Celestial Manoeuvres

Asteroids may be destructive, but they may also have triggered the creation of life on earth. Maura Imbert explores the phenomenon

Great Leonid meteor shower of 1933. Taken from an old plate by Maura Imbert.

At the time of writing this article last July, the Earth had just had three relatively close shaves with asteroids. An asteroid named 2001 YB 5, about 300 metres in diameter, whipped by the Earth on January 7, 2002, a few weeks after it was discovered by astronomers. 2001 YB 5 passed at a distance of about 600,000 kilometres (roughly twice the distance to the Moon) — too close for comfort.

Two months later there was a general alarm at the unnoticed — and even closer — passing of the asteroid 2002 EM 7 within 461,000 km of the Earth. Because it had come from the direction of the Sun, the asteroid was not noticed until four days later, in spite of NASA’s monitoring. This chunk of space rock was approximately 70 metres long, and its disintegration in the atmosphere would have released sufficient energy to flatten a modern city — or a West Indian island! Asteroid 2002 EM 7 is considered slightly larger than the asteroid which exploded over Tungusta, in an isolated region of Siberia, in 1908. The Tungusta asteroid flattened thousands of acres of forest, though, while reindeer herds were decimated, there was no loss of human life. If it had exploded over an inhabited region, it would have caused widespread destruction, similar to that caused by the atomic bomb dropped on Hiroshima in 1945. The orbit of 2002 EM 7 has been calculated, and although NASA has reported that there could be a collision with the Earth in 2093, the odds of such a collision are currently 1 in 10 million.

This may not be the case with the 1-km-wide asteroid 1970 DA, which may pass very close to the Earth on March 16, 2880 — how close is still not known. This date can be calculated with great precision as the asteroid has been monitored since it was discovered in 1970, and its current orbit has been mapped very accurately. This orbit, however, is influenced by gravitational tugs from other celestial objects, and it even gets a push from sunlight. Asteroids absorb radiation from the Sun and then later release it. The emission of energy, however slight, in a single direction, gives the asteroid a nudge in orbit in the opposite direction. This solar push is called the Yarowski Effect. Over the centuries, these tiny nudges add up, and they could mean the difference between a miss and a collision with Earth. The Yarowski Effect can be used to avert a collision, however, because as asteroid 1970 DA gets closer in the coming centuries, these solar effects can be used to change the path of the asteroid. It has been suggested that the asteroid’s surface could be dusted with either chalk or charcoal, depending on the desired directional change, or that it could be wrapped in a solar sail by a spacecraft — a safer option for orbit-change than blowing it up with a nuclear device! This means our distant descendants should not have to fear a collision with asteroid 1970 DA.

But not too long after scientists had published these asteroid-evasion techniques, they were shaken by the undetected passing of an asteroid at the uncomfortably close distance of 125,000 km (75,000 miles) — less than a third of the mean distance between the Earth and the Moon — on March 14, 2002. This asteroid was about the size of asteroid 2002 EM 7, and was travelling at a velocity of almost 40,000 km per hour. Workers on the LINEAR Search Programme in New Mexico did not detect it until last March 17.

A miss may be “as good as a mile”, but many asteroid-detection personnel must be looking over their shoulders these days. A new dimension has recently been added to unexpected asteroid impacts on the Earth, as it has been postulated that warring nations with nuclear capabilities may mistake the impact for a hostile nuclear detonation and press that “red button”!

While the current thrust is the early detection of near asteroids with a view to preventing collisions with Earth, it must be remembered that such collisions in the distant past may have been the very triggers which initiated life on our planet. Liquid water, carbon-based molecules and a suitable environment are essential elements for most forms of life on Earth, and comets (most of them “dirty snowballs” around a rocky core) and many asteroids contain the elements on which life on Earth is based. Both celestial bodies, however, can also cause the extinction of terrestrial life, and it is believed that the extinction of the dinosaurs and many other animals some 65 million years ago was due to the impact of a large asteroid about 10 km in diameter. This impact is considered responsible for the 100-km-wide crater at Chicxulub on the Yucatan Peninsula. While not all scientists believe that this asteroid impact caused the extinction of reptiles and other lifeforms, recent evidence obtained from 65-million-year-old fossilised leaves supports the hypothesis. Researchers have found there was a sudden rise in the carbon dioxide content of the Earth’s atmosphere 65 million years ago, which led to catastrophic global warming and the extinction of many life forms. It is suggested that the impact of a large asteroid, vaporising billions of tonnes of limestone rock, would suddenly release vast quantities of carbon dioxide into the atmosphere.

Previously, it had been thought that the impact of such a large asteroid on the Earth would cause the ejection of so much dust into the atmosphere that the Sun would be obscured for a number of years, with the consequent death of both vegetation and plant-eating animals. Asteroids are rich in the element iridium, which is not found on Earth in significant quantities, and the widespread deposit of asteroid dust 65 million years ago was indicated by the discovery of layers of iridium-rich clay between limestone strata in many parts of the world. These limestone strata were found, by geological dating, to be 65 million years old.

All the rocky bodies in the solar system, including the Earth, are pockmarked with craters, indicating the numerous collisions which occurred in the early history of planetary and asteroid formation. Many of the craters on Earth have been obscured by vegetation or eroded, but they can be detected in satellite photographs.

While such massive impacts with the Earth are now very rare occurrences, an estimated 300 tonnes of extraterrestrial rock and dust fall on our planet every day. The Earth’s atmosphere protects us from this space debris, most of it the size of pebbles or rice grains, which are vaporised high in the atmosphere as meteors or “shooting stars”. Some of this space debris has resulted from the break-up of a comet, and this cometary disintegration is responsible for swarms of particles that continue to orbit the Sun in the comet’s original path. If the Earth’s orbit intersects this, a meteor shower occurs, and if the intersection occurs near a swarm of particles, the shower becomes a “meteor storm”, with thousands of meteors visible per hour.

The meteors travel on parallel paths through the atmosphere, appearing to diverge from a certain point on the celestial sphere due to an optical effect also characteristic of railway tracks. This point is called the “radiant” and the meteor shower is named after the constellation in which the radiant is situated.

Swift, bright Leonid meteors radiate from the constellation Leo in November, debris from Comet Temple burning up in the Earth’s atmosphere. There are often spectacular Leonid showers every 33 years when the Earth’s orbit intersects particularly dense particle swarms; the most noteworthy shower occurred in 1866, when a quarter of a million meteors were observed from one viewing station. There were also spectacular showers in 1933, 1966 and 1999, and the 1999 storm was repeated in 2001 when the Earth passed through remaining filaments of the 1999 particle swarm; thousands of meteors per hour were seen in many parts of the world.

Here in Trinidad, many observers classified the 2001 meteor shower as the best they had ever seen, with fireballs arching across the sky in the early morning hours of November 18. Most experts consider that 2002 will be the last year to see a Leonid “meteor storm” for at least three decades and that it will be a spectacular show with thousands of meteors per hour visible in favoured locations during the period November 16 to 20. November 17 will favour the Far East and Australia, and November 19 best suits Europe, North Africa, Brazil, North America and the Caribbean. Meteors are best seen after midnight. It is advisable to be alert every night during the peak period to avoid missing what may be a breathtaking celestial show.