Re: Interstellar Propulsion idea using an Asteroid and a few comets!

From: Ian Stirling (root_at_mauve.demon.co.uk)
Date: 08/23/04 

Date: 23 Aug 2004 11:41:38 GMT

In sci.space.policy AA Institute <abdul.ahad@ntlworld.com> wrote:
> Ian Stirling <root@mauve.demon.co.uk> wrote in message news:<411e4269$0$7250$ed2619ec@ptn-nntp-reader03.plus.net>...
>> In sci.space.policy Alfred A. Aburto Jr. <aburto@sbcglobal.net> wrote:
>> <snip>
>> > Yes, there is reason. The Oort Cloud of comets go well beyond the
>> > heliopause.
>> > The Oort Cloud of comets may extend out to 100,000 AU --- maybe 3 light
>> > years from the Sun. This is a good fraction of the way to the nearest star.
>>
>> How are you going to find dark bodies at such great distances?
>
> That's a very interesting point. The theoretical Oort cloud is thought
> to exist at some 30,000 to 50,000 AUs out from the Sun. This is way
> way beyond the 11,500 AUs (the "Ahad radius"!) determined as a
> limiting distance for *solar* illumination of objects.
<snip>

Your problem is photons.

> However, since we are talking futuristic technology here, a generation
> starship could be kitted up with powerful, wide beam lasers mounted on
> its exterior body, which could be used to temporarily act as long
> range "flash lights" that selectively light up the AsterCom starship's
> forward path and illuminate any oncoming target comets / ice balls.
> Since these objects are likely to be "icy" in their compositions at
> that distance, away from any star (with high albedos) they would show
> up for thousands of miles up a ahead in the torch beam, lighting up
> the ship's path like an airport runway lit at night for an aircraft
> coming into land.

Thousands of miles at 1000Km/s = a few seconds.

Let's say that you'r emitting a gigawatt of laser light over a 30 degree
beam, for an hour, and want to detect a 10m object.

This is 10^12J, or about 10^27 photons. (green light).
You will need at least a couple of photons from a target to detect
it, as otherwise there is no way of telling it from background radiation.

Let's assume that the recieving telescope is 10Km wide.

Or an apature of about 10^8m^2.

So, we need a photon flux of about 2*10^-8 photons/m^2 to detect something.

At 10,000Km, an object subtends 2*10^-4 radians, or 4*10^-8 of the beam.
So it'll reflect 10^19 photons.
>From the POV of the object, the dish subtends 10^-3 radians, or about 10^-7
of a hemisphere.
So, it'll reflect 10^12 photons into the dish, easily detectable.

But, a thousand times further away (a day away), it's not.

Space is BIG.

Relevant Pages

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