The debris scattered by comets that pass by the Sun in very elongated orbits is so thin that the meteor showers produced are difficult to detect.
From a new meteor shower study published in Icarus magazine, researchers now report seeing this danger in the debris of some comets that pass close to Earth's orbit every 4.000 years or so.
A meteor shower
"Our studies presented us with a situation of danger from comets that orbited close to the Earth around 2000 BC," says the astronomer and lead author. Peter Jenniskens of the SETI Institute.
Jenniskens is in charge of the project Cameras for Allsky Meteor Surveillance (CAMS), which observes and triangulates visible meteors in the night sky using video surveillance cameras in low light conditions to measure their trajectory and orbit.
CAMS, danger detection networks from space
CAMS networks currently exist in nine countries, led by the paper's co-authors.
In recent years, new networks in Australia, Chile and Namibia have significantly increased the number of triangulated meteors. The addition of these networks produced a better and more complete picture of meteor showers in the night sky.
"Until recently, we only knew five long-term comets as the parent bodies of one of our meteor showers," said Jenniskens, "but we have now identified nine more, possibly 15 more."
Comets comprise only a small fraction of all impactors on Earth, but researchers believe they caused some of the largest impact events in Earth's history. They can be very large and their orbits are such that they can produce a high speed impact.
In the future, with more observations, we may be able to detect weaker rains and track the orbit of mother comets on even longer orbits.Peter Jenniskens
How the CAMS network works
Each night, the CAMS network determines the direction from which comet debris enters Earth's atmosphere. Maps are created on an interactive celestial sphere (published on http://cams.seti.org/FDL/) showing the meteor showers as colored spots.
"These are the shooting stars that we see with the naked eye," Jenniskens said. "By plotting their direction of approach, these maps show the sky and the universe around us in a very different light."
A data analysis found that long-term meteor showers can last for many days.
"This was a surprise to me," says Jenniskens. "It probably means that these comets have returned to the solar system many times in the past, while their orbits have gradually changed over time."
The data also revealed that the most fragmented meteor showers are probably the oldest.
"This could mean that larger meteoroids fall apart into smaller meteoroids."