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.
The whole “stealth spaceship” discussion is quite interesting. Mr. Chung is doing an excellent job compiling useful information for artists, writers, and anyone interested in rocket ships at Project Rho, and the arguments against space stealth presented at his web site are basically correct, but I can’t help but feel that something is missing- a discussion of the detectors used to locate our would-be stealth spaceships.
There are two ways to detect a spaceship- we can either detect the radiation a spacecraft emits, or bounce our own radiation off of the spacecraft and detect the returning radiation- i.e. radar and lidar. To detect these radiations, a space cruiser will need telescopes operating in a wide variety of wavelengths (infrared to detect radiated heat, optical, possibly x-ray and gamma ray to detect fusion and antimatter engines) and radar dishes. The question to ask is how good are sensors (i.e., how far out can they detect a certain object), and how might we try to conceal ourselves from enemy sensors? Is it even possible?
Spaceships emit plenty of radiation. Rockets produce lots of waste heat and a visible exhaust plume. Powerful rocket drives like fusion and antimatter engines produce brightly visible exhaust trails and hard radiation, as well- a terawatt burn is easily detectable even at a distance. Even while the jets are cool, anything operating at a temperature above a few degrees above absolute zero produces thermal radiation- including the comfy crew cabins and heaters maintaining devices at operating temperatures- which will be detectable by sensitive infrared sensors. Electrical equipment produces weak RF radiation. Our ships also reflect light near stars, and must avoid heating up too much in sunlight- which means shiny surfaces.
I doubt there is much a spaceship can do about its thermal signature- even outside the sensor-friendly environment of space, heat is a big problem for tanks and aircraft. On Earth, these craft can try to blend in with the background by cooling their exhaust, but in space the background is only 2.7 degrees K unless you are directly in front of a planet or star. The amazing technology demonstrated by Spitzer shows that the cryogenic infrared detector can be shielded from room temperature components on the same spacecraft. Perhaps the thermoscope could be mounted outside the spacecraft, carefully shielded by insulating pylons and reflective materials so it can scan for the heat signatures of interloping Martian rocket ships. Space forces will place sensitive detectors in orbit, passively scanning the skies for a heat signature that would give away an approaching alien invasion force. Sadly, I don’t know how to calculate the maximum range at which a particular spaceship might be detected with modern technology- I’ll have to find out.
Radar will be popular for detecting incoming spacecraft, missiles, and shrapnel. Detection is not the same as a target lock (getting a firing solution on an enemy ship), and again radar will probably be invaluable for targeting enemy ships. Here, we might actually be able to do something to obscure our signature- angular surfaces for bouncing away radar, radar absorbent materials, black surfaces, etc.- but there will be limits. Attempts to remain “stealthy” could interfere with the spacecrafts thermal management, energy generation, and propulsion, as NROL-15 must contend with. Furthermore, future detectors will probably be better than what we have today, and many more observers will be trying to pinpoint a “stealth spaceship” than a few receivers on the ground, including space based observation platforms. We’ve already seen stealth technology fail on Earth- an F-117 Nighthawk was eventually outfoxed and shot down over Yugoslavia. A single error, like firing a thruster or deploying a weapon too soon, might give away a stealth spaceship as it sneaks up on its target, even assuming stealth technology will work in space.
It would be interesting to see stealth tech in space- I imagine black, angular spacecraft using various sophisticated technologies to mask their radar and thermal signatures, but I’m still unsure about space stealth. Even with stealth fighter style technology, like NROL-15 presumably uses, no ship could ever completely hide its signature. Also, simply firing your rockets could give you away. This makes me wonder if the crew of a stealthy space fighter would end up helpless- incapable of maneuvering, turning on a toaster, or deploying sensors without compromising their stealth. D’oh!!
You say that if the alien stealth spaceship can get close enough to launch its missiles before being detected, its stealth will pay off- but if it has to conduct a thrifty engine burn many millions of miles away and cruise for 6 months to avoid being spotted by thermoscopes (at which point our space battleship is many millions of miles away from where it had hoped to ambush us), or just can’t avoid being given away by the radiated heat as it draws within several thousand kilometers of our craft, will it win the battle?
Then again, perhaps this is an overly simplistic picture, and space stealth will prove practical- or even just a bit of obfuscation and trickery to keep the nasty aliens from spotting a stealth drone tailing their mothership. After all, no sensor is perfect, and the basic idea of stealth is not making yourself totally invisible, but finding how your enemy detects you and masking those signatures. It’ll be interesting to see whether or not stealth spaceships are practical or not. NROL-15 changes things, a bit like finding those plans for one-man space fighters in military papers just after the hard SF fans debunked the “space fighter” yet again.
I always thought it was highly amusing that cloaked Romulan and Klingon ships are given away by their thermal and EM signatures in Star Trek, not to mention the exhaust plasma from the impulse engines…
I always thought the problem was twofold for stealthing it.
1.Waste heat. Heat sinks (and directed radiators, do those work?)
2.Engines. Any sort of conventional rocket is impossible to disguise.
I think I may have found a way around 2. Specifically a concept called a magnetic sail.
The idea is that you have a giant loop of superconducter wire dragging your ship along like a hybrid parachute/sail. The usual mode for this is using it to make a plasma-based solar sail, but wikipedia(I know, right) mentions in passing the possibility of thrusting directly against nearby magnetospheres, like Earth’s and the Sun’s. Practically a reactionless drive, at least where there is a magnetosphere to push off of.
The idea of achieving reactionless propulsion by interacting with planetary or stellar magnetic fields is actually something that my graduate research group looked at quite a bit! There are several ways to do it, including electrodynamic tethers (which is what I suspect you found on Wikipedia). There are some very real ideas that are within our current technological capabilities.
Heat sinks delay the problem – but a delay may be good enough to stealth a spacecraft when required. Directional radiators are certainly another way to manage thermal radiation from the vehicle. In fact, current spacecraft don’t radiate heat isotropically – and many of them are built to radiate in a particular direction. For example, the Space Shuttle radiated most of its thermal energy out from the open cargo bay doors. The Space Station carries deployed radiators that have to be angled away from the Sun to work most efficiently.
Actually, rocket plumes are very difficult to detect. You cool your engine with the cryogenic propellant, shoot it out the nozzle where its heat is converted into kinetic energy (because that’s what a rocket nozzle does) and you’re all set to thrust cool and quiet.
The ship radiates energy, sure, but its surface can be as cold as you want. Radiate 10 kW out of a tennis ball, it’ll be hot and bright. Radiate the same 10 kW out of a 100m sphere and it’ll be very cold and almost invisible. Also, you can use a very small amount of cryogenic propellant to absorb a lot of waste heat and boil it off into space.
And of course the ship’s hull is a faceted mirror, as reflective as possible. Solar radiation doesn’t heat up the ship, it is reflected in a very thin cone. And yes, the navigator has to make sure that the cone doesn’t intersect enemy sensor installations.
So ships can be extremely hard to detect by any sensor setup, both while thrusting and while coasting.