It is not circular in the literal sense shown on ancient maps of the Earth, before we understood Earth to be a sphere. Rather, Gliese 581g spins at the same rate as it orbits its star, so its sun is always in the same place in its sky. Heat from the red dwarf, distributed by the circulation of the atmosphere, keeps a circular region under the star warm enough to melt ice into liquid water. Thus, the habitable regions fall entirely within a disc under the constant light of the red star. Outside this region, water freezes – and the further one goes out onto the ice, the more inhospitable it gets. Travel to the far side of the planet is about as difficult as traveling from the Earth to the Moon – and so, to the inhabitants of Zarmina, their world might as well be a circle ringed in ice.1
This artist’s concept, based on image mapping from our recent interstellar probes, depicts the habitable region of Zarmina:
For discussion of Zarmina, some reference points and directions are necessary. The circular boundary of the map is the ice line: beyond this point, water is certain to freeze. The center of the circle thus defined is the substellar point. When standing here, the red dwarf Gliese 581 is directly overhead. This image shows Zarmina oriented with is orbital plane horizontal. The planet has a south magnetic pole pointing roughly towards the top of the page, and so the “top” and “bottom” of this map become the cardinal directions north and south. East and west take on their usual definitions.
Gliese 581g is approximately three and a half times the mass of Earth. It is tidally locked to its star, meaning that one side always faces its Sun just as one side of the Moon always faces the Earth. Gravitational tides from the star also have the effect of pulling the rocky surface of the planet into an oblong shape, like a rugby ball. Since our probes reached the Gliese 581 system,2 we determined that the planet has a tiny orbital eccentricity (from perturbations by the other planets in the system) which causes a periodic shift in the gravity force on the planet: slightly east to slightly west, and back again, every Zarminan day (about 37 Earth days). The combination of the periodic variation in stellar tide and the fact that the ocean is more mobile than rock makes dry land much more common in the center of the disc than near the edge, as we see in the map.3
Two space-fighter games recently came out in quick succession. Both are free, downloadable fan-made takes on popular franchises, and both show very high production values. The first is Wing Commander: Saga, based on the 1990s-era Wing Commander space simulator games.* The second is Diaspora: Shattered Armistice which lets you hop in the cockpit of your favorite fighter from Battlestar Galactica, accelerate out a launch tube, shoot up some Cylon raiders while flying sideways, and then burn in for a combat landing.**There’s also a recent article on the Foreign Policy web site about carriers in space. So now I’m thinking about that favorite military sci-fi trope: the space carrier!
Whether it makes sense, from a military, technical, or economic point of view, to build a carrier vessel to launch smaller fighting craft is a complex argument. (The FP article discusses more of this than I will here.) The major reasons to do so would be the same reasons why we build naval aircraft carriers now: the ship provides a base of operations for the aircraft, and allows them to participate missions that they could not perform on their own. That’s the sort of argument that even a far-flung space military would go for – if backed up with plenty of supporting evidence – but whether their space carriers launch single-seat fighters, small-crew attack ships, or robotic drones is up for grabs. I think that we can’t completelyanswer that question without knowing more about the reasons for this space military’s existence and the socioeconomic conditions during the Space War!
Let’s just suppose that it makes sense to have some kind of mother ship carrying some kind of smaller craft in a space military. I’m going to take a couple examples of carriers from military science fiction and grade them on what they do well and what they don’t. My examples are going to illustrate some common types of space carriers in media: space carriers from Star Wars, space carriers from the 2004-2010 TV series Battlestar Galactica, and space carriers from from the “Wing Commander” games.
In the wee hours of last Monday (Eastern US time), a jubilant Mission Control erupted at the successful landing of the Mars Science Laboratory “Curiosity.”
Curiosity has demonstrated some amazing technological feats. Now, that portion of its mission is nearly over, and the rover will go over to science operations. The hair-raising, fist-pumping, frenzy-whipping part is done – but it’s been great practice!
While the MSL entry, descent, and landing system may seem harebrained and silly, it is in fact quite conservative and driven by fundamental engineering decisions. The engineering triumph of this system demonstrates to me how spacecraft engineers can set extraordinarily technically ambitious goals and achieve them in dramatic fashion. The JPL engineers who devised it are the types of people who design a device to last for three months and find it still happily ticking away six or eight or more years later. This thing was going to work. The toughest part was probably selling the concept to the NASA brass!
