Studying asteroids that approach Earth

On June 30, 1908, a large explosion occurred near River Tunguska in Russia in the early hours of the day. There were no causalities but eye witnesses reported a bright blue flash, followed by a thunder and shattering of glass window panes. Almost 10 years later, scientists led an expedition. They observed some unusual features like the trees that appeared to have been torched, standing erect with no branches.

The team was disappointed on not having found a crater or a meteorite.

Since then, a lot of research has gone into this ‘impact’ - it assumes importance on being the sole test case for the hypotheses of impact.

Newspaper reports that appeared during the days after the impact were collected to put together the after effects - an earthquake on Richter Scale 5 was recorded. The shock waves had been detected. The devastated area was about eight km wide. The atmospheric effects also had been reported. People from about 100 km away had seen a bright blue cylindrical fire column descending on near the horizon, just before the earthquake shook the buildings. The ‘impact’ is a matter of serious concern for all of us. A study of this kind is important for us to survive. Several hypotheses have been worked out. While one states it was comets, another one says it was a sudden release of natural gas from the earth itself.

The opportunities for these studies are rather rare. An asteroid collision in Chelyabinsk on February 15, 2013 gave another opportunity – the asteroid just about the size of a small aircraft and it exploded without creating a crater. The effect of the impact was 500 kilotonnes in the airburst.

Eugene Shoemaker, an American geologist and one of the founders of the field of planetary science, who proposed the impact theory of lunar craters had made estimates of energy of the impact too. The effects on the atmosphere, on the biosphere are well studied. It is established that an impact was responsible for the mass extinction of the dinosaurs 65 million years ago. The impact crater is identified with a huge circular region named the KT boundary near Mexico.

The importance of the study of these asteroids which approach the earth was realised some time ago and special investigatory projects were initiated. Near-Earth Asteroid Tracking (NEAT), Lincoln Near- Earth Asteroid Research (LINEAR) and similar ones employ robotic telescopes to survey the sky for fast moving body near the earth. The near-earth asteroids can be too tiny to be seen even with moderate sized telescopes. A registry of the potentially hazardous near-earth asteroids (PHNEAs) is maintained and these objects are tracked.

The list of PHNEAs is growing fast with the advent of new detectors and new survey telescopes. There are still a million asteroids to be identified that could enter into this list. The distance of approach is usually given in units of LD, which stands for lunar distance (384,000 km). For example, the asteroids designated 2014 BS5 will move past the earth at a distance of 3.12 LD on July 23. This is about 30-50m in diameter (if it is circular) and will be a good target for the study using Radar imaging.

Thus, the PHNEAs do pose a risk to earth, and the consequences of an impact can be serious. We have the technological capability to detect. Newer ideas on tracking and deflecting dangerous ones away from impacting earth are being experimented.

There are only a handful of asteroids whose crash on to earth was known. Some of them had names like the Lost City, Pribram, Peekskill and Innisfree. They were very small and their impact did not do a great damage. However, there are some larger bodies which come to be known every time they near earth once in every three to five years The closest approach of Apophis, which is about 370 km away, is predicted for April 13, 2029 when it is likely to come within a distance of 36,000 km.

June 30 is observed as Asteroid Day and it commemorates the anniversary of the Tunguska asteroid impact.

(The author is with Jawaharlal Nehru Planetarium, Bengaluru)