Aliens are coming. How soon do we find out?
March 28, 2011 8:10 PM   Subscribe

If an alien spaceship was approaching earth, how soon would we detect it?

I know it's a bit of a silly question, but we hear so much about UFO sightings and it makes me wonder. If an alien spaceship was coming towards us, wouldn't we be aware of it long before it was visible to the naked eye? That is assuming that it doesn't have some kind of cloaking technology to keep it invisible to us, of course.
posted by giggleknickers to Science & Nature (16 answers total) 5 users marked this as a favorite
Semi-realistically speaking, it depends heavily on the size of the ship. We can't even see Pluto clearly, and that's a (relatively) huge chunk of rock that we know where to look at. A ship would be (presumably) pretty small, not necessarily highly reflective, not necessarily terribly bright (depending on the kind of engine it used), and we wouldn't be looking for it.

On the other hand, once it was pretty close to the Earth, it would be pretty obvious. There's simply not that much stuff between, say, Mars orbit and Earth; we'd most likely pick it up and our first assumption would be that it's a fast-moving asteroid.
posted by Tomorrowful at 8:19 PM on March 28, 2011 [4 favorites]

Not enough info to answer. How big is it? How fast is it going? Does it look like a meteor?
posted by pompomtom at 8:19 PM on March 28, 2011

Response by poster: Hmmm, I was thinking that it would show some kind of flight pattern that suggested intelligent maneuvering, so I'm operating under the assumption that we wouldn't mistake it for a meteor.
posted by giggleknickers at 8:21 PM on March 28, 2011

Ignoring all the many explanations why UFOs are bullshit, unless it does something really weird like emit vast quantities of energy (in spectra we are actually watching), explode spectacularly, or disgorge little green men looking for our leader, we wouldn't. Sorry for the buzzkill.
posted by kjs3 at 8:24 PM on March 28, 2011 [1 favorite]

Hmmm, I was thinking that it would show some kind of flight pattern that suggested intelligent maneuvering, so I'm operating under the assumption that we wouldn't mistake it for a meteor.

Nah, our alien ship pilots would easily be able to run the math on where they should aim to intercept the Earth, so it'll look just like a meteor headed straight for us. Not much maneuvering. On the other hand, it'd probably spend a fair bit of time accelerating or decelerating to match speed with the Earth, so we'd hopefully notice either A) the fact that it appears to be influenced by forces other than gravity, which doesn't happen to dumb rocks, or B) the engine plume or plasma exhaust or whatever it is the little green people are using for thrust.
posted by Tomorrowful at 8:25 PM on March 28, 2011 [3 favorites]

My aliens would only decelerate at the last second. They've got interstellar travel, why limit them to our puny g-force tolerance? Also, they were just passing through and didn't notice us until the last second. They did all their acceleration a long time ago, and are storing their exhaust for reasons of stealth (these are not the friendly, glowy-fingered aliens).
posted by pompomtom at 8:44 PM on March 28, 2011

Best way to do it would be to put seismographs on the moon, for when it rumbles when a mothership passes near, and also just a bunch of listening stations in far orbit, so that we could hear the whine of their engines. Also wait and see if it tries spinning to dodge enemy pursuers.

Really though there are too many factors to factor in. The most efficient way for an alien ship to come to earth would just be to intercept with it after doing a slingshot from another planet's orbit, so it would just look like any other interstellar body, though it could be doing some out-of-the-ordinary speeding up and slowing down. And if it used any number of science-fictional faster-than-light/spacetime-bending drives, we wouldn't see it until it was already there.
posted by tumid dahlia at 8:51 PM on March 28, 2011 [1 favorite]

Most of the sky isn't under continuous observation. There's a reason why the majority of new comets are found by hobbyists.
posted by Chocolate Pickle at 9:28 PM on March 28, 2011 [1 favorite]

Best answer: As others have said, there are far too many factors you've left open in the question. There are a few obvious areas of detection, however:
  • Albedo: essentially, how shiny is the ship? Obviously, this also depends on the size of it: but a bright, shiny object would likely be caught by the Spacewatch program or an amateur astronomer, especially if it approached from sunward.
  • Occlusion: as the craft gets closer, it will hide stars behind itself as it passes between them and the Earth. A series of astronomy photos of the right area of the sky placed in a blink comparator (or its modern equivalent) would show the craft's transit.
  • Transmission: if the craft was transmitting on the radio spectrum, the SETI program would likely pick it up, and be able to tell (from the doppler shift in the signal) that the source was moving towards us.
  • Relativistic effects: if the craft was moving at a significant percentage of the speed of light, it would gain apparent mass, and possibly produce visual effects that could be seen in telescopes, such as micro-gravitational lensing.
About half of Neal Stephenson's Anathem has a plot that might interest you in this regard... there are a ton of hard to semi-hard SF books with similar themes.
posted by Bora Horza Gobuchul at 9:30 PM on March 28, 2011 [8 favorites]

If the spacecraft is not decelerating, we have as much chance to observe it as an asteroid of similar size and albedo (reflectivity). Somewhat less, as asteroids tend to burn up in the sun's light, and teh alien craft presumably would not.

