Blocking Star Light Much Closer to Home: Pluto Occultations

Today’s blog is by guest blogger Dr. Jay Pasachoff.  Jay is Chair of the International Astronomical Union’s Working Group on Eclipses and is Field Memorial Professor of Astronomy at Williams College. He has viewed over 50 solar eclipses, and is an expert on both their use for scientific observations and their use for public education. Jay has has authored and co-authored  several astronomy books including the Field Guide to the Stars and Planets. Today he’ll be talking about a different way of planetary bodies blocking out star light that’s a little closer to home. He’ll be discussing what we can learn from studying dwarf planet Pluto blocking out (or occulting) the light from distant background stars.

In Sunday’s New York Times had an Op-Ed piece ( “Praise of not knowing” by Tim Kreidler) that praised the observations of Pluto’s surface from the Hubble Space Telescope but said that nothing more would be found out about Pluto until NASA’s New Horizons mission gets there in 2015.  The latter statement is flat wrong, since three independent groups are studying Pluto and its atmosphere by watching it go in front of, “occult,” distant stars.

As I write, I am in Pasadena, en route to Hawaii for a rare double event: On  Wednesday/Thursday night, June 22/23, both Pluto and its moon Charon will occult a 13th magnitude star, each occultation lasting a minute or so and separated from each other by 12 minutes.  On Sunday/Monday night, June 26/27, Pluto will occult a star, and over a much narrower path, its small moon Hydra might also occult a star.  But to see those occultations, we have to be in a particular set of places on Earth, those over which the shadow of the object in starlight passes.  Since the stars are so far away, their light is essentially parallel and the shadows of the objects on Earth are the same as the sizes of the objects.

With such a long lever arm as the distance from Earth to Pluto, the positions of the solar-system objects and the stars have to be known to better then a tenth of an arcsec.  When I joined this game at the invitation of Jim Elliot of MIT about 10 years ago, the predicted paths could change by 1000 km or more a few days before the event; in 2002, we had a mad scramble to move our equipment 1000 km north a couple of days before the predicted time and date.  Now, the models used by our MIT, Lowell Observatory, and U.S. Naval Observatory colleagues to make the predictions are more accurate, though they are being refined up to the last days with new observations taken in Flagstaff.  A few weeks ago, on May 22, a fainter star’s shadow was predicted to pass over northern regions of Earth, with the centerline off the Earth above the north pole.  We were, barely through clouds, able to detect the event for about 100 s at our home site at Williams College in Williamstown, Massachusetts, using the 24″ (0.6 m) telescope that is usually used only for students in the survey courses.  (The detection, through fairly thick clouds, required careful subtraction of background and ratioing with a nearby, brighter star, carefully and expertly carried out by my Williams College colleague Dr. Steven Souza, and then remeasured by my Williams College colleague Dr. Bryce Babcock.)  The detection gives us more confidence than we had in the accuracy of the prediction.

Jim Elliot, who died in March, was a friend of mine from graduate school in the 1960s.  He came to fame in the 1970s when he, accompanied by junior colleagues Ted Dunham and Doug Mink, discovered the rings of Uranus by their sequential occultations of a distant star, observing from NASA’s Kuiper Airborne Observatory, an instrumented airplane.  It turned out that my fast-readout equipment used to study total solar eclipses was similar to the equipment he used to study stellar occultations, and a partnership was born.  My students and I first tried to help him discover rings around Neptune when we were in Indonesia for the eclipse of 1983, since a Neptune occultation observed a few days later.  We were clouded out.  In 1997, Babcock, student Tim McConnochie ’98, and I tried to observe the atmosphere of Triton as it occulted a star from Australia.  We got data, but no occultation showed from our site; the prediction had moved even farther north than we had thought a few days in advance, when we made a mad dash 1000 km north from Siding Spring Observatory to a telescope of the University of North Queensland in Toowoomba—the farthest-north telescope in Australia at that time.

In 2002, an occultation of a star by Pluto was predicted, and we brought our equipment to the University of Hawaii’s 88″ (2.2 m) telescope on Mauna Kea.  MIT colleagues were observing from the nearby NASA IRTF 3-m telescope.  Our data were so good that I was hooked on occultation studies, and I have been working with Jim and his MIT team ever since.  My colleagues at Williams Steve Souza and Bryce Babcock and a series of undergraduates have worked with me in these studies.

