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IsaacKuo
2013-Jan-04, 11:11 PM
A recent Centauri Dreams post discusses the paper The Effect of Probe Dynamics on Galactic Exploration Timescales by Duncan Forgan of the University of Edinburgh.

http://www.centauri-dreams.org/?p=26083
http://arxiv.org/abs/1212.2371

This paper investigates the possibility of exploring the galaxy using interstellar probes, where the probes start off with very low Voyager-like speeds but build up to on the order of 1000km/s via repeated free gravity assist slingshots with each stellar encounter. Each probe visits many star systems, passing close to each star so its gravity will bend its trajectory toward the next star.

They find that mass produced probes from a single source system are sufficient to explore the galaxy a billion years. I'd quibble that much faster times are possible if white dwarf assists are included (for more common and not as hazardous as neutron stars/black holes). But leaving such quibbles aside, it's a good concept.

But here's the interesting thing about this method of exploring the galaxy--it leaves no lasting evidence behind, and it offers a compelling reason to not leave anything behind. Each probe needs to retain its speed and conserve its maneuvering fuel so it can visit many star systems rather than just one.

So how might we detect these hypothetical ET flyby probes? Well, if there were just one ET civilization that did it billions of years ago, it's hopeless. Even if there's any evidence left to be found, it's in the form of long dead probes drifting through interstellar space. But if there are a bunch of these flyby probes cruising through the galaxy today, we might have a chance.

The key feature of these probes is that they fly close to each star, so they can use the star's gravity to deflect them toward the next star. SOHO or other solar observations might detect such a probe as a tailless comet. But, instead of slowly diving into the sun at a mere 500km/s, these would be zooming by at a blistering 1000km/s.

Such an ET flyby probe would be almost certainly artificial in nature based on its incredible speed alone. There just aren't many natural ways for something to reach such speeds, and for such a thing to just happen to graze by the Sun would be too much a coincidence.

Further evidence could solidly prove its artificial nature. The incoming and outgoing trajectories could be traced to the previous and the next stars. For both of these to be aimed perfectly at nearby stars would be implausibly unlikely by chance.

Unfortunately, the galaxy would have to be teeming with these probes for us to be lucky enough to see one within the next millenium. But, it wouldn't hurt to look just in case. After all, it piggybacks on existing data gathering for Sun research/sun-grazing comet research.

IsaacKuo
2013-Jan-04, 11:17 PM
...umm...Sorry about the wall of text. Here's a brief summary:

1) Paper describes idea of ET probes which visit multiple star systems using solar flyby slingshots.

2) I propose using SOHO to look for these, since they'd look like super fast tailless comets.

3) I discuss that they would be definitively artificial in nature rather than natural.

Paul Wally
2013-Jan-04, 11:50 PM
...umm...Sorry about the wall of text. Here's a brief summary:

1) Paper describes idea of ET probes which visit multiple star systems using solar flyby slingshots.

2) I propose using SOHO to look for these, since they'd look like super fast tailless comets.

3) I discuss that they would be definitively artificial in nature rather than natural.

It looks like a very difficult problem. Perhaps some more information is needed, so I'll ask a few questions:

Do these probes use any energy sources for propulsion and communications?

Is their size comparable to that of a comet?

You see, I'm firstly wondering how any objects of too small a size would be detectable. But if they are too small, then the only other option I see is to look at some kind energy emissions either for communications or propulsion or both.

A.DIM
2013-Jan-05, 12:08 AM
It's a neat idea Isaac, but it strikes me as being as vanishingly unlikely as SETI. That it piggy backs on existing science is great but as you say the galaxy would need be teeming with them, similarly advanced civs (to be employing such tech) all sending their probes within a fairly narrow window of time.

