Andrew Yee
January 5th 06, 12:03 AM
Office of Public Affairs
Williams College
Williamstown, Massachusetts
Contacts:
Dr. Amanda Gulbis, MIT
Telephone: 617.253.1785
Dr. Jim Elliot, MIT
Telephone: 617.253.6308
Dr. Jay Pasachoff, Williams College
Telephone: 413.597.2105
EMBARGOED until January 4, 2006, 13:00 EST.
Astronomers Seize Rare Opportunity to Measure Distant Charon
Being in the right place at the right time gave a group of Massachusetts
research astronomers a unique opportunity to study Pluto's largest moon
Charon. The resulting measurements, to unprecedented accuracy, of Charon's
size and possible atmosphere provide insight into the way this distant
world may have formed.
On July 10, 2005, astronomers from the Massachusetts Institute of
Technology (MIT) in Cambridge and Williams College in Williamstown
observed the light from a star as it disappeared behind Charon and
reappeared on the other side -- an event known as a stellar occultation.
Occultations provide important information about the size of remote
bodies, as well as the makeup of their atmospheres (if they have them).
According to team member Jim Elliot, a professor in MIT's Department of
Earth, Atmospheric, and Planetary Science and in the Department of
Physics, observations of a stellar occultation like this one have been
made only once before, from South Africa in 1980. "We have been waiting
many years for this opportunity," he said. "Watching the star vanish as it
was blocked by Charon was spectacular."
Although the star disappeared behind Charon for less than a minute, data
from the observations provided considerable information about this tiny
moon. In a paper released in the January 5, 2006, edition of Nature, the
MIT-Williams collaboration determined Charon's radius to be 606 +/- 8 km.
For perspective, this radius is roughly twice the width of Massachusetts
with an error of only 5 miles. The size was combined with mass
measurements from Hubble Space Telescope data to establish a density for
Charon of 1.72 g/cm3. This density, roughly 1/3 that of the Earth,
reflects Charon's rocky-icy composition.
What makes this achievement so remarkable is that the observation could
only be made from a narrow, 650-mile wide region in South America. The
MIT-Williams observers were located at four telescopes in Chile and one
telescope in Brazil for the event.
The largest telescope employed by the consortium was the 8-meter Gemini
South Telescope on Cerro Pachón. The observations utilized the Acquisition
Camera, a guider instrument that is typically used for telescope pointing
and target selection, as a high-speed photometer. Portable camera systems
constructed by the MIT-Williams group were mounted on the other
telescopes: the 6.5-meter Clay and 2.5-meter du Pont at Las Campanas
Observatory in La Serena, Chile, the 0.8-meter at the Observatório Cerro
Armazones in Anofagasta, Chile, and the 0.6-meter at Observatório Pico dos
Dias, Itajubá, Brazil. Observations were successful at all stations
excluding Brazil, which was clouded out.
Jay Pasachoff, Professor of Astronomy at Williams College and a
collaborator in the effort, praised the team doing the work. "It's
astounding that our group could be in the right place at the right time to
line up a tiny body three billion miles away," he said. "The successful
observations are quite a reward for all of the people who helped predict
the event, constructed and integrated the equipment, and traveled to the
telescopes."
Observations taken at a high rate, 10 frames per second, from the
6.5-meter Clay telescope (which was built by a collaboration of
institutions including MIT) detected subtle optical effects caused when
the starlight passed the edge of Charon's disk. By analyzing these
effects, known as diffraction fringes, the MIT-Williams team concluded
that any atmosphere on Charon is less than one millionth the density of
Earth's atmosphere. Their analysis provided very strict limits on the
amounts of various gases that could be present. Three years earlier, the
team previously used the technique of stellar occultation to study Pluto's
thin atmosphere, showing that it was subject to slight global warming.
The results of the observations argue against the theory that Pluto and
Charon were formed by the cooling and condensing of the gas and dust known
as the solar nebula. Instead, astronomers think that Charon was formed in
a collision between two objects early in the formation of the solar
system.
"Our observations show that there is no substantial atmosphere on Charon,
which is consistent with an impact formation scenario," said Nature lead
author Amanda Gulbis. "We also find that Charon contains roughly 10% less
rock by mass than Pluto. This difference suggests that either, or both,
objects involved in a Charon-forming collision had concentrations of
heavier materials in their cores." A collisional formation like this has a
parallel in theories for the formation of the Earth-Moon system.