So, now we’ve got reinforcing knowledge that we can aim for the stars and hit them (well, planets, anyway). Let’s set out with some crazy-ambitious goals! And let’s set out for some places that let us answer fundamental questions.
This is my core disagreement with the NASA Decadal Survey, which prioritizes a Martian sample return mission above all else: such a sample return will advance the sub-sub-field of Martian geochemistry an incremental amount. This is not an ambitious enough goal to meet our demonstrated engineering capability! I don’t want to discover evidence that some place may have been habitable sometime in the distant past – I want to go someplace where we discover life because it’s staring right back at us.
Not so long ago, I proposed a mission concept for a subsurface probe to Jupiter’s ice moon Europa. Europa is intriguing because we already know that it has liquid water, and we already know that it has a strong energy source from Jovian tides – both of which are key ingredients for life as we know it! Even better, there are certain surface features on Europa, which – if our best models for how those features form are correct – are conduits from outer space to the ocean beneath. I suggested that we might develop a space vehicle that conducts a high-wire act above one of these exposed ice fractures, dropping probes down into the ocean below.
When I started my original series of posts on space battles, I speculated about what a combat-spacecraft designer might want to do in order to make a vehicle that could avoid enemy detection:
It would make sense to build their outer hulls in a faceted manner, to reduce their radar cross-section. Basically, picture a bigger, armored version of the lunar module.
Almost immediately, I got some feedback pointing me to the “Project Rho” website, which declares quite bluntly that “there ain’t no stealth in space.” The argument goes basically like this: any device you put on a spacecraft has to obey the second law of thermodynamics, which means that it generates waste heat. This heat will raise the temperature of your spacecraft well above the background temperature of ambient space (about 2.7 Kelvin). Therefore, the spacecraft will radiate and will be visible to infrared sensors, no matter what. Therefore, stealth combat spacecraft are impossible.
This argument is fundamentally sound. The principles are correct: you can build a detector that could locate any spacecraft. What I don’t like about this argument is its implied definition of the word “stealth” as “total invisibility.” Yes, it is possible that the detector you build will locate a stealthy space-fighter eventually. That clock is always ticking. But your adversary’s stealthiness can still pay off – if they get to launch their missiles before you spot them!
Later on, when I revisited space-battle physics, I went into a little more detail about possible stealthing technologies for spacecraft. In another post, I thought about some of the thermal concerns our hypothetical space-fighter designer would run into in trying to make the fighter hard to detect.
But the proof, as they say, is in the pudding.
There’s a military aphorism (Wikipedia tells me that Helmuth von Moltke is responsible) that battle plans never survive contact with the enemy. I suspect that, for all anyone’s speculations about what can, cannot, will, will not, or might happen in space combat, if we ever did find ourselves in a space war we would very quickly learn an entirely different set of guiding principles. Whether or not stealth spacecraft are possible will be apparent then, after the fact, no matter what arguments we make today.
However, we can get some insight by asking the question: do any stealth spacecraft exist today?
The answer, as it turns out, is “yes.”
Weather permitting, we are coming up on the launch of a Delta IV Heavy – a gargantuan behemoth leviathan giant of a rocket – carrying a National Reconnaissance Office “spy” satellite with the cryptic designation of NROL-15. Quoting a civilian military space analyst, AmericaSpace reports that the vehicle
is likely the No. 3 Misty stealth version of the Advanced KH-11 digital imaging reconnaissance satellite. It is designed to operate totally undetected in about a 435 mi. high orbit.
The article includes some description (or speculation?) about the physical appearance of the stealth spacecraft, too:
Looking somewhat like a stubby Hubble space telescope stuffed in an giant F-117 stealth fighter with diverse angles to reflect radar signals in directions other than back to receivers on the ground, Misty 3 is also covered in deep black materials designed to absorb so much light that it can not be tracked optically from the ground.
These design aspects are a huge challenge for a satellite that must also deploy solar arrays to generate electrical power and have reflective surfaces to reject heat. … The satellite may actually change shape to reflect heat when not over hostile countries trying to break its cover.
Apparently, there may also be some tricky maneuvering by the launch vehicle – to disguise the final orbit trajectory of the satellite. There is some speculation at the end of the article about the various options the vehicle might take to pull off that feat of obfuscation.
The bottom line for science fiction: cloaking devices are probably not going to work. But are stealth spacecraft possible or not? Well…we’re already doing it.
If you follow space news, you’ve likely seen one of the articles on this event. Woah!