None the less, I certainly agree we'll never find them if we don't look.
Thanks for sharing!

neilzero
2013-Jan-05, 12:45 AM
Your analysis seems correct = a tailess comet at twice usual comet speed. Has SOHO found any like that? My guess is we are looking carefully for intersteller comets with hyberbiolic orbits. Even a minor orbit change anomoly would be a surprise if there is no tail. How small can SOHO see a million kilometers from the Sun's photosphere, if there is no tail? Would a thin shiny platimum skin survive the infrared one million kilometers from the photosphere? How about a skin made of Halfnium which has a higher melting point There would be a tail if the probe has an ablative coating. Does SOHO do chemical anaysis of the tail? Neil

TooMany
2013-Jan-05, 06:14 PM
A speed of 1000 km/s is not so fast given interstellar distances. It is an interesting idea, kind of a star ship in a pinball machine .

How much could the trajectory be "bent" by a flyby say a couple of solar diameters from the Sun? A comet at 500 km/s is close to escape velocity, so maybe at 1000 km/s you would get about a 90 degree change in direction? Would 1000 km/s be cruising speed or top speed near the Sun? At that speed a transit would last about 20 minutes, but would the thing be detectable if just say 100 meters across? Wouldn't it be more likely to be seen the way earth crossing asteroids are observed against a dark background rather than by SOHO?

publiusr
2013-Jan-05, 07:10 PM
Now here is where having a dense star cluster is a big help. A star cluster gives you the "forty acres to turn the big rig around" as it were in that you could not only use gravity assists in (relatively) short order, but can change direction while in the cluster. It might be better to look at clusters in that they may serve as a railway turntable.

If I wanted a probe near the galactic center, I might go to the cluster first, come screaming out of that--and do a close pass and dump speed near the galactic core BH.

Gomar
2013-Jan-07, 03:48 AM
Just one problem with alien probes. How would they get back to their home planet to relay the
info collected? If probes are sent on 10,000 year old missions, the alien civilization, or nation, or
even the planet might not even exist. So, what's the point of probes if you cant learn or get any
info from them?
Perhaps, some probes could send messages back, or even videos, but for how long and how far out?
There is a point at which signals cant be pick up.

However, here is a bright idea:
What if these probes hand the info over to the next world they find? If they locate an intelligent
civilization possessing science, then all the knowledge learned from thousands of years of flying
around is just passed on. Ifcourse, the probe learns from each planet, gets a message, or videos,
or data, and keeps flying to the next world.

Ifcourse, planet Earth has not (atleast that we know of) been visited by any alien probes.

djellison
2013-Jan-07, 06:24 AM
2) I propose using SOHO to look for these, since they'd look like super fast tailless comets.

You could also use STEREO and SDO.

But how much chance do they have. They have not, to my knowledge, discovered any asteroids (which would be a similar challenge and we KNOW they exist)

STEREO and SOHO have spotted a few known asteroids - but nothing of astronomical merit.

IsaacKuo
2013-Jan-07, 03:53 PM
You could also use STEREO and SDO.

But how much chance do they have. They have not, to my knowledge, discovered any asteroids (which would be a similar challenge and we KNOW they exist)

STEREO and SOHO have spotted a few known asteroids - but nothing of astronomical merit.
SOHO has, in fact, discovered over a thousand comets so far:

http://sungrazer.nrl.navy.mil/index.php?p=introduction

Both SOHO and STEREO have coronograph sensors which block direct light from the Sun's disc in order to study the corona. The reason SOHO tends to discover comets first is simply a matter of data download details (SOHO's data is more realtime).

IsaacKuo
2013-Jan-07, 04:53 PM
A speed of 1000 km/s is not so fast given interstellar distances.
Going faster does not really gain much, for this concept. Two downsides to going faster are that you spend less time in each system, reducing the quality of data, and you have to go further to reach the next star system within the reachable deflection angle.

How much could the trajectory be "bent" by a flyby say a couple of solar diameters from the Sun? A comet at 500 km/s is close to escape velocity, so maybe at 1000 km/s you would get about a 90 degree change in direction? Would 1000 km/s be cruising speed or top speed near the Sun?
The hyperbolic excess velocity is related to perihelion velocity and escape velocity by this equation:

v_inf^2 + v_esc^2 = v^2

So, a v_inf of 1000km/s and v_esc of 500km/s corresponds to a perihelion velocity of 1120km/s.

