By David Whitehouse
Science editor, BBC News website
For only the second time, astronomers have captured Pluto's mysterious moon, Charon, passing in front of a star.
Experts want to work out how round Charon is
Four telescopes spread across South America watched out for the rare event on the night of July 10-11.
The so-called occultation will allow any atmosphere that the moon may have to be probed.
"We have been waiting many years for this opportunity," said Professor James Elliot, of the Massachusetts Institute of Technology, US.
In addition to assessing whether Charon has an atmosphere, the team expects to get a new, accurate value for Charon's radius and to determine how round it is.
Right place, right time
Professor Jay Pasachoff, of Williams College, Massachusetts, US, said: "It's amazing that people in our group could get in the right place at the right time to line up a tiny body four billion miles away. It's quite a reward for so many people who worked so hard to arrange and integrate the equipment and to get the observations."
MIT team leader James Elliot headed the group at the Clay Telescope at Las Campanas Observatory in Chile. Using its 6.5m mirror, the researchers were able to observe split-second changes throughout the event, which lasted less than a minute.
From just how the light dimmed and brightened, the MIT-Williams consortium will look for signs that Charon has an atmosphere.
It has very little mass, so has little gravity to hold in an atmosphere, but it is so cold (being some 40 times farther from the Sun than the Earth, and thus about 4 billion miles away) that some gases could be held in place by the small pull of Charon's gravity.
Other telescopes around Chile used by the MIT-Williams consortium included the 8m Gemini South on Cerro Pachon, the 2.5m DuPont Telescope at Las Campanas Observatory, and the 0.8m telescope at the Cerro Armazones Observatory of Chile's Catholic University of the North near Cerro Paranal.
The team had searched for a distribution of telescopes along a north-south line in Chile because the predictions of the starlight shadow of Charon were uncertain by several hundred kilometres.
Since the star that was hidden is so far away, it casts a shadow of Charon that is the same size as Charon itself, about 1,200km in diameter. To see the event, the distant star, Charon, and the telescopes in Chile had to be perfectly aligned.
It is too soon to tell if Charon has an atmosphere
The images from the three telescopes in Chile, including the Clay Telescope, and one in Brazil, were taken with new electronic cameras and computer control equipment.
Analysis of the data is underway but it is too soon to tell if Charon has an atmosphere.
The results from the recent observation will be presented at the 2005 American Astronomical Society's Division of Planetary Sciences meeting to be held in Cambridge, UK, in September.