[ExI] Spacecraft (was MM)

[Alfio Puglisi]

On Mon, Jan 3, 2011 at 6:47 PM, Keith Henson <hkeithhenson at gmail.com> wrote:

> On Mon, Jan 3, 2011 at 5:00 AM,  Samantha Atkins <sjatkins at mac.com> wrote:
> >
> > On Jan 1, 2011, at 5:39 PM, Keith Henson wrote:
> >
> >> On Sat, Jan 1, 2011 at 1:19 PM,  Samantha Atkins <sjatkins at mac.com>
> wrote:
> >>
> >>> On Jan 1, 2011, at 2:40 AM, Keith Henson wrote:
> >>>
> >>>> On Fri, Dec 31, 2010 at 11:07 PM,  Samantha Atkins <sjatkins at mac.com>
> wrote:
> >
> > <snip>
> >
> >>
> >>>>>
> >>>>>> Based on
> >>>>>> Jordin Kare's work, this takes a flotilla of mirrors in GEO.
>  Current
> >>>>>> space technology is good enough to keep the pointing error down to
> .7
> >>>>>> meters at that distance while tracking the vehicle.  The lasers
> don't
> >>>>>> need to be on the equator so they can be placed where there is grid
> >>>>>> power.  They need to be 30-40 deg to the east of the lunch point.
> >>>>>>
> >>>>>
> >>>>> Uh huh.  What is the max distance you are speaking of?
> >>>>
> >>>> Around one sixth of the circumference 40,000/6, 6,666 km.
> >>>
> >>> That amounts to about 0.002 MOA tracking a rocket through atmosphere.
> >>
> >> MOA?
> >>
> >
> > Miinute of arc.
>
> Ah.  The Hubble, which has been up for 20 years and is based on
> technology at least 10 years before that, has a pointing accuracy of 7
> milliarcseconds.  A milliarcsecond is about 5 x 10^-9 radians, so 7
> would be about 35 x 10-9 rad.  At the end of a 36,000,000 m radius,
> the error would be ~1.3 m
>

You are finally touching points where I can contribute something :-)

Typical pointing accuracy for ground-based telescopes are on the order of 1
arcseconds, sometimes worse. The Hubble doesn't do much better (some
references I saw speak of 0.2-0.5 arcsec, which I would regard as very
good). The number you quote (7 milliarcsec) is for pointing *stability*,
with feedback from a guide star. Hubble has the advantage of working outside
the atmosphere, in diffraction-limited mode. A normal telescope inside the
atmosphere has worse stability (20-30 milliarcsec or more), because the
guide star feedback is smeared by atmospheric aberrations. A telescope
equipped with an adaptive optics system (AO) can again reach a stability in
the single-digit milliarcseconds range.

All these numbers are for fixed, low-speed objects whose trajectory can be
computed in advance.

Therefore a laser-propulsion device cannot work without an AO-enabled launch
system, which can keep your laser beam focused on your target and will track
its motion. This supposes that the payload can give optical feedback about
its position, but I imagine that the ablation process will make it quite
bright :-)

Such AO systems are now commonplace on big telescopes, but work in the
milliwatt regime instead of GW, since they just reflect starlight :-)
Laser-guided AO systems, where a laser is shoot up in the sky to create a
"fake" guide star, are starting to work right now, and if I remember
correctly some of them are planning an adaptive launch mirror. Such lasers
are on the order of 10 watts. I don't have the foggiest idea of what happens
when the power is scaled by a factor of 10^9. Having seen the safety systems
for such lasers, I would imagine that a multi-GW laser will be treated like
a nuclear-testing ground. At least, I would keep the same distance.

Alfio


> Keith
>
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