Re: starship-design: HIGHLY OPTIMIZED TOLERANCE

[Steve VanDevender <stevev@efn.org>]

STAR1SHIP@aol.com writes:
 > Does the group have any thought, ideas, methods or machines to solve the 
 > problem or know of others attempt or solution to answer the question "How far 
 > is that star?" with any accuracy and given plus or minus values. It would 
 > seem reasonable to be sure of the distance before starting the journey

We know reasonably accurate distances to nearby stars via parallax
measurements.  In fact, the distance unit "parsec" is defined as the
distance at which an object will appear to change angular position by
one arcsecond when viewed from opposite positions in the Earth's orbit
around the Sun perpendicular to the line between the Sun and the distant
star.  One parsec is approximately 3.26 light years.  Sub-arcsecond
position measurements of stars has been possible for a long time (since
the 1800s, I think).  Dividing one parsec by the measured angular
parallax of a star in arcseconds gives the distance to the star, but
obviously this amplifies the uncertainty in the parallax measurement,
making parallax suitable for measuring the distance to only fairly close
stars, so far meaning the ones within about 100 light years.

More recently the European Space Agency's HIPPARCOS project used an
orbiting satellite to make high-precision parallax measurements of
thousands of stars to previously unobtainable precision.  A web page
with information and links to the catalogues obtained from the
satellite's measurements is at:

http://astro.estec.esa.nl/Hipparcos/hipparcos.html

Unfortunately HIPPARCOS didn't achieve its intended orbit and while it
was possible to complete its mission in the orbit it attained, the
accuracy of its measurements isn't quite as good as was hoped.  There
are proposals for even more accurate parallax-mapping satellites, which
could conceivably accurately measure the distances to most of the
visible stars in our galaxy.

Parallax is in fact the only currently known method of measuring
distance to astronomical objects that has solidly understood
uncertainty.  Although we know the distance to stars within about 10-20
light years to an uncertainty of a few percent (and the closer the star,
the smaller the proportion of uncertainty), all of the other methods
used to estimate the distances to farther objects are based primarily on
postulated luminosity properties of various stellar objects -- if you
know exactly how bright something is supposed to be, then you can tell
how far away it is.  For example, Cepheid variables (named after a
prototype in the constellation Cepheus) are believed to have a
relationship between their period of variability and their instrinsic
luminosity.  Assuming this is true, then by measuring the period of a
Cepheid, you know how bright it is in absolute terms; then by measuring
how bright it appears to us, you would know how far away it is.  Since
Cepheids are pretty bright, they can be observed even in nearby
galaxies.  But so far there's no way to calibrate the period-luminosity
relationship of Cepheids, as the nearest Cepheids are too far away to
measure accurately by parallax.  Similarly it's postulated that Type Ia
supernovae have a constant luminosity; they result when a white dwarf in
a binary star system gradually accretes mass from its companion until it
becomes massive enough to undergo core collapse, so they should always
go supernova once they reach a particular mass.  Again, however, no
nearby Type Ia supernovae have been observed and the estimates of their
instrinsic luminosity have been guessed primarily by astrophysical
theory.

The end result is that while we know the distances to nearby stars to
within a few percent, we don't know the distances to nearby galaxies to
any better than 20%, maybe worse, and this inaccuracy is a problem for
cosmologists.  If you have some background in astronomy, an astronomy
professor at the University of Oregon (where I work) named Greg Bothun
has written a book _Modern Cosmological Observations and Problems_ about
the current state of cosmology.  It's rather dense reading but very
educational, especially as he makes it clear what we _don't_ know about
some of the fundamental issues in cosmology.