice Fracture Explorer

Europa, the second Galilean moon of Jupiter, has been my favorite planetary body for a long time. The reason I like Europa so much is that it’s a world whose orbital dynamics with Jupiter, its orbital resonances with the other Galilean moons, and its own rigid-body dynamics have a strong hand in creating its surface features – and giving it the potential to harbor life. It’s one of perhaps two or three extraterrestrial places in the Solar System where we might hope to find life. Europa is also easier to get to than Enceladus or Titan. As such, I think it ought to be one of the highest-priority exploration targets for robotic space probes. (Human exploration would be nice, too, but if you think radiation exposure on the way to Mars is hard, you don’t even want to consider putting people in the Jovian system!)

Thanks to magnetometer measurements and images from the Galileo mission, it’s pretty much established at this point that Europa has an icy outer shell over a global liquid ocean, with a rocky core on the inside.* The only question is how thick that ice shell is – I’ve read estimates ranging from 10 meters to 100 kilometers, with a pretty high confidence of ones to tens of kilometers. The ice shell gives rise to a number of interesting surface features. A particularly cool sort of feature, found with global extent across Europa, is the double ridge.

A prominent double-ridge feature on Europa, most likely a crack in the icy shell

Planetary scientists have a number of models for how these double ridges form, and they generally seem to agree that the ridges mark the locations of cracks in the ice crust. One especially well-established model suggests that these cracks occur when Jupiter raises tides in Europa’s ocean – just like how the Moon raises tides in terrestrial oceans, but much stronger, because Jupiter is frakking huge compared to Earth’s moon. Europa’s ice crust bulges out over the ocean’s tidal swell and then cracks under the incredible stress. (I like to take a moment to think about the mindbogglingness of that statement: the whole moon’s surface cracks. I’ve stood on a frozen pond when a crack pings through the foot or so of ice on top of the water – Just imagine standing on Europa when this happens!) Once a crack forms, the tides don’t go away. As Europa rotates, about once every three and a half Earth days, the tides periodically lever these cracks apart and squeeze them back together again. In this model, every time the cracks gape open the subsurface ocean gets exposed to space. The surface water boils and rapidly crusts over with ice, and when the cracks get smushed closed, all this ice gets crushed up and forced to the top and bottom of the crack, forming the ridges. The ridges appear in pairs because the crack opens up again after that. These double-ridge features are mounds of crushed ice flanking passages into Europa’s ocean!

Dr. Richard Greenberg is a planetary scientist who thinks that these cracks in the ice shell might be potential sites for life to take hold. Unlike the rest of the subsurface ocean, they get exposed to sunlight, which means that photosynthesis could take place. The periodic in-and-out forcing of the crack would also drive strong currents, which is another energy source Europan life could use. (Those aren’t the only energy sources: other possibilities include thermal gradients in the water, volcanic vents on the ocean floor, or even induction as Europa travels through the Jovian magnetic field.) Of course, that life would also have to adapt to the crack opening and closing once every 3 1/2 Earth days!

Europa's possible ice-fissure biosphere (from New Scientist; click for full article)

We do at least know, from the Galileo mission, that these cracks often have accompanying veneers of organic (e.g. carbon-based) molecules and salts splashed onto the ice surface. This is why the cracks appear as brown stripes in large-scale context images. The crack/veneer combination suggests that there are organic molecules and salts in the Europan ocean, and that those compounds get pumped to the surface through these cracks.

So, let’s take stock: Europa is the only extraterrestrial world with a global liquid water ocean, there is a definite possibility for life in that ocean, and these double-ridged cracks are a possible gateway into the alien biosphere.

Well, then, let’s go diving! Read on for my concept system architecture for an ambitious Europan ocean-exploring mission, which I call the Ice Fracture Explorer.

Continue reading The Ice Fracture Explorer

Projecting Space Battle Physics

When I wrote my original article on the physics of space battles, and the accompanying short story, I made the creative decision to speculate on how space battle technologies and tactics would play out if we built from the present day – or, at least, the very near future. The obvious thing to look at next is what a more distant future might hold – so, I’ll embrace my status as That Space Battle Physics Guy!

