Threatening Asteroid Aids Planetary Prognosticators

A radar image of asteroid 29075 (1950 DA), acquired on March 4, 2001, from Arecibo observatory in Puerto Rico. At the time, the asteroid was 7.8 million kilometers from Earth (22 times the Moon's distance). This is not a true photograph but rather a map of echo time (vertical axis) versus Doppler shift (horizontal axis).
NASA/JPL image, courtesy Steven Ostro.
A kilometer-size asteroid, whose whereabouts have been unknown since just after its discovery 52 years ago, has suddenly reemerged as an object that may pose a significant threat to Earth in the distant future. Astronomers at Lowell Observatory rediscovered the wayward object, known as 1950 DA, by accident on New Year's Eve 2000, and three months later a team of radar astronomers pinged it from Goldstone, California, and Arecibo, Puerto Rico. When orbital dynamicists combined the high-precision radar tracks with the half-century-long photographic record, they realized that 1950 DA will make three close brushes with Earth in the centuries ahead.

The Earth-crossing asteroid 29075 (1950 DA) takes 809 days to orbit the Sun. NASA has a Java applet to show the asteroid's orbit from any perspective.
Sky & Telescope diagram (source: NASA/JPL).

One of those, on March 16, 2880, is the most worrisome.
As detailed by Jon D. Giorgini (Jet Propulsion Laboratory) and 13 colleagues in tomorrow's issue of Science, that collision probability might be as high as 0.33 percent — or 1 in 300 — which represents a risk 100 times higher than that for any other known asteroid. (Another 1-km object, designated 2002 CU11, has a 1-in-30,000 chance of striking Earth on August 31, 2049.)

However, notes coauthor Steven R. Chesley (JPL), "The impact risk is not the story here, because we can say almost unequivocally that it's not going to hit Earth." The real story, he says, is how having such a precise orbit has allowed dynamicists to push the realm of impact prediction so far into the future. Typically impact probabilities can't be computed reliably for more than about 100 years in advance, beyond which gravitational perturbations and other forces make it impossible to predict an object's exact location. But, Giorginio notes, "Whenever we get radar data, it reduces orbital uncertainties and opens up a huge window into the future." His team's 900-year-long prognostication was able to include such subtle and esoteric effects as the Sun's mass loss and galactic tides.

A spinning body radiates the most heat from its afternoon side, creating a slight thermal imbalance called the Yarkovsky effect. Over time an asteroid rotating in the same sense as its motion around the Sun is gradually accelerated and pushed into a wider orbit. Conversely, a retrograde spinner is doomed to spiral inward toward the Sun.
Sky & Telescope illustration.

One important factor that can't be calculated is termed the Yarkovsky effect. Sunlight-warmed objects radiate excess energy as heat, but on a rotating asteroid more energy is emitted from the hotter "afternoon" face than from the cooler "morning" face, causing a radiational imbalance that very gradually nudges the body one way or another, depending on which way it's rotating. For 1950 DA (now officially numbered 29075), knowing the spin direction is crucial: if it's prograde, like Earth's, then the chance of a collision in 2880 remains as high as 0.3 percent; if it's "retrograde," like Venus, then a direct hit can't possibly occur.

Unfortunately, even though Czech astronomer Petr Pravec (Ondrejov Observatory) has deduced that the asteroid spins in just 2.1 hours, no one knows where its axis points or what direction its turning. But since the close encounter won't happen until 2880, there's little urgency. Besides, notes dynamicist Andrea Milani (University of Pisa), even if the odds worsened, we could use the Yarkovsky effect to our advantage. In what he terms the "sweet solution," Milani envisions coating the asteroid with a white substance to alter its response to the Sun and thus its orbital evolution. "A back of the envelope computation shows that only about one ton of white dust, such as powdery sugar, could be enough!" says Milani.

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