A rock that crashed through the roof of a house in New Jersey proved to be the real thing — a chunk spalled from a 4.5 billion-year-old asteroid.
Suzy Kop didn't expect to find a visitor from outer space in her father's bedroom when she stopped by the house on Monday, May 8th. On the floor lay a black, mango-size rock, its broken surface exposing a creamy interior. When she looked around and spotted two holes in the ceiling she knew something wasn't right. Some 15 minutes before Kop arrived (around 12:30 p.m.) there was a report of a brilliant fireball in the area. As it happened, the heavy end of that falling star crashed through the roof of the Hopewell, New Jersey home. The meteorite struck the floor with such force that it rebounded and punched a second hole in the ceiling before landing back on the floor. Finally becalmed, its long interplanetary journey had come to an end.
Kop described the stone as still warm to the touch, which was interesting because most meteorites arrive cold (even frosty) after a long journey through the vacuum of space. Unsure what to do with the rock, she called the Hopewell police for assistance. According to the Associated Press, a hazmat team arrived to examine the cosmic trespasser for signs of radioactivity and check on the family. For the record, meteorites are no more radioactive than a banana you'd buy at a grocery store. Technically, all rocks are radioactive since they contain fissionable elements like potassium, uranium and thorium — but at vanishingly small levels. Studies have shown that most meteorites are in fact much less radioactive than Earth rocks.
Meanwhile, the police contacted the nearby College of New Jersey and got a hold of Shannon Graham, an assistant professor of physics, to help determine the object's identity. Kop and Graham connected, and on Wednesday (May 10th), Kop brought the mystery rock to the College for analysis. Graham, with help from Nathan Magee of the physics department, and colleagues, confirmed its cosmic origin based on visual inspection, density measurements, and close-up examination with an electron microscope.
A first analysis indicates it's likely an LL6 chondrite, a stony meteorite with low iron content that frequently displays a cement-like color when fresh. A thin rind of fusion crust from atmospheric heating covers much of the outer surface. At some point on the way down — and certainly after piercing the roof and hitting the floor — pieces broke off to expose the rock's interior. Like almost all meteorites, chondrites date back to the formation of the solar system 4.6 billion years ago. Holding one in your hand conjures visions of a time before there was even a planet to stand on. The LL group is the least abundant of the ordinary chondrites and likely originated from hot asteroid interiors. The heat homogenized the rock's minerals and blurred the borders between individual chondrules.
Some wondered whether the space rock might have been related to the Eta Aquariid meteor shower, which is active this time of the year. Meteors from the shower derive from dust released by Halley's Comet during its cyclic visits to the inner solar system. But by noting the direction of the meteorite's flight (see diagram above), we can definitively say there is no connection. It entered the atmosphere traveling east to west. At the time of entry, the radiant of the Eta Aquariids — the spot in the sky from which meteors emanate during the shower — stood about 20° high in the southwestern sky, directly opposite the meteoroid's arrival trajectory.
Also, the material that comprises meteor showers is typically the size of sand grains and friable. A big one might reach the size of a chocolate chip or pea. To date, not a single meteorite of the 71,688 classified and named meteorites has been shown to originate from a meteor shower. While it's possible we may have a few cometary meteorites in our collections, the vast majority are fragments from asteroids.
As an interesting aside, the Japanese probe Hayabusa returned samples from the asteroid 25143 Itokawa in June 2010. Analyses identified the specks as LL5 and LL6 chondrite material — the same as Earth's latest cosmic visitor.
Some are already calling the Hopewell fall the "Titusville meteorite," after the location of the nearest post office. But according to the Guidelines for Meteorite Nomenclature, naming meteorites after a post office site is a common misconception. Other factors such as the actual fall location or geography can play roles. We'll just have to sit tight until the Nomenclature Committee of the Meteoritical Society makes the call.
One description that will forever stick to the Hopewell fall, at least among meteorite collectors, is hammer stone. That's the name given to any meteorite that strikes a human-made object such as a home, car, fence, dog house and/or an animal. Most collectors place a high value on hammer stones because of the amazing stories that result from these encounters as well as the rarity of a space rock hitting a human structure. The New Jersey fall produced the second hammer of the year. The first one fell on January 23rd near Muskogee, Oklahoma, when a 388-gram stone was found on the roof of a horse barn. For more on 21st-century meteorite falls and hammers, check out Mike Gilmer's Recent Meteorite Falls.
Dedicated meteorite hunters have been out in force since the news first broke. Combing the landscape for black rocks, they're guided by maps made using Doppler-radar facilities in the area, which can detect meteorite fragments falling through the air. As an incoming meteoroid streaks over a landscape, it commonly breaks into pieces due to thermal shock from the atmosphere. These disperse to form an oval-shaped strewn field beneath and near the meteoroid's path. Fragments are sorted according to size — small meteorites typically fall at the near end of the oval and the largest chunks at the terminus.
Every rock that hurtles from the heavens brings us closer to a better understanding of how the solar system grew from dust into this beautiful, baffling and bountiful place. Keep 'em comin'!