Pictures of the Earth taken from the International Space Station are endlessly fascinating. Sometimes the locations are obvious, and sometimes not so much.

Phil Plait Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

And sometimes we get a mystery. I have one of those for you today.




This started when I got an email from Nahum Mendez Chazarra, who had been going through pictures taken by astronauts from the ISS. In a batch taken on July 15, 2014, at about 11:57 UTC, he found three in a row that showed a curious thing. Here’s one of them:

Photo by NASA

Spectacular! The ISS was south of Australia at the time, so the green glow is the aurora australis, the southern lights. The ISS solar panels stick into the shot from the upper right (seen nearly edge on), and the Earth dominates below.

But look to the Earth’s limb, just below and to the right of the brightest part of the aurora, and just above the solar panel. See that streak? It’s clearly some sort of moving object.

It’s in the two pictures taken just before this one as well. I added them together and zoomed in on the object so you can see it better:

Photo by NASA






Some things to note: The first picture had an exposure time of 0.2 seconds, the second one was 0.4 seconds, and the third 0.8. Measuring the length of the streak, it looks like the object is moving at a constant apparent velocity (the last streak is four times longer than the first, and twice as long as the second, as you’d expect from the exposure times).

Here's an animation I made to show the motion more obviously:

Chazarra suspected it was a meteor, burning up in the atmosphere below the ISS. At first I disagreed, thinking it might be a satellite. But then I wondered … so I sent a note to my friend Jonathan McDowell, who is an expert on things in orbit. He noted it was consistent with a satellite or a meteor, and added it could also be a small bit of debris much closer to the station; for example, a piece of ice just a few dozen meters away.

Arg! How to distinguish between these?




Well, one way would be to look at pictures taken just before and just after this set. So I did, and found that the object is not in pictures taken just three seconds earlier, nor is it in the next set taken three seconds later!

If it were a satellite or a piece of debris moving at a constant speed, then I’d expect it to be in at least the first picture taken after this set of three, down in the lower right. I looked carefully; it’s not there. That makes a satellite or piece of debris less likely (though still more likely than some alternatives). Also, note how in the picture above it crosses over the face of the Earth; that means it must be in a lower orbit than ISS. If it were up higher then it could never be seen against the Earth like that. The ISS is at a height of about 415 kilometers (260 miles), which is pretty low. There aren’t many satellites orbiting appreciably lower than that height. This doesn’t preclude it being a satellite, but a priori it makes it less likely.

Photo by NASA

That leaves meteor. That does fit most of what we see; it appears suddenly, disappears just as suddenly, and moves at a relatively constant rate. If it were small bit of rock it wouldn’t necessarily flare up and get hugely brighter, which has been seen before when a Perseid meteor burned up as seen from ISS:

The object does seem to be brighter in the longer exposure, which is interesting. Since it’s moving, each pixel should be about a constant brightness; a longer exposure just means the streak is longer, not brighter. If it’s actually brighter per pixel, that means the object itself was getting brighter, as a meteor would. However, a longer exposure also means the Earth and other stationary background objects get brighter, and their light would add digitally to the object’s, making it look brighter even if it isn’t.

Arg again!

In the end, I’m leaning toward this being a meteor, but I cannot be positive. It’s still something of a mystery, as promised.

So, BABloggees: What do you think? What did I miss? Is there more (or less) here than meets the eye? I think that throwing this out to the Hive Mind might bring some insight to the puzzle.