Detection of spacecraft at large distances

[Jaziel]

According to NASA the Space Shuttle required 12 GW to get it off the launch pad. So if a hypothetical rocket at the Second Lagrange Point (where the JWT is heading for right now) was producing the same amount of watts, all other things being equal, how far away in space could this output be detected? For example, could the rocket's exhaust be detected here on Earth, whether visibly or by other means? Someone (not here at Physics Forum) believes it would be visible as a 14th magnitude starlike object, but I don't have the maths to verify this claim, unfortunately. (This is for a short semi-SF story, which has stalled because of this quandary).

 

[hutchphd]

With apologies your question makes very little sense. For instance Voyager 1 has a 22.4-Watt transmitter and we are still tracking it to interstellar space. It depends upon how the power is used and how it is directed.

 

[phyzguy]

Are you asking about detecting radio signals from the object? Or are you asking about detecting the object visually with a telescope?

 

[DaveC426913]

I thought the OP was not too vague.

...could the rocket's exhaust be detected here on Earth, whether visibly or by other means?

 

[Jaziel]

Hi

Yes, sorry if I wasn't being clear enough. I meant the rocket exhaust, or even heat radiation. My apologies again. Thanks for getting back to me anyway. Jaz.

 

[berkeman]

What does rocket exhaust look like in space (past the atmosphere)? It's probably only visible in IR, right? JWST could probably see it...

 

[DaveC426913]

(This is for a short semi-SF story, which has stalled because of this quandary).

It should not stall the story. If the story requires it then let it be so. It's plausible enough to pass.

The job of a writer is not to be hamstrung by math (especially math that no one could conclusively refute); the job of a writer is to write a story that's believable.

 

[Melbourne Guy]

It should not stall the story. If the story requires it then let it be so. It's plausible enough to pass.

I'm with @DaveC426913 on this, @Jaziel. It is plausible for the ship and / or it's exhaust to be detected visually or in IR.

If this story is set 'about now', you can even used ATLAS as the method of detection. The Asteroid Terrestrial-impact Last Alert System scans the sky and even provides you some measure of 'how far' your ship can be detected: one day's warning for a 30-kiloton 'town killer', a week for a 5-megaton 'city killer', and three weeks for a 100-megaton 'county killer'.

As for math. Avoid it! Just decided how far out you need it to be and assume some kind of telescope has the resolving power to notice.

 

[hutchphd]

The visibility will be most akin to an outgassing comet. If lit by the sun against a dark background it should be quite apparent. I remember gettting into my car to get out of the city to view comet Hale-Bopp (in 1997). When I happened to look up I noticed the comet 10 degrees up from the street lamp! So much for my country sojourn..
lI am reminded of the Falcon 9 launch from Vandenburg at dusk. With the exhaust in sunlight at altitude, my friends on the left coast said was pretty impressive.



/

 

[Rive]

What does rocket exhaust look like in space (past the atmosphere)?

Falcon9 second stage is not really impressive in this regard:

Anyway, A.C. Clarke already set a notable precedent in 2061: Odyssey Three, for a spacecraft exhaust being visible from Earth so this should not stall any work.

 

[Rive]

Another (somewhat actual ) reference: visible light reflecting on a distant object...

 

[Astronuc]

According to NASA the Space Shuttle required 12 GW to get it off the launch pad.

About 83 to 85% of the power for the Space Shuttle launch is provided by the solid rocket boosters (SRBs), which are attached to the Shuttle's main fuel tank, which holds the liquid hydrogen and oxygen for the Space Shuttles three rocket motors. The SRBs are expended at 127 s, so they do not participate after 2 minutes. At sufficient altitude, the Shuttles main motors can continue to propel the Shuttle and fuel tank further without the atmospheric drag. The main engines operate for a total of about 8.5 minutes, or about 6.5 minutes after the SRBs are spent.

As for the thermal and/or plume signature, whether or not one could see the hot propellant or plume depends on the temperature and cross-sectional area of the nozzle exhaust, and the orientation of the rocket with respect to the observer. L2 point is about 930,000 miles away, so one can estimate the solid angle viewed by dividing the cross-sectional area of the nozzle by the area of the sphere based on the distance to the object.

https://www.nasa.gov/topics/universe/features/webb-l2.html

A rocket expending 12 GW for a long period of time would have to be fairly substantial with many Shuttles-worth of hydrogen propellant.

 

[Jaziel]

Hi

Thanks for the helpful (and encouraging) responses. I guess the issue began when I read a paper by Dr Robert Zubrin (see link below) about the detection of alien spacecraft , in which he restricted his detection criteria to four kinds of non-FTL propulsion: fisson, fusion, antimatter and magnetic sails. Reading it, I was dismayed to learn that detection of such craft from Earth space could range from fractions of light-years to hundreds of parsecs. Add to this the technical nature of the paper and it had me questioning the accuracy of my SF story, insofar it as it applies to spacecraft detection. The story's premise has a crippled alien spacecraft operated by a benign mainframe AI (and its robotic iterations) decelerating into the solar system as a result from a corrupted emergency procedure. It is essentially a rescue story, one which as well as bringing out the best and worst in humankind, also deals with our ongoing relationship with AI.

