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Asteroid 2007 WD 5
Asteroid 2007 WD5 - A Close Encounter with Mars
A portion of the orbit diagram for Asteroid 2007 WD5 showing it to be both a "Near-Earth" asteroid and a "Mars Crosser". NASA Image - click to enlarge. |
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On November 20th, 2007, astronomers with the NASA-funded Catalina Sky Survey
discovered a new near-Earth asteroid. This object, designated Asteroid 2007 WD5, was
worthy of concern. Based upon its magnitude it was estimated to be about 50 meters (164 feet) across and traveling at about 28,000 miles per hour.
At the time of its discovery, it had already passed within 7.5 million kilometers (5 million miles) of earth. That close approach occurred on November 1, 2007. It streaked past Earth undetected.
The 50 meter diameter and tremendous speed of this asteroid would produce major damage if it were to hit Earth on a future orbit. An asteroid
this big would hit with a force exceeding any atomic explosion that people have produced. It would also blast a
hole about the size of Arizona's famous Meteor Crater - nearly one mile across.
Astronomers immediately placed Asteroid 2007 WD5 on
their "watch list" and began calculating its orbit around the sun.
They wanted to find out if future orbits would bring it dangerously close to Earth.
Initial Orbit Calculations
Arizona's "Meteor Crater", an impact structure formed about 50,000 years ago when an asteroid about the same size as 2007 WD5 hit the Earth. NASA Image. |
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NASA has a team of astronomers who detect and track asteroids and comets passing
close to Earth. This Near Earth Object Observation Program, commonly called "Spaceguard,"
calculates the orbits of all known near-Earth objects to determine if
they are on a future collision path with our planet.
Astronomers with the Spacewatch program at Kitt Peak, Arizona and the
Magdalena Ridge Observatory in New Mexico began tracking the asteroid. This tracking
gave them important information about its path through space. They used this data
to determine that the asteroid was not an immediate threat to Earth; however, it
could potentially impact Mars. With minimal data they estimated that it would pass
by Mars on January 30th with about one chance in seventy-five of hitting the planet.
Refining the Asteroid's Orbit
As the asteroid zipped off towards Mars, astronomers on Earth were carefully
searching their space image archives. They were looking for pre-discovery images of Asteroid 207 WD5.
They were in luck. Andy Puckett, a recent Ph.D. from the University of Chicago (now at the University of Alaska at Anchorage), found an image taken on November 8, 2007 - about two weeks
prior to its discovery. This image showed Asteroid 2007 WD5 as a faint dot of light.
This image allowed astronomers to accurately determine the position of the
asteroid in space at a specific instant in time. The new data enabled them
to refine the path of the asteroid's orbit.
A 4% Chance of Hitting Mars!
New calculations allowed astronomers to determine that Asteroid 2007 WD5 had about
one chance in 25 of hitting Mars!
An artist's representation of possible locations of Asteroid 2007 WD5 on January 30, 2008.
NASA image. |
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At that time their best prediction had the asteroid missing Mars by a mere 50,000
kilometers (30,000 miles). However, this prediction was highly uncertain. The
orbit of the asteroid was not known with a great degree of accuracy.
Astronomers knew that the asteroid would approach Mars through a very long and very
narrow window. This window was about a 400,000 kilometers (250,000 miles) long but
only about 600 kilometers (350 miles) wide. A portion of the window is illustrated in the artist's
image at right.
Steve Chesley, a scientist at NASA's Jet Propulsion Laboratory estimates
that an object the size of Asteroid 2007 WD5 hits Mars about once
every 1000 years. He believes that this type of impact produces an explosion equivalent to about three megatons
of energy and yields a significant crater.
As Asteroid 2007 WD5 approached Mars in early January, 2008, the uncertainties in astronomers' calculations were significantly reduced and on January 9th their estimated probability of an impact was only 1 in 10,000 - an almost certain miss! Why the big decrease in the impact odds? It wasn't sloppy math - it is a matter of probability. As the asteroid got closer to the planet astronomers could more accurately calculate it's path relative to the path of Mars. Its like throwing a baseball at a target, at first it looks like it might hit and then as it gets really close you can tell with greater certainty if a hit will happen.
The important lesson in this story is the knowledge that there are lots of asteroids in space, many of them pass close to the earth, and although impacts are rare they certainly do happen. There is also a lesson in numbers. If an impact event has a 4% probability at first, that probability will most likely decrease to a very low number and then to zero as the object gets closer to its target. In other words, a 4% probability of a hit is a 96% probability of a miss.
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