ALMA Pinpoints Pluto to Help Guide New Horizons Spacecraft
6 août 2014
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 telescopes here on Earth and in space, astronomers are still working out Pluto’s exact orbit around the Sun. This lingering uncertainty is due to Pluto’s great 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. The dwarf planet was discovered in 1930 and takes 248 years to complete one orbit around the Sun.
“With these limited observational data, our knowledge of Pluto’s position could be wrong by several thousand kilometres, which compromises our ability to calculate efficient targeting manoeuvres for the New Horizons spacecraft,” said Hal Weaver, the New Horizons project scientist and a member of the research staff at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, USA.
The New Horizons team made use of the ALMA positioning data, together with newly analysed visible light measurements stretching back nearly to Pluto's discovery, to determine how to perform the first such scheduled course correction for targeting in July.
To prepare for these important milestones, astronomers need 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 great distances is very challenging. Normally, distant stars are used by optical telescopes 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 is as on-target as possible.
The most distant and most apparently stable objects in the Universe are quasars — very remote galaxies with brilliant nuclei. Quasars, however, can appear very dim to optical telescopes, making accurate measurements difficult. But, due to the supermassive black holes in their centres as well as emission from dust, they are bright at radio wavelengths, particularly the millimetre 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 Charon by picking up the radio emission from their cold surfaces, which are at about -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.
“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 mission principal investigator Alan Stern of the Southwest Research Institute; Stern is based in Boulder, Colorado. “We thank the entire ALMA team for their support and for the beautiful data they are gathering for New Horizons.”
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Southern Observatory (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) 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.
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