A possible near-future space fighter radiating excess heat between battles

I think that extrapolating or projecting space battle technologies forward in time is a difficult thing to do, even for the cleverest science fiction geeks. I say this for two reasons: first, aside from some general trends, it’s hard to predict exactly where technology will go in the next ten or twenty or fifty years; second, nobody gets to play this game against a live opponent – and that’s really how combat tactics and technology develop. Still, given the trends, it’s fun to speculate! Physics won’t change radically for quite some time, so we have some direction in which to proceed.

I’m going to proceed from the assumption that “spacecraft” are different from launch and reentry vehicles. Let’s take some possible combat spacecraft systems, think about the related problems that spacecraft engineers try to solve, and see what might (!) happen if the aliens wait till we have some operational space colonies before they invade…

Continue reading Projecting Space Battle Physics

A fleet to realize the new vision

I think that President Obama’s vision for NASA holds a great deal of promise. However, I seem to be in the minority – with people from Senators with NASA-associated districts to Stephen Colbert to Jesus Diaz on Gizmodo talking about the “end” of the human space program. I often wonder why they don’t see what I see. Obama has both increased the NASA budget and explicitly stated that he wants more astronauts flying in the coming decade than ever before, so he clearly is not trying to “cancel the human spaceflight program.” Given that, it seems straightforward to me that the NASA centers will still need to train astronauts, build vehicles, and conduct mission operations; NASA vehicles will still push the boundaries of capability, and NASA astronauts will explore the Solar System beyond Earth space. The only difference is just that astronauts won’t get to those new vehicles atop Ares launchers, but rather perched on something like the Falcon 9 – which is much, much closer to operation – and our targets are more ambitious. So why the enormous gap in opinion among space exploration proponents? And what might NASA administrator Charlie Bolden do to consolidate support?

I think the problem is that, without a NASA launch vehicle, critics have a hard time envisioning how the new generation of NASA astronauts will get around and what they will do. There won’t be any dramatic Space Shuttle or Saturn V launches – instead, the astronauts will be…”taxiing.” And they will taxi up to…what, exactly?

President Obama wants humans to leave the Earth-Moon system by 2025, get to Mars orbit by 2030, and develop the capability to live and work in space indefinitely. Here’s where Administrator Bolden could step in. NASA systems engineers and artists could crank away and produce concept studies to suggest a new fleet of NASA crewed vehicles. By starting right in on the design of new vehicle concepts, and setting explicit deadlines for their launch and operation, the new NASA vision could become more clear and exciting. The public will start to see what I see – a NASA program that develops dedicated space exploration vehicles, which carry astronauts for months at a time on journeys to deep space, asteroids, and other planets. Clearly, that is no end of the human spaceflight program. It’s the next step.

Below the break, I’ll outline such a possible concept vehicle fleet.

Continue reading A fleet to realize the new vision

Fixed an error in an LRO image

Phil Plait of Bad Astronomy posted a few days ago about caved-in lava tubes on the Moon. This isn’t really new news, but it’s still pretty darned cool news. He posted some images of the cave. However, I found a major, glaring error in the LROC image data.

I fixed it.

Lava cave - fixed!

Seriously, though…those sites are perfect premade Moon base locations. Imagine a team of astronauts putting an inflatable dome over the hole in the roof, belaying down there, putting inflatable endcaps a few tens of meters down the lava tube in each direction, spraying expandable foam sealant into all the crevasses, and using some ISRU atmosphere generators to pump the tube full of oxygen.

a nifty thought experiment: the Earth with rings

One of the most majestic and awe-inspiring structures in the Solar System is the Saturnian ring system. My sister sent me this video, which imagines what that same ring system would look like around the Earth – and what it would look like in our sky when viewed from the surface. The result is pretty wonderful to imagine:

However, sciency guy that I am, my very first thought on seeing this video translocate the Saturnian rings around the planet Earth was, “Hey! The Cassini Division’s still there!”