When reduced, the 2002 data showed that Pluto’s atmosphere had warmed a bit and had doubled in density since the previous occultation 14 years earlier, a surprise since Pluto had passed perihelion in 1989 and was (and is) moving farther from the Sun each day.  Apparently, the kind of thermal lag that makes our terrestrial daytime temperatures peak closer to 4 pm than to noon was operating on Pluto as well.  The key question was and is whether any atmosphere would remain around Pluto when a spacecraft got there, or would it snow out by then.  Subsequently, NASA launched its New Horizons mission to Pluto, run by Alan Stern and colleagues at Southwest Research Institute in Boulder, Colorado, and administered at the Johns Hopkins University Applied Physics Laboratory.

By showing that Pluto was still warming, our MIT-Williams team’s data indicated that the atmosphere would probably remain warm enough by 2015 for the spacecraft to detect it nicely.  Since then, we have observed several other occultations, taking data at about 4 times per second (4 hertz = 4 Hz), which indicate that the temperature and density changes have leveled off, though they have not yet started decreasing.

Based on our success, we put in a joint equipment-grant request to NASA, which, when granted, allowed us to buy six special cameras (3 for MIT and 3 for Williams College), which we linked with GPS for time signals and computers in systems each called POETS: Portable Occultation, Eclipse, and Transit System.  We have used ours in solar eclipses in Greece in 2006, Russia in 2008, and China in 2009, as well as on Sacramento Peak for the Mercury transit of 2006, so POETS has lived up to its name.  We have worked closely with these cameras since then, linking with Jim Elliot, Mike Person, and Amanda Gulbis at MIT (Gulbis is now at the South African Astronomical Observatory in Cape Town, though retains a visiting appointment at MIT).

In 2005, from several telescopes in Chile, our MIT-Williams team captured an occultation of a star by Pluto’s moon Charon.  Whereas the dimming of starlight by Pluto is gradual because of Pluto’s atmosphere, the dimming by Charon was abrupt, showing that Charon has no atmosphere.  Both our 2002 Pluto occultation and the 2005 Charon occultation results were published first in Nature, a sign of their importance.  Two competing groups, one headed by Leslie Young of Southwest Research Institute, a former student of Jim’s, and another headed by Bruno Sicardy of l’Observatoire de Paris at Meudon in collaboration with Thomas Widemann also of that French observatory, also make similar measurements, and we often vie with them for access to telescopes in given spots.

In 2010, we were able to accomplish a long-term goal of Jim’s, to detect an occultation by a Kuiper belt object beyond Pluto.  We keep astrometric track of a dozen or so of the largest KBOs, to look for potential occultations.  When 2002 TX300 was to pass a star, we were ready. I had arranged for Wayne Rosing and colleagues of the Las Cumbres Global Telescope to use their Faulkes 2-m telescope on Haleakala, Hawaii.  Bryce Babcock and our student Katie DuPré ’09 were observing with a POETS from a 16″ telescope at Leeward Community College on Oahu, and Steve Souza took one of our fancy POETS to the Big Island.  Unfortunately for us, NASA was crashing its LCROSS mission into the Moon only about 55 minutes after our predicted KBO event, and we couldn’t get any of the major telescopes on Mauna Kea to do our task–which turned out to be more interesting and important than what they were doing.  In the event, Souza aggravated an old shoulder injury and a local University of Hawaii at Hilo student who had been assigned to work with him used his own telescope and CCD from a parking lot at the Onizuka “mid-level facility” halfway up Mauna Kea.  When the data were reduced, Faulkes had beautiful data with about 50 points of dip in total intensity because of the occultation, the Big Island data had only two low points (given their slower cadence) but that was enough to confirm the occultation and to set limits on the orientation of the image, and the Oahu data were of high quality but showed no dip.  With those observations, again published in Nature, we concluded that the KBO, which has the asteroid number 55636, was only 143 ± 5 km in radius.  Had we known it was so small, it wouldn’t have been on our list, since we would have had little hope of detecting it occulting anything!  Anyway, to be so small and to have its observed brightness, it has to have 90% albedo, making it the brightest known thing in the solar system.  That it is bright was not a surprise because it was thought to be in the family of the dwarf-planet Haumea, which is covered with ice.  But for the ice to be that bright rather than darkened over the billions of years of time since its formation was a surprise.

