How to Weigh an Asteroid — and Why You Should Care

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The near-Earth asteroid Eros is seen from the NEAR spacecraft at a distance of 127 miles, March 3, 2000.

Of all the things you don't give a hoot about, the weight of asteroid 1999 RQ36 would probably rank high. But suppose you knew that the giant space rock is predicted to have eight close — and potentially deadly — encounters with Earth from 2169 to 2199. Suppose you knew that calculating its exact weight and mass will help scientists better track its path, not to mention determine how bad the damage would be if we actually did get clobbered. Would you care then?

If so, be happy that investigators affiliated with NASA have indeed figured out how to take the measure 1999 RQ36 and have successfully calculated its more or less exact weight. That raises two questions: what exactly does an asteroid weigh and how in the world did the astronomers figure it out in the first place?

Asteroid 1999 RQ36, which measures 1,800 ft. (560 m) across, is what's known as a near-Earth asteroid, one that, as its name suggests, does not stay safely within the massive river of rubble between Mars and Jupiter known as the asteroid belt. Rather, it follows an egg-shaped orbit around the sun — swooping as close as 83 million miles (133 million km) to the solar fires and soaring as far as 126 million mi. (202 million km) away. That takes it directly across Earth's rounder, 93 million mi. (150 million km) orbit twice on each trip: once on the asteroid's way in and once on its way out. That, in turn, is more than enough to earn the rock a place in NASA's Near-Earth Objects Observation program — better known as Spaceguard — a sort of watch list that keeps a telescopic eye on any asteroid that poses even a theoretical threat to the planet.

The first step in determining the weight of 1999 RQ36 was determining its precise trajectory over time. The path an orbiting object follows will be partly determined by its mass and partly by the gravitational tug of any objects nearby. In the case of 1999 RQ36, those objects include the sun, moon, planets and other asteroids. Using data gathered in 2011 by the Aricebo Observatory in Puerto Rico as well as other observations conducted in 1999 and 2005 by the Goldstone Observatory in California's Mojave Desert, astronomer Steve Chesley of NASA's Jet Propulsion Laboratory in Pasadena minutely tracked 1999 RQ36's orbit over the past 12 years. His next step was to tease out all of the gravitational effects to determine how the rock would be flying based on its mass alone. Once he did that, he found that the path of 1999 RQ36 deviated from what the mathematical model said it should be by a cumulative 100 mi. (160 km).

In the vastness of space, 100 miles over 12 years is less than a rounding error — it's practically nothing at all. But practically nothing is not absolutely nothing, and clearly something was nudging the asteroid. The likely explanation was a phenomenon known as the Yarkovsky effect — the faint propulsive power produced when an object like an asteroid absorbs sunlight and re-emits it as heat. When Chelsey re-crunched his numbers, he found there was a Yarkovsky effect pushing the asteroid alright, but an almost surreally tiny one.

"At its peak," he says, "when the asteroid is nearest the sun, the Yarkovsky force on 1999 RQ36 is about half an ounce — around the weight of three grapes."

Even a three-grape degree of exactitude, however, was still not enough for Chesley to conduct his weight calculations accurately. For that, he would need additional data from observations NASA's Spitzer Space Telescope has made of 1999 RQ36. Spitzer sees in the infrared, which is another way of saying it measures heat. Knowing the temperature of the asteroid would provide an indication of how thick its regolith — or fine, dusty covering — is. That would help refine the Yarkovsky calculations further.

Planetary scientist Josh Emery of the University of Tennessee, Knoxville, analyzed the Spitzer data and passed the information onto Chesley — and with temperature, orbital and size data in hand, he could finally calculate the asteroid's weight. The answer: 60 million metric tons — or about 66 million U.S. tons. That's an awful lot deadly ordnance to be flying free, but spread across a body so large, it means that 1999 RQ36 has roughly the density of water. "It's more than likely a very porous jumble of rocks and dust," Chelsey says. Low density is good news in the near-Earth asteroid game, since it means that any eventual technology designed to destroy and disperse the rock would be likelier to succeed.

NASA is hardly done with 1999 RQ36. In 2016, the space agency plans to launch an unmanned probe that will land on the asteroid, collect a sample and return it to Earth. That will provide more details about the make-up of the rock, as well as insights into the origin of the solar system itself. Astronomers will be pleased to have the little sample in hand for analysis — provided, of course, that that one scrap is all of 1999 RQ36 that ever makes it to Earth.