118 PART 1^ |^ EXPLORING THE SKY
sunscreen. In fact, it could not observe from Earth orbit because
Earth is such a strong source of infrared radiation, so the tele-
scope was sent into an orbit around the sun that will carry it
slowly away from Earth as its coolant is used up. Named after
theoretical physicist Lyman Spitzer Jr. who originally suggested
that space telescopes would be useful, it has made important
discoveries concerning star formation, planets orbiting other
stars, distant galaxies, and more.High-Energy Astrophysics
High-energy astrophysics refers to the use of X-ray and gamma-
ray observations of the sky. Making such observations is diffi cult
but can reveal the secrets of processes such as the explosive deaths
of massive stars and eruptions of supermassive black holes.
Th e fi rst astronomical satellite, Ariel 1, was launched by
British astronomers in 1962 and made solar observations in the
ultraviolet and X-ray part of the spectrum. Since then many
space telescopes have made high-energy observations from
orbit. Some of these satellites have been general-purpose tele-
scopes that can observe many diff erent kinds of objects.
ROSAT, for example, was an X-ray observatory developed by
an international consortium of European astronomers. Some
space telescopes are designed to study a single problem or a
single object. Th e Japanese satellite Hinode, for example, stud-
ies the sun continuously at visual, ultraviolet, and X-ray
wavelengths.atmosphere is the lack of seeing distortion. It can detect fi ne
detail, and, because it concentrates light into sharp images, it can
see faint objects.
Th e telescope is as big as a large bus and has been visited a
number of times by the space shuttle so that astronauts can
maintain its equipment and install new cameras and spectro-
graphs. Astronomers hope that it will last until it is replaced by
the James Webb Space Telescope expected to launch no sooner
than 2013. Th e Webb telescope will carry a cluster of beryllium
segments that will open to form a 6.5-m (256-in.) mirror once
in space.
Infrared Astronomy from Orbit
Telescopes that observe in the far-infrared must be protected
from heat and must get above Earth’s absorbing atmosphere.
Th ey have limited lifetimes because they must carry coolant to
chill their optics. Th e Infrared Astronomical Satellite (IRAS) was
a joint project of the United Kingdom, the United States, and
the Netherlands. IRAS was launched in January of 1983 and car-
ried liquid helium coolant to keep its telescope cold. It made
250,000 observations and, for example, discovered disks of dust
around stars where planets are now thought to have formed. Its
coolant ran out after 300 days of observation.
Th e most sophisticated of the infrared telescopes put in
orbit, the Spitzer Space Telescope, like IRAS, is cooled to −269°C
(−452°F). Launched in 2003, it observes from behind a
HubbleSpitzerWebb■ Figure 6-22
The Hubble Space Telescope orbits Earth
only 569 km (353 mi) above the surface.
Here it is looking to the upper left. The
James Webb Space Telescope, planned
to replace Hubble, will be over six times
larger in collecting area. It will not have
a tube but will observe from behind a
sun screen. The infrared Spitzer Space
Telescope orbits the sun slightly more
slowly than Earth and gradually falls
behind as it uses up its liquid helium
coolant. (NASA/JPL-Caltech)