The significance of that gap between Saturn’s A and B rings is that it’s one of the most clear markers of the interaction between Saturn’s moons and the rings. All of the various gaps and spaces between the rings come from orbital resonances between the rings particles and various moons. If, for example, a ring particle orbits twice around Saturn for every orbit of the moon Mimas, then Mimas will pump energy into the orbiting particle and it will move into a higher-energy orbit with a larger semimajor axis – thus clearing a space in the rings (for the 2:1 Mimas resonance, the Huygens Gap).

That made me wonder just what a Terrestrial ring system would look like. We have only one moon, but it’s incredibly massive compared to the Earth. In fact, the Earth/Moon system has the largest moon-to-planet size ratio, by any measure, in the Solar System. (Sorry, Pluto/Charon!) Our single moon compared to Saturn’s dozens means that our ring system would be much more orderly, with many fewer and much more regularly spaced gaps. However, the huge size of the Moon means that the weaker resonances would have a stronger effect. The Saturnian rings show evidence of weak resonances all the way out to the double digits – like, say, 9:14 resonances – so I’d argue that weaker-still resonances would still be visible in the Earth-Moon system.

So, I wrote a little Matlab script. Clearly, this was more important today than getting my work done.

As in that video, I placed the outer limit of my hypothetical Terrestrial ring system at the Roche Limit, ~2.86 Earth radii from the center of the orbit. This is the innermost limit at which a fluid satellite could hold itself together, by its own self-gravity, against being ripped apart by tidal forces fromt he Earth. Outside this limit, the rings could start to aggregate together into moonlets. I bounded the inside of the ring at 1.59 Earth radii on the inside, coinciding with the definition of the outer limit of the exosphere. Even in low Earth orbit, atmospheric drag would eventually cause ring particles to fall into the deeper atmosphere, so I felt this would be a good value to pick to ensure that the ring would have a long enough lifetime to persist for millions or billions of years.

I started my script with a ring opacity of 100% at all radii and put a fuzzy boundary on the ring system at either end. Then I had Matlab calculate the orbital radii of every ring-Moon resonance from 1:1 to 100:100 using Kepler’s Third Law. For each resonant semimajor axis that fell between the Roche limit and drag limit, I subtracted a narrow Gaussian from the ring opacity as a function of radius. Since my big 100×100 matrix of resonances had some repeats (like 3:4 and 6:8), several of these Gaussian functions would add together and decrease the ring opacity further, crudely estimating the effect of stronger resonances. Finally, I lowered the albedo and tweaked the color of the rings from what they are at Saturn, to make them look more like they’re made of rock rather than ice, which sublimes away in space at our distance from the Sun. This is what I got:

THe Earth's hypothetical rings
The Earth's hypothetical rings

Earth's Rings in a more Moon-like color
Earth's Rings in a more Moon-like color

The rings in this image go around the Earth’s equator, inclined 22 degrees with respect to the field of view because of the Earth’s obliquity. Sadly, my Matlab graphics cannot handle casting the shadow of the rings onto the Earth, and I had to Photoshop in the shadow of Earth on the rings for effect. Still, pretty cool looking. Here’s the punchline: the ring system viewed from directly above the ring plane, with a white background so you can easily see the pattern:

From directly above the ring plane and backlit
From directly above the ring plane and backlit

You can see that the lunar resonances don’t start to have a major effect until about halfway through the ring system. This pattern, and the coloration, are mainly what that video was missing.

Of course, I don’t have the complete story, either. Again, our Moon is huge and that will do even more to the rings’ shape. The Moon’s orbit is inclined 5 degrees to the Earth’s equator, so the tidal torques from the Moon should make the rings precess around the Earth with a one-month period. (That precession would lag the Moon, so we wouldn’t always see the rings piercing the Moon in our night sky.) In addition, I suspect that the lunar tides would twist the rings a bit, pulling them into a spoked configuration like Cassini has seen at Saturn.

It’s definitely fun to think about how these rings would look from vantage points on the Earth. Actually, since my ring system starts well above low Earth orbit, I have to wonder what they would look like to spacewalking astronauts…