How noticeable is an interstellar spaceship in cruise?

  • #1
From Pirx´s Tale:
[B]Pirx[/B] said:
The screens were blank. A routine shower, I thought. But the noon bulletin was far from routine: Luna’s long-range trackers had traced the swarm to another system!
It was the second such swarm in astronautical history. Meteoroids travel along elliptical paths gravitationally tied to the sun like yo-yos; an alien swarm from outside the solar system, from somewhere in the galaxy at large, is regarded as a sensation, although more by astrophysicists than by pilots. For us, the difference is one of speed. Swarms in our own system travel in circum-terrestrial space at speeds no greater than the parabolic or the elliptical; those from outside may—and, as a rule, do—move at a hyperbolic velocity. Such things may send meteoritologists and astroballisticians into ecstasy, but not us.
The radiotelegraph operator was fazed neither by the news nor by my lunchtime lecture.
What actually was found inside the swarm, though:
At exactly twenty-two kilometers, the other ship began to outstrip the Pearl. From now on the values would increase, which meant we were in the clear. All this time my eyes had been glued to the range-finder. I shifted my gaze back to the radar screen.
What I saw was not a ship but a flying island. From twenty kilometers away, it now measured about two fingers in width. The perfectly symmetrical spindle had become a disk—better, a ring!
I know what you’re probably thinking: an alien encounter. I mean, a ship measuring twenty kilometers in length…? An alien encounter. A catchy phrase, but who believes in it? My first impulse was to tail the thing. Really! I even grabbed the stick—then held back. Fat chance I’d have with all that scrap in tow. I heaved out of my seat and climbed a narrow shaft to the small, hull-mounted astrodome atop the cockpit. It was conveniently stocked with telescope and flares. I fired three in quick succession, aiming for the ship’s general radius, and tried to get a sighting in the glare. An island, yes, but still hard to locate right away. The flash blinded me for a few seconds, until my eyes adjusted to the brightness. The second flare landed wide, too far away to do any good; the third, just above it. In that immobilizing white light, I saw it.
Only a glimpse, really, lasting no more than five or she seconds, because I was using one of those exceptionally bright flares that fade very fast. But in the space of those few seconds, I saw, looking down at an angle through my night glasses, whose eighty-power lenses brought it to within a few hundred meters, an eerily but sharply illuminated mass of metal. So massive, in fact, it barely fit into my field of vision. Stars showed in the center. A sort of hollow, cast-iron, spaceborne tunnel, but—as I noticed in the last glimmerings of light—somewhat squashed, more tire-shaped than cylindrical. I could see straight through the core, even though it wasn’t on the same axis; the monster stood at an angle to my line of vision, like a slightly tipped glass of water.
There was no time for idle contemplation. I fired more flares; two failed to ignite, the third fell short, the fourth and fifth made it stand out—for the last time. Having crossed the Pearl’s tangent, it sheered off and quickly widened the gap—one hundred kilometers, two hundred, three hundred—until it was completely out of eye range.
[QUOTE="Pirx"]There are times when the human eye can behave like a camera lens, when a momentarily but brilliantly cast image can be not merely recalled but meticulously reconstructed as vividly as if viewed in the present. Minutes later, I could still visualize the surface of that colossus in the flare’s afterglow, its kilometers-long sides not smooth but pocked, almost lunar in texture; the way the light had spilled over its corrugated rills, bumps, and craterlike cavities—scars of its interminable wandering, dark and dead as it had entered the nebulae, from which it had emerged centuries later, dust-eaten and ravaged by the myriad bombardments of cosmic erosion. I can’t explain my certainty, but I was sure that it sheltered no living soul, that it was a billion-year-old carcass, no more alive than the civilization that gave birth to it.[/QUOTE]
But the conclusion was:
[QUOTE="Pirx"]I sat down and estimated the probability of a sighting through Luna’s giant radiotelescope, the most powerful radioastronomical unit in the system. Powerful, yes, but not powerful enough to pick up a target of that magnitude at a distance of four hundred million kilometers. Case closed. I tore up my computations, got up, and quietly retired to my cabin, feeling as though I had committed a crime. We’d been visited by an intruder from the cosmos, a visit that occurs, who knows, maybe once in a million years—no, once in hundreds of millions of years. And because of a case of the mumps, because of a man named Le Mans and his convoy of scrap, and a drunken halfbreed, and an engineer and his brother-in-law, and my negligence—it had slipped through our fingers, to merge like a phantom with the infinity of space.[/QUOTE]
No evidence whatever, in short.
The Earth of Pirx´ Tale could not spot a dead ship - an asteroid-like object - at a distance of 400 million km. It was only through chance presence of a spaceship illegally passing within 22 km that it was spotted.
The Earth of 2017 could not spot an asteroid-like object passing at 24 million km, but could and did spot it on the way out, at 30 million km.
How easy would it be to spot, in 2017, an object as described? Passing Earth at 400 million km at 90 km/s?

Answers and Replies

  • #2
A 22km spaceship would be bigger than most asteroids in the solar system, and bigger than a lot of planetary moons
It should certainly be visible at Mars distance, probably Jupiter too.
It might not be easily resolved as far away as Saturn though.
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  • #3
A 22km spaceship would be bigger than most asteroids in the solar system, and bigger than a lot of planetary moons
It should certainly be visible at Mars distance, probably Jupiter too.
It might not be easily resolved as far away as Saturn though.

Right. The 400 million km specified by Pirx is the distance between Earth and Mars when Mars is on far side of Sun.
And 22 km is almost as big as Phobos, bigger than Deimos

Would an observer on Earth be able to resolve Phobos, though?
  • #4

Would an observer on Earth be able to resolve...

An arc second is π/648000 radians, round to 5 x 10-6 which makes a radius 2 x 105 longer than something one arc second wide. A telescope with a resolution of 1 arc second would be able to resolve 2 objects separated by 22km at 4.5 x 106 kilometers.

Two fingers (2cm) at one meter away blocks an object 22km away. Because of AAA theorem the object's diameter is 440m.

What you want for this case is not "power" in the telescope but resolution. θ = 1.22 λ/D If luna has a telescope imaging with 1 micron radiation (infra-red) and and a 10 km dish then θ = 1.22 x 10-10 radians. So able to resolve 2 objects 1 meter apart at 8 million km. With 440 m diameter separation the distance stretches to 3.5 billion km. For a radio telescope using 1mm radio wave (300 gigahertz) the objects need to be at less than 3.5 million km. It is possible that the slackers have not put a dish larger than 1km on the moon. :cry:

Should be able to use several satellites orbiting Earth as an interferometer. That would be 14,000 km diameter. They could aim radio waves at the target to get the brightness up. Radio scopes orbiting Lagrange points would make an interferometer 1 au diameter.

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