Let say some light years away there is an earthlike world. Thousands (millions?) of years ago they sent out a Voyager like probe. The probe is happily cruising along at 20km/s. The probe passes right by our humble planet. Would we even notice?

Given the size and the speed - would one have to be directly looking for a probe travelling that fast to notice it or would it pass by leaving us oblivious to life on another world for another however many years?

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Spock Jenkins of the Vulcan Jenkins'.

Yes, you would have to be looking for it. It would be electronically dead and very small. Remember, cometary debris comes along now and then too. It almost certainly would be missed.

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Disclaimer: Avatar is not an official NASA image and does not imply any specific interplanetary or interstellar capability.

The Leif Ericson Cruiser

We might miss it. More likely it would be recorded as one of the hundred tiny asteroids and comets that pass close to Earth each year. If the solar panels were brightened from reflected sunlight we might marvel that so little mass would be even 25th magnitude.
Likely it would pass Earth at 60 km/sec if it came from outside our solar system. It would pick up speed as it's galactic orbit fell toward our sun. That fast would attract our attention, except it would only be close to Earth a few hours, and the flash of reflection from the solar panels would last seconds at the most. Neil

We would miss it unless it came within a few thousand miles of Earth AND got lucky as far as photographic evidence.

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Forming opinions as we speak

Well, in order for it to pass at only 20 km/s coming from outside the SS it would have to perform some kind of deceleration burn. I´d guess such burn would leave an observable EM signature, especially in radio.

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What brings us together is stronger than what pulls us apart

" ISS ruptured by space debris" but thats just fiction. we would not be aware of this pioneer type space probe whistling past Earth. If you take the next step down this line of thought what would be the chances of detection if the space craft is not transmitting a signal. Almost 0. Isn't reality booring?... But if it did a retro burn and entered Earth orbit it would get our attention.

I figured as much. I stumbled upon this site a year or two ago after reading a couple hypothetical light speed ideas and other astronomy news and getting all excited thinking we were near some break throughs. I'm very much the uninformed novice - but logical enough that I didn't come in expecting to know anything. You are correct, I did find out how boring reality is - although it's strange how something like astronomy and the geological history of our planet can be so boring and amazing at the same time.

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Spock Jenkins of the Vulcan Jenkins'.

I drew a manga where advanced humans discover a Voyager-type probe in the Oort Cloud of a system with an Earthlike moonlet, visit and get chased by a MIB-type group. More humor than anything else, especially the assistant of the astronomer who found the human ship, thinking it was a meteor.
"This could be the most important moment of either of our lives, Ansi."
"Yeah, but it could also be the last moment of our lives, Dr. Safi. They could be brain-eating aliens like in the movies!"

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I want to go back to the moon.
I don't care which rocket you use, whichever one you pick, I'll like it, I swear.
"If you think the LHC will create black holes, you might as well believe Hobbits are at the bottom of your garden."- Dr. Mike Inglis
Rovers forever! - ToSeek

You mean, "Ameobas."

I totally agree.

I will find myself reading a paper on Permian strata and thinking "OhYeah, only 251 mya" and then thinking about supper in 30 minutes. I don't think we would see it unless it bit us in the butt.

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily available to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

"Quaerendo inventis"

I still believe that the chances of us or them noticing each other...even if looking right at us, are negligeable.

1. All life wouldn't evolve at the same time, nor at the "right" time.

2. No life below technological could be detected from very far away without a probe. And we haven't had any probes landing here even.

3. Distances forbid it... even with science fiction, the likelyhood of someone passing by us near or faster than lightspeed and detecting us is NILL.

4. We just aren't interesting enough for anyone to look at. ..if they are THAT advanced, they would have encountered our types several times, and just shrugged.

In another thread someone mentioned the intersting concept that a probe (Like Voyager) that had been travelling in space for a very long time would accumulate a LOT of dust and debris on its surface. Eventually it would become the "core" of it's own asteroid... And you wouldn't even recognize its nature unless you cracked it open.

It probably depends on the trajectory and distance from earth.

The U.S. Space Surveillance Network (SSN) is an array of electro-optical sensors that monitors objects in deep space: http://en.wikipedia.org/wiki/US_spac...llance_network

It can detect orbital space debris the size of a bolt or screw. It's likely a much larger object like a space probe in a near-earth trajectory could be detected further out.

This paper indicates the limiting sensitivity of one optical SSN sensor is about magnitude 18, which is pretty dim:

http://www.mitre.org/work/tech_paper..._deepstare.pdf

However mere detection would mean little. It would just be another space debris track -- a few pixels cataloged. We have no quick-reaction capability to rendezvous or inspect it more closely.

