A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

The occult sciences

Last weekend I had my first experience with the occult sciences.
Maybe I should rephrase that.
Last weekend I did my first occultation science. That’s what I meant.
Occultations are interesting events that can be seen here on earth. They are like miniature total eclipses except that instead of the sun being blocked, it is a star. And instead of the moon doing the blocking it is something else, an asteroid, a planet, a Kuiper belt ice ball. You know an occultation is occurring when a star suddenly disappears and then reappears seconds to minutes later. Something dark must have moved in front of the star.
Scientifically, occultations provide a unique glimpse at the dark object that is passing in front of the star. If you measure how long the star disappears and you know how fast the object was moving, you have just directly measured the size of the object. Or at least measured the size of the object across one line. To really measure the full size of the object you need more than one line. To do that, you station astronomers in something resembling a north-south string over the full expected size of the object. Everyone watches and carefully times the event, and then you combine all of the information to find out the real size and shape of the object. If you’re lucky, you might even detect that the star does not blink out, but fades out, instead. This fading shows the atmosphere of the object. If you’re even luckier, you might see a second disappearance of the star a little before or after the main event. You would have just discovered a moon of your object.
The occultation last week was by a large Kuiper belt object. Kuiper belt objects are so far away and appear so small from our point of view that the probability of one of them covering up a star at any point in time is quite small. Astronomers carefully track these Kuiper belt objects and carefully measure positions of stars over and over in the hopes that one of them will be found to occult.
Sometimes these predictions can be made months ahead of time and astronomers can prepare for the event. Sometimes, like for the one last week, no one knew for sure that the occultation would occur until a last set of careful measurements of the position of the star occurred a few weeks before. Suddenly it appeared that this occultation would be visible across much of North America and that the path would go over some of the major observatories: McDonald, Kitt Peak, Palomar, Lick.
With only two weeks to prepare, though, it is tough to suddenly get a telescope. All of the large telescopes are fully scheduled months in advance, but there sometimes some observatories have smaller telescopes that can be made available at shorter notice if you know the right person.
At Palomar, the right person to know if you want to observer on the brand-new 24-inch robotic telescope is me. I’ve been constructing this new telescope for an embarrassingly long time now, but it is almost finished and ready for real scientific observations. One of its major long-term projects is to monitor Saturn’s moon Titan for signs of major storm activity. But the telescope is still not quite ready yet; we hope to really have it finally commissioned by October.
But when we heard that this occultation was potentially going to be visible from Palomar we decided it was worth going up and trying to use this little telescope even though it was not quite ready.
We arrived Saturday afternoon for the Sunday occultation. Emily Schaller – my now former graduate student (who moved to Hawaii last week to begin a new position as a Postdoctoral Fellow at the Institute for Astronomy at the University of Hawaii) – and I left Pasadena at noon, stopped once for coffee, and arrived at Palomar Observatory at around 3pm. We went right to the small dome of the 24-inch telescope, unlocked the door, and peered inside a bit apprehensively. No one had even been in side for the past few months as we were waiting for the final control systems to be finished. We knew that there was a moderate chance that something would have broken over this time period and the telescope simply would not work. We knew that last winter the dome had leaked. What would we find?
To our relief, everything looked fine. We plugged the telescope and the computer that controls it in and double checked that we could, at least, move things. We could! We set to work to get things going. We had brought some new software up on a laptop to control some important auxiliary functions. But we had forgotten to check if the laptop control ports were compatible with the telescopes, and, of course, they weren’t. We’d have to drive back down the mountain on Sunday to the electronics store and then pray we could get them to work on Sunday.
But still, we could at least try to make sure we could do some basic things, like point to things in the sky.
We did a few daytime pointing tests and, to our sudden horror, realized that the telescope did not move the way it was supposed to. When we said go north, it went south. East was west. Looking carefully through the software we eventually realized that someone the telescope was confused about who it was. It thought it was its [bigger] sister telescope in southern Arizona. Somehow the control software had been switched. The sister telescope had enough different parameters (like which way was east and west) to know that this would never work.
Frustrated, we went to dinner with all of the other astronomers who were at Palomar for the evening, and we brainstormed about how we might fix things. By the end of dinner we had decided that no fix was possible; we needed the right software.
We were in luck, though. Another of my graduate students was awake and looking at her email and realized what we needed and, more importantly, realized where we the software was. We copied it over tested things out, and realized that we were in business.
