Space is a big place. And things move, fast. So imagine if you're a spaceship on a decade-long journey to reach a tiny world billions of miles away. Sure, orbital dynamicists have a very accurate view about where the planets will be at any given time, but it helps to have a precision lock on a celestial body's location so you don't miss your flyby encounter... or even hit it.
As NASA's New Horizons mission is set to make its historic flyby mission of Pluto on July 14, 2015, a powerful ground-based observatory is currently refining the dwarf planet's location. Why? Well, as it orbits so far away (at least 40 times the distance the Earth orbits the sun) and its orbital period is so long (it takes 248 Earth years for Pluto to complete one orbit), astronomers' best measurements of Pluto's location could still be thousands of miles out.
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"With these limited observational data, our knowledge of Pluto's position could be wrong by several thousand kilometers, which compromises our ability to calculate efficient targeting maneuvers for the New Horizons spacecraft," said New Horizons Project Scientist Hal Weaver, from the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., in an NRAO press release.
A margin for error this large could become problematic for a space probe that is currently flying through interplanetary space at 10 miles per second (36,000 miles per hour), so it needs to make fine adjustments as it gets closer to its target.
To provide a more precise view on Pluto's location ahead of a scheduled New Horizons course correction in July, mission scientists have enlisted the help of the Atacama Large Millimeter/submillimeter Array (ALMA), located in Chile.
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To measure distances throughout the Cosmos, astronomers traditionally use the method of astrometry to track the motions of the planets, for example, relative to the "fixed" stars many light-years beyond. But when we're talking about threading an interplanetary needle 40 AU away, a more precise form of astrometry is needed as even stars drift regardless of their distance from us.
Using quasars - ancient active galaxies that can be spied over 10 billion light-years away by the most sensitive radio observatories on the planet - ALMA can provide this mindblowing precision (as these objects are truly fixed in the Universal landscape), adding a new dimension of positioning data to back up decades of optical observations of the distant world.
"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.
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During this campaign, ALMA has been able to detect the radio emissions being generated by the frozen Pluto and moon Charon and has recorded the pair's orbital locations several times since November 2013. As these observations occurred at different times in the Earth's orbit around the sun, the tried and tested method of parallax can be applied to this high-precision method of observation.
"By taking multiple observations at different dates, we allow Earth to move along its orbit, offering different vantage points in relation to the sun," said Fomalont. "Astronomers can then better determine Pluto's distance and orbit."
It's nice to know that even when you're a spacecraft barreling through the solar system, when communications take 4 hours to traverse interplanetary space, telescopes back on Earth have your back, making sure you arrive in the right place at the right time.