NG Field Notes: For Round 2, Pluto Chasers Trying a New Trick

The following is an update from Leslie Young (currently in the Philippines) and Marc Buie (currently in Australia) on efforts to capture two back-to-back occultations of Pluto. With one event in the bag, the team now has about three days to refine their predictions for the next round of observations. The above gallery includes field shots from Marc as well as from team members Eliot Young working at the Hale A’a Observatory in Hawaii and John Stansberry stationed at the Bosscha Observatory on the island of Java. -V

We got the June 23 event. Now what?

All across the globe, from Australia to Nauru to Hawaii to Paris, we spent the last day focusing on the prediction of the June 27 occultations. One goal of the June 23 occultations was to improve our June 27 predictions. We’ve done this, and it turns out that the shadow paths are not where we thought they’d be when we bought our plane tickets. The latest predictions are posted at Bruno Sicardy’s site.

How certain are we this time? We think they’re good to tens of kilometers, but this is the first time we’ve tried using a precursor occultation to pinpoint a second event. This means that, in any case, we’d like to find out how well this trick really works.

The latest prediction has the southern edge of Pluto’s shadow a few hundred kilometers north of Hawaii and the Marshall Islands, with Cebu City as a graze, probing only the very upper atmosphere. The shadows from June 23 and 27 cut across the Earth at different angles; Cebu was one of the most southern of our sites for the June 23 event, and the most northern for June 27. What’s our strategy for Pluto? We plan to observe where we are, and contact observers in Japan.

The latest prediction of the Hydra shadow is now well off of Australia. How did this happen? It’s a question of “one giant step south, two small steps north.” The current Hydra shadow is within a few hundred kilometers of where we thought it would be back last fall, but back in the fall, we all knew that this was very uncertain. The big error was the uncertainty in Hydra’s position relative to Pluto—about 2,000 kilometers of uncertainty. New Hubble Space Telescope images and analysis by Marc Buie and Dave Tholen moved Hydra’s predicted shadow path a few thousand kilometers south. This is the “giant step south.” In fact, the best prediction from last March had Hydra’s shadow going right down the center of Australia. Wow! Great!

Just in the last month or so, we got new measurements of the star. Don’t we know where the stars are? Well, yes and no. Pluto only subtends a hundred milliarcseconds, or about a tenth of the typical smear of star through a typical professional telescope. If we work hard, we can measure stars to better than that, say 5 to 20 milliarcseconds. The problem is that stars move, and you have to either keep remeasuring them or know how to correct for their motion. A few of the occultation groups remeasured the stars earlier this summer, and this moved the June 23 shadows south and the June 27 shadows north. This was the first of the two “small steps north.” When we all got on the airplanes a week ago, the Hydra shadow was predicted to be just on the northern edge of Australia, near Darwin. Darwin is harder than Alice Spring: The weather isn’t as good, and it’s a longer drive for most observers. This prediction still had the uncertainty in Pluto’s ephemeris, the position of Pluto itself, which we planned to refine with the June 23 observations.

A day before the June 23 occultation, we thought Hawaii and Mexico might be north of both the Pluto and Charon shadows, while in fact they showed glorious victory and saw double occultations. This means that the Pluto ephemeris correction pulls the June 23—and the June 27—shadows north. This is the second “small step north” that puts the predicted track back where it started last fall!

The current Hydra shadow now might cross Hawaii, the Marshall Islands, and Malaysia. Where we already are observing, we’ll look to double-up with other telescopes 2 to 30 kilometers away, to make local grids. In other words, we’ll try to find ways so if one telescope gets a Hydra event, we’ll have a good chance of seeing it from two telescopes. For Australia, we’ll ask the question, “What if the old star positions are more accurate than the updated ones?” and observe near the shadow predicted from March.

(The Pluto occultation research is supported by NASA’s Planetary Astronomy program, NASA’s New Horizons Mission, the National Geographic Society’s Committee for Research and Exploration, and the Southwest Research Institute.)

Changing Planet