Pluto has recently received considerable attention, with NASA's New
Horizons mission to be launched in January 2006, the discovery of two new
moons, and the discovery of several Kuiper belt objects that are
Pluto-sized (or even larger). The success of the MIT-Williams team in
observing the Charon occultation bodes well for their ability to observe
occultations of different stars by these newly discovered objects.
The so-called "10th planet" (2003 UB313), recently discovered by
scientists from Caltech, is a prime candidate for stellar occultation
observations. Although this object is approximately twice as far away from
the Earth as Charon, it is thought to be twice as large. 2003 UB313 thus
covers the same angular extent in the sky as Charon, just as the Moon and
the Sun appear to be the same size although the Sun is physically larger.
"We are eager to use the occultation technique to probe for atmospheres
around large Kuiper belt objects," remarked Jim Elliot, who has been
observing stellar occultations by bodies in the solar system for more than
three decades.
Members of the MIT team were Jim Elliot, Amanda Gulbis, Michael Person,
Elisabeth Adams, and Susan Kern, with support from undergraduate Emily
Kramer. The Williams College team included Jay Pasachoff, Bryce Babcock,
Steven Souza and undergraduate Joseph Gangestad.
The article describing this research is "Charon's Radius and Atmospheric
Constraints from Observations of A Stellar Occultation," by A.A.S. Gulbis,
J.L. Elliot, M.J. Person, E.R. Adams, B.A. Babcock, M. Emilio, J.W.
Gangestad, S.D. Kern, E.A. Kramer, D.J. Osip, J.M. Pasachoff, S.P. Souza,
and T. Tuvikene. It will appear in the January 5, 2006, edition of Nature.
A team led by French astronomer Bruno Sicardy and a team led by American
astronomer Leslie Young also observed the occultation from telescopes in
South America. American astronomer David Tholen discussed the significance
of the various results in a "News and Views" introductory article in the
same issue of Nature as the MIT-Williams and Sicardy-team articles.
The MIT-Williams electronic equipment and expeditions are supported by
grants from NASA.
Williams College
Williamstown, Massachusetts
Contacts:
Dr. Amanda Gulbis, MIT
Telephone: 617.253.1785
Dr. Jim Elliot, MIT
Telephone: 617.253.6308
Dr. Jay Pasachoff, Williams College
Telephone: 413.597.2105
EMBARGOED until January 4, 2006, 13:00 EST.
Astronomers Seize Rare Opportunity to Measure Distant Charon
Being in the right place at the right time gave a group of Massachusetts
research astronomers a unique opportunity to study Pluto's largest moon
Charon. The resulting measurements, to unprecedented accuracy, of Charon's
size and possible atmosphere provide insight into the way this distant
world may have formed.
On July 10, 2005, astronomers from the Massachusetts Institute of
Technology (MIT) in Cambridge and Williams College in Williamstown
observed the light from a star as it disappeared behind Charon and
reappeared on the other side -- an event known as a stellar occultation.
Occultations provide important information about the size of remote
bodies, as well as the makeup of their atmospheres (if they have them).
According to team member Jim Elliot, a professor in MIT's Department of
Earth, Atmospheric, and Planetary Science and in the Department of
Physics, observations of a stellar occultation like this one have been
made only once before, from South Africa in 1980. "We have been waiting
many years for this opportunity," he said. "Watching the star vanish as it
was blocked by Charon was spectacular."
Although the star disappeared behind Charon for less than a minute, data
from the observations provided considerable information about this tiny
moon. In a paper released in the January 5, 2006, edition of Nature, the
MIT-Williams collaboration determined Charon's radius to be 606 +/- 8 km.
For perspective, this radius is roughly twice the width of Massachusetts
with an error of only 5 miles. The size was combined with mass
measurements from Hubble Space Telescope data to establish a density for
Charon of 1.72 g/cm3. This density, roughly 1/3 that of the Earth,
reflects Charon's rocky-icy composition.
What makes this achievement so remarkable is that the observation could
only be made from a narrow, 650-mile wide region in South America. The
MIT-Williams observers were located at four telescopes in Chile and one
telescope in Brazil for the event.