I’ve like to contribute just a couple things to the wild speculation at this point. The MIT Technology Review article concludes that asteroid mining is the only possible thing of interest in space – but really, that is just one writer’s blog. I want to point outthat there are other possibilities:
Space-based solar power systems: either a constellation of satellites or a system of stations on the lunar surface that collect solar energy and beam it back to Earth, with the potential to provide inexpensive (after the initial investment!), reliable electricity to anywhere on the globe. Phil Plait at Bad Astronomy correctly identified this as a possibility. Tom Jones’ involvement makes me think this possibility less likely, though.
Lunar mining: not only are there potential resources on asteroids, but there are some on our nearest planetary neighbor! While the Moon had higher gravity than an asteroid – requiring a little more than a token kick to lift return vehicles – its proximity makes it a more reachable target.
Water mining: outer solar system moons are often covered with water ice laced with minerals or organic compounds. A robot could land on the surface, cut out blocks of ice, and thenshove them Earthward. I’m not sure there is an economic case for this activity, but I wouldn’t rule it out as a bad idea for all time.
My bet is that they are going for asteroids or the Moon, but I think space power systems are a potential line of business for Planetary Resources. Maybe they plan on becoming a general space-based utility company!
Well, since I just had some discussion about orbits and other fundamental physical concepts in science fiction, here’s a short scene I’ve been sitting on. It’s set in the Cathedral Galaxy, and I’m not quite sure what I want to do with it yet.
The Kite stretches his solar wings wide, spanning over five hundred meters. He fans out his array of electromagnetic membranes, thermal structures, transceiver antennae, and weapon emitters, flourishing. The Kite’s voice booms out over the electromagnetic spectrum, mingling with the others in the Coliseum, as they announce themselves to the assembled spectators:
“In salute, we die and live by the will of the Imperium!”
The Kite pulls one solar wing out from the light flux to tack. He wheels around, scanning and assessing his competitors. He catalogues their capabilities but pays special attention to their faces – distended from all the grafts and alterations, stone-gray and glassy-eyed from the environmental treatments, yet still faces. The younger competitors growl and sneer at him, while the more experienced repay his cool appraisal in kind. Today, The Tiger and The Worm worry him.
Silence falls across the EM bands, leaving The Kite with only the intermittent discharges from the Coliseum walls. His stomach (though no longer really a stomach) lurches in anticipation. A moment drags on in the flickering silvery shell of the Coliseum, buried in the sparse mist of an orange nebula. This could be the day, thinks The Kite, when I die. Again.
The Kite pulses an electromagnetic field, launching himself away from the spherical inner surface of the Coliseum. The others do the same. Continue reading In the Arena→
Decadal surveys and other prioritizations of potential NASA exploration missions often rank one thing very highly: a sample-return mission from Mars. However, I think there are some much more scientifically interesting, technologically challenging, and engaging to the public mission proposals out there. This is one: a Titanian UAV!
The idea is to send an airborne vehicle to Saturn’s moon Titan which would fly around the moon, observing surface features from its high vantage point. A powered flyer, as opposed to a balloon, has the advantage of being able to travel to a specific location: such as the moon’s liquid lakes!
The proposal team uses some clever mission planning approaches to handle the limitations of the aircraft: for example, using glide phases to hoard power for downlink sessions. Their nominal mission duration is one year: a year of exploring another planet from the air, a year of images and science data depicting a world of lakes, rivers, ice, and rain. The full proposal is online here.
I find the idea exciting, and I hope that NASA’s governing councils soon prioritize exploration of those extraterrestrial locations most likely to harbor life – like Europa, Enceladus, and Titan.
Ah, I’ve only been out a few months, but I already miss some things about being in grad school! For instance, I miss all the crazy brainstorming of new and wild space systems, missions, and technologies. No doubt you, dear reader, also miss my crazy brainstorming: after all, that is how I ended up writing blogs about space battles or missions to Europa or what the Earth would look like with rings or the science of Avatar. Now I have an industry job where people tend to care more about “affordability” and “reliability” and “performance,” than they do about harebrained schemes to drop space probes into the Europan ocean.
But, fear not, intrepid reader who has been sticking it out hoping for another crazy notion to appear here! You see, my research group at Cornell is still working at churning out wild ideas. And you can participate!
