Pluto Remains Goofy

Posted Oct. 11, 2008 – Two years ago the International Astronomical Union (IAU) officially defined the word “planet” in a way that gave Pluto the boot. It was quick and seemingly final. NASA’s illustrated card containing all the planets, published for educators, was immediately reissued with Pluto airbrushed out, as if in an old Stalinist photo.

The demotion of Pluto to a “dwarf planet,” as if to join all the other Disney dwarfs, sparked considerable public controversy. Most folks even now lament Pluto’s disappearance; many astronomers have protested the IAU’s decision, as well. But many other planetary astronomers continue to support it.

I’m in the latter group. When we learned in 1978 just how tiny Pluto really is – only a bit more than half the diameter of our moon – and recall that its orbit has a wildly different appearance from all the other planets, we wondered if it truly belonged with the rest. But we stayed silent. Why disturb the long-established order? What was the harm?

But then new bodies started to be found beyond Pluto. Some virtually matched Pluto’s size, and finally astronomers found an even larger one. Suddenly we had Eris, Sedna, Quaoar, and now Makemake, and there was no end in sight. Now we had a problem. If Pluto remained a planet, and these others match or exceed Pluto’s size, and are just as round, they would have to be planets too. We’d have 13 planets, then soon 40, and eventually hundreds. Kids would no longer be able to memorize them.

It made far more sense to create new, separate categories. There’d be eight big planets, all orbiting within 7 ½ degrees of the Earth-Sun plane, with orbits that are more circular than oval. Then we’d have a separate “dwarf” class for much smaller bodies with very elliptical orbits. There’d be a third category for objects with too little weight to even be round in size, like the asteroids or the Kuiper Belt Objects (KBO’s) that probably number in the hundreds or even thousands, beyond Pluto.

Works for me. But some folks won’t give up. That’s why, in recognition of the need for further scientific debate on planet definition, more than 100 scientists and educators representing a wide range of viewpoints converged for three days at the Applied Physics Laboratory of Johns Hopkins University for “The Great Planet Debate: Science as Process” conference, sponsored by NASA, the Planetary Science Institute, The Planetary Society, and other heavy hitters.

Steve Maran, the official spokesperson, whom I know personally, wrote that “different positions were advocated, ranging from reworking the IAU definition (but yielding the same outcome of eight planets), replacing it with a geophysical-based definition (that would increase the number of planets well beyond eight), and rescinding the definition for planet altogether and focusing on defining subcategories for serving different purposes.”

It was almost a food fight. Neil Tyson of New York’s Natural History museum thought that the very word “planet” has outlived its usefulness and should be discarded, and replaced with a new term.

Renu Malhotra, a University of Arizona astronomer said, “I think the IAU made a mistake getting into the business of defining a widely used word, ‘planet,’ and sowing confusion thereby. Scientifically, the useful discussion would be about categories of planets (e.g., gaseous planets, rocky planets, dwarf planets, icy planets. . .and an individual celestial body may fall into more than one category). This approach would address the main practical problem of nomenclature without confusing the public about ‘planet’ itself.”

Still others thought the present situation is the best compromise.

The windup? The conference ended and no consensus was reached. Nothing will change. Too bad Mickey Mouse wasn’t around to sum it all up: Goodbye Pluto.

Terminal Velocity

Posted Jan. 3, 2007 – Terminal Velocity – It’s so catchy, they used it as a movie title. But “terminal velocity” is an important science concept that affects meteors, mankind, and the mass-extinctions that keep changing our planet. Plus it’s fun.

People who bewilderingly take up sky-diving learn that you don’t just keep falling faster and faster. By spreading arms and legs, a jumper stops picking up speed when she reaches 120 mph. It’s air, of course, that slows things down. Everything has its own terminal velocity: For raindrops it’s 23 miles an hour give or take. That’s the speed of falling rain, in case you ever wondered. (Actually it varies with the size of the drops. A light mist’s particles might fall at only a half inch per second, which is why clouds, made of tiny droplets, can just hang there).

There is no terminal velocity on airless bodies like Mercury or the Moon. On those places, meteoroids captured by gravity keep gaining speed, up to a maximum that happens to equal that world’s escape velocity, the speed necessary to wrench free from its gravity in a single upward blast. On Earth it’s 25,000 mph. In other words, if there were no air and you FELL to Earth from a great distance, even beyond the moon, you’d hit the ground at that same escape velocity speed of 25,000 mph. Throw a coin up and then catch it. The speed you tossed it exactly matches the speed it travels when it lands back in your hand. Gravity is like that. Symmetrical.

At first a falling person or meteor keeps gaining speed. After each second of falling, a plummetting stone or person goes another 22 miles an hour faster. Two seconds of dropping, achieved by falling from five-stories, causes a rock to hit the ground at 44 mph. The speed would just keep increasing, up to that maximum of 25,000 mph, if we had no atmosphere. We’ve already seen that air slows skydivers to 120 mph, a speed reached after falling 500 feet or 50 stories. That’s still fast, of course: The fatal human impact velocity ranges from 15 to 38 mph, so it’s hardly news that we humans can easily die in a fall. But that’s not true of all animals.

Some mammals like cats and squirrels have non-lethal terminal velocities. They can generally fall from any height and survive.
Here’s where we get back to astronomy. An incoming meteoroid can easily weigh a ton as it strikes our atmosphere; that was the estimated weight of the intruder that broke into dozens of fragments over a Chicago suburb on March 26, 2003. One piece invaded a teenager’s bedroom and broke a mirror. But it could have been much worse.

Meteoroids start out at a sizzling 7 to 44 miles per second relative to Earth. Fortunately, if the meteoroid weighs less than 8 tons — and nearly all of them do — air friction robs it of ALL its original speed. At a height of about 10 miles or 50,000 feet, it slows to just 2 or 3 miles per second, where it no longer glows. Nonetheless this 7,000 mph velocity, 3 to 6 times faster than a bullet, gives a one-pound meteor enough kinetic energy to easily destroy a jetliner. It hasn’t yet happened, but it could.

Continuing downward, now dark and unobservable, the meteoroid’s encounter with increasingly thick air slows it to a terminal velocity of about 240 mph. This is its final speed as it strikes the ground. That’s the speed at which nearly all meteorites land, plus or minus 20%. That’s still plenty fast – usually enough to pierce a roof and end up on the floor of some room. Buildings are penetrated every year or two in North America alone. Just since 2002, meteors have entered seven homes including two in the United States.

If the meteoroid weighs over 100,000 tons, our atmosphere won’t slow it down in the slightest: It slams into the ground at full cosmic velocity. This isn’t good, as the dinosaurs learned 65 million years ago. Yet even then, it’s not all doom and gloom. The
big ones shake up the biosphere, change the course of evolution, and create new bio-adventures. We mammals now rule the Earth solely because a single impactor had enough mass to make it immune to — terminal velocity.