Several light-years away, an alien species has just discovered Mars. They deem it a candidate for colonization, so they launch a Voyager-like probe to get a closer look. They have no idea that there's life in our solar system; they just want to examine Mars more closely for signs of life and see if there's any more potentially habitable planets in the system.

The probe isn't landing, just flying by Mars and perhaps ending up in a stable orbit around the Sun.

This raises two questions:

  • Would we (modern humans) notice this probe? What conditions would have to be met to detect it?
  • Would the aliens notice us? Presumably, the aliens would notice some signs of life if they pointed the probe's instruments at Earth, but would they notice our technology?

Assume the aliens have a technology level similar to us, and ignore the problems of getting the probe to our solar system.

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    $\begingroup$ A question of luck? If the alien probe happens to fly by the sensors of a Mars orbiter, we see it. If not, not. $\endgroup$
    – o.m.
    Commented Jul 6, 2015 at 19:33
  • $\begingroup$ But if it's in a stable orbit (Around the sun or Mars), then we have a bunch of slim chances, which might multiply out to a good chance. $\endgroup$
    – ItsAmy
    Commented Jul 6, 2015 at 19:38
  • $\begingroup$ If it's not stealthed, it'd be fairly easy to detect through radar reflections. $\endgroup$
    – jamesqf
    Commented Jul 6, 2015 at 19:57
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    $\begingroup$ What's the tech level of the aliens? That will change answers a lot. If they are using rockets and radio, then yes we would see them. $\endgroup$
    – Tim B
    Commented Jul 6, 2015 at 21:08
  • $\begingroup$ @TimB The question does specify a tech level similar to ours. Of course, the year isn't specified, but I think we can safely assume the OP intended present day tech. $\endgroup$
    – Frostfyre
    Commented Jul 6, 2015 at 21:52

6 Answers 6


Assuming the launch was done in our own pre technological era (with current technology, we "might" be able to send a probe to Alpha Centarui arriving a thousand years from now) so we didn't see the exhaust or heat signature, the probe is coasting in at interstellar speeds. Even interplanetary velocity is pretty huge (an object can stay in solar orbit until it passes about 72 kilometres/sec velocity), so if the intention is a flypast and using a tight beam laser or similar system to communicate with the home world, it is quite possible the object could fly through the solar system and not be noticed, or briefly tracked and noted as an anomaly ("Excessive warmth in a rogue interplanetary body on a hyperbolic orbit" might be the title of the paper written about this).

Once the probe powers up, however, it's game on. To save mass, they might deploy a "Mag sail" in the far reaches of the solar system and use the solar plasma as a braking medium. This will result in a "radio howl" as the plasma is violently disturbed by the magsail, and radio astronomers will be able to calculate the rate of deceleration by the frequency shift as the probe slows down, and then work out the new trajectory and orbit. If the aliens use a light sail, the astronomers will be treated to a vast mirror image of the Sun coming at them from deep space, and once again examining the spectral shift of the image they will be able to determine a lot about the probe. Using a rocket engine of some sort simply amplifies the issue and makes things even easier for the Earth to determine something is out there and start making plans.

Of course, in the last 100 or so years of the journey, the probe itself would be aware of technological activity coming from the Earth in the form of radio and television transmissions. Even though they might not be able to interpret then, the power, patterning and strange appearance/disappearances of the transmissions due to the Earth's rotation would be difficult to miss. I note there seems to be no provision by the builders of the probe as to what actions to take in that event. If they are paranoid enough, then simply shutting down the systems and doing an unpowered flypast (as per paragraph one) might be sufficient.

This also presents an interesting quandary for the Earth. If we really did detect a probe arriving in the Solar System, we would have very few options. Trying to send a probe or spaceship to intercept it is going to be beyond current technology (it took 10 years of intricate orbital manoeuvres to meet up with a comet), so we would be mostly passive observers of the probe. The various powers might try to communicate with the probe, and certainly some sort of Earth probe might be sent or (if a suitable one is close enough) diverted to try and look at it. If the Earth is paranoid enough, there might be some discussion of how to destroy the incoming probe. This would be fairly easy in physics terms, if a bucket of sand or ball bearings could be released in the path of the probe the high velocity of impact would vaporize it. The problem would be just getting the bucket into place.

Long term, Humans would try to get to Mars to see the probe if it has gone into orbit, as well as send expeditions to Mars to understand what the probe is looking for. For the Aliens, they will be seeing that Mars is being rapidly settled and claimed by the Humans, so once again, we would need to know if the builders had programmed any sort of contingency plan for something like this.