The deflection angle is calculated by:

sin(theta/2) = 1/(1+2*v_inf^2/v_esc^2)

So, in this case we have a deflection of

sin(theta/2) = 1/(1+2*1000kms^2/500kms^2) = 1/9

theta/2 = 0.11 radians

theta = 0.22 radians = 13 degrees

For a v_inf equal to v_esc, we get a deflection of 39 degrees.

If you want a 90 degree deflection, you need a v_inf a bit less than half of escape velocity.

Note that white dwarf stars are common, and they have escape velocities far greater than a sun-like star. With a typical escape velocity on the order of 3000km/s, sharp deflections are easy enough.

At that speed a transit would last about 20 minutes, but would the thing be detectable if just say 100 meters across? Wouldn't it be more likely to be seen the way earth crossing asteroids are observed against a dark background rather than by SOHO?
SOHO detects comets via its LASCO coronagraph sensors. These sensors block the Sun's disc in order to study the much fainter corona. This is ideal for detecting sungrazing comets, because they are extremely brightly lit against a dark background.

Now here is where having a dense star cluster is a big help. A star cluster gives you the "forty acres to turn the big rig around" as it were in that you could not only use gravity assists in (relatively) short order, but can change direction while in the cluster. It might be better to look at clusters in that they may serve as a railway turntable.

If I wanted a probe near the galactic center, I might go to the cluster first, come screaming out of that--and do a close pass and dump speed near the galactic core BH.
Really no reason to go through that trouble. If you just want to get a probe to the galactic core, you could use a powered flyby of a white dwarf star (available all over the place, not just at clusters), and get faster velocities.
There are also plenty of plausible methods of providing much faster relativistic velocities...these aren't really relevant to this concept because such speeds come with downsides for multiple flyby missions.

Just one problem with alien probes. How would they get back to their home planet to relay the info collected?
There are a number of possibilities, which the paper does not delve into. The most obvious is to radio back information. This does not actually require extremely long distance comms if the probes can relay data between each other. Another possibility is to simply collect the data and wait to dump it whenever the probe passes nearby another probe or the home system.

If probes are sent on 10,000 year old missions, the alien civilization, or nation, or even the planet might not even exist. So, what's the point of probes if you can learn or get any info from them?
Again, the paper does not delve into these questions. But there's no particular reason to expect a priori that the alien civilization will die out. The aliens in question may even have individual lifespans which are indefinite, so the individual aliens can indeed expect to get back info from them. Even without long lived aliens, the probes in question can begin to provide data soon after launch.

Perhaps, some probes could send messages back, or even videos, but for how long and how far out? There is a point at which signals cant be pick up.
Actually, with radio communications there's no inherent reason why the signals cant reach all the way across the galaxy. It's a question of sufficiently large size. But even with small probes, it's possible to just gather data and wait for returning near the home system. This means waiting for the data about twice as long--which may or may not be a big deal depending on the mission specifics.

However, here is a bright idea: What if these probes hand the info over to the next world they find? If they locate an intelligent civilization possessing science, then all the knowledge learned from thousands of years of flying around is just passed on. Ifcourse, the probe learns from each planet, gets a message, or videos, or data, and keeps flying to the next world.
That's a cute idea. It implies the probes have some capability of detecting an "intelligent civilization", though. It would be simpler for such probes to just transmit their data upon request. Like Vger, it's just designed to wait for a particular radio command data queries, and replies with the requested data. If some alien civilization is smart enough to figure out the communications protocol, then they can reap the spoils, sure.

Ifcourse, planet Earth has not (atleast that we know of) been visited by any alien probes.
If flyby probes have passed by Earth, we'd have no way to tell unless it took place within the last century and we were lucky enough to see it. Otherwise, it would just be another dot in the sky.

