The cold surface of Pluto and its largest moon Charon as seen with ALMA on July 15, 2014.
Credit: NRAO/AUI/NSF
Animated image of ALMA data showing the motion of the moon Charon around the icy dwarf planet Pluto.
Credit: B. Saxton (NRAO/AUI/NSF)
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) are making high-precision measurements of Pluto's location and orbit around the Sun to help NASA’s New Horizons spacecraft accurately home in on its target when it nears Pluto and its five known moons in July 2015.
Though observed for decades with ever-larger optical
telescopes on Earth and in space, astronomers are still working out
Pluto's exact position and path around our Solar System. This lingering
uncertainty is due to Pluto's extreme distance from the Sun
(approximately 40 times farther out than the Earth) and the fact that we
have been studying it for only about one-third of its orbit. Pluto was
discovered in 1930 and takes 248 years to complete one revolution around
the Sun.
“With these limited observational data, our knowledge
of Pluto’s position could be wrong by several thousand kilometers, which
compromises our ability to calculate efficient targeting maneuvers for
the New Horizons spacecraft,” said New Horizons Project Scientist Hal
Weaver, from the Johns Hopkins University Applied Physics Laboratory in
Laurel, Maryland.
The New Horizons team made use of the ALMA
positioning data, together with newly analyzed visible light
measurements stretching back to Pluto's discovery, to determine how to
perform the first such scheduled course correction for targeting, known
as a Trajectory Correction Maneuver (TCM), in July. This maneuver helped
ensure that New Horizons uses the minimum fuel to reach Pluto, saving
as much as possible for a potential extended mission to explore Kuiper
Belt objects after the Pluto system flyby is complete.
To
prepare for this first TCM, astronomers needed to pinpoint Pluto's
position using the most distant and most stable reference points
possible. Finding such a reference point to accurately calculate
trajectories of such small objects at such vast distances is incredibly
challenging. Normally, stars at great distances are used by optical
telescopes for astrometry (the positioning of things on the sky) since
they change position only slightly over many years. For New Horizons,
however, even more precise measurements were necessary to ensure its
encounter with Pluto would be as on-target as possible.
The
most distant and most apparently stable objects in the Universe are
quasars, galaxies more than 10 billion light-years away. Though quasars
appear very dim to optical telescopes, they are incredibly bright at
radio wavelengths, particularly the millimeter wavelengths that ALMA can
see.
“The ALMA astrometry used a bright quasar named
J1911-2006 with the goal to cut in half the uncertainty of Pluto's
position,” said Ed Fomalont, an astronomer with the National Radio
Astronomy Observatory in Charlottesville, Virginia, and currently
assigned to ALMA’s Operations Support Facility in Chile.
ALMA
was able to study Pluto and its largest moon Charon by picking up the
radio emission from their cold surfaces, which are about 43 degrees
Kelvin (-230 degrees Celsius).
The team first observed these
two icy worlds in November 2013, and then three more times in 2014 --
once in April and twice in July. Additional observations are scheduled
for October 2014.
"By taking multiple observations at different
dates, we allow Earth to move along its orbit, offering different
vantage points in relation to the Sun," said Fomalont. "Astronomers can
then better determine Pluto's distance and orbit." This astronomical
technique is called measuring Pluto's parallax.
"We are very
excited about the state-of-the-art capabilities that ALMA brings to bear
to help us better target our historic exploration of the Pluto system,"
said New Horizons Principal Investigator Alan Stern of the Southwest
Research Institute in Boulder, Colorado. "We thank the entire ALMA team
for their support and for the beautiful data they are gathering for New
Horizons."
The National Radio Astronomy Observatory is a facility
of the National Science Foundation, operated under cooperative
agreement by Associated Universities, Inc.
ALMA, an
international astronomy facility, is a partnership of Europe, North
America and East Asia in cooperation with the Republic of Chile. ALMA
construction and operations are led on behalf of Europe by the European
Southern Observatory (ESO), on behalf of North America by the National
Radio Astronomy Observatory (NRAO), and on behalf of East Asia by the
National Astronomical Observatory of Japan (NAOJ). The Joint ALMA
Observatory (JAO) provides the unified leadership and management of the
construction, commissioning and operation of ALMA.
New Horizons
is the first mission to the Pluto system and the Kuiper Belt of rocky,
icy objects beyond. The Johns Hopkins University Applied Physics
Laboratory (APL) manages the mission for NASA’s Science Mission
Directorate; Alan Stern, of the Southwest Research Institute (SwRI), is
the principal investigator and leads the mission. SwRI leads the science
team, payload operations and encounter science planning; APL designed,
built and operates the New Horizons spacecraft. New Horizons is part of
the New Frontiers Program managed by NASA's Marshall Space Flight Center
in Huntsville, Ala.
For more information, visit http://pluto.jhuapl.edu.
Contact:
Charles Blue,
NRAO Public Information Officer
(434) 296-0314;
cblue@nrao.edu
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