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    $\begingroup$ Any estimations on the ratio of probe mass to "magsail" area? I've not heard of this kind of device and I'm curious. $\endgroup$
    – Green
    Commented Jul 6, 2015 at 23:39
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    $\begingroup$ You might start with this wikipedia article: en.wikipedia.org/wiki/Magnetic_sail. A magsail built with near future technology is expected to have a mass/thrust ratio of 600 kg/N, which isn't much, but since you would be doing this on a constant basis as you fall towards the Sun, it adds up. $\endgroup$
    – Thucydides
    Commented Jul 7, 2015 at 2:04
  • $\begingroup$ I love this answer because it considers several different options for the probe to decelerate, as well as the scenarios of both stable orbit and flyby. $\endgroup$
    – ItsAmy
    Commented Jul 7, 2015 at 3:01
  • $\begingroup$ At any rate, whether we'll see the probe or not, let's hope the aliens will never find us. $\endgroup$
    – Christine
    Commented Jul 7, 2015 at 16:54
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    $\begingroup$ Re-read johannes's answer. The SSME could be detected from as far away as Pluto, and the orbital manoeuvring thrusters seen from as far away as the asteroid belt. A ship or probe com in in at interstellar velocity would have hundreds to thousands of times the energy in the exhaust in order to slow down in any reasonable time. $\endgroup$
    – Thucydides
    Commented Jul 8, 2015 at 1:10

Assuming the aliens have technology similar to us, yes, we would definitely notice them.

The only way to slow down from interstellar space is to send energetic mass ahead of you in the direction that you're going. Space is a vacuum, you can't just stick out a parachute and slow down as you approach the system. There is a little atmosphere on Mars, but the inter-system distances are so huge that you can't hope to use Mars' tiny atmosphere to aerobrake.

The aliens would have to load the ship with enough fuel to accelerate for up to half of the trip from their system to ours, then turn around in the middle and blast the solar system with the energetic exhaust. All the while, they'd be radiating any heat from inefficiency of their engines and system into space with glowing radiators. All of this would be occurring in the otherwise cold, dead, silent darkness of space: Easily detectable with a telescope.

As the Rocketpunk Manifesto describes:

In space, everyone sees everything.

Project Rho further describes some current-tech examples in There Ain't No Stealth In Space:

This means the exhaust is so intense that it could be detected from Alpha Centauri. By a passive sensor.

The Space Shuttle's much weaker main engines could be detected past the orbit of Pluto. The Space Shuttle's manoeuvering thrusters could be seen as far as the asteroid belt. And even a puny ship using ion drive to thrust at a measly 1/1000 of a g could be spotted at one astronomical unit.

As of 2013, the Voyager 1 space probe is about 18 billion kilometers away from Terra and its radio signal is a pathetic 20 watts (or about as dim as the light bulb in your refrigerator). But as faint as it is, the Green Bank telescope can pick it out from the background noise in one second flat.

This is with current off-the-shelf technology. Presumably future technology would be better.

But you said to ignore the problems of getting the probe to our solar system, which is conveniently when it would be most noticeable, so check the bit about the Voyager 1 probe and its 20 watt radio signal. Voyager 1 is just coasting, so the whole issue about blasting rocket or ion exhaust at us is moot. Perhaps you're just going to coast silently by and blast your Unobtainium-built probe backwards at unimaginable acceleration out of a rear-facing, hundred-mile-long railgun as you zoom past Mars, where it can fall down with an Unobtainium parachute.

The probe still needs to communicate with the mother planet to let it know whether life is possible, so it has to emit radio energy of enough power so that the home planet would see it. We can't even really see the signature of a Martian-sized exoplanet occluding the sun; the probe will need to send out unimaginably powerful signals for the home planet to detect it. We are currently searching for the faintest possible signals of intelligent life; there's no way we'd miss recording a communication from our own system destined for a system light-years away with our same technology. Unless, of course, it blew out all our receivers...

  • $\begingroup$ But what if the signal being sent back to the alien homeworld was a laser, or otherwise very tightly focused? Then wouldn't we have to be in its path to detect it? (Which brings up another question: Do Voyager and other real-life probes have such tightly-focused beams?) $\endgroup$
    – ItsAmy
    Commented Jul 6, 2015 at 20:17
  • $\begingroup$ Yes, we could miss it. A laser with very short pulses such that the pulsed power is not noticable against the background can only be detected (and robustly read) if the receiver knows the cadence as well as the frequency and exactly where to look. Optical seti is in its infancy and certainly does not cover all the possibilities, nor have we even thought of all the possibilities. $\endgroup$
    – JDługosz
    Commented Jul 6, 2015 at 20:17
  • $\begingroup$ Now I'm curious. This was a great first post -- Welcome to the site, by the way! -- so why did it get downvoted? $\endgroup$
    – Frostfyre
    Commented Jul 6, 2015 at 21:50
  • $\begingroup$ I don't think they would use rockets to slow down when entering the solar system. If they managed to detect Mars, they surely also detected some of the gas giants. So I would expect them to plan on using slingshots to slow down. Enough delta-v for a direct approach does not sound realistic. $\endgroup$
    – kasperd
    Commented Jul 6, 2015 at 22:20
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    $\begingroup$ "you can't just stick out a parachute and slow down as you approach the system" Haha, kind of funny that the current accepted answer suggests the probe might do just that. (A magsail is basically a giant parachute designed to work with solar wind instead of regular wind.) $\endgroup$
    – Ajedi32
    Commented Jul 7, 2015 at 13:50

For just a flyby, we'd really have to be lucky and looking in the right place at the right time. For comparison, it's estimated there are approximately 100 near-earth objects larger than 1 km in diameter that we have yet to discover, and those are objects we are actively looking for, for obvious reasons. There are tens of thousands of spacecraft-sized near-earth objects we don't know about. Even if we saw an alien probe, we would assume it's an asteroid, confirm its orbit, put it into a database, and give it no further thought.

