194
Analysis of the images revealed
that the mirror was the wrong
shape around the edge. The error
was tiny—about 2 millionths of
a meter—but enough to send the
light captured by the outer part
of the primary mirror to the wrong
area of the secondary mirror, creating
serious aberrations in the images.
This was a worrying moment for
Spitzer and his team, as it seemed
as if HST might be about to prove
an embarrassing failure.
Corrective vision
If Hubble was to fulfill its potential,
it needed corrective elements
added to its optical system. In
effect, it was given a pair of
eyeglasses. The problem with
the primary mirror was precisely
calculated by analyzing the
telescope’s images. The solution
was to add carefully designed
mirrors in front of Hubble’s
instruments so that the light
entering them from the main mirror
was correctly focused. Two sets
of these mirrors were fitted during
a crucial service mission to HST
in 1993. They worked perfectly.
HST could at last be put to work,
and the results were astonishing.
Astronauts serviced HST four
more times after 1993 and for the
last time in 2009, in one of the
final shuttle missions. The shuttles
were retired in 2011, after which
it would not be possible to service
HST again. However, that final
service added significant upgrades,
which mean that HST may remain
in use until 2040.SPACE TELESCOPES
Ultra deep, ultra clear
Despite its shaky start, HST has
surpassed all expectations. The
telescope has made 1.2 million
observations to date during its
3-billion-mile (5-billion-km) journey
around Earth. Despite traveling at
17,000 mph (27,000 km/h), it can
pinpoint a position in space to an
accuracy of 0.007 arc seconds—
which is like hitting a penny coin
from 200 miles (300 km) away.
It can resolve an object that is
0.05 arc seconds. NASA likened
this to standing in Maryland and
viewing two fireflies in Tokyo,
Japan. Astronomers worldwide
began booking HST’s time to see
objects of interest. The archive of
everything it has seen—totaling
100 terabytes and counting—can
be viewed on a public website.
Many of HST’s observations
have looked deep into space—
and far back in time. In 1995,
the Deep Field image focused
on an empty patch of space,
one 24-millionth of the total sky.
Combining 32 long exposures
revealed a number of unknown
galaxies that were 12 billion light-
years away—light that began itsUS astronaut Andrew Feustel uses
a power tool to repair the Hubble
Space Telescope during a servicing
mission in 2009.Nature has thoughtfully
provided us with a universe in
which radiant energy of almost
all wavelengths travels in
straight lines over enormous
distances with usually rather
negligible absorption.
Lyman Spitzer Jr.