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Far Out!
Mission planners want to place the James Webb
Space Telescope in an orbit well beyond Earth's Moon
at a place called the Second Lagrange Point (L2) — a
challenging orbit for a NASA space astronomy mission.
The planners have several good reasons for their
choice. At that distance (940,000 miles, or 1.5 million
km) from Earth, the telescope would be protected
from stray light and heat, yet would remain in an orbit
that makes operations and communications easy. Just
as important, Earth wouldn't obstruct the telescope's
view.
Climate Control
Although an L2 orbit offers several advantages, the
most important is its location out beyond Earth. The
new telescope will be sensitive to infrared light, which
warm objects emit more efficiently. That means the
infrared detectors in these instruments must operate
at very cold temperatures (about -375 degrees Fahrenheit,
or 40 Kelvin or -233.3 degrees Celsius).
Without cooling, these instruments wouldn't be able
to see beyond the radiation they generate on their own.
Size Considerations
Because the James Webb Space Telescope must
maintain its trim size (15,000 lb, or 6,800 kg) to fly
on a medium-sized rocket, it can't carry enough of
its own coolant. A cold orbit around the Sun offers
an ideal solution. The optimal engineering solutions are
an easier-to-keep-cold orbit around the Sun and — as
an essential feature of the telescope — a deployable,
lightweight sunshield to further block heat and stray
light.
Rugged Design
At this distant orbit, the Webb telescope is too far from Earth to have the protection of our planet's magnetic field, which blocks high-energy cosmic rays. Cosmic rays can interfere with the telescope's signals or even build up electrical charges that can create the equivalent of small lightning strikes on the telescope. Such sparks can hurt sensitive equipment or damage the telescope's materials. Webb has been engineered to take this into account, with extra shielding for detectors and conduction areas in the sunshield to prevent voltages from accumulating.
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Who was
Lagrange?
French
mathematician and theoretical physicist Joseph Louis Lagrange (1736-1827)
discovered the different stable orbital positions in our solar
system, which we call Lagrange points. |
| The
position of the Second Lagrange Point (L2)
of the Sun-Earth system, where JWST will orbit
around the Sun at roughly constant distance from
Earth over the year.
Visit our Telescope Gallery to see an animation of the L2 orbit.
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| Webb's distant orbit places it far beyond the Earth. The far-away orbit helps keep the telescope cold, a necessity for a telescope that looks at infrared light.
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