Making history: 15000 NEAs and counting…
21 October 2016
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.
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.
The "Charlois Dome" at the Observatory of Nice and the 50 cm refractor presently hosted. [Credits: Observatoire de Nice]
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.
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.
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.
The Catalina Sky Survey dome at Mt. Lemmon and the 1.52 m reflector. [Credits: Catalina Sky Survey, University of Arizona]
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.
An NEO over Antarctica
09 September 2016
Asteroid 2016 RB1 has hit the news because of a peculiar close passage on 7 September 2016 at 19:20 CEST. About the size of a cottage, the asteroid flew past our planet at an altitude of 34000 km, roughly the same as the so-called "geostationary ring" where most telecommunication satellites reside. Yet it posed no hazard neither to our planet nor to the satellite operators. Despite having been discovered only 24 hours before closest approach, the orbit became quickly so well constrained to ensure that the computation of the incoming flyby had the necessary accuracy to rule out any Earth impact solution. As a matter of facts already in the morning of 7 September 2016 RB1 was present in our close approaches list but not in the updated Risk List, which ranks the objects for which a non-zero impact probability is detected.
The danger to geostationary satellites would be possible (although very unlikely) only if the closest approach distance were reached near the Earth equatorial plane, where the geostationary ring is located. But 2016 RB1 was heading to the South pole, scoring the minimum altitude just over Antarctica. This had an interesting consequence: it is well known that a polar encounter geometry causes the gravitational pull of our planet to change the asteroid orbital inclination and 2016 RB1 was no exception – but with a peculiar behaviour.
The asteroid nodes were switched, meaning that while before the 7 September encounter the asteroid used to cross the ecliptic in that point coming from above, now the orbit is turned upside down after Earth gravity bended upward the asteroid trajectory. The effect is clearly shown in the diagram above, which has been computed using the NEO Coordination Centre orbit visualizer. Zooming into the Earth encounter geometry is also displayed thanks to a forthcoming new functionality, which will be soon available on our website.
The top panel shows the geometry of the 2016 RB1 Earth encounter. The bottom images show the flyby diagrams: the red arrow indicates the distance from the Earth at closest approach, the green arrow shows the direction of motion [Credits: ESA/NEOCC].
The NEOCC has timely alerted its network of collaborating telescopes to attempt follow-up observations, which were successfully carried out by the team led by Daniela Lazzaro using the OASI telescope located in Nova Itacuruba, Brazil. The excellent quality of the images obtained by Filipe Monteiro can be appreciated in the animation below showing the motion of 2016 RB1 in the 7 September morning sky.
2016 RB1 animation based on images taken at the OASI observatory [Credits: F. Monteiro, H. Medeiros, P. Arcoverde, D. Lazzaro, R.Souza, T. Rodrigues]