Given the articles you've not doubt seen about asteroids coming very close (in astronomical terms) to Earth before being detected, it would be easy to miss a craft that was passing the Earth without actively decelerating.

Here's an article about the detection of two "house-sized" asteroids in near-Earth orbit. In it, NASA's Near Earth Program manager explains
"Things like this happen every day that we simply don’t know about because we don’t have the telescopes large enough to find them or surveys that are looking full-time," he said. "This demonstrates the system's working on some level, but we need larger telescopes and more of them to find objects that are coming this close."
However, if the craft is decelerating, it needs to expend energy proportional to its mass times the acceleration it wants to cancel.

If it does that with chemical rockets, that will produce light we'd easily see, light that would show up in an area of the sky where light isn't supposed to be. (A craft could attempt to orient itself to be between the Earth-side observer and an existing light source (i.e., a star), but this would be difficult to maintain, the more so as it got closer to Earth, and it would be difficult to mimic the particular frequency of the expected light source.)

Similarly, if the craft is nuclear powered, this would produce a characteristic illumination, which would be even more obviously brighter and "out of place" than chemical rockets.

If the craft used a solar sail to brake, this too would reflect light, and a lot of it, as the sail would have to be much larger than the craft, kilometers or hundres of kilometers of a big light reflector.

It might be possible to decelerate using another technique, such as launching rocks or ice or water instead of rockets, but as these would be of lower energy (and thus harder to spot), you'd need to launch many many more of them, meaning your initial mass (and thus, presumably volume) would need to be correspondingly larger and this more easily spotted from Earth.

The craft might instead decelerate using various "gravity slingshots" (Oberth maneuvers) around Earth (as our probes of the Solar System have used). This still requires some chemical (or other form of) propulsion, to position the craft for this.

So basically, if they slow down to match orbits with Earth, they have to expend energy to do so, and that will produce "noise" that we can observe, if we're looking.
posted by orthogonality at 9:59 PM on March 28, 2011

Just off the top of my head:

We can scratch off FTL because it's impossible and undetectable until it's on top of us. So let's assume a slow ship with engines pointing at the sun in a deacceleration burn. Another possibility is a solar sail.

Everything out to geosynchronous orbit is tracked via radar by multiple space agencies. On top of that, you have a low-res nuke monitoring system looking for unexplained gamma bursts. If it puts out or reflects EM, there's a reasonably high chance of detection.

Outside of Earth orbit, there's the Gamma-ray bursts Coordinates Network (GCN) which would probably detect anything using using fusion, fission, or antimater. Many of these detectors are omnidirectional and use the timing between widely-spaced detectors to estimate location. Depending on how much energy the UFO spits out and for how long, detection in the outer solar system or interstellar space is likely.

Visible and radio? Depends on how much radiation it's spitting in our direction. A solar sail would be bigger than a comet and much more reflective. Reactive engines would vary depending on the mass of the object. In both cases you're looking at sky surveys and the equivalent of blink plates.

At least in the foreseeable future, we have eyes on the sun, so anything using the sun as a way to dump momentum might pop up there.

Note also that it took 40-odd years to get something like MESSENGER to Mercury because it took more energy to get there than it did to shoot the Voyagers out of the solar system. Anything trying to get into Earth orbit from interstellar travel will have to burn an asston of energy to dump an asston of momentum.
posted by KirkJobSluder at 10:21 PM on March 28, 2011

In Battle: LA, the aliens were thought to be meteors. There were a lot of questions as to why the "meteors" weren't detected until they hit Earth's atmosphere. Then they realized the meteors were deaccelerating after hitting the atmosphere and that multiple meteor clusters were falling off the shoe of major seaside cities. Time to send in the marines.
posted by Brandon Blatcher at 4:27 AM on March 29, 2011

There is some monitoring of Near Earth Objects. In fact, there was a recent New Yorker article about it. If an NEO did something that violated the laws of physics, we might know. Look up the WOW signal. We don't listen to everything, but we listen, and anomalies get noticed.
posted by theora55 at 8:35 AM on March 29, 2011

Given all these explanations of what we would notice, I am thinking these are good arguments to use in the "UFOs are fake" conversations: If the sighted UFOs were really alien, surely we would have noticed their initial approach to Earth atmosphere, not just their hovering movements once they are already here, right?
posted by CathyG at 8:48 AM on March 29, 2011 [1 favorite]

Space War: Detection "There ain't no stealth in space!"
posted by Tom-B at 2:33 PM on March 29, 2011 [1 favorite]

At any given moment, half the sky is unobservable from the surface of the earth -- because it's on the sun side of the planet and you can't see the stars through the blue sky.

You wouldn't be able to see an approaching spaceship from that side, either.
posted by Chocolate Pickle at 7:28 PM on March 29, 2011

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