ALMA shows where Pluto-bound spacecraft should be heading

Many years ago, in January 2006, NASA launched an unmanned spacecraft to Pluto. The craft is called New Horizons. It is the fastest moving spacecraft ever. But the dwarf planet Pluto is very far away. It takes New Horizons more than nine years to get there. In July 2015, it will zip past Pluto and study the small, frozen world and its five moons.

There is one problem, however. Astronomers do not know exactly where Pluto is. Of course they have photographs of Pluto in the sky. But its position in space is not known accurately enough. It’s like looking at a distant lamp post in the night. You can see it’s there, but you don’t know its distance or its precise position to the nearest millimeter. In the case of Pluto, the error may be a few thousand kilometers.

For the New Horizons team, it is very important to accurately know where Pluto is. In July 2014, New Horizons had to carry out a small course correction, to set it on the right path to its encounter with Pluto. For this, the team needed the best possible knowledge of Pluto’s position.


The Atacama Large Millimeter/submillimeter Array (ALMA) came to the rescue. In November 2013, and again in April and July 2014, ALMA observed millimeter waves from the cold surface of Pluto. The big telescope array also measured millimeter waves from a very remote galaxy called J1911-2006. By measuring how Pluto moves with respect to the galaxy, it was possible to determine the dwarf planet’s distance and position very accurately.

Thanks to the ALMA observations, Pluto’s place in space is now known twice as precise as before. The New Horizons team thankfully used the ALMA observations to plan the spacecraft’s course correction in July. Next year, when New Horizon will finally arrive at Pluto, we will know how accurate ALMA’s measurements have been!

The dwarf planet Pluto is a small, frozen world of rock and ice at the very edge of our solar system. Pluto takes 248 years to complete one orbit around the sun. It has one big moon, called Charon, and four little moons. So far, it has never been visited by any spacecraft.

The ALMA measurements of Pluto have been collected and analyzed by a team led by Ed Fomalont of the National Radio Astronomy Observatory in Charlottesville, Virginia. The New Horizons team is led by Alan Stern of the Southwest Research Institute in Boulder, Colorado.