A new web page offers an overview of known Near Earth Asteroids (NEAs) which may have close encounters with the Earth in the next 200 years. NEAs present a clear and present danger to the world’s population. Once every 200 years a medium-sized NEA (diameter 40-1000 m) hits the Earth and causes serious local damage. Once every 2 million years a NEA, comparable in size with the one which terminated the era of the dinosaurs, impacts and causes fatal global damage. 85% of the NEAs with diameters larger than 1000 m are known to date; of the potentially hazardous smaller ones, down to 25 m, only 1% is chartered.
Every day some 100 tons of interplanetary dust rains on the Earth atmosphere, tiny fragments of asteroids and comets, observed as meteors or "shooting stars". A few times per year a car-sized asteroid hits the Earth atmosphere and burns in a fraction of a second as a blinding fireball. Impacting objects larger than some 25 meter burn only partially and their remains reach the surface of the Earth: meteorites. Once every 200 years the Earth is hit by a NEA with a diameter of 40 m to 1000 m. Such NEAs can cause very serious local damage. If the impact happens in one of the oceans, huge tsunamis can affect even larger areas.
Once every 2 million years the Earth is hit by a NEA larger than 1000 m in size. Some 65 millions years ago, a 10-km sized NEA impacted near Chicxulub (Yucatán, Mexico), leaving a crater with a diameter of some 265 km and blowing enough dust in the atmosphere to end the era of the dinosaurs. For the USA the issue of potentially hazardous NEAs is a serious matter. Initially NASA was charged to detect and track 90% of all NEAs with diameters larger than 1000 m. Later that limit was lowered to 140 m. And soon the limit may be lowered again, to 40 m or 25 m.
Hunting for NEA’s
Of the NEAs larger than 1000 m about some 800, or 85% of the estimated number, are presently known. Of the NEAs with diameters between 1000 m and 140 m some 6200 are known, about 25% of the estimated number. The estimated number of NEAs with dimensions between 140 m and 25 m is about 11 million, of which only 1% is known. In particular in the USA a number of observatories is hunting for NEAs.
Since the seventies a small number of astronomers of Leiden Observatory were actively involved in asteroid detection. In the eighties the Dutch/UK/USA infrared space observatory IRAS observed over 4000 asteroids, including some NEAs. Since January 2010 the USA infrared space observatory WISE discovers numerous asteroids and tens of NEAs per month.
The International Astronomical Union (IAU) presents on its web site a page headed "Near Earth Asteroids (NEAs)". That web page offers a chronological overview of milestones of NEA research and of fly-by dates of NEAs which passed and pass Earth closer than the distance Moon-Earth in de period A.D. 1900 – 2200. The web page, composed and maintained by SRON senior scientist Karel A. van der Hucht, demonstrates that much work still has to be done. "The astronomical community needs to step up its number NEA observatories considerably if we wish to have a more or less complete data base of NEAs with sizes down to 25 m within the next 20 years," says Van der Hucht.
Van der Hucht represents the IAU in the United Nations Committee on the Peaceful Uses of Outer Space (UN-COPUOS), which has a keen interest in NEA inventory and options for mitigation. De IAU web page lists over 500 relevant milestones of NEA-research and events, referring to the original sources. See http://www.iau.org/public/nea/.