Quantum Rocketry

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!

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.

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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…

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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.

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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.

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

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

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…

I’ve been thinking all summer about NASA’s Constellation program. In general, I think it’s great to be getting out of low Earth orbit…but I think the strategic goals of the Vision for Space Exploration and the technical solutions in Constellation are somewhat lacking. My experience this summer at Johnson Space Center, and the Augustine Commission’s generally open mind, have made me very hopeful – I’ve noticed that the people at JSC aren’t as gung-ho Orion-is-the-best-thing-to-hit-space-since-John-Glenn as I feared they would be. (That seems to happen more at Marshall Spaceflight Center, where they are building Ares, and in the mind of Mike Griffin, who has called anybody who wants the current architecture to change “ignorant.” Note that that includes famous people like Buzz Aldrin and a lot of top-flight engineers.)

The current Constellation architecture is a result of basically two factors:

(1) A knee-jerk response to the Columbia disaster. NASA wanted safety during reentry, and it decided that capsules never burn up in reentry. (Never mind that while Shuttle had two major disasters in ~130 missions, Apollo had two major disasters in ~20. The tildes are because both numbers depend a little on how you count.)

(2) The Vision for Space Exploration, which sets a “Moon sorties first, then Mars sorties, and I guess we can support ISS until it’s just barely finished” mission for NASA.

That first item is a terrible point to be feeding directly into strategic planning and design work, because it limits creativity and ingenuity. Right now, Constellation is operating under the assumption that it should use as little new technology as possible. Compare that to the Apollo design days, when engineers designed their spacecraft with materials that had yet to be invented. The second item takes away from Constellation the kind of grandiose vision that could really attract attention and acclaim to NASA. We have gotten complacent with our space program; it seems routine to the ordinary Earthling. That’s the exact opposite of what we need to nuture strong public support of the space program (and science and technology in general): astronauts, flight controllers, scientists, and engineers need to be heroes again.

So where do I think NASA should go, and what should it do to get there? First of all, let’s take a look at the capabilities that NASA has now.

NASA has an incredible low-Earth-orbit capability. The Shuttle lets us bring a lot of mass into orbit. It also lets us bring a lot of mass down from orbit, which is crucially important for the Space Station’s science architecture and is a capability that Orion ignores. We also have a lot of experience with rendezvous, capture, and docking, from the assembly of ISS, Hubble servicing missions, and the like – going all the way back to Gemini – also a versatile capability virtually ignored by Orion. Finally, ISS has given us a lot of long-duration spaceflight experience.

NASA is also very good at getting robots to and operating them on Mars. Sure, several missions failed, but each of those did so because of a specific technical reason which all have known fixes. The last four major missions (Phoenix, MER1 and 2, and MRO) were all – or still are! – rousing successes.

Now let’s look at what’s problematic about today’s space program. The Shuttle is horrendously expensive to operate. It is also unsafe, not because its design is inherently flawed, but because each launch requires a total refurbishment of the vehicle and the vehicles are effectively 20-30 years old.

Here’s what I think NASA ought to do strategically, and (without doing any detailed trade studies, because though I’m a Ph.D. candidate in spacecraft engineering, I am acting as but a humble blogger here) how I think it could get there.

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