JULY 23 NORTHEAST FIREBALL PINPOINTED
from Sky & Telescope, August 3
It now appears that July 23rd's dazzling daylight fireball punched through the atmosphere over central Pennsylvania and may have scattered meteorites over the rugged woodlands of Sproul State Forest. Defense Department satellites tracked the meteoroid's flare for several seconds beginning at 6:19:11 Eastern Daylight Time. The path began over Scranton (75.6 deg. W, 41.5 deg. N) and ended 140 kilometers to the west over the town of Williamsport (77.3 deg. W, 41.3 deg. N), during which it dropped in altitude from 82 to 32 km. Despite occurring in daylight, the meteor was bright enough to be spotted by eyewitnesses from Canada to Virginia.
In its final moments the fireball created a deafening sonic boom that shook the ground. Meteor expert Peter Brown (Los Alamos National Laboratory), who is analyzing the satellite records, told Sky & Telescope, "I can almost guarantee that this object broke up." He says that reconstructing the object's orbit and flight path are proving difficult because the entry velocity is uncertain, though it's probably in the "asteroidal" range of 17 to 20 km per second. Brown believes that whatever remains of the incoming object probably fell in an elongated pattern up to 30 km long.
The meteoroid's size is also still a guess. The satellites' visible and infrared sensors recorded 1.3 billion joules of luminous energy, which corresponds to a kinetic- energy wallop equivalent to 3,000 tons of TNT (one- fifth that of the Hiroshima bomb). Meteoroids in this energy range strike Earth roughly 10 times each year. If it was stony, as most meteorites are, such an object would have weighed 30 to 90 tons and been the size of a car. However, Brown says acoustic and seismic data argue for much less kinetic energy and, in turn, a much smaller object. "I'd hoped to have had some meteorites recovered by now," Brown concludes, but the many uncertainties diminish that possibility. "That's why I'm here in New Mexico instead of heading for Pennsylvania."
DETECTION OF BOLIDE FROM SPACE
Air Force Technical Applications Center Public Affairs Office 1030 South Hwy A1A Patrick AFB, Fla. 32925- 3002 Phone: (321) 494-4404
News Release 002 July 27, 2001
On July 23, at 22:19:11 UTC, several DOD satellites recorded the bright flashes of a fireball (bolide) occurrence lasting more than three seconds. The optical waveform was analyzed, determined to be non- nuclear, and consistent with past observed bodies. The location of this event was reported to be in the eastern US.
IR sensors aboard US DOD satellites detected the impact of a bolide over the Eastern US on 23 July 2001 at 22:19:11 UTC. The object was traveling roughly East to West. The object was first detected at an altitude of approximately 82km at 41.5 North Latitude, 75.6 West Longitude, and tracked down to an altitude of approximately 32 km at 41.3 North, 77.3 West. The impact was simultaneously detected by space based visible wavelength sensors operated by the US Department of Energy. The total radiated energy was approximately 1.27 X 10^12 joules.
METEORITES DON'T POP CORN
NASA Science News (July 27, 2001)
A fireball that dazzled Americans on July 23rd was a piece of a comet or an asteroid, scientists say. Contrary to reports, however, it probably didn't scorch any cornfields.
July 27, 2001: Every few weeks, somewhere on Earth, a fiery light streaks across the sky casting strange shadows and unleashing sonic booms. Astronomers call them fireballs or "bolides." They're unusually bright meteors caused by small asteroids that disintegrate in our planet's atmosphere. Often they explode high in the air like kilotons of TNT -- blasting tiny meteorites far and wide.
It happens all the time, say experts, but usually no one notices. We live on a big planet, after all, and very little of Earth's surface is inhabited by people. Most debris from space falls unseen over oceans or sparsely- populated land areas -- or during times when sky watchers simply aren't paying attention.
Last Monday was different, however. On July 23rd hundreds of thousands of people were looking when, unexpected, a fireball appeared over the US east coast. It was 6:15 p.m. local time. The Sun hadn't set, but onlookers had no trouble seeing the fireball in broad daylight. Witnesses from Canada to Virginia agreed that the colorful fireball was brighter than a Full Moon, and some saw a smoky trail lingering long after it had passed.
