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ALMA guides New Horizons spacecraft to Pluto

Jenny Winder, News Writer
Aug 10, 2014, 14:24 UTC

Sen—Astronomers are using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to make high-precision measurements of Pluto's location and orbit to help NASA’s New Horizons spacecraft accurately home in on its target.

Pluto, discovered in 1930 takes 248 years to complete one orbit around the Sun. Astronomers are still working out the dwarf planet's exact orbit around the Sun, due to Pluto’s great distance (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.

“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.

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 when it nears Pluto in July 2015.

The team first observed these two icy worlds in November 2013, and then three more times in 2014. Additional observations are scheduled for October 2014.

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This picture shows the cold surface of Pluto and its largest moon Charon as seen with ALMA on 15 July 2014. Image credit: NRAO/AUI/NSF

Astronomers need to pinpoint Pluto’s position using the most distant and most stable reference points possible. 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, remote galaxies with brilliant nuclei. These can appear very dim to optical telescopes, 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 and currently assigned to ALMA’s Operations Support Facility in Chile.

ALMA studied Pluto and Charon by picking up the radio emission from their cold surfaces, which are at about -230° C.

A mini movie from images taken by New Horizons in July shows Charon and Pluto orbiting their common centre of gravity. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

“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. “We thank the entire ALMA team for their support and for the beautiful data they are gathering for New Horizons.”

Last month, New Horizons took several images showing Pluto and its largest moon Charon from a distance of between 429 million and 422 million km (267 million to 262 million miles). The shots were taken from 19-24 July with the probe’s best telescopic camera, the Long Range Reconnaissance Imager (LORRI) and Charon makes alost one complete orbit around Pluto in that time.

The moon orbits approximately 18 000 km (11 200 miles) above Pluto’s surface.