Whether it was a space rock or a probe with a message inscribed on the side, we'd never know the difference -- even IF it was detected.

What makes you think there are movies about brain eating ameobas on Vita'minc?

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I want to go back to the moon.
I don't care which rocket you use, whichever one you pick, I'll like it, I swear.
"If you think the LHC will create black holes, you might as well believe Hobbits are at the bottom of your garden."- Dr. Mike Inglis
Rovers forever! - ToSeek

As others have said, we'd miss it. Something that small would be invisible unless it were very, very close, and the flyby would be blindingly fast; we're talking crossing the Earth-Moon distance in less than two hours.

Hence, as Sagan himself once said, IIRC, any civilization capable of finding--let alone capturing--the Voyagers will almost certainly be far more advanced than we are, all the more so because close flybys to any star are extremely unlikely.

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Ian Safi would certainly agree. That was mostly the model for his character, with some Milo Thatch (From Atlantis: The Lost Empire) and Nikola Tesla mixed in. Oh, and green skin.

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I want to go back to the moon.
I don't care which rocket you use, whichever one you pick, I'll like it, I swear.
"If you think the LHC will create black holes, you might as well believe Hobbits are at the bottom of your garden."- Dr. Mike Inglis
Rovers forever! - ToSeek

Considering we saw it a month before, what measures can we take to capture it? I suppose hitting anything with 20 km/s would shred it into dust. Can we send a mini nuclear engine to give it enough momentum from the opposite direction to slow it down? Or make it orbit the earth.

It would be interesting to see the methods we can use to capture such an object.

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A slight inclination of the cranium is as adequate as a spasmodic movement of one optic towards an equinine quadruped utterly devoid of any visionary capacity.

The U.S. Space Surveillance Network (SSN) is an array of electro-optical sensors that monitors objects in deep space: http://en.wikipedia.org/wiki/US_spac...llance_network

It can detect orbital space debris the size of a bolt or screw. It's likely a much larger object like a space probe in a near-earth trajectory could be detected further out.

Not quite. The SSN is a collection of radar and electro-optical (EO) sensors. The EO sensors are primarily the GEODSS units. They're capable of tracking a volleyball-sized object out at geosynch but nothing as small as a bolt or screw. While they sometimes are used to survey NEOs further out, that isn't their mission. They might detect a Voyager-sized spacecraft if it passed fairly close to the Earth but only if it happened while the sensor was enjoying a clear night. GEODSS doesn't work during daylight and it can't see through clouds.

The best chances of detecting a Voyager-sized object would be from those sensors working full time to track NEOs. Even then, it'd have to come pretty close to the Earth to be detected.

I didn't mean to imply they could detect a small screw at GSO distance.

As stated, SSN consists of both electronic and optical sensors. According to this document some of the radars can detect 1 cm objects (presumably in LEO) http://www.fas.org/spp/military/prog...ack/mccall.pdf

I think the recorded track data of some of these radars were used after the Columbia disaster to evaluate small pieces (a few cm across) that drifted away from the vehicle while in orbit. The goal was to compare the radar data for these to the probable size of an RCC tile fragment.

The OP didn't say what trajectory, so many are possible. If it came pretty close -- up to a few hundred miles, the liklihood of detection is greater.

Agreed the SSN mission is not deep space detection, so it's unclear if a random Voyage-size space probe that came (say) within GSO distance (22,000 mi) would be detected. The radar resolution capability is probably there -- if they knew ahead of time. Whether a random Voyager-size passage at GSO distance would be detected, I'm not sure. I tend to doubt it.

We don't detect all near-earth asteroids, so it seems unlikely we'd detect a much smaller space probe, even if it passed closer than the moon.

But -- mere detection means little. The question implies if it was detected, we'd learn something: ("look there goes an alien probe").

In reality, even IF detected it would be a few pixels (optical or radar), IOW "there went something". Even IF it was detected, we'd never know whether it was a space rock or an alien probe with a message inscribed.

I didn't mean to imply they could detect a small screw at GSO distance.

Unfortunately, GEODSS doesn't have that kind of resolution at any distance.

I think the recorded track data of some of these radars were used after the Columbia disaster to evaluate small pieces (a few cm across) that drifted away from the vehicle while in orbit. The goal was to compare the radar data for these to the probable size of an RCC tile fragment.

None of the SSN phased array radars would've been able to track pieces falling off of Columbia, at least not after it cross the US coastline. Beale quite likely could've tracked Columbia earlier in the reentry but that might've been before pieces started falling off. None of the other phased arrays look in the right direction. It's possible that the Fence might've seen a few pieces when Columbia crossed its coverage. However, the Fence only looks straight up and can only see things that pass through it.