One issue is trying to calculate when detection is first achieved. This has a direct bearing on the plot (set about 50~75 years in the future), so the timing is important. I had thought that since it's possible to determine the absolute magnitudes of small celestial objects like asteroids, that the same rule could also apply to spacecraft propulsion systems. Yes, asteroids shine by reflected light, whereas a spacecraft generates its own light - at least that part of it emitted in the visible spectrum. But it's all radiation in the end, whether reflected or generated, yes? So I naively assumed a spacecraft 's exhaust would also have an absolute magnitude. For simplicity I chose the known output of the Space Shuttle to serve as a useful metric, one that could be scaled up and down according to need. Yes, I could turn a blind eye and invent an answer, but I don't want to go down that road. One reason why I admire writers like Arthur C Clarke is because he took the trouble to get his facts correct. That's what gives his best SF stories, both long and short, their authenticity. Obviously I'm not comparing myself to writers like ACC! But I still want to take a leaf out of his book where possible and do it right. I just wish I was better at maths, that's all!

PS. While I can't locate the Zuprin paper, I am able to offer a review of it in the link below.https://www.centauri-dreams.org/2012/08/20/to-detect-a-starship/

 

[hutchphd]

Is this the Zubrin paper in question?

"The paper is Zubrin, “Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft,” Progress in the Search for Extraterrestrial Life, ASP Conference Series Vol. 74 (1995). Available online."

I will give it a read.

 

[berkeman]

I meant the rocket exhaust, or even heat radiation.

The story's premise has a crippled alien spacecraft operated by a benign mainframe AI (and its robotic iterations) decelerating into the solar system

Ah, so that's why a spacecraft that is inbound would point its engine exhaust in the direction where Earth observatories could see the heat signature, right?

 

[DaveC426913]

OK, it's alien. Which means alien technology. Which means not even the harshest pedant can tell you the exhaust couldn't be as visible as necessary for the story.

Heck, it could be visible in all sorts of ways. It could pulse regularly. It could alternate between EM frequencies. Etc.

 

[Astronuc]

Ah, so that's why a spacecraft that is inbound would point its engine exhaust in the direction where Earth observatories could see the heat signature, right?

A decelerating spacecraft wouldn't necessarily be pointing toward the earth. It could pointing toward the sun, or elsewhere. If the aliens are sneaky, they'd decelerate on the opposite side of the sun from earth.

Ostensibly, if there are aliens outside the solar system, they would have tremendous capability to travel intergalactic distances, so they have very advanced technology. If they go to the trouble, perhaps they would by-pass earth.

 

[stefan r]

Use black body radiator as a reference. You have several degrees of freedom. The temperature of the ship can be adjusted by making it a larger radiating surface. If you want it to be easy for characters to spot make it glow red like an electric stove.

The story from Oumuamua should help. It was quite detectable by many telescopes but it was not detected until someone did. Then it was already moving further away.

Which part of the sky makes a huge difference too. People usually do not aim telescopes toward the Sun because it destroys astronomy equipment and could light fires. The doughnut region around the Sun has a bunch of dust (zodiacal dust) that obscures objects. That means many observations that could continue do not because the astronomer can get good data on some other target. Most of the sky is has a seasonal window where no one is looking.

An AI attempting to stop is likely to flyby the Sun and then Jupiter so that it can utilize the Oberth effect.

Apparent magnitude changes by distance squared. The Sun is magnitude -27 and 4 x 10²⁷ W. At perihelion flyby a 4 GW ship is dimmer by a factor 10¹⁸. A magnitude difference of 45 so should see it as magnitude 18. The Earth-Sun Lagrange point 1 and 2 are fairly close to 100x shorter distance, and therefore 10,000x brighter. I would suggest magnitude 8.

Earth-Sun L2 is not a likely target. I don't know your storyline. Consider engines off while approaching near Earth space. Nothing noteworthy about the object except that it will make a close pass of Luna. Lighting up the engines while passing Luna changes that to close Earth flyby. Lighting up the engines increases the visibility by a very large amount. It abruptly just another thing drifting around the solar system to an object of interest.

 

[Melbourne Guy]

One reason why I admire writers like Arthur C Clarke is because he took the trouble to get his facts correct.

Yes and no. Time's Eye (written with Stephen Baxter) is an example where the science took a back seat to the story. Which is not to say you shouldn't go for accuracy. Orbital mechanics, for instance, is often poorly described in novels, but it is clear that you can decide at what distance the alien ship is noticed and weave a credible explanation of why it was seen then.

