Trail of crumbs discovered from potentially hazardous comet
The February Eta Draconids appear to originate from a long-period comet that passes close to Earth’s orbit.
July 29, 2011
The Central Bureau issued a telegram July 10 for Astronomical Telegrams of the International Astronomical Union (IAU) announcing that a stream of dust from a potentially dangerous comet impacted Earth for a few hours last February 4.
This +2 magnitude February Eta Draconid was filmed by Peter Jenniskens with one of the low-light-level video cameras of the Cameras for Allsky Meteor Surveillance (CAMS) station in Mountain View, California, February 4, 2011. NASA-Ames
“This particular shower happens only once or twice every 60 years,” said Peter Jenniskens from the Search for Extraterrestrial Intelligence (SETI) Institute, Mountain View, California. “The stream of dust is always there, but quite invisible just outside of Earth’s orbit. Only when the planets steer the dust in Earth’s path do we get to know it is there.”
Since last October, the SETI Institute has teamed up with Fremont Peak Observatory in San Juan Batista, California, and UCO/Lick Observatory just east of San Jose, California, in monitoring the night sky with low-light video cameras in an effort to map the meteor showers in the sky over the San Francisco Bay Area. They triangulate the meteor trajectories and determine their orbits in space.
The IAU keeps score of showers that were claimed to exist in the past and now has a list of more than 300 showers that need confirmation. Only 64 showers have been established so far. Jenniskens’ goal is to establish many more.
While reducing the Fremont Peak and Mountain View station observations from February 4, Jenniskens discovered a handful of meteoroids that arrived at Earth from the exact same direction in the sky. They came from the direction of the star Eta Draconis, and the shower is now recognized by the IAU as the February Eta Draconids. This was the first new shower discovered in the Cameras for Allsky Meteor Surveillance (CAMS) project.
The meteoroids in question were moving on an elongated orbit, typical of that of long-period comets such as Hale-Bopp. Unlike Hale-Bopp, this one passes close to Earth’s orbit. Long-period comets rarely come back to the Sun, and if any one is on a trajectory to hit Earth, we could have little warning.
Now, Jenniskens has found the trail of crumbs of such a comet, which passed close to Earth’s orbit the last time it was near the Sun. That could have been only a few hundred years ago, or many thousands. At that time, the comet released a cloud of dust that is now returning. Some dust grains return earlier than others, depending on how elongated their orbit was, and the result is a continuous stream of returning dust grains. That stream is detected only when it encounters Earth, when the meteoroids cause a brief 2-hour meteor shower.
“Earth gets hosed typically only once or twice every 60 years by such streams,” said Jenniskens. “Only when Jupiter and Saturn are back at their original positions do they steer the dust trail in our path. The trail wags in and out of Earth’s path much like the Sun moves around in response to the motion of these heavy planets.”
The February Eta Draconids follow a short list of other such known showers, which include the November 22 Alpha Monocerotids, which were last seen in 1995, and the September 1 Aurigids, which created a spectacular shower in 2007. Jenniskens predicted the return of those showers.
Now that the February Eta Draconid shower has been discovered, Jenniskens is confident that a next return can be predicted. He teamed up with Finnish astronomer Esko Lyytinen to investigate. Lyytinen calculated a possible return in 2016 or 2023, after that not again until 2076.
Future observations of this shower may bring other information about the comet that caused this stream of meteoroids, which is a potential danger to Earth. “If the meteoroids can hit us, so can the comet,” said Jenniskens, “We don’t know whether the comet has already passed us by or is still on approach.” To get some advance warning, one could look along the measured orbit to those spots where the comet could arrive at Earth’s orbit on a future February 4 date.
“Even then, chances are very small that the comet will actually hit us, as such impacts are rare in Earth’s history,” Jenniskens added.