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Comment by talmand | Hacker News Reader

How can that be? Space is big, we cannot even properly comprehend how big it is. I'm talking just in our immediate vicinity. If I took, say, three of the world's largest ships, put them together to form one ship and then placed it in a random spot somewhere within the orbit of Pluto; how long would it take you to find it? Keep in mind I'm not talking within the orbital plane, I'm saying within the sphere of the solar system.

Unless someone is watching 100% of the sky for even the slightest miniscule changes, I imagine you could get mighty close (granted, close is Pluto) without being detected.

Isn't most of the information we have about newly discovered far away objects is how they interacted with other known far away objects?

But, of course, there's the given that if we've advanced far enough to engage in space combat then we've likely created technology to help in detection.

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It's two things basically:

-You can do a full survey in about 5 hours with one telescope these days.

-Ships are very bright, don't look anything like stars and CCDs are very sensitive.

http://www.projectrho.com/rocket/spacewardetect.php has a good break down.

I've been trying to get a sense of how effective these scans are. Asteroid 2005 YU55 was discovered 28 December 2005. It's about 400m in width, which is the length of the biggest vessel we have. When it was discovered, it was 0.8AU from the Earth, and its orbit is inside that of Mars.

This tells me that 6 years ago we did not know all of the +300m asteroids within the orbit of Mars. How much better are we now? I read the page you linked to, but that didn't list actual numbers based on asteroid searches.

There's an estimated 4,000,000 asteroids in the solar system of size 300m or larger. I sincerely doubt that a telescope survey of 5 hours finds 1 million+ asteroids each night, otherwise that number would be pinned down a lot better. (It can't find all 4 million because at 5 hours there's still substantial portions of the sky unseen.)

Asteroids don't pump out waste heat. Things get a lot more obvious when heat differentials are big.

I was commenting on Talmand's scenario; take "three of the world's largest ships, put them together to form one ship and then placed it in a random spot somewhere within the orbit of Pluto; how long would it take you to find it?"

Using the formula in the "Refrigeration" section of the page linked to by Avshalom, the "maximum range a ship running silent with engines shut down" (assuming diameter = 500 m and temperature = 290K is about 500 million km, or about 3.3 AU. That's well within the orbital radius of Jupiter.

That's assuming the entire asteroid is at 290K. Pluto averages about 40 AU out, given a 0.056% chance that a room-temperature asteroid would be detectable.

I'm also uncertain about the reasoning in the page linked to by Avshalom. I think that assumes a 3K background, and I think you also think there's a big heat differential. However, the average temperature of the moon is something like -25C, and if an asteroid were closer to the sun, then the difference in surface temperature between an internally heated vs. sun heated asteroid is well within the errors in measuring the physical properties of the asteroid.

You can even work it out for yourself, using the equation at http://en.wikipedia.org/wiki/Standard_asteroid_physical_char... . I worked it out to:

  166C at Mercury orbit: http://www.google.com/search?&q=(((1-0.1)+*+(3.827E26+watts)+%2F+(0.9+*+Stefan-Boltzmann+constant+*+16+*+pi+*+(0.3+*+150000000+km)**2)))+**+(1%2F4)

  90C at Venus orbit: http://www.google.com/search?q=(((1-0.1)+*+(3.827E26+watts)+%2F+(0.9+*+Stefan-Boltzmann+constant+*+16+*+pi+*+(0.6+*+150000000+km)**2)))+**+(1%2F4)

  5C at Earth orbit: http://www.google.com/search?q=(((1-0.1)+*+(3.827E26+watts)+%2F+(0.9+*+Stefan-Boltzmann+constant+*+16+*+pi+*+(1.0+*+150000000+km)**2)))+**+(1%2F4)

(The values for albedo and emissivity are not well known, so there is a wide error range in this calculation.)

Still, that's enough to show that the heat differentials inside of the Earth's orbit are not that big.

You are using averages where the min and max are extremely wide apart. For example, the Moon is not actually at -25C, it is either +130C or -110C (or transiting between). We know the time when those temps occur, so we can look for other objects not at those temps at that time. Same goes for the other objects.

I am definitely using averages because if you have an ship which is 300+ meters across then you can leave the top few meters for insulation, so the surface of your ship has the expected extreme temperature ranges.

I should restate what I said: instead of "that's assuming the entire asteroid is at 290K", I mean, "except the insulation layer", which you need anyway to keep the near-surface area livable.

If you need 20 meters for that, then your living volume is 21 million cubic meters, or a reduction of about 20%.

That's not to say that there's no contribution. More that I believe the calculations from that linked-to page ignore the difficulties of differentiating between the internal heat (cooled through black-body radiation) from surface heat from the sun.

For example, from Stefan-Boltman, cooling goes as T * * 4, which means if you have a cooling radiator on the sun-side which is at 150C, or 20C above "normal", then it's about 6 times more effective than a -90C radiator on the cold side. I suspect it's easier to stop the 20C delta on the cold side than the warm side.

I'm in the US, how exactly would I complete my survey with one telescope over five hours if I wanted to see the full sky of, say, Australia?

Well yes, there are quite a few telescopes that constantly scan the sky for tiny changes. And it's not just the size of the ship, but the burning fuel from its engines as it navigates near our solar system.

Good replies, but I'll combine my thoughts here.

Ships CAN be bright, we have the tech now to make them dark.

True, burning fuel is a big problem. Especially if they were traveling rather fast towards us and they flipped over to reverse thrust to slow down. But what if you don't burn fuel as a means of propulsion?

I'm going with the assumption if that if someone wished to do something bad, say invading a neighboring solar system, they would take precautions in being detected.

The point was never that the ships themselves can be bright, it was that the radiation is easily visible.

The article was also operating within the realm of theoretically available technology. There are very few propulsion systems that don't emit hot matter out the back (like solar sails) and the article addressed them suitably.

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