On to Pluto!

by Josh Gelernter
We haven’t been to the moon lately, but we haven’t given up exploring.

There has been some fanfare this week over the 45th anniversary of Apollo 11. If the celebration has made you despair at the listless state of today’s space program, you may be comforted by another astronomical date, a sort of pre-anniversary: One year from now, for the first time in human history, a probe will reach Pluto. The probe is a compact-car-sized NASA spaceship called New Horizons. It left Earth in 2006. Pluto’s a long way away.

Pluto used to be our smallest, remotest planet, the ninth planet from the sun. In 2006, the International Astronomical Union redesignated Pluto as a “dwarf planet.” Despite the indignation of everyone who grew up with a now-obsolete solar-system mnemonic, the demotion was not without justice. According to the IAU, a planet ought to “clear its neighborhood” — that is, it should be the dominant massive object in its orbit. In fact, Pluto is less than a tenth of the total mass following its path around the sun. Earth, for reference, is very nearly 100 percent of the mass in our orbit. Pluto is smaller than Earth’s moon. It’s smaller than Saturn’s largest moon, and Neptune’s, and all four of Jupiter’s major, “Galilean” moons.

So, Pluto is a dwarf. But, in a year, it might get a promotion — along with its largest moon, Charon — to a binary dwarf-planet system. Charon is big enough, at more than 10 percent Pluto’s mass, that the center of its orbit is a point outside Pluto’s body — a point that Pluto orbits too, the system’s center of mass, its “barycenter”. In essence, the two orbit each other.

(Though, of course, that’s true of all astronomical bodies with satellites, in much the way you have to lean backwards if you spin around holding something large at arm’s length. The barycenter of the Earth–moon system is not the center of the Earth; the barycenter of the sun–Jupiter system is actually outside the body of the sun. Fun facts, but back to Pluto:)

We don’t know if Pluto and Charon will end up being called a binary system because we don’t know a whole lot about them. We know their masses and orbits, but not much else. Pluto is a mystery; nothing manmade has ever come near it. New Horizons, which is still a year from its destination, has been the closest manmade object to Pluto since 2011. Not just the closest at the moment, but the closest ever. New Horizons was (is) the fastest spacecraft ever launched, hitting 37,000 miles per hour as it left Earth — and it has taken eight and a half years to get as close to Pluto as it is now. Pluto is so distant that our best photographs show it as a smudged disc. At its closest approach to the sun, it’s two and three-quarter billion miles away; four and a half billion miles at its furthest. But one year from now, we’re going to get a close-up look with a car-sized cosmic camera. What a time to be alive.

And that’s not all. Pluto isn’t the only dwarf planet; there are four others, and about three dozen “probables.” Three of the four confirmed dwarves are beyond Pluto’s orbit, but the fourth is right here in the inner solar system, orbiting in the asteroid belt between Mars and Jupiter. How about that? Its name is Ceres, and — like Pluto — for a time after it was discovered (in 1801) it was considered a planet. It was demoted to an asteroid, then promoted to a dwarf planet. Like Pluto, it has been photographed only as a pixilated blur. But in just eight months, NASA’s Dawn spaceship will enter a Ceres-centered orbit.

So, we haven’t had a manned mission to the moon since the end of the Apollo era in 1972. Certainly, we ought to go back; certainly, things in space have been better. But in less than a year’s time, we will have first-ever looks at two brand-new worlds. And that should take your breath away.

— Josh Gelernter is a writer in Connecticut.

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