The Astronomy Book

305

See also: Radio astronomy 179 ■ Space telescopes 188–95 ■ A digital view of the skies 296 ■
Gravitational waves 328–31 ■ Lagrange (Directory) 336

THE TRIUMPH OF TECHNOLOGY

L2 is a place in space where the
gravity of the sun and Earth work
together to pull an orbiting object
around the sun at the same rate as
Earth, making one orbit every year.
This means the JWST will be largely
in the shadow of Earth, blocking
out any heat pollution from the
sun and allowing the telescope
to detect very faint infrared sources
in deep space. NASA claims that
the telescope could detect the heat
of a bumblebee on the moon.

Heat seeker
The JWST’s vast primary mirror
is seven times the area of Hubble’s
and, instead of polished glass, the
mirror contains 18 hexagonal units
made from beryllium for maximum
reflection. The 270-sq-ft (25-m^2 )
mirror is too large to be launched
flat, so it is designed to unfold
once in orbit.
To pick up the faint heat
signatures of the most distant stars,
the telescope’s detectors must
always be extremely cold—never
more than –370°F (–223°C). To
accomplish this, the JWST has
a heat shield the size of a tennis
court. Again, this is folded away
for launch. The shield is made from
five layers of shiny plastic that reflect
most of the light and heat. Any heat

that penetrates the top layer is then

radiated sideways by successive

inner layers so that almost nothing

reaches the telescope itself.

First light

The light waves from the first stars

to form have been stretched as

they shine through the expanding

universe, changing them from

visible light to infrared, so they

The light from the first stars has been

shining through expanding space.

Infrared is mostly

invisible from

Earth’s surface.

The expansion has

stretched the light into

infrared wavelengths.

are a prime target of observation

for the JWST. At the same time,

this ultra-sensitive eye on the

infrared sky has three other main

goals. It will investigate how

galaxies have been built over

billions of years, study the birth of

stars and planets, and provide data

about extrasolar planets. NASA

hopes that the telescope will be

in operation for at least 10 years. ■

JSWT will not be exactly at the L2 point, but will circle around it in

a halo orbit. Lagrange points are positions in the orbit of two large bodies

where a smaller object can keep a stable position relative to those two large

bodies. There are five L points in the orbital plane of Earth and the sun.

To see the first stars,

a giant infrared

telescope must be

sent into space.

L1

L4

L2

L5

L3

Earth’s orbit

Moon’s orbit

JWST’s orbit