The largest telescope employed by the consortium was the 8-meter Gemini
South Telescope on Cerro Pachón. The observations utilized the Acquisition
Camera, a guider instrument that is typically used for telescope pointing
and target selection, as a high-speed photometer. Portable camera systems
constructed by the MIT-Williams group were mounted on the other
telescopes: the 6.5-meter Clay and 2.5-meter du Pont at Las Campanas
Observatory in La Serena, Chile, the 0.8-meter at the Observatório Cerro
Armazones in Anofagasta, Chile, and the 0.6-meter at Observatório Pico dos
Dias, Itajubá, Brazil. Observations were successful at all stations
excluding Brazil, which was clouded out.
Jay Pasachoff, Professor of Astronomy at Williams College and a
collaborator in the effort, praised the team doing the work. "It's
astounding that our group could be in the right place at the right time to
line up a tiny body three billion miles away," he said. "The successful
observations are quite a reward for all of the people who helped predict
the event, constructed and integrated the equipment, and traveled to the
telescopes."
Observations taken at a high rate, 10 frames per second, from the
6.5-meter Clay telescope (which was built by a collaboration of
institutions including MIT) detected subtle optical effects caused when
the starlight passed the edge of Charon's disk. By analyzing these
effects, known as diffraction fringes, the MIT-Williams team concluded
that any atmosphere on Charon is less than one millionth the density of
Earth's atmosphere. Their analysis provided very strict limits on the
amounts of various gases that could be present. Three years earlier, the
team previously used the technique of stellar occultation to study Pluto's
thin atmosphere, showing that it was subject to slight global warming.
The results of the observations argue against the theory that Pluto and
Charon were formed by the cooling and condensing of the gas and dust known
as the solar nebula. Instead, astronomers think that Charon was formed in
a collision between two objects early in the formation of the solar
system.
"Our observations show that there is no substantial atmosphere on Charon,
which is consistent with an impact formation scenario," said Nature lead
author Amanda Gulbis. "We also find that Charon contains roughly 10% less
rock by mass than Pluto. This difference suggests that either, or both,
objects involved in a Charon-forming collision had concentrations of
heavier materials in their cores." A collisional formation like this has a
parallel in theories for the formation of the Earth-Moon system.
Pluto has recently received considerable attention, with NASA's New
Horizons mission to be launched in January 2006, the discovery of two new
moons, and the discovery of several Kuiper belt objects that are
Pluto-sized (or even larger). The success of the MIT-Williams team in
observing the Charon occultation bodes well for their ability to observe
occultations of different stars by these newly discovered objects.
The so-called "10th planet" (2003 UB313), recently discovered by
scientists from Caltech, is a prime candidate for stellar occultation
observations. Although this object is approximately twice as far away from
the Earth as Charon, it is thought to be twice as large. 2003 UB313 thus
covers the same angular extent in the sky as Charon, just as the Moon and
the Sun appear to be the same size although the Sun is physically larger.
"We are eager to use the occultation technique to probe for atmospheres
around large Kuiper belt objects," remarked Jim Elliot, who has been
observing stellar occultations by bodies in the solar system for more than
three decades.
Members of the MIT team were Jim Elliot, Amanda Gulbis, Michael Person,
Elisabeth Adams, and Susan Kern, with support from undergraduate Emily
Kramer. The Williams College team included Jay Pasachoff, Bryce Babcock,
Steven Souza and undergraduate Joseph Gangestad.
The article describing this research is "Charon's Radius and Atmospheric
Constraints from Observations of A Stellar Occultation," by A.A.S. Gulbis,
J.L. Elliot, M.J. Person, E.R. Adams, B.A. Babcock, M. Emilio, J.W.
Gangestad, S.D. Kern, E.A. Kramer, D.J. Osip, J.M. Pasachoff, S.P. Souza,
and T. Tuvikene. It will appear in the January 5, 2006, edition of Nature.
A team led by French astronomer Bruno Sicardy and a team led by American
astronomer Leslie Young also observed the occultation from telescopes in
South America. American astronomer David Tholen discussed the significance
of the various results in a "News and Views" introductory article in the
same issue of Nature as the MIT-Williams and Sicardy-team articles.
The MIT-Williams electronic equipment and expeditions are supported by
grants from NASA.