Gegenschein is a "faint brightening of the night sky" at the anti-solar point. A naked eye limiting magnitude of about 7.6 might enable an observer to make out gegenschein.

The Wikipedia article on gegenschein suggests that interplanetary dust at the Sun-Earth L2 point might be responsible for gegenschein.

The James Webb Space Telescope will also be at the Sun-Earth L2 point.

Does this imply that the same interplanetary dust that may be responsible for gegenschein will affect the JWST?


The Wikipedia article cites "Zdeněk (1962)" for the statement that the dust responsible for the Gegenschein has a possible concentration at L2. I haven't been able to obtain that paper, but I can't really see why that would be the case, since L2 is not dynamically stable.

However, the dust consists of millimeter-sized grain (see e.g. this APOD image), which is quite large for dust. Such large grains probably has a highly anisotropic "phase function", with a high preference for backscattering. Since when you look from Earth toward L2 you have the Sun in your back, you will thus see an increased brightness compared to other directions, even though the density of that dust is the same.

If this is the case, then placing JWST in L2 is no problem (except when it looks exactly away from the Sun, but that would be a problem anywhere in the ecliptic).

  • $\begingroup$ The Zdeněk article in new scientist books.google.co.uk/… It is mostly about the possiblity of communication satelites at the Earth Moon L2 point. The gegenschein is only mentioned in an aside. $\endgroup$
    – James K
    Mar 17 '17 at 17:35

Not likely. The sun shade will always be pointed towards Earth and the sun. Thinking about the design, that means the telescope will never be pointed in a direction where it will have gegeschein in its field of view, because otherwise the heating from IR radiation from Earth would cause it problems. The term for the angle between where you're looking and where the sun is is "solar elongation". I don't know what the ranges of solar elongation JWST will be limited to, but I'm sure it excludes a substantial angle around $180^\circ$.

The Spitzer space telescope is similarly limited in the parts of the sky it can view at any one time by the need to keep the sun behind it's shade.

  • $\begingroup$ You indicate that JWST will be pointed away from the Earth/Sun, but that is exactly the direction of the source of the Gegenschein. So why wouldn't it see this source of light if it is looking right at it? $\endgroup$
    – zephyr
    Mar 13 '17 at 13:12
  • $\begingroup$ Gegenschein is confined to a spot near $180^\circ$ from the direction of the sun. JWST is limited to observing some band of the sky around $90^\circ$ from the sun. Pointing the telescope in a direction that it could observe gegenschein would expose it to heat radiation from the Earth that would warm up the telescope more than acceptable. $\endgroup$
    – Sean Lake
    Mar 13 '17 at 20:17

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