Orbiting object with oscillating brightness

Question

Yesterday was quite a good night to watch the sky using my 8 inch dobsonian (until moonrise at least). I was lucky enough to catch some satellites flying into my sight as I was searching for some deep sky objects. One of those satellites was a little different though.

I followed it a bit and noticed a period oscillation in its brightness. When I first saw it, it was at its brightnesses peak point, going fainter from there until it eventually vanished into darkness. I thought it might have set over the horizon, without any more sunlight for it to reflect, but a few seconds later it became visible again. I've seen three full cycles of it going from invisible, to visible, to invisible again, before I lost it.

The object went overhead from east-northeast to west-southwest and according to my sky tracking app it appeared to be a Cosmos 1400 rocket stage. Approximate position was Frankfurt/Germany, at about 11pm-12am local time. My question is, how come the satellite's brightness is oscillating so periodically?

My initial thought was that the satellite (rocket stage) is rotating and only reflective on one of its "sides".

Am I right with my initial assumption? If not, how else could it be explained?

Answer 1

There are actually a couple things that can account for changes in brightness of a satellite, but for an oscillation like that (especially for a rocket body) it's almost certain that the object is tumbling.

This site offers pictures and analysis (some of the pictures/charts are even rocket bodies from Cosmos satellites). To quote their explanation of periodic flasing:

Many satellites do not have a constant brightness, they give off flashes at (usually) regular times. This flashing behavior is caused by the rotation of the satellite around its rotation axis. The satellite's metallic surfaces act as mirrors for the sun (specular reflection). Objects with a diffusely reflecting surface will also show varying brightness since the observer will see a changing amount of light reflecting area of the rocket as it tumbles about in its orbit.

I'd say that pretty well covers tumbling objects, but as for "how else could it be explained" (taken to be asking "what else causes flashes from satellites") Iridium Flares come to mind. Iridium satellites have large, flat antennas that are reflective, and active iridium satellites are stabilized to ensure the antennas point in a useful direction. The combination of known orbits and orientation means that flares are predictable, and this site even has a tool to tell you when the next flares will be (enter a lat/lon in the url).

The same concept applies to other satellites (orientation + reflectivity results in a bright flash when the sun and observation point are lined up properly), but iridium is especially easy to see due to how many satellites are in the iridium constellation and how bright the reflection from them is.