IsaacKuo
2013-Jan-07, 05:09 PM
It's a neat idea Isaac, but it strikes me as being as vanishingly unlikely as SETI. That it piggy backs on existing science is great but as you say the galaxy would need be teeming with them, similarly advanced civs (to be employing such tech) all sending their probes within a fairly narrow window of time.
Why do you consider SETI as vanishingly unlikely? Given the uncertainties involved, it's hard to assign any a priori probabilities to SETI.

As for this concept, there is no need for more than one civilization to be sending them, nor is there any need to be sending them within a specific narrow window of time. Once the probes are on their way, they might continue doing their job for extremely long timescales--providing regular updates rather than just visiting the galaxy once.

As with SETI, the uncertainties are so high that it's hard to assign any a priori probabilities.

Your analysis seems correct = a tailess comet at twice usual comet speed. Has SOHO found any like that?
No. So far, no interstellar comets have been detected by any means (except for some comets barely above escape velocity--consistent with a solar system comet on its way out due to a Jupiter interaction).

My guess is we are looking carefully for intersteller comets with hyberbiolic orbits. Even a minor orbit change anomoly would be a surprise if there is no tail. How small can SOHO see a million kilometers from the Sun's photosphere, if there is no tail? Would a thin shiny platimum skin survive the infrared one million kilometers from the photosphere? How about a skin made of Halfnium which has a higher melting point There would be a tail if the probe has an ablative coating. Does SOHO do chemical anaysis of the tail? Neil
I don't know how small a tailless comet can be and still be detected by SOHO. But plausibly, these probes may be pretty large. Unlike Voyager, they would not typically fly very near planets. If the goal is to study planets and not just the host stars, then they'd need some decent sized optics to do so.

Also, there's a question of power and comms. Depending on how long they're supposed to operate, the most appropriate power sources may be solar and/or ISM heating. Solar power is only available in brief bursts, but these times coincide with when data gathering is done--could be quite useful for radar sensors. ISM heating is less obvious. It essentially gets energy from the gravity flyby maneuvers, and it slowly converts the probe's kinetic energy into heat as the probe collides with ISM. Either way, the desirable form factor for the probe is broad and thin--a large sheet, like a solar sail. Being a solar sail also solves the problem of how to do course corrections without eventually running out of propellant.

So, plausibly these probes could be pretty big and easy for SOHO to see, even if they're pretty lightweight.

djellison
2013-Jan-07, 05:49 PM
SOHO has, in fact, discovered over a thousand comets so far:.

Comets have huge big bright tails.

Asteroids - which is what I said - do not. They are hard to find - as would a large extra-solar spacecraft. I would suggest that the thing to look at isn't speed - but size / magnitude. Are the things you are looking to find even theoretically identifiable in the data from these spacecraft. Could - SOHO, STEREO or SDO, for example, see a 100 meter asteroid at 1 solar radius from the sun?

I suspect not.

IsaacKuo
2013-Jan-07, 06:14 PM
Comets have huge big bright tails.
Tailless comets do not have huge big bright tails. Most of SOHO's comets which aren't from the Kreutz group are tailless.

djellison
2013-Jan-07, 07:06 PM
They have comas. They have a brightness that belies their diminutive size. A visiting spacecraft would not.

Stop using comets as the benchmark. Use asteroids.

IsaacKuo
2013-Jan-07, 08:25 PM
Sungrazer comets are presumed to be comets, regardless of whether or not there is any tail or visible coma.

http://home.earthlink.net/~tonyhoffman/SOHOfaq.htm#realcomets

Are all “SOHO comets” actually comets (as opposed to asteroids or other bodies)?
Probably, but we don’t know for sure. Most are clearly cometary fragments. Some of SOHO’s Kreutz fragments, and most of the objects belonging to the new groups, appear more or less stellar, with no visual cometary characteristics.
[...]
In an informal discussion at IWCA III, Dan Green said that classifying the ones with no obvious cometary characteristics (apart from following a Kreutz, Meyer, or Marsden orbit) as comets is bending the rules.