As for 1 cm resolution, there's only one radar with that resolution and that's only under special circumstances in a particular operating mode. I hesitate to go into the operational details.

Radar coverage out at geosynch is pretty limited. Only a few of the radars can track things out that far and most of them are mechanical trackers instead of phased arrays. This is important because of the coverage areas for different types of radars. Depending on power availability and the range, a phased array radar can set up a "fence" to track whatever passes through. A mechanical tracker can really only detect whatever passes through the area where it's pointing. It's kind of like looking at something through a soda straw - your field of view is really limited. Unless the mechanical tracker is tasked to scan an area (typically by sweeping the antenna in a bow-tie or spiral pattern), the chances of detecting something new and unexpected are really, really slim. An object could pass undetected as close as a degree or two of the radar's boresight by being outside the beamwidth.

In reality, even IF detected it would be a few pixels (optical or radar), IOW "there went something". Even IF it was detected, we'd never know whether it was a space rock or an alien probe with a message inscribed.

If someone were able to track it long enough to determine the trajectory, they might discover that it came from outside the solar system. That would make it very interesting but wouldn't necessarily mean it was an alien probe.

why are you everywhere telling that everything is booring...NOT everyone needs hyperdrives and 200 suernovas per day in the universe to be amazing

Just to be clear, I was talking about post-crash evaluation of radar data taken while Columbia was in orbit. The goal was to see if any small object (such as an RCC panel) separated while in orbit.

A small object, likely about 9" x 10", was detected by various space surveillance assets including the SSN, the U.S. Navy Space Surveillance System, and the Air Force PAVE PAWS radar. The details are discussed in the CAIB report.

However this very case illustrates the limitations of current capability. The object was only found after the fact, only because they knew to look for it, and it required exhaustive sifting through mountains of data, including specialized supercomputer radar cross-section analysis.

To turn this around a little, if something(?) wanted to approach covertly, they could??

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily available to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

"Quaerendo inventis"

if the probe started broadcasting its data back to its originating star as it made or soon after the fly-by... could we detect that?

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plenty of woo, at the hotel hoagaland...

SETI would possibly detect part of the data transmission, but it would be very weak assuming the probe aimed it's very narrow beam laser and/or microwave beam at it's home planet, the beam would all but surely miss Earth, however about 1% of the beam energy is typicaly scattered in many other directions.
Other than the several projects simular to SETI, I don't think anyone is looking for random signals from space. Neil

Yes, could we? I have a SF story where the hero recieves a ransom call at him house. He dials the unknown call number but gets nothing, because the villian is calling from another solar system (They're both something very much like Time Lords.) An alien spaceship that's an ally of our hero nearby intercepts the call and traces it with their "advanced alien tech".

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I want to go back to the moon.
I don't care which rocket you use, whichever one you pick, I'll like it, I swear.
"If you think the LHC will create black holes, you might as well believe Hobbits are at the bottom of your garden."- Dr. Mike Inglis
Rovers forever! - ToSeek

Dept of Defense space surveillance maintains close watch on orbital objects for national security reasons and for tracking space debris. Many thousands of small orbital debris objects are tracked and cataloged, since this facilitates identifying new objects. It also helps avoid collisions with manned orbital vehicles like the shuttle and ISS.

It seems unlikely a large object (say the size of a large unmanned space probe) could orbit the earth within several thousand miles for a long time and not be detected, since much smaller objects are detected regularly.

However as already stated, the main DOD space surveillance emphasis on earth orbit, not beyond. I doubt a large space probe passing at 1/2 lunar distance would be seen.

Also the radar assets are at known (or knowable) locations. If an incoming sophisticated robotic or crewed extra-terrestrial space vehicle wished to avoid detection, it could theoretically just avoid entry paths covered by those radars.

Whats with you? where did I say that?
Go back and read what I said. I have a passion for astronomy and science. If I appeared negative or you think I judge as boring, No. I do not. I enjoy a different point of view.
I am also sure that a passing space probe would or could be detected if it came within a lunar distance of Earth. But would detecting it tell us anything we would want to know. No it would not. As I said previously ' if it did a burn and put itself into orbit. We would know'.

There should be objects going past our solar system faster or slower than everything else here. Ejected comets, asteroids and....planets should swing through our system every so often. However, since no scientist as observed anything extrasolar coming into our system, it is very likely that such and event is rather rare and if an alien voyager like probe pass us, it would be very, very, very, very rare and a coincidence that at this point in time the probe pass earth and not say, a billion yeaars ago.

For all we know an alien probe carrying mircrobes crash into Earth or Mars, seeding the world(s)[ATMS] 4 billion years ago.

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The problem is the word "rare".

Rare for our short existence. And the lack of evidence if it doesn't cause something we can go back and find.