On 19 Oct 1999 11:15:53 +0200 Anders Sandberg <email@example.com> writes:
> Larry Klaes <firstname.lastname@example.org> writes:
> > If a starship composed of a 1,000 kilometer-wide sail being pushed
> > at nearly the speed of light by a very powerful laser beam in the
> > terrawatt range was headed towards Earth - and was, say, less than
> > 1/2 a light year away - how would it appear from our planet in
> > various spectrums and wavelengths, especially optical?
> I don't think the ship would be visible, the angular diameter would
> be around 10^-14 degrees. But since the laser would be rather spread
> out, it would be visible as laser light coming from the departure star.
> It might be hidden by the starlight, but would likely show up as a
> strange peak in the emission curve.
Half a light year is 4.727x10^15 meters. An object 1000 km wide, oriented to subtend the maximum angle, would appear 2.115x10^-10 radians wide at that distance, or 1.2 x 10^-8 degrees.
A one terawatt beam could exert 750 pounds of force on a black sail, and 1500 pounds on a white sail.
A one terawatt beam on a 1000 km wide square sail would be one watt per square meter of sail. Sunlight would be that strong at a distance of about 37 AU from the Sun.
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