The Physics of the James Webb Space Telescope


There’s actually another good reason to use infrared light for the JWST: It’s difficult to get an unobstructed view of far-away celestial objects thanks to the gas and dust that are the detritus from old stars. These can scatter visible light more easily than they can infrared wavelengths. Essentially, infrared sensors are able to see through these clouds better than visible light telescopes can.

Since the JWST is observing in the infrared spectrum, scientists will need everything to be as dark as possible around the telescope. That means that the telescope itself needs to be extremely cold to avoid emitting its own infrared radiation. This is one reason it has a sunshield. It will block the sunlight from the main instruments so they can stay cold. It will also help blot out excess light so the telescope can pick up the comparatively dim light from exoplanets as they orbit 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 the JWST Look Back in Time?

Light is a wave that travels really, really fast. In just a second, light could go around the circumference of the Earth more than seven times.

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

(Actually, everything you see is in 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 for the JWST to be able to see more than 13 billion years into the past, to the point in the evolution of the universe when the first stars were being formed. That’s just awesome, if you think about it.

What Is a Lagrange Point?

The Hubble Space Telescope is in low Earth orbit, which is nice because it has been possible for astronauts to service it when needed. But the JWST is going to be much farther away, at the L2 Lagrange point. But what the heck is a Lagrange point?

Let’s consider Hubble orbiting the Earth. For any object moving in a circle, there needs to be a centripetal force, or a force pulling it towards the circle’s center. If you swing a ball on a string around your head, the force pulling it towards the center is the tension in the string. For Hubble, this centripetal force is the gravitational force due to its interaction with the Earth.

As an object moves farther away from Earth, the strength of this gravitational force decreases. So, if the telescope moved into a higher orbit (a larger circular radius), the centripetal force would decrease. In order to stay in a circular orbit, Hubble would have to take longer to orbit. (We would say it has a lower angular velocity.)


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