The communications are probably easier to pick up, although spacecraft tend to have relatively narrow band transmitters for efficiency reasons. You can pick up their signals fairly easily, but mostly you need to know what to listen for. Your best bet of detection would be in the case of some sort of interference. Perhaps the alien probe is doing routine scans and amplifies and retransmits any interesting signals it receives, causing interference with our equipment on or around Mars. That we would notice fairly quickly.

  • $\begingroup$ This is the best answer. $\endgroup$
    – Tony Ennis
    Commented Jul 8, 2015 at 4:52

In order for us to detect it on Mars, it would need to do something that would make us notice because remember, space is really big. The more energetic the event, the more likely we will detect it. A long deceleration burn to put the probe into orbit around Mars would have a good chance of tripping some IR scanning satellite somewhere. Deorbiting the probe in Mars' atmosphere would look indistinguishable from a meteor and likely ignored.

Edit: After reading the "There ain't no stealth in space paper", any kind of deceleration burn would assuredly be detected.

There are satellites that take pictures of the surface of Mars with sufficient resolution to locate various Mars rovers. If the alien's surface probe was as big or bigger than the US Curiosity probe then we should see it after a few months or a year. If the probe has a larger orbital component then we have a decent chance of detecting the orbiter based on its radio transmissions to home (though this may not work if the orbiter is using a tight beam radio transmission.)


I think it's utterly ludicrous that a spacefaring alien race only a few lightyears away from us thinks that Mars is habitable and don't think that Earth is, or even that they can sneak up on us.

First off: Earth has been sending out electromagnetic radiation for at least a century by now. That means that our alien race likely has been sending them out just as long since they're at a similar tech level. So we would have known about them for a while by now. Similarly, they would have known about us for ages by now, at least good enough that they've watched all our series except anything up to, say, 2010 (so anything before Game of Thrones), assuming they're 5 years from us.

Additionally, they can calculate that these radio waves are coming from Earth, not Mars, and that nothing is coming from Mars except for a few weaker signals from our older probes, like Spirit and Opportunity.

Finally, even without these signals, Earth would have been a far better candidate to send a single life detecting probe to. It's right in the middle of the habitable zone around Sol, while Mars is really on the outer edge of that zone. If the aliens, like us, are carbon-based lifeforms, it's likely that they need oxygen and hydrogen to live, which they detect massive amounts of on Earth using spectrometers. If they aren't carbon-based, they still would likely view Earth as more interesting because of the more varied spectrometry of our planet. Mars has about 96% CO2, 2% argon, %2 Nitrogen and traces of other gasses. Earth has 78% nitrogen, 21% oxygen and 1% of traces of other gasses. The only way they would see Mars as more promising is if they breathe CO2.

  • $\begingroup$ For your first complaint: Hey, people (and aliens) make mistakes. But more importantly, your comment about "sneaking up": The original question never specified that the aliens were trying to sneak up, just that they wouldn't be actively broadcasting their arrival, since they don't know that we're an intelligent species. $\endgroup$
    – ItsAmy
    Commented Jul 7, 2015 at 2:59
  • $\begingroup$ Space is big. It's entirely plausible that, at the time the probe was launched Mars seemed a better candidate for life than Earth. $\endgroup$
    – Taemyr
    Commented Jul 7, 2015 at 11:57
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    $\begingroup$ There's no reason to think that Mars couldn't be more habitable to these aliens than the Earth is. All of your assumptions on habitability are based around our own species. $\endgroup$ Commented Jul 7, 2015 at 14:57
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    $\begingroup$ They wouldn't "know all about us" from watching our TV signals, even assuming the signal strength was strong enough to get a coherent signal, because the way radio waves are encoded for TV broadcast is strongly related to some very specific technical details regarding early color TV hardware (pre-HD) or to Earth-specific digital protocols designed by committee for acceptable compromises, not for technical excellence. They might be able to tell that these signals mean something, but they wouldn't be able to resolve the moving pictures without having lived among us. (Or spied on us.) $\endgroup$ Commented Jul 7, 2015 at 19:39

Years ago, when Art Bell hosted Coast To Coast, one guest was a conventional mainstream scientist dealing with space research - I can't remember his exact name or area of expertise. While he discounted UFOs and alien visitation claims, he did get Art's attention when he said that one of his mainstream colleagues was surmising that alien probes may have already visited our solar system. He didn't say who that was or why he had that feeling. I don't think he had any direct physical evidence - ie. spacecraft debris. However, I wonder if his guess what tied to the theory of panspermia - life spreading through meteors. However, it could also hitch a ride on alien space probes.

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    $\begingroup$ Can you provide a source for this? $\endgroup$
    – Vincent
    Commented Jul 7, 2015 at 0:56

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