Even if you're unhappy with calling them sungrazer comets, that's what they're called and that's what these hypothetical ET probes would look like, at first blush.

djellison
2013-Jan-07, 09:27 PM
and that's what these hypothetical ET probes would look like, at first blush.

On what basis are you making such an assertion?

IsaacKuo
2013-Jan-07, 09:46 PM
On what basis are you making such an assertion?
On the basis of the orbital characteristics. It would be headed almost straight for the Sun, just like a sungrazer comet.

Initially, it would look like one of the handful of unaffiliated tailless sungrazer comets detected by SOHO per year. The first sign of it being something special would be the fact that it's moving much faster than expected, unless it happened to be coming in at an angle roughly 30 degrees or less from the Earth-Sun line (this would reduce its transverse speed to less than half it's true speed, so it might still be a normal comet based on speed). Even in that case, it would be highly noteworthy due to its survival past perihelion.

djellison
2013-Jan-08, 01:28 AM
You're still not understanding me. What makes you think that from 100 million miles away - an alien spacecraft will look like a comet or at least be as easy to see as a comet?

Size. Albedo. These are the things I'm trying to get it.

IsaacKuo
2013-Jan-08, 02:36 AM
You're still not understanding me. What makes you think that from 100 million miles away - an alien spacecraft will look like a comet or at least be as easy to see as a comet?
Many of the sungrazing "comets" detected by SOHO lack a visible tail or coma, and are starlike (in the sense of being indistinguishable in size from a dot). They are presumed to be comets, and called comets for convenience as much as anything else.

Size. Albedo. These are the things I'm trying to get it.
The size and albedo of these hypothetical probes is not known, a priori. I gave technical reasons why they might plausibly need to be very large, but it really depends on what capabilities they're supposed to be designed around.

According to this page, the comet detection limit of SOHO's coronagraph is typically around 8th magnitude:

http://sungrazer.nrl.navy.mil/index.php?p=news/the_shy_comet

For a low albedo asteroid, this would correspond to a diameter around 65 km - 150 km. However, such a low albedo is not plausible in these circumstances. With the sunlight so intense, it will get hot enough to glow at visible wavelengths like an incandescent light bulb filament anyway--the result being the same as high albedo. This means 8th magnitude corresponds to a diameter around 33km.

So, for example, I suggest that the probes might use carbon based refractory photovoltaics, which could combine very high efficiency with high temperature tolerance and solar sail capability (Geoffrey Landis's grey solar sail concept). These would be dark when far from the Sun, capable of absorbing sunlight efficiently at multi-AU distances to power long range communications and perhaps radar sensors. But when close to the Sun, they'd heat up and start glowing from the heat.

IsaacKuo
2013-Jan-08, 03:19 AM
Oops! I forgot that absolute magnitude assumes the target is at 1AU from the Sun! In fact, the target is more like 0.01AU from the Sun, reducing the size from 33km down to a mere 330m. This is on the order in size of Forward's proposed Starwisp probe.

According to this page, the comet detection limit of SOHO's coronagraph is typically around 8th magnitude:

http://sungrazer.nrl.navy.mil/index.php?p=news/the_shy_comet

For a low albedo asteroid, this would correspond to a diameter around 65 km - 150 km. However, such a low albedo is not plausible in these circumstances. With the sunlight so intense, it will get hot enough to glow at visible wavelengths like an incandescent light bulb filament anyway--the result being the same as high albedo. This means 8th magnitude corresponds to a diameter around 33km.

djellison
2013-Jan-08, 04:13 AM
So - how many 330m size asteroids have been discovered using those spacecraft - or even observed at all.

IsaacKuo
2013-Jan-08, 09:17 AM
So - how many 330m size asteroids have been discovered using those spacecraft
As far as I can tell, it's unknown. Unless a sungrazer comet shows a tail or a visibly large coma, there's typically not going to be anything to distinguish what exactly each specific one is.

- or even observed at all.
Well, plenty of smaller asteroids have been detected, when passing close to Earth.