Because the telescope was not actually ready to be used yet, we had to do some very low-tech things to get it to work right. First, we found nice bright Jupiter up in the sky. Then we used a hand paddle to get the telescope pointing in approximately the right direction. Then I stood up on a ladder, looking down the barrel of the telescope, trying as hard as I could to line up on Jupiter while Emily took continuous pictures with the telescope’s digital camera. We finally meandered around enough that we found it (it helps that Jupiter is so bright that when you get even moderately close to the right place you can see the glow off to one side). Once we were at Jupiter, the telescope was smart enough to know the rest of the sky, so we quickly pressed a few buttons and the telescope automatically slewed to where our occultation was going to be the next night. We weren’t sure how accurate the slew was going to be, but, to our surprise, the star that was going to be occulted was right there in the center just as it was supposed to be. This might work!
We spent the next 2 hours pretending like it was Sunday night and doing exactly what we were going to do that night. Everything worked well except for the occasional problem we had when we forgot that one thing not quite finished yet on the telescope is the dome control software. We had to move the dome by hand to following the moving sky. Sometimes we forgot. We vowed to do better the next night.
The next morning we woke up and drove down to San Diego to pick up some computer equipment. On the drive back up the mountain we looked up at the sky and groaned. Summer thunderstorm clouds had completely covered the sky while we were going. It was possible that they would abate as the sun went down, but they looked pretty bad.
We got back up to the telescope, installed the new equipment, tested it, and realized, again to our thorough surprise, everything was going to work perfectly. Before dinner time, we finally stuck our heads out of the dome to see what the sky looked like. It was hopeless. The sky was 100% covered, and the possibility of observing at all that night seemed very very remote.
We went to dinner in sour moods and lingered over our deserts longer than usual, knowing that looking outside was going to make matters worse.
But we were wrong. When we finally forced ourselves to look, the sky was miraculously clear. Not a single cloud. I have no idea how it so thoroughly cleared itself in under 45 minutes. We ran back to the 24-inch, opened the dome (we had closed it, fearing thunderstorms!), and waited for it to get dark enough to find Jupiter. As soon as it was visible in the twilight glare, we swung the telescope, pointed it up, and punched in the coordinates of the star. Again, on the screen, was just the right field. It looked pretty crummy though; everything seemed too faint. Ah! The dome! We turned the dome in the right direction and everything looked fine.
It was 8:30pm. The occultation was predicted to begin in an hour, so we started acquiring the data, meaning that we took a picture of the star every 4 seconds (which makes many many pictures of the star). At about 9pm clouds suddenly appeared north of where we were, but we quickly realized they were heading even further north. Still safe. At 9:20pm we took one final look outside: not a single cloud. We then crowded in front of the computer screen to watch our pictures come in. At about 9:26 we started thinking that the star was getting fainter. But really? We made some very rough instant measurements and thought: yeah. Maybe. By 9:27 we were sure. Every single image showed the star consistently fainter. It stayed that way for 4 full minutes before getting back to normal bright again. We had seen it! At 9:40pm we sent a quick email to the other astronomers who were observing around the country. The subject line was “Subj: Report from Palomar: We saw it!”
Over the next hour other reports came in. Many observatories were clouded out, but a handful got good data. A quick comparison revealed that Palomar had, I think, been right down the center, giving the longest of all possible occultations. An even more careful look at the data revealed that the occultation was certainly not sudden; we had without a doubt detected an atmosphere around this Kuiper belt object.
We went to sleep, exhausted but thrilled. Heading back we realized that the sky was 100% covered in clouds again. We had just snuck in some clear skies at the right time.
Enough people had collected good data that useful information would come out of these observations. We would get a nice measurement of the atmosphere and whether or not it has changed recently. Looking at the atmosphere was one of the main hopes of the observations. The Kuiper belt object is currently receding from the sun and many astronomers suspect that its atmosphere will soon freeze out. Of course, only the very largest few Kuiper belt objects even have atmospheres, but this one has been known to have had an atmosphere for a while. The Kuiper belt object we were studying was Pluto.


  1. I wonder if the result will be the same if Pluto occults a star in ~2030. How long will this dwarf keep its atmosphere as it pulls away from perihelion? -- Kevin Heider

  2. Wow. You were really lucky the skies cleared for you. Congrats on a job well done.

    Now you can relax and enjoy today's XKCD:

  3. Hopefully these new results will help New Horizons decide at what rate the atmosphere is freezing to the surface.

    I like the cartoon. -- Kevin Heider

  4. How frustrating that so few occultations are occuring on Earth. So many TNOs to study... Far more occultations would be accessible from space close to Earth, although at a higher cost!

    If you Google "transneptunian occultation space" you will find this: An occultation-chasing mission is envisaged for a preliminary study (a paper at Europlanet congress from: Poncy / Martinot/ Petit / Roques and Sicardy).