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The serious asteroid watchers know the big ones by name, diameter, composition, orbit, spin and potential troublesomeness.
There’s Apophis, for example, a rock bigger than the Pentagon, which on April 13, 2029, will pass close enough to the Earth to be seen with the naked eye and quicken the pulse. There’s Toutatis, mountain-sized tumbling and, fortunately, at a safe distance. There’s 2005 YU55, a quarter of a mile wide, which passed the Earth at roughly moon distance a little over a year ago.
And now comes 2012 DA14, a rock the size of an apartment building that on Friday will buzz the Earth at a distance of about 17,200 miles. It poses no hazard. It won’t generate the slightest breeze or disturb a single mote of dust meandering in a sunbeam.
DA14 is too small to see with the naked eye. Though it will pass just inside the orbit of communications satellites, the chance that it will obliterate anyone’s favorite TV show is vanishingly remote.
Scientists point out that we have never detected an asteroid this large passing so close to our planet. The key word here is “detected.” Astronomers have honed their ability to see things that just a few decades ago would have been invisible. The universe keeps coming into focus, and it’s messier out there than the ancient stargazers could have imagined.
“Thirty years ago, we would not have been able to find the thing,” said Michael Busch, a planetary astronomer at the National Radio Astronomy Observatory in Socorro, N.M.
Busch estimates that an object this size passes this close to Earth every decade or two, on average. He is part of a team of researchers that will use four telescopes, arrayed from California to New Mexico, to bounce radar signals off DA14 in an attempt to understand how it is spinning.
The precise size and shape of the rock are unclear. In telescopes, DA14 is just a bright dot. That will change Friday as astronomers observe it coming and going.
DA14 isn’t an old asteroid. It likely is the remnant of a collision in the asteroid belt within the past few million years, Busch said. Such rocks have limited life spans: As sunlight strikes their uneven surfaces, they spin faster and faster, until finally they explode, a process known as the YORP effect (for the scientists who studied it — Yarkovsky, O’Keefe, Radzievskii and Paddack).
Most asteroids are in the famous belt between Mars and Jupiter. Some asteroids are big enough to have their own moons. At least two asteroids orbit the sun in the opposite direction of everything else in the solar system; they’re wrong-way drivers. Some asteroids spin neatly on a single axis; others, such as Toutatis, tumble chaotically, as if someone had knocked them silly.
Any asteroid that comes within 50 million kilometers — about 31 million miles — of the Earth’s orbit is categorized as a near-Earth object. There are hundreds of thousands of NEOs, and scientists believe they’ve now mapped roughly 95 percent of the largest NEOs, the ones that would be very hazardous to human civilization should one strike the planet. None of the asteroids found so far is on a collision course with Earth.
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