Where the heck is Pluto? Call ALMA

ALMA radio telescope image showing the icy dwarf planet Pluto (bottom) orbited by its largest moon Charon. Pluto’s 1,430 miles in diameter; Charon is half as big at 750 miles. Credit: B. Saxton NRAO/AUI/NSF

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.

New Horizons’ Long Range Reconnaissance Imager (LORRI) made this  movie of Pluto and almost one full rotation of its largest moon, Charon. The 12 images were taken July 19-24, from a distance ranging from about 267 million to 262 million miles (429 million to 422 million km). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

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).

The 66-dish radio telescope array known as ALMA located in the Atacama Desert of northern Chile. They detect radio waves with wavelengths measured in millimeters and sub-millimeters (far infrared and microwaves) given off by everything from planets to the energy released in the aftermath of the Big Bang. Credit: Christoph Malin

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.

These two multiple-exposure images from NASA’s Hubble Space Telescope show Kuiper Belt objects – icy asteroids beyond the orbit of Neptune – against a background of stars in the constellation Sagittarius. The two are roughly 4 billion miles from Earth. NASA is using the Hubble to pinpoint potential candidates for flybys after next July’s Pluto visit. Click to learn more about the program. Credit: NASA, ESA, SwRI, JHU/APL, New Horizons KBO Search Team

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.”

Artist’s impression of how the surface of Pluto might look, according to one of the two models that a team of astronomers developed to account for the observed properties of Pluto’s atmosphere. The image shows patches of pure methane on the surface. At Pluto’s average distance of 3.7 billion miles, the Sun appears about 1,000 times fainter than on Earth. Credit: ESO

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.

Sources: 1, 2, 3

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

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