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Artist conception of the James Webb Space Telescope.

Artist conception of the James Webb Space Telescope. (NASA GSFC/CIL/Adriana Manrique Gutierrez)

The James Webb Space Telescope, launched Dec. 25, will shortly arrive at the destination from which it will begin its in-depth examination of the distant universe. However, unlike its predecessor, the Hubble Space Telescope, the James Webb Telescope won’t be orbiting Earth.

Rather, it will orbit the sun in an area of space known as a Lagrange point, where the tricky overlap of gravity from the sun and Earth and centrifugal forces from its orbit will allow the telescope to maintain its position relative to the orbiting Earth almost — but not quite — effortlessly.

The Earth-sun orbital relationship produces five Lagrange points, which scientists number L1 through L5. Placing the telescope at L2 — perpetually in Earth’s shadow, shielded from the sun’s radiation and glare — will allow it to look farther, while also keeping its sensitive instruments at the super-cold temperatures they need to operate.

In the 18th century, Italian French mathematician Joseph-Louis Lagrange determined that celestial bodies’ overlapping gravity wells would produce areas where the pull of gravity and the push of centrifugal force would offset. A small object at such a point would not fall toward the sun or Earth but would remain more or less stable. These are the Lagrange points.

Ordinarily, an object orbiting closer to the sun’s gravity well would have a faster orbit. An object orbiting farther away from the sun would have a slower orbit.

In addition to the sun’s gravity, though, Earth’s gravity also pulls on objects at these points, slowing the orbit of objects at L1 and speeding up objects at L2. Objects at these points orbit the sun at “Earth speed” as they balance between the gravity wells.

The James Webb Space Telescope will orbit L2. It won’t be the first space observatory to come here: Several have set up shop around L2, including the European Space Agency’s Herschel and Planck space observatories in 2009, NASA’s own Wilkinson Microwave Anisotropy Probe, which operated here from 2001 to 2010 and China’s Queqiao satellite that allows communication with the Chang’e 4 probe on the moon’s far side.

This particular Lagrange point is not completely stable: The interweaving pulls of gravity between Earth, the moon, the sun and other forces will require the James Webb Space Telescope to make occasional, minor adjustments to maintain its orbit around L2. The spacecraft is designed to carry enough propellant to make these adjustments for at least 10 years. However, NASA reports that a smoother-than-anticipated journey to the Lagrange point will leave the James Webb telescope with additional fuel, extending its useful life span.


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