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The Man Who Knew Comets

In 1986, the European spacecraft <a href="">Giotto</a> looked into the heart of Halley's Comet as it approached the sun. Data from Giotto's camera were used to generate this enhanced image of the comet's potato-shaped nucleus, measuring roughly 15 kilometers across.
Halley Multicolor Camera Team/Giotto Project/ESA
In 1986, the European spacecraft Giotto looked into the heart of Halley's Comet as it approached the sun. Data from Giotto's camera were used to generate this enhanced image of the comet's potato-shaped nucleus, measuring roughly 15 kilometers across.

Imagine stepping into an elevator and bumping into Lou Reed or Maya Angelou or Paul Newman. What would you do? What would you say? And what if the legend you bumped into on your ride up the 32nd floor was none other than the father of the comet (kind of)?

That was the dilemma I faced back in the day as a young graduate student at Leiden University in the Netherlands. On my way up to my office after lunch I stepped into the elevator and — BOOM — there he was — Jan Hendrik Oort — one of the greatest astronomers of the 20th century.

He was an old man by this time. But his eyes were still bright and curious. I was stunned. I had an impromptu, private audience with a guy who helped shape modern astronomy. My legs got wobbly. I broke out in a sweat. My brain froze. All I could think to say was:

"Dude, totally awesome cloud."

Luckily I held off from that comment and muttered something like, "Good afternoon." My first instinct was however true to the man's greatness.

Jan Oort made many contributions to astrophysical knowledge. But he is best known for his discovery of a vast swarm of comets surrounding the sun. The Oort Cloud, as it came to be called, was the source of the last week's much-ballyhooed sun-grazing Comet ISON. Oort's discovery was important for a lot of reasons, but most of all it anticipated the great, ongoing revolution in our understanding of planetary systems.

A century or so ago the solar system was just a bunch of planets orbiting the sun. Sure, there were some asteroids and comets, but none of that really mattered. Once you got past Neptune, you were done with solar system. What Oort realized was that the orbits of comets implied there was a lot more solar system than the region occupied by the planets.

There are two kinds of comets. Short-period comets (like Halley's) take less than 200 years to loop around the sun in their highly elongated orbits. (Halley takes 75 years to orbit the sun, reaching as far out as 35 times the Earth's orbital radius, which is beyond Neptune.)

Long-period comets are another story. When they're seen it's usually for the first time (in recorded history). They seem to come from all directions (whereas short-period comets move in the same orbital plane as the planets). Tracing the orbits back out into deep space, Oort reasoned that the long-period comets must come from a kind of cosmic storage locker.

Oort's "cloud" was a quasi-spherical distribution of comet nuclei (mountains of ice and rock). The cloud surrounds the sun at distances between 50,000 to 100,000 times the Earth's orbit (that's almost a quarter or more of the way to the nearest star).

Distant gravitational tugs from other stars, or clouds of gas, as the sun rotates around the center of the galaxy are enough to occasionally perturb an Oort Cloud comet onto a long journey towards the sun. The inward trip could take as long as 100,000 years, after which the comet may, or may not, survive its solar encounter and head back into space.

While the Oort Cloud is a cool idea in and of itself, what really matters is the recognition that the solar system is so much more than planets. Our sun was born from a cloud of gas and dust that collapsed under its own weight about 5 billion years ago. The Oort Cloud is some of that early construction debris, flung outward into the freezing depths of near-interstellar space via collisions (perhaps with larger proto-planets of the era).

That was what the wizened old man in the elevator had seen before everyone else. He glimpsed the true meaning of comets like ISON without the aid of computers or digital databases. He saw to the very edges of the solar system at a time when no one even knew such an edge existed. That's why I was dumbstruck.

I still am.

You can keep up with more of what Adam Frank is thinking on Facebook and on Twitter: @AdamFrank4

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Adam Frank was a contributor to the NPR blog 13.7: Cosmos & Culture. A professor at the University of Rochester, Frank is a theoretical/computational astrophysicist and currently heads a research group developing supercomputer code to study the formation and death of stars. Frank's research has also explored the evolution of newly born planets and the structure of clouds in the interstellar medium. Recently, he has begun work in the fields of astrobiology and network theory/data science. Frank also holds a joint appointment at the Laboratory for Laser Energetics, a Department of Energy fusion lab.