Pluto’s Fifth Moon

Poor little Pluto! Every third-grader’s favorite planet got officially downgraded to a “dwarf planet” in 2006, and nothing’s been the same since. Although planetary scientists have known for more than 20 years that Pluto is just one of the larger of the objects comprising the Kuiper Belt, in the public’s mind those same scientists were just disrespecting her.

But Pluto’s made something of a comeback since then. Even before the reclassification in August 2006, a spacecraft was launched that January on its way to Pluto. It’s a long way out there! Even though it was the fastest human-made object ever to leave Earth (it passed the moon in 8.5 hours; it took the Apollo astronauts three days), it will not arrive at Pluto for another three years, in July 2015.

And Pluto has been adding moons at a pretty good clip too. Of course, they have always been there. It’s just that with a spacecraft approaching, we’ve turned the Hubble telescope toward Pluto to spy out the neighborhood. Pluto has one large moon Charon, which is actually large enough for the two objects to qualify as a binary—two objects orbiting a common center of mass that lies outside either of those objects. Here is a diagram showing the two orbits of Pluto and Charon. Pluto is the larger of the two and nearer the center.

Charon was actually discovered in 1978, long before the sharp-eyed Hubble was launched. Here is the discovery image:

Charon is the “bump” on the side!

Compare this to the latest image from the Hubble that discovered the fifth known moon of Pluto, P5 for now:

The black strip in the center is actually a different image superimposed on the larger one. Pluto and Charon must be blocked out to prevent their relative brightness from obscuring the images of the dimmer small moons.

This newly discovered moon could possibly alter the planned trajectory of the New Horizons spacecraft as it flies past Pluto. There is really no concern that the spacecraft will collide with a moon—scientists can steer with amazing accuracy to avoid any such calamity. It is just that the more moons there are in the system, the greater the likelihood of debris that can’t be seen in advance. The spacecraft will be moving at more than 30,000 miles per hour relative to Pluto, and at that speed a collision with a pebble could destroy it.

But there will be time and opportunity to adjust the trajectory as needed. In early May 2015, the spacecraft will be close enough for its cameras to exceed the Hubble in the detail they can show. There is plenty of fuel on board to shift its path if necessary. And how long will it take for the signals from the craft to make it back to Earth, traveling at the speed of light? Around 4.5 hours.

Among the artifacts carried on the spacecraft is a compact disc with over 400,000 names stored on it, one of which is mine. Even cooler, I think, is the one ounce of ashes of Clyde Tombaugh, the Kansas farm boy who discovered Pluto in 1930. Clyde and I (he more than I) will have made to 33 astronomical units from Earth!