"Contrary to some reports this was not a meteor shower," says Donald Yeomans, manager of NASA's Near Earth Object program at the Jet Propulsion Laboratory. Meteor showers happen when Earth passes through the debris trails of comets and countless thousands of cosmic dust specks burn up in Earth's atmosphere. At the heart of Monday's fireball, however, was a solitary object -- perhaps a small asteroid or a piece of a comet.
Hundreds of eyewitness reports collected by the American Meteor Society establish that the fireball was moving on an east-west trajectory that carried it directly over the state of Pennsylvania. "It was traveling perhaps 15 km/s (34,000 mph) or faster when it exploded in the atmosphere with the force of about 3 kilotons of TNT," says Bill Cooke, a member of the Space Environments team at the Marshall Space Flight Center. If this was a rocky asteroid, then it probably measured between 1 and 2 meters across and weighed 30 or so metric tons.
"Asteroids that size enter Earth's atmosphere every month or so," says Yeomans.
"The pressure wave from the airburst shattered some windows in towns west of Williamsport," Cooke continued. "Breaking glass requires an overpressure of about 5 millibars (0.5 kPa), which means that those homes were within 100 km of the explosion."
No one knows if any sizable fragments of the object survived the blast. But if they did, the meteorites probably landed in the wooded, hilly terrain west of Williamsport -- perhaps in one of the many state parks of that area.
Says Bob Young of the State Museum of Pennsylvania: "One of our planetarium staff was told that the little northern Pennsylvania town of Trout Run was destroyed by the meteor! The witness was about 100 miles away when she heard the tale from her hairdresser." Other reports credit the fireball for scorching a cornfield in Lycoming County, PA, and littering the countryside with burnt rocks.
In fact, says Yeomans, it's unlikely that any substantial meteorites reached the ground. Atmospheric friction would have reduced most of the fragments to dust. Even if fragments did survive, he added, they wouldn't burn cornfields because -- despite their fiery appearance in the sky-- freshly- fallen meteorites are not hot.
Objects from space that enter Earth's atmosphere are -- like space itself -- very cold and they remain so even as they blaze a hot-looking trail toward the ground. "The outer layers are warmed by atmospheric friction, and little bits flake away as they descend," explains Yeomans. This is called ablation and it's a wonderful way to remove heat. (Some commercial heat shields use ablation to keep spacecraft cool when they re- enter Earth's atmosphere.) "Rocky asteroids are poor conductors of heat," Yeomans continued. "Their central regions remain cool even as the hot outer layers are ablated away."
Asteroids move faster than the speed of sound in Earth's atmosphere. As a result, the air pressure ahead of a fireball can substantially exceed the air pressure behind it. "The difference can be so great that it actually crushes the object," says Cooke. "This is probably what triggered the airburst over Pennsylvania."
Small fragments from such explosions lose much of their kinetic energy as they heat the atmosphere via friction. They quickly decelerate and become sub- sonic. Dusty debris from airbursts (and ablation) can linger in the atmosphere for weeks or months, carried around the globe by winds. Walnut- to baseball-sized fragments might hit the ground right away at a few hundred kilometers per hour.
"Small rocky meteorites found immediately after landing will not be hot to the touch," says Yeomans. They will not scorch the ground or start fires. On the other hand, notes Cooke, "if we got hit by something large enough to leave a crater, the fragments might be very hot indeed." A stony meteorite larger than 50 meters might be able to punch through the atmosphere and do such damage -- but that's far larger than the object that flew over Pennsylvania.
No one knows what kind of space debris caused the July 23rd fireball. It might have been a small piece of an icy comet, in which case it's unlikely that anything larger than dust grains survived. It might also have been a rocky asteroid -- the most likely candidate -- or perhaps a nickel-iron meteorite. "Iron objects are more likely to survive a descent to Earth," says Yeomans, "but they are rare."
It's possible that fragments will never be found, notes Cooke. "We still don't have a precise trajectory for this object," he explains. "And so much of the targeted area (in central Pennsylvania) is heavily forested -- searching for debris will be like looking for a needle in a haystack."
Or should that be a needle in a cornfield?
"I suppose it's possible that some ablative fragments fell into that field," says Cooke, "but it is strange that only a small area was affected. I doubt it's a good candidate impact site."
"I wouldn't start looking there either," agrees Yeomans. "That scorched cornfield story sounds a little too corny for me...."