 

[Drakkith]

One issue is trying to calculate when detection is first achieved. This has a direct bearing on the plot (set about 50~75 years in the future), so the timing is important.

You're in luck! Orbital mechanics is complex enough that virtually anything you decide will be plausible. Want the exhaust to be visible as the craft burns its engines just outside the Moon's orbit? Plausible. IR detection as the craft decelerates on the far side of the Sun? Plausible thanks to the existence of space probes that don't orbit the Earth. Sudden detection 50,000 km from Earth? Plausible.

 

[Jaziel]

Thanks again for the instructive comments. They have really got me thinking!

Apparent magnitude changes by distance squared. The Sun is magnitude -27 and 4 x 1027 W. At perihelion flyby a 4 GW ship is dimmer by a factor 1018. A magnitude difference of 45 so should see it as magnitude 18. The Earth-Sun Lagrange point 1 and 2 are fairly close to 100x shorter distance, and therefore 10,000x brighter. I would suggest magnitude 8.

Stefan, just a clarification: a perihelion flyby means viewed at a distance of 1 AU - e.g. from Earth orbit? So if that works out at magnitude 8 for a 4GW output, I take it then that its apparent brightness would progressively diminish over increased distance, that it would be measurable in terms of apparent magnitude, in fact?

"The paper is Zubrin, “Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft,” Progress in the Search for Extraterrestrial Life, ASP Conference Series Vol. 74 (1995). Available online."

Thanks, hutchphd. Yes, that's the paper, the one that caused all the trouble. A complicating factor in it is Zubrin having chosen as examples to illustrate his analysis alien starships massing a million tonnes, and applied the corresponding power outputs. Having such massive starships makes sense given the context, of course, but it's overkill for my present needs. I guess scaling is the answer here.

You're in luck! Orbital mechanics is complex enough that virtually anything you decide will be plausible. Want the exhaust to be visible as the craft burns its engines just outside the Moon's orbit? Plausible. IR detection as the craft decelerates on the far side of the Sun? Plausible thanks to the existence of space probes that don't orbit the Earth. Sudden detection 50,000 km from Earth? Plausible.

The story's plot has the alien craft first detected while it's approx 1000~2000 AU from Earth space. PS. It would in time pass through the Kuiper belt but for the fact it's entering the inner solar system from the apparent direction of the Large Magellanic Cloud (this is intended as a riff on A.C. Clarke's "Rendezvous with Rama").

I had no idea that the JWT could be seen at a distance of half a million km. I assume there's no thrust involved, that it's simply coasting to its destination. That makes the sighting of it all the more remarkable.

Again many thanks

 

[Rive]

The story's plot has the alien craft first detected while it's approx 1000~2000 AU from Earth space.

By my opinion that would be quite stretched, especially since as you have said, the plot is less than 100 years in the future. I think you would need a very directional output (and Earth being caught in the cone) and still some big luck to have the right instrument looking at the right direction. No exactly impossible, but definitely needs some technobabble to make it work.

Maybe stray EM waves from the coils of a pulse-type fusion drive or something like that (it is technobabble already!). And an extended SETI program on the receiving end, maybe? Something like that might work, with decent amount of handwavium.

 

[berkeman]

The story's plot has the alien craft first detected while it's approx 1000~2000 AU from Earth space.

If detection is very difficult at the distance you want for the plot to work best, you could add some other event in that general area of the sky that is more noticeable and would have caused several telescopes to have been aimed that way. For example...

https://www.physicsforums.com/threa...xplode-to-ordinary-type-ii-supernova.1011032/

 

[DaveC426913]

add some other event in that general area of the sky

Good idea.

Or something closer, say, an asteroid or comet.

That would give them an even better reason to be looking for dim objects in motion against the background of stars.

 

[berkeman]

I think the aliens would probably avoid trajectories that would take them close to any obvious extra objects like that. But they could get surprised by a short-notice event like the supernova starting behind them...

 

[DaveC426913]

I think the aliens would probably avoid trajectories that would take them close to any obvious extra objects like that. But they could get surprised by a short-notice event like the supernova starting behind them...

Perhaps, but now you're second-guessing the intent or ability of the aliens in the author's story.
We have no reason to assume they're being sneaky.

(Not to mention the fact that the aliens are 1000AUs out. They'd have to know there's an object of particular interest in line-of-sight between them and Earth - an object that's 900+AUs closer to the system than they are.)

 

[hutchphd]

Aliens are always sneaky. Humans are nothing if not tribal. Why do you think we call'em aliens

 

[Nik_2213]

IIRC, 'The Mote etc...' by Niven / Pournelle had Moties' solar sail's doppler spotted from a great distance.

Something like a Bussard ram-rocket, which seems unfeasible in our 'Local Bubble', may be practicable in less denuded space, and visible from afar.

An Orion-derivative, popping micro-fusion 'kilo-tonnes' with metronomic precision, is likely to be noticed by hypothetical 'neighbours'...