Check out this message from Zac, who was starting his Ph.D. as I was on my way out:
Zac has set up a page on KickStarter, which you can jump to by visiting KickSat.org. The idea behind KickSat is to make a bare-bones 10x10x10 cm CubeSat which contains hundreds or thousands of microchip-sized satellites called Sprites and will deploy them all in low Earth orbit. The KickStarter platform means that, if you want, you can sponsor your very own Sprite – Zac has even defined a sponsorship level at which you get to write your own flight code for the tiny spacecraft to run in orbit!
The spacecraft, which each could fit comfortably in the palm of your hand, are very simplistic as far as spacecraft go – they consist of solar cells to charge a little bank of capacitors, a teeny TI processor for a brain, and a little antenna. These are proof-of-concept spacecraft, and are actually derived from three test units which my lab group sent up to the Space Station on the last launch of the Space Shuttle Endeavour! In the future, they hope to integrate other sensors onto the chips to give Sprites more capabilities. One of the ideas batted around during lab meetings that I consider a personal favorite: put “lab-on-chip” detectors on a Sprite to look for characteristic organic compounds (like nucleic acids!) and program them to simply send a chirp back if they get a positive result. Send a million Sprites to Mars, and listen to the peeps – and then you know where on the Red Planet the next big flagship mission has just got to go!
Imagine if you got the shot at writing the flight code. If you could put a solar cell in space and make it beep, what could you measure? How creative can you get in getting the Sprite’s whisper of a radio signal to carry information? Could you receive enough data to tell how fast the chip is spinning and seeing the Sun, or how much average power it has to work with, or how long it lasts before an errant proton from the solar wind blasts your Sprite out of the sky? The chance to put your own code on a spacecraft, even such a simplistic one, offers a lot of learning opportunities.
(Incidentally: one question that Zac and his research advisor, Dr. Mason Peck, get a lot is some variation on: “Hey, paint flecs moving at orbital velocity are enough to crash through the Space Shuttle windows. Aren’t these Sprites going to become dangerous space junk?” The answer is that yes, the Sprites could be hazardous as long as they are in orbit; but the orbit that KickSat will reach is going to be within just enough of the Earth’s atmosphere that all the Sprites will get dragged down in a couple days. The special property Sprites have that influences this fast orbital decay – and other effects – is a high surface-area-to-mass ratio.)
KickSat has already reached its minimum fundraising goal to start building hardware. However, the project is still looking for more backers to secure a commercial launch opportunity, which will offer more certainty than applying for a free launch program through NASA. But if Zac gets to about $300,000 of funding, he thinks that will be enough to start looking at new technologies to shrink the Sprite chips down to even smaller sizes – and offer even more capability in the future!
Cool stuff. I’m glad to see the Cornell Space Systems Design Studio keeping the wild space ideas flowing!
I can’t help but contrast this animated system with the SLS announcement from NASA. It illustrates my criticism of recent NASA policy perfectly: at Congress’s behest, the space agency has stopped innovating.
It’s not a super-heavy-lift launch vehicle that will enable expansion of the human exploration program beyond flags-and-footprints missions or the long-term development of space. Instead, it’s the fantastically easier access to space afforded by a rapidly reusable launch system like that presented by Musk. The control technology and hardware for such a system exists already; I hope to see test flights in a few years. With only a little luck, they’ll happen before the first SLS is supposed to take off.
An article appeared today on NASA.gov about the detection of “free-floating planets.” These planets may have formed around a central star, like the planets in our Solar System did, but due to some gravitational interaction during their star system’s formation the planets escaped their stars. These Jovian planets, which may outnumber stars in our galaxy, are now doomed to endlessly wander the cosmos under perpetual starry night skies.
Naturally, this notion tripped my sci-fi circuits.
We live in an age in which new planetary systems are being discovered at an incredible rate. We are getting closer and closer to the ability to detect other Earth-like worlds around other stars. In fact, just a few days ago a study found that certain climate models of Gliese 581d (that would be potential-planet Zarmina‘s until-now-slightly-less-sexy sister) may support a liquid water cycle.
So what would it take for one of these free-flying, starless planets to be habitable?
The immediate answer that may come to you, the average person, is, “Joe, you are crazy.” But wait a moment!
All life requires is an energy input and certain chemicals, right? Well, all sorts of chemicals exist in gas planets. And there are plenty of possible energy inputs from the gas dynamics going on in their atmospheres – not to mention magnetic fields and other esoteric stuff like that that Earth life generally doesn’t incorporate into its metabolism.
