There are thousands of satellites and spent rocket parts in low earth orbit (LEO). Mostly near the lines of dawn and dusk they can be in sunlight while observatories below are still viewing the night sky, as are the observatories like Hubble (also in LEO).

They are moving roughly 7 km/sec and as seen from Earth that motion will be roughly a half-degree per second, depending on distance.

Considering the number of observatories imaging the sky at any moment and the number of objects in LEO, streaks across images must be happening quite regularly.

While that would strongly impact a one-hour exposure on emulsion, these days solid state imaging is used and these are probably regularly buffered to provide the dynamic range necessary to capture dim objects with very bright images in the same field.

While a fraction of a second streak by a satellite will not normally completely obliterate a long exposure, it is still a problem and it seems would have to be managed in an accepted and systematic way.

Question: How do observational astronomers manage streaks and other artifacts from objects in LEO? Since these things are (usually) carefully tracked and their trajectories predictable, do observatories, or at least observers, ever plan ahead for these events, or schedule to avoid them? Or is it just handled automatically in post-processing?

For subtext, see astronomer Alex Parker's Tweet (image shown below) and then astronomer Benjamin Pope's more recent Tweet about the Humanity Star, as well as his response "Oh god why would you do this to us astronomers"? response here.

Also see:

Within Stack Exchange:

From Alex Parker's Tweet:

enter image description here


1 Answer 1


Simple - as you say, in order to get lots of dynamic range in deep astronomical images, you generally need to split your exposure time down into a number of sub-exposures.

When you combine these images, you don't just add them up, you "median stack" them (take the median on a pixel by pixel basis) or perform a slightly more sophisticated flux-weighted averaging that kicks out the outliers.

This process gets rid of any moving objects in the final image as well as cleaning up all the cosmic rays.

The problem with this reflector that has been launched is that it will be the equivalent of a very bright star. It will totally saturate any single image it appears in and might actually be bright enough to damage, or at least leave a persistent image, in some infrared detectors.

  • $\begingroup$ Is there somewhere where I could read more about median-stacking and outlier-filtering? I can certainly imagine how one might do it, but is there a standardized flow written up somewhere to make sure people don't get too creative or even do it wrong? Also, you wouldn't want to artificially force a steady state upon the data, from time to time real astronomical time-dependent events happen! There are also asteroids - would they be flagged by this process, or suppressed? There must be some SOP that's evolved over the years and become standardized. $\endgroup$
    – uhoh
    Commented Jan 27, 2018 at 16:51
  • $\begingroup$ If you think there is something to expand upon with regard to intentional highly reflective spacecraft and the problems they might cause, I could certainly split that off and ask it as a separate question. $\endgroup$
    – uhoh
    Commented Jan 27, 2018 at 16:53
  • $\begingroup$ @uhoh No, there is no standardised process. What I have described deals with asteroids too. $\endgroup$
    – ProfRob
    Commented Jan 28, 2018 at 0:31
  • $\begingroup$ OK, I'll try to do some more reading then, thanks! $\endgroup$
    – uhoh
    Commented Jan 28, 2018 at 3:21

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