In less than five weeks, New Horizons will zip past the Pluto-Charon system in a brief but historic encounter. Given the huge interest in Pluto, it’s fair to ask: Why won’t mission planners let the probe hang out a while?


Above: Our first glimpse of Pluto and Charon in color (Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

The simple reason is that New Horizons can’t make a stop at the Pluto-Charon system. It’s a constraint that has as much to do with engineering as it does with basic physics.


Above and below: Pluto’s position as of today, June 9, 2015. (NASA/New Horizons)

In order to get New Horizons to Pluto in a reasonable amount of time (in this case 9.5 years), NASA had to get the probe moving very, very fast. And a probe on the move can be difficult to slow down.

After its launch from Cape Canaveral on January 19, 2006, the probe entered into an escape trajectory featuring a speed of 16.26 kilometers per second (58,536 km/h; 36,373 mph), setting a new record for the highest launch speed of a human-made object flung from Earth. New Horizons’s encounter with Jupiter offered a subsequent gravitational assist that increased its speed by an additional 4 km/s (14,000 km/h; 9,000 mph). Once at the Pluto-Charon system, the spacecraft will pass through at a velocity of about 13.8 km/s relative to the dwarf planet (49,680 km/h; 30,800 mph).


That’s obviously a lot of momentum. To get New Horizons into Pluto’s orbit, mission planners would have to reduce its speed by over 90%, which would require more than 1,000 times the amount of fuel the probe can carry. That’s a technologically unfeasible proposition. And so, the probe will have no choice but to zoom past Pluto, feverishly snapping pics and taking measurements before being flung outward towards the Kuiper belt.

Which is a pretty neat consolation prize. The New Horizons mission will be far from over after its July 14 encounter with Pluto.


[ NASA’s New Horizon Mission ]