We have a variety of websites describing our work on occultations.  Last year, I set up a site put together by our Williams College student Caroline Ng ’11.  It has links not only to our own work but also to the websites of Mike Brown at Caltech and Mike Person and others of the MIT group. When I was on sabbatical at Caltech in 2008-9, I worked with Mike Brown and his group on some mutual events of Haumea and its moon Namaka, with the most interesting and pleasant of our attempts when I was hosted by Meg Schwamb at the 200″ (5 m) Hale telescope at Palomar for one of the predicted 1% events.

Now we are en route to Hawaii tomorrow for this week’s occultations.  Maps and details of the predictions can be found here, and more detailed information can be found on  this site. Mike Person now heads the group at MIT.  Carlos Zuluaga at MIT keeps the predictions up to date.  Amanda Bosh is now a scientist in the group there.

For the June 22/23 event on payday loans uk (June 23 UT but June 22 in Hawaii), the star is a magnitude 14.4 in the standard, UCAC2 catalogue.  The revised, recent predictions seem to have shifted the prediction south, so that Hawaii is slightly off the main predicted path, to its north.  The island sites are still within the uncertainty for both Pluto and Charon, so we can still hope to see the events.  One MIT person, Amanda Zangari, has gone to Cairns, Australia, to see if it goes that far south. The event is particularly exciting because if we capture both Pluto and Charon nearly simultaneously, we can find out about the system’s internal orbits with higher precision than before, perhaps allowing a refinement of the center of mass and thus the masses and densities of each object. Another excitement is to be the first deployment for an occultation of the NASA/German SOFIA airplane, the successor to the Kuiper Airborne Observatory that has been so long in being readied. Mike Person is Jim Elliot’s successor as co-Investigator, and Ted Dunham is also a co-Investigator.  They will fly to the centerline of the latest prediction, which, as of today (Sunday before the events) is off the coast of Mexico.

For the June 26/27 event (June 27 UT but June 26 in Hawaii), the star is magnitude 13.6, a couple of magnitudes brighter than most of the stars we have observed being occulted, so the data would be particularly low-noise.  In addition to the occultation of Pluto itself, whose southern limit is predicted to pass through the Hawaiian islands, the tiny Pluto moon Hydra is to be occulted, though that narrow path’s prediction now passes north of the Hawaiian islands.  We have arranged for telescopes in Yunnan, China, in Japan, Taiwan, and Thailand to observe with us, and MIT’s Matt Lockhart is en route to Yunnan with one of our POETS cameras.  We have Australian sites still observing as well, just in case the actual path is hundreds of kilometers south of the predictions.

Bryce Babcock will observe at Leeward Community College with Mohanan Kakkala there, our alumnus Eric Pilger ’82, and my summer student Shubhanga Pandey ’13.  I will observe at Windward Community College with Joe Ciotti and Marvin Kessler of the faculty there and their students, as well as with my summer exchange student, Keck Northeast Astronomy Consortium Wesleyan student David Amrhein ’12, with MIT undergraduate Stephanie Sallum ’12, and with Robert Lucas from the University of Sydney.  Our work is supported by a research grant from NASA’s Planetary Astronomy Division.

In about a week, if all goes well, we will know a lot more about the Pluto system.


5 responses to “Blocking Star Light Much Closer to Home: Pluto Occultations”

  1. Angel says :

    Thank you for this thrilling insight into your work. The hunt for star shadows is such an artful way of doing science. It would be a great to see it as a subplot in the next James Bond movie ;-).

  2. Gregg Weber says :

    I remember an article in either Astronomy, Sky & telescope, or possibly Scientific American about the results of an occultation observed in Oklahoma and Texas somewhere in the late 1960’s or early 70’s. The writer realized that pluto was going to occult a star and when and where it would be seen on earth. It was in colaboration with many amateur astronomers, each with a telescope and a radio tuned to WWV for proper timeing. Things went good but some reported that the star blinked off and on twice. The article showed Pluto and a smaller “moon” nearby. I haven’t seen anything of this event in any other report even though I know I read of it in a real science magazine.

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