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telescope, the earth’s atmosphere blurs that object a little bit, preventing you from seeing the smallest details. This blurring was the reason that we were never certain at first how big the things we kept finding were; their little disks were blurred out enough that they all looked the same. Antonin had been hired to work on a project to fix this problem. The trick is to take a powerful laser and shoot it out the front of the telescope into space. The light from this laser traverses the earth’s atmosphere, and then, right when it is about to shoot out the top, it encounters a thin layer of gas that has been burned off from asteroids as they enter the earth’s atmosphere. The laser light is precisely tuned to bounce off the asteroid gas and return back to the earth. If you then take your telescope and point it right at the location of the laser, you see a little spot of light—an artificial star!—in the sky. The real trick then happens. You take that picture of the laser beam, which has been distorted by the earth’s atmosphere, and you bounce it off a fun-house mirror that is warped precisely so that the picture of the laser is as sharp as you know it is supposed to be. And then you do it all over again one-hundredth of a second later, using a different fun-house mirror shape, as the roiling of the earth’s atmosphere distorts the laser beam differently. If you can get all of the laser light going in the right place and the computers calculating fast enough and the fun-house mirrors warping accurately at your command, you can then take a nice long picture of the sky, and you will see a beautiful pinpoint of a laser beam, just as you shot it. That would be a lot of work just to see a laser beam; but if you pointed the laser beam directly at something else in the sky that you really cared to look at, you would also be perfectly correcting the light coming from that object, too.

One of the very first times that Antonin and the rest of the team hard at work at the Keck Observatory got all of the pieces in place and pointed up to the sky to test things out, they looked at Santa—and they found that Santa has a moon. We named it Rudolph.

Discovery of moons is extremely helpful, since it means that you can weigh the object using simple high school physics. Once you know how far away the moon is from the object and how fast it goes around the object, you’ve got all the information you need. Now that we knew that Santa had a moon, we only had to observe it a few more times and we would know how fast it was moving and how far away it was.

For me, all this meant was a lot of waiting. The first ticking clock was, strangely enough, that of our own moon. Because the laser system that Antonin was working on was still experimental, no one wanted to potentially waste the most valuable observing time. The laser was allowed to experiment on the telescope only when the moon was full—when it was bright time and many of the astronomer’s favorite targets in the sky were washed out by the glare of the moon. So though we knew about the moon of Santa quickly, we didn’t get our second look until twenty-nine days later, when the earth’s moon came around full again.

After twenty-nine days Rudolph had moved, but we now had no idea if it had gone around several times and come back or if this was still its first revolution. We had to wait twenty-nine more days to get our next look. At this point it was close to where we had seen it the first time. This would all make sense if the moon took about fifty days to go around Santa, but I couldn’t be certain for twenty-nine more days. The fourth time we saw it, we knew for sure: Rudolph goes around Santa every forty-nine days. It took one final measurement for confirmation to make sure we had the distance down, too. Combining the time period of the orbit with the distance from Rudolph to Santa allowed us to know that Santa weighs only one-third as much as Pluto. It was a relief, almost. We had finally been careful not to get our hopes up too much.

Just because there was Santa’s moon to track and due dates to