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Yesterday I spotted the Terra Satellite (EOS AM-1) with my binoculars. This is the 22nd satellite I've spotted. It all looked like usual but there was something very different: the satellite was shinning with a reddish tint.

At first I thought that is was just an illusion, but when Terra passed in front of Zeta Cassiopeiae and Lambda Cassiopeiae (both blue stars) I saw the color contrast clearly enought.

Why is Terra reddish? The other satellites I've seen had no remarkable color. My guess is that it has to do with the gold insulating foil covering the satellite but this seems to good to be true. Does anyone here has ever seen this object and noticed that peculiarity? Are there better examples of this?

uhoh
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Swike
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  • Do you have a picture of how red it is? – Oscar Lanzi Aug 11 '19 at 00:45
  • @Swike it's possible you could capture an approximate color by taking a few second or few minute exposure with a separate, fixed camera looking at the sky, and analyzing the RGB values in the resulting trail, but of course looking at them with binoculars is probably more fun! There's always an option of using a low dispersion prism in front of the camera to spread the colors out a bit. If you every try that you can be the first person to answer Has anyone seen an actual spectrum of a satellite made by an amateur photographer? ;-) – uhoh Aug 11 '19 at 01:25
  • That would be awesome indeed. I'm going to try to get a picture of it with some planning and see if I can show it in a background field of blue stars. I will keep you informed. In principle the spectral data is not my first priority but also an awesome proposal. – Swike Aug 11 '19 at 13:15

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As seen in the photos below from https://terra.nasa.gov/about Terra is reddish, so the answer to

Why is Terra reddish?

is exactly what you have suspected; because it is wrapped in a thermal protection film that is reddish in color.

However, the answer to a more interesting question:

Why am I surprised that Terra appears reddish?

would have several components.

  1. Many spacecraft are overwrapped in a neutral color film that appears silvery or various shades of gray. The engineering choice is related to thermal management of the whole spacecraft, where incoming visible and near-infrared light from the Sun carries about 1360 Watts in every square meter, visible and thermal infrared heating from the Earth, and the rest of space which offers almost no heating but readily accepts the spacecraft's thermal radiation which cools it. For some reason Terra is wrapped in this particular film, which is reddish in color.

    Terra carries ASTER the Advanced Spaceborne Thermal Emission and Reflection Radiometer which images a wide range of wavelengths including thermal infrared out to 11.65 microns. It uses a 1.2 Watt Stirling cryocooler that reaches down below 80 K (1, 2, 3) and keeping the cryogenically cooled infrared sensor and optics sufficiently cold may have required this particular thermal overwrap material.

  2. It is sometimes difficult to judge the color of star-like objects. Color perception is a broad science, but there are at least two things at play here. For dimmer star-like objects it becomes difficult to judge the color because of the way our vision system works, especially in the central area of vision (e.g. rods and cones). It may be that you have looked at other satellites that had colors that were not neutral and simply didn't notice the color, or did but dismissed it (e.g. expectation bias or choose another from this list of cognitive biases). At the same time, when viewing a bright star-like object using good light-collecting binoculars, some spacecraft with a discernible color may have been too dazzlingly bright in your dark-adapted eyes, so you may have missed some non-neutral colors before.

Images from https://terra.nasa.gov/about In reflected sunlight the spacecraft will probably appear roughly this color at night under ideal observing conditions, but see above for some of the ways that one can be thrown off.

enter image description here enter image description here

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uhoh
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    Wow! Such an awesome thing that one can spot differences between satellites and get some insight on them just with binoculars. I wonder if there are more "special characteristics" one could get from other satellites (just like flares contain information of the geometry of the object). – Swike Aug 11 '19 at 13:13
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    Even the ISS looks like a orange-reddish fast moving star (with the naked eye) due to the massive solar panels. – Magic Octopus Urn Aug 13 '19 at 16:25
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    @MagicOctopusUrn interesting! It may vary in color a lot depending on the Sun-Target-Observer angle, relative orientation of the ISS, and even the orientations of each of the individual solar panels; 1, 2 – uhoh Aug 13 '19 at 22:04