Photographing the asteroid Apophis

Never heard of it

99942 Apophis is an asteroid whose orbit - somewhat worryingly - intersects the Earth's. It's defined as a hazardous object. Apophis orbits nine times for every eight Earth orbits, and because of where the intersection is located, the danger period is every eighth April.

Wait, what?

The intersection is on the ascending node, meaning Apophis approaches the Earth from "below" (assuming Earth's northern hemisphere is "up"). This means Apophis is visible from the Southern Hemisphere before the encounter, and it falls below the horizon, becoming visible from the Northern Hemisphere after the encounter.

There's a super close (but safe) pass coming up in April 2029, meaning about now is the last encounter before then.

So of course I want a photo

In preparation for writing an article about the asteroid, I wanted to actually get a photo of it. Because it's not a very close pass, it wanders by rather than zooms, and so I had a good couple of weeks' window of opportunity. However, weather is always a problem, and I had to take opportunities I was given.

The night of 23-24 February looked OK. It was a three-quarter waxing Moon, but I was hoping that Apophis (which is very faint at magnitude 16 or so) would still be visible in the glare. I took myself to the dark sky site and set up.

Just how do you get a shot?

My problem was that I didn't know exactly where to aim my scope. Because I have an equatorial mount, I can track stars all night without the aim drifting or the field rotating at all. I wanted a series of photos with Apophis drifting across the frame. if I knew where Apophis was going to be, say 10pm and 4am, I could aim my scope so the photo would have both these points in them. So where - exactly - was Apophis going to be?

Stellarium, my normal planetarium program, showed me an orange line through the stars. Here is the program's prediction for where Apophis would move on the evening between about 10pm and 4am. The asteroid would drift past a ninth-magnitude star from right to left over the course of the night.

Frankly, I wasn't confident that Stellarium had it right. Another program, Cartes du Ciel, has the ability to refresh the orbits of asteroids on request, but it's more complicated than Stellarium and I've never bothered learning it. Perhaps I should have.

A shot in the dark

In the absence of other information, I decided to pick a point about half way along the orange line and centre my image there. It would give me the best chance of hitting it. My field is wide, about 1.8° by 1.4°, so I was reasonably confident it'd fall in there - somewhere.

The search begins

I began taking continuous 2-minute images, as the nearby Moon prevented longer exposures. My automated set-up paused only to refocus every hour, and to perform a meridian flip at about 1:20am. I retired to my tent for a few hours.

After a nap I began comparing the latest image to the first, trying to see a dot that moved. Nothing. It was dark, my eyes were bleary from sleep and the screen was dim. After a while I began to think I'd either missed it, or Apophis was dimmer than I'd guessed. 

Failure is always an option

Just before 4am the Moon had set and the sky had darkened, so I decided to take a 20-minute exposure. If Apophis were there, it would show up as a bright line against the unmoving stars. After the exposure had downloaded I opened it and began scanning.

Got it!

And there it was, not all that far from the predicted position.

After my "happy astronomer dance", I looked back at the previous images. Now knowing where to look, I was able to track back and found Apophis was in all my frames as a dim dot. It was moving between frames as I expected, I just hadn't found it.

Um, we're not that sure?

It was also on about the path Stellarium had predicted, but it had shown up on the scene quite a long time after the predicted time. The orbital path was close, but not very close.

I wondered why, but then I remembered how close Apophis is coming to the Earth in 2029.

I didn't wonder too much.

Perhaps I don't really want to know the answer.