Commentary Magazine

Flying over Pluto

Screen grab/NASA

When I first became fascinated by astronomy, in the early 1950’s, Pluto had only been discovered two decades earlier. But except for its orbital period (248 years), its average distance from the sun (39.4 astronomical units or about 3.6 billion miles) and its 17-degree inclination to the ecliptic (the plane of the Earth’s orbit), we knew absolutely nothing about it. It was merely a very dim white dot moving slowly through the black sky of outer space. With a magnitude of only 14.2, Pluto requires, at the very least, a 10-inch telescope to see.

We knew remarkably little about the other planets, too, despite the fact that they are far closer and far brighter than tiny Pluto (which was officially downsized to a mere dwarf planet in 2006). The planet Venus is the brightest object in the night sky after the moon. And it comes closer to earth than any of the other planets, a mere 25 million miles or so. But because of its featureless cloud cover, we didn’t know how fast it rotated on its axis. When we found out in the early 1990’s, thanks to the Magellan spacecraft, astronomers were stunned. While Venus revolves around the sun in 227 days, it rotates on its axis only once in 243 days. More, unlike every other planet, it does so in retrograde, revolving clockwise as seen from above its north pole. Early in the history of the solar system, Venus must have had a massive collision with another planetesimal that drastically altered its rotation.

In the 1950’s we knew of only twelve moons of Jupiter. Today we know of 69. The four big moons, discovered by Galileo in 1609, were, like Pluto, merely white dots and we knew almost nothing about them. Astronomers assumed that they would be dead worlds, like earth’s giant moon, and resemble each other closely. But when they were imaged close up in 1979 by the Voyager spacecraft, they turned out to be very unalike. Io, the innermost moon, is a constantly changing collage of reds, yellows, and browns. One astronomer said it looked like a big pizza. Jupiter’s immense tidal forces knead Io as it orbits in a mere 1.7 days. This heats the moon’s interior, making Io the most volcanically body in the solar system. Europa is covered with ice and many think an underlying ocean may harbor life.

The science of astronomy has been utterly revolutionized in my lifetime. Far larger telescopes, massive computers, and, of course, the dawning of the space age in 1957 has multiplied our knowledge of the solar system by several orders of magnitude.

If you want an inkling of how much planetary astronomy has advanced in sixty-odd years, just compare that dim little white dot that was Pluto in the early 1950’s with this stunning, computer-generated video that NASA has created from the photographs made by the New Horizons spacecraft as it flew over Pluto in July of 2015, after a 9 1/2-year voyage from Planet Earth.

We live in an age of wonders.

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