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2009-07-29 06:18:42
Charles Q. Choi
Special to SPACE.com
SPACE.com charles Q. Choi
special To Space.com
space.com Tue Jul 28, 10:45 am ET
The recent bruising Jupiter received from a cosmic impact is a violent reminder
that our solar system is a shooting gallery that sometimes blasts Earth.
Still, what are the odds of a cosmic impact threatening our planet?
So far 784 near-Earth objects (NEOs) more than a half-mile wide (1 km) have
been found.
"If an object of about the same size that just hit Jupiter also hit Earth it
was probably a typical cometary object of a kilometer or so in size (0.6 miles)
it would have been fairly catastrophic," explained astronomer Donald Yeomans,
manager of NASA's Near-Earth Object program office at the Jet Propulsion
Laboratory in Pasadena, Calif.
Scientists have ruled out the chances of an Earth impact for all of these 784
large NEOs. Still, lesser objects also pose a risk, and researchers estimate
more than 100 large NEOS remain to be found.
Small risk
Billions of years ago, impacts were far more common. Our moon retains a record
of the pummeling it and Earth took: the moon's craters remain, while on Earth,
most scars of ancient impacts have been folded back into the planet or
weathered away.
Today's solar system is far less crowded, and in fact Jupiter, having more mass
and gravity, scoops up a lot of the dangerous objects, as does the sun.
Currently just one NEO of all the objects scientists are tracking poses any
significant chance of hitting the Earth 2007 VK184. If this roughly
425-foot-wide (130 meters) asteroid hit our planet, it would strike with an
energy of roughly 150 million tons of TNT, or more than 10,000 times that of
the atom bomb dropped on Hiroshima.
Roughly 100 telescopic observations made so far suggest that 2007 VK184 has a
1-in-2,940 chance of hitting Earth 40 to 50 years from now. However, if the
past is any guide, further observations to refine computations of its orbit
very likely will downgrade its probability of hitting Earth to virtually
nothing, Yeomans said.
Of remaining concern are the NEOs that we do not see. Researchers suspect about
156 large NEOs 1 kilometer in diameter or larger remain to be found, and when
it comes to dangerous NEOs in general, "when we get down to 140 meters (460
feet) or larger diameter objects, we think we've discovered about 15 percent of
them, and with 50 meters (164 feet) or larger diameter, we've discovered less
than 5 percent of them," Yeomans explained.
On average, an NEO roughly a half-mile wide or larger hits the Earth roughly
every 500,000 years, "so we're not expecting one anytime soon," Yeomans
explained.
"For 500 meters (1,640 feet), we're talking a mean interval of about 100,000
years," he added. "When you get down to 50 meters, the mean interval is about
700 years, and for 30 meters (98 feet), about 140 years or so, but by then
you're getting down to a size where you won't expect any ground damage, as they
burn up in the atmosphere at about 25 meters (82 feet) in diameter and smaller,
probably for an impressive fireball event."
When it comes to truly monstrous NEOs some 10 kilometers (6.2 miles) or larger,
of the size thought to have helped kill off the dinosaurs, "that's a 100
million year event, and in fact, I don't think there is anything like that we
see right now," Yeomans said. "The largest near-Earth object that can actually
cross the Earth's path, Sisyphus, has a diameter of 8 kilometers (5 miles), and
the largest that is termed a potential hazard is Toutatis, which has a diameter
of approximately 5.4 km (3.35 miles)."
Keeping watch
There are currently four teams worldwide actively looking for both large and
small NEOs, Yeomans said. "We're concentrating on the large ones for now, but
hopefully with the next generation of search, we'll be more efficient in
finding the smaller objects, to find 90 percent of the total population of
potential hazards larger than 140 meters," he added.
Keeping an eye on NEOs might not just be healthy for humanity, but also help
lead us out into space.
"They're easy objectives to get to, and asteroids have significant metal
resources that can be mined, while comets have significant water resources for
space habitats or travel," Yeomans said. "If you want to build a habitat in
space, you're not going to build it all on the ground and launch it up, since
that's too expensive you want to go up and look for resources instead."
Furthermore, asteroids and comets are among the objects that have changed the
least since the birth of the solar system roughly 4.6 billion years ago, and
might reveal vital clues behind the mysterious process.
"They may well have delivered the water and carbon-based molecules to Earth
that allowed life to form, so they're extremely important for study in that
direction," Yeomans added.