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The number of known near-Earth asteroids (NEAs) has just surpassed the threshold of 15000. That is a 50% increase over the number known in 2013, when we posted a similar news item on our portal for the crossing of the 10000 threshold. The discovery rate in the last few years has been extremely good, with an average of more than 30 new discoveries per week. Just a couple of decades ago this number would have been more than what was typically discovered in a full year. And even in 2012 the average rate was about half the current one.

The "Charlois Dome" at the Observatory of Nice and the 50 cm refractor presently hosted. [Credits: Observatoire de Nice]

So, what happened in the recent past to change things so much? The main jump in discovery rate, in the late Nineties, was the result of the installation of the first dedicated asteroid surveys.

These were mainly funded by the US following the direction of the US congress to discover 90% of the asteroids larger than a kilometre (including the so called “dinosaur-killers”) as quickly as possible. This goal was eventually reached a few years ago, thus representing a significant milestone in humanity's search for the most dangerous asteroids.

Its relevance can be fully appreciated in a historical perspective. Back in 1898, when the first NEO (Eros) was found, luck still played a major role. Auguste Charlois, one of the most prolific asteroid hunters of that time, barely spotted it from the Observatory of Nice and could not confirm his observation because of bad weather. The very same night Eros was found by Gustav Witt from the Sternwarte Berlin who scored only 2 discoveries in his whole career. In the following 34 years only three more NEOs were added and none of them posed a hazard because their orbits, much like Eros’, were just grazing that of the Earth. Then, on 24 April 1932 Apollo came, with a perihelion well inside the orbit of our planet: impact monitoring was to begin.

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Today the two main discovery projects in the world, the Catalina Sky Survey in Arizona, USA and the Pan-STARRS project in Hawaii, USA, jointly account for about 90% of the new NEOs. For many years, the Catalina Sky Survey with its multiple telescopes led the world effort to discover asteroids. In 2014, the Pan-STARRS survey took the lead after it became almost exclusively dedicated to the NEO search, increasing their own discoveries by a factor of about 3.

The Catalina Sky Survey dome at Mt. Lemmon and the 1.52 m reflector. [Credits: Catalina Sky Survey, University of Arizona]

What's the future going to be then? Is there room for further improvement? A few new players will likely be coming into the game over the next decade, with the promise of revolutionizing the field once again. The next few years may see the beginning of the operation of both the Large Synoptic Survey Telescope (LSST) in Chile and ESA's new fly-eye telescope.

They have different operational modes: LSST will be able to observe smaller objects further away, while the fly-eye telescope will have a very large field of view and be able to cover more of the sky each night. Coupled with proposed space-based survey capabilities, these future assets may give us the almost complete sky coverage and depth needed to be sure that as many incoming objects as possible are identified and studied before they are any risk for impact.