June is a time for beach parties and barbecues. It is a time for wildflowers and warm breezes and slow summer evenings. It is a time when our sun-orbiting planet cuts across a trail of cosmic debris, scattered by a monster comet, that one fine day may visit a holocaust upon us.
So says astrophysicist Victor Clube of Oxford University. He’s been studying the Taurids--a meteor shower that strikes Earth each year in late June and again in November. The Taurids are not nearly as spectacular as, say, the Perseids of August; in June they’re not even visible, because they approach from the dayside of the planet. But Clube and his co-workers think the Taurids are underrated. The meteor stream, they say, includes rocks so large--as much as a mile across--that to see one up close would be to lose a city, a continent, or more.
The evidence comes in part from history. In the early morning of June 30, 1908, a huge fireball exploded in the sky above Siberia with the force of a 20-megaton nuclear bomb, leveling 400 square miles of remote forest around the Tunguska River. The glow lit up the sky as far away as Western Europe. The Tunguska object, Clube says, was a 150-foot comet fragment--one of the Taurids.
Another near miss may have happened in 1178. One night in late June, according to a monk named Gervase of Canterbury, eyewitnesses saw a flaming torch on the upper horn of the new moon, which thereupon throbbed like a wounded snake. The throbbing could have been a dust cloud kicked up by a meteorite; a 13-mile-wide and apparently fresh crater named Giordano Bruno seems to be in the same spot as the flaming torch. If so, then the Earth narrowly missed being hit by a meteorite about a mile across--large enough to have devastated a continent.
The moon appears to have been pounded again by the Taurids as recently as 1975, when seismometers left behind by Apollo astronauts picked up the impact of a huge swarm of boulders. The onslaught started on June 22.
Clube and his colleagues believe a single cosmic marauder lies behind all these events. Meteor showers are debris shed by passing comets, and usually the pieces are no more than a few inches across. If Clube is right, however, the Taurid meteor stream includes some large chunks: in addition to the historical impactors, Clube says, there are half a dozen full-size asteroids whose orbits place them squarely in the stream.
Clube and his colleagues argue that the Taurids’ range of orbits indicates they were all shed by a huge comet, originally 100 miles across or more, that entered the inner solar system some 20,000 years ago. The comet’s orbit took it inside that of Mercury, close to the sun. By 10,000 years ago it was desiccated and brittle, and since then big chunks have been breaking off each time it passes the sun. One of those chunks, Clube thinks, is a comet called Encke. But the core object itself may still be out there. We suspect that the source of the Taurids is in an orbit similar to Encke’s, going round the sun every 3.39 years, says Clube. We think we’re on the verge of finding it.
Clube believes his killer comet sends bursts of Tunguska-size objects our way every few thousand years or so, and that in the past the dust clouds raised by such impacts have plunged Earth into cosmic winters and perhaps even full-blown ice ages. But he has yet to convince most astronomers that his historical evidence amounts to more than coincidence. It’s quite possible some of the June events fit in with a single object, but I think Victor may have turned it into a bit of a conspiracy theory, says Brian Marsden of the Harvard-Smithsonian Center for Astrophysics.
What many astronomers do agree on, though, is that the risk of a large impact, while perhaps not as great as Clube suggests, is worth taking seriously. A committee of NASA experts has recommended that a global network of telescopes be set up to hunt for potential impactors; a second committee is expected to urge NASA to develop a way of nudging an incoming rock out of our way, perhaps with a neutron bomb. After a decade of growing awareness that Earth has been blasted in the past, a consensus seems to be emerging: if we don’t want to go the way of the dinosaurs, the danger of an impact--in June or any other month--is one we should not ignore.