Physics of the James Webb Space Telescope


In fact, there is another good reason to use infrared light for JWST: it’s hard to get an unobstructed view of distant celestial objects thanks to the gas and dust that are remnants of old stars. They can scatter visible light more easily than infrared wavelengths. In essence, infrared sensors can see through these clouds better than visible light telescopes can.

Since JWST observes in the infrared spectrum, scientists will need to keep everything as dark as possible around the telescope. This means that the telescope itself must be extremely cold to avoid emitting its own infrared radiation. This is one of the reasons why it has a sunscreen. It will block sunlight from the main instruments so they can stay cool. It will also help remove excess light so that the telescope can pick up relatively dim light from exoplanets as they circle around their much brighter host stars. (Otherwise, it would be like trying to see in the dark while someone shines a flashlight in your face.)

How does JWST look back in time?

Light is a wave that travels very, very fast. In just one second, light could orbit the Earth more than seven times.

When we observe celestial bodies, we must take into account the time it takes for light to travel from an object to our telescope or eyes. For example, light from the nearby star system Alpha Centauri takes 4.37 years to reach Earth. So if you see it in the sky, you are literally looking 4.37 years into the past.

(In fact, all you see is the past. You see the moon about 1.3 seconds in the past. When spotted closest to Earth, Mars is three minutes in the past.)

The idea is that JWST can see more than 13 billion years into the past, to the point in the evolution of the universe when the first stars formed. That’s just great, if you think about it.

What is Lagrange’s point?

The Hubble Space Telescope is inside low Earth orbits, which is nice because it was possible for astronauts to service it as needed. But JWST will be much further, at the L2 Lagrange point. But what the hell is Lagrange’s point?

Consider Hubble orbiting the Earth. For any object moving in a circle, there must be a centripetal force, or a force that pulls it toward the center of the circle. If you swing the ball on the wire around your head, the force that pulls it towards the center is the tension in the tendon. For Hubble, this centripetal force is the gravitational force due to its interaction with the Earth.

As the object moves away from the Earth, the strength of this gravitational force decreases. Thus, if the telescope were to move into a higher orbit (larger circular radius), the centripetal force would decrease. To stay in a circular orbit, Hubble would need to orbit longer. (We would say it has a lower angular velocity.)



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