But forget gas-giant balloon-life. Suppose we constrain our notion of habitability to the usual anthropocentric meaning: liquid water on a rocky surface.
In order for a rocky planet to have liquid surface water, it needs two things: heat and pressure. (Pressure so that the water doesn’t just sublimate or boil off into space, and heat so that it doesn’t freeze.) The “pressure” part we can take care of by giving our rocky world an atmosphere. However, we need a heat source – not only to keep the water from freezing, but to keep the atmosphere itself from freezing onto the planet, too. How do we get this heat source? Radioactive heating from the planet interior isn’t going to warm the surface to 273 K. Stars are all going to be too far from these planets to do any good. Emission nebulae are way too cold and rarified, even if the planet is right in the middle of them. The planet is going to pretty efficiently radiate away any heat inputs before that energy goes into heating ice to make water. (I suppose we could stick the planet right in the way of a black hole’s polar jet or some other source of hard radiation for our energy source – but then we’re back to getting really alien alien life. Fun to think about! And what happens to those alien civilizations that thrive on a dark planet bathed in X rays when their planet finishes traversing the zone of hard radiation?!)
I’m pretty convinced that liquid surface water is not going to appear on any free-flying rocky planets. Unless…
Suppose, when a Jovian planet got ejected from its birth star system, it carried its moon system away with it. Maybe some heat can come off of that gas giant and hit the moon! It’s not going to be reflected light, though, because there’s no star to provide bright enough light. No, the energy will have to come from the Jovian itself. This condition means that we’ll have to look at something like brown dwarfs: astronomical bodies that are just slightly too small to ignite under their internal pressure and turn into the hydrogen fusion furnaces that are stars. But they do have some fusion going on in their dense cores.
Take Teide 1, the first brown dwarf to have its existence confirmed. It has a surface temperature of around 2500 K, a luminosity of about 0.001 Lsun, and a radius around 0.1 Rsun. Suppose that a rocky (Earth-density) satellite orbits Teide 1 at its Roche limit, the closest orbital radius it can have without tides tearing the moon apart into a pretty but uninhabitable ring. (By a quick calculation, I get about 337,000 km for Teide 1 – coincidentally close to the Earth-Moon distance.) At that distance, the moon would receive around 1 million watts per square meter from the Jovian. If that’s the input power, the Stefan-Boltzmann law gives the output radiation of the planet in equilibrium. With a couple assumptions about albedo (Earthlike) and assuming that the moon receives incoming radiation over its cross-sectional area but radiates out over its entire surface (and that it’s the size of Earth’s Moon), my quick hand scratchings give a surface temperature near 50 K. Hmm…no liquid surface water there.
But there’s another possible heat input to a moon around a gas giant: the tides of the Jovian world.
Consider Jupiter: it has four big moons, and Jupiter raises such huge tides on these moons that the rocky mini-worlds actually flex, generating heat from friction. On Europa, this tidal heating in its central rocky part is sufficient to melt the inner bit of its water-ice coating into an ocean. Heck, scientists combing Galileo probe data just determined that tidal heating is sufficient to keep pretty much all of Io’s interior molten. That world is made of lava, with a thin crusty shell. And it’s all because the moon orbits a gas giant in a resonance with some other moons. the interaction between their orbits keeps the tidal energy coming.
So let’s give our moon some companions and an orbital resonance. Solar radiation is negligible compared to tidal heating even for Jupiter, so we know that that could give our moon liquid water…at least under the surface, like Europa.
But add an atmosphere, and you get an insulating blanket around the moon’s surface. More internal heat stays trapped on the moon’s surface instead of radiating away into space. I haven’t done the calculations, but if tidal heating can liquify rock on Io I bet it could be enough to melt Europa’s ice layer all the way through for slightly different orbital parameters. And with an atmosphere, the moon gets pressure to keep that liquid water from boiling. Like Titan. Put Titan where Io is…and what do you get? I’m not sure, but it would be really interesting. And it wouldn’t require the Sun.
Cool, huh? It certainly hasn’t been confirmed, and I don’t have a detailed model, of course, but I think the theoretical grounds exist for these free-flying dark planets to have liquid-water surfaces. Imagine vacationing on a beach next to a steaming ocean that is basically a global-scale hot spring, where it’s perpetual night and every couple (Earth) days you see the shadowy form of the gas giant loom overhead, visible more because of the stars it blocks out than from any external light source, except for the occasional immense spark of lightning through its clouds…