Few spacecraft travel 10 astronomical units during their entire mission. But with New Horizons already logging more than twice that distance on its way to Pluto, coming to within 10 AU of its main target is akin to entering the home stretch.
An astronomical unit is the average distance between Earth and the sun, about 93 million miles or 149 million kilometers. At around 4:55 Universal Time on Feb. 11 (or late tonight in the U.S.), New Horizons crosses to within 10 AU of the Pluto system. To the team that has guided the piano-sized probe since its launch in January 2006, that means approach distances that used to be marked in billions of miles can be counted in millions, and astronomical units to go are listed in single digits.
Even time seems much shorter, now that the mission has entered the final three-year segment of its nine-plus year interplanetary trek from Earth to Pluto. The voyage culminates in the historic flight past Pluto and its moons on July 14, 2015, though encounter operations start several months earlier. ["Three years from now, encounter operations will already have begun, and we'll be beginning the exploration of frontier planet Pluto and its system of moons," says Alan Stern, New Horizons mission principal investigator from the Southwest Research Institute.
Not that spacecraft itself is marking any of this, mind you. New Horizons is currently in hibernation, more than 2.1 billion miles (nearly 3.5 billion kilometers) from Earth, racing outbound at 34,000 miles per hour. Operators at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., will rouse New Horizons in late April for a comprehensive, two-month-long systems and instruments checkout.
The Fast Track
New Horizons covers nearly a million miles of space a day - and that pace is evident when you check the spacecraft's path. New Horizons was 20 astronomical units from Pluto in November 2008; today that distance is 10 AU (which, for scale, is slightly more than the average space between Saturn and the sun). In October 2013, New Horizons will be just 5 AU away. It reaches a single AU in March 2015, with the Pluto encounter campaign in full swing.