Rosetta traveled hither and yon across the inner solar system for 10 years to finally reach its target comet. NASA’s New Horizons probe, bound for Pluto, will take nearly as long. Launched in 2006, it will fly past Pluto and its system of five known moons in July 2015.
To arrive at a distant destination like Pluto, you need to know exactly where it’s at or risk blowing right past it. At 3 billion miles away and traveling at an average speed of 10,440 mph (16,800 kph) around the sun, Pluto’s on the move.
While we can measure its position in the sky with great accuracy, it’s a bit trickier to measure a precise distance. If we had, say, 500 years, we could track Pluto for two complete revolutions around the sun and know its orbit and position with precision. But who has that kind of time?
Instead, Marc Buie, New Horizons co-investigator, dug through the archives at Lowell Observatory in Flagstaff, Arizona, where the dwarf planet was discovered in 1930. There he uncovered records and photos of Pluto made by astronomer Carl Lampland dating back to its discovery date and spanning 21 years. Combining these positions with high-accuracy modern measurements, Buie was able to nail down Pluto’s current position to within about 620 miles (1000 km).
That may sound well off the mark until you consider that Pluto lies some 2.8 billion miles (4.5 billion km) away. Not bad but not quite good enough to send a spacecraft to take pictures and measurements at close range. That’s where ALMA stepped in.
The Atacama Large Millimeter/submilliter Array (ALMA) consists of 66 separate, moveable radio telescopes. When all their data streams are combined, the array can resolve details and pinpoint positions of celestial objects with 10 times the accuracy of the Hubble Space Telescope.
The New Horizons team used ALMA’s observations of Pluto combined with the old, re-analyed positions to perform the first scheduled course correction for the spacecraft last month.
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.
Astronomers normally use background stars to pinpoint the positions of objects in the solar system, since the stars shift position only over many years. For greater precision, the ALMA team chose instead to use 10-billion-light-year-distant quasars, brilliant beacons of radiation powered by black holes in the cores of remote galaxies, for reference. The more remote the object, the more stable its position in the sky.
“The ALMA astrometry (position measurements) 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 currently working at ALMA in Chile. “By taking multiple observations at different dates, we allow Earth to move along its orbit, offering different vantage points in relation to the Sun,” he said. “Astronomers can then better determine Pluto’s distance and orbit.”
Pluto is so far away that the sun looks like a blazing pinpoint of light in its sky. Surface temperatures there hover around 382 degrees below zero Fahrenheit (-230 C), only 43 degrees above absolute zero. Bright areas on its surface appear to be covered in nitrogen and methane ice. Give me the chills just thinking about it, yet, like you, I’m excited we’re almost there.