Skeptics in the Pub explore spaaaaace


Tonight’s Skeptics in the Pub guest was Sarah Kendrew, from Oxford University’s Astrophysics group, talking about the James Webb Space Telescope (JWST).

One of the best parts of attending talks is the spin-off knowledge you pick up. Obviously you hear about the actual subject matter, which can be interesting in its own right, but in the process you reveal gaps in your own knowledge you hadn’t really bothered filling before.

So, the telescope is named after James Webb:


(picture from the NASA archives)

Not a scientist, such as Edwin Hubble, William Herschel or Johannes Kepler who also have space telescopes named after them but instead the NASA Administrator during the 60s when the Mercury, Gemini, Apollo projects were underway. One of those people that the public never hear about, unlike Buzz Aldrin or Neil Armstrong, but who was key to ensuring JFK’s vision of a man on the moon became reality.

[[Tangent #1 – did you know that the form of the Slough bus station is inspired by local lad Herschel’s famous infra-red experiment?]].

The JWST is looking for very faint infra-red light and any heat from the telescope itself would swamp the instruments so it needs to be kept very cold. A large shield will be fitted to block light from the Sun, Earth and Moon but the challenge is finding somewhere in space where all three are in the same rough direction. This is where the the Lagrange points come in.

The mathematician Joseph-Louis Lagrange found five solutions to the three-body problem – that is, how can three bodies orbit each other yet stay in the same position relative to each other? These are as follows:

  • L1, L2, L3 lie on a line drawn through the Sun and Earth.
  • L4 leads the Earth by 60 degrees
  • L5 lags the Earth by 60 degrees

So an object, such as a telescope or asteroid, occupying the L1 Lagrange point will remain in orbit around the Sun and at the same distance between Sun and Earth due to the gravitational forces of these much larger bodies partially cancelling each other out.

[[Tangent #2 – The Earth doesn’t really orbit around the Sun. The Earth is affected by the Sun’s gravity and vice versa so there is a barycentre that they both orbit around. It just happens that this point is within the volume of the Sun; same for the Moon orbiting the Earth where the barycentre is not at the Earth’s centre but instead three-quarters of the way to the surface.]]   

The JWST will be moved to the L2 Lagrange point (which puts it in an orbit outside that of the Earth) as this will mean the Sun, Earth and Moon can all be blocked by the shields, reducing interference from heat and allowing a larger uninterrupted view of space.


(Picture by Andrew Moise)

It doesn’t stop there – once you’ve got your head around how gravitational forces create these points of relative stability, you then have to consider objects in Lissajous orbits about these points – which explains how you can have multiple objects at the same single point in space. Don’t let anyone tell you rocket science isn’t hard.

One thing I noticed about the event was the disappointing balance in the attendees – predominantly middle-aged men. Neither my wife or daughter were interested in coming along and there seemed very few women in the audience but we had a smart young woman explaining all about infra-red astronomy. There’s so much work still to be do done before we make science interesting and accessible enough for everyone. 

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