The 2016 WJ1 precoveries: back to the future
09 December 2016
The word "precovery" has entered the astronomical jargon rather recently but it quickly grew in popularity among asteroid hunters. It refers to the finding of an archival observation of an object which was not recognized as such because the corresponding image originally served other purposes. Hence the name "precovery", which explains why we may have direct measurements of an asteroid position at a time well ahead its discovery date. Searching for precoveries is a powerful technique for NEO impact monitoring. It allows to extend the observational arc of an object leading often to a dramatic improvement in its orbit knowledge without the need of additional telescope time. Although extremely rewarding, it is a tricky business which requires peculiar skills. The past trajectory of the asteroid (including uncertainties) must be assessed in order to start searching and retrieving candidate images from astronomical databases all over the world. A careful inspection looking for a faint spot close enough to the expected location is then carried out.
The case for 2016 WJ1 is no exception, but with a further taste of complexity: a chain of precoveries brought the NEO Coordination Centre to catch a view of the asteroid position as far back as July 2003. 2016 WJ1 was discovered on 19 October 2016 by the Catalina Sky Survey. When impact monitoring systems were run , the object turned out to be rather peculiar, with more than 200 dates within the next century when an impact could not be excluded with certainty. However, the highest probability for any of these events was quite low, about 1 in 80 000.
The left panel shows the 2003 precovery image of 2016 WJ1: the asteroid appears a faint spot between two bright stars. The right panel shows the orbit of 2016 WJ. The orbit of 2016 WJ is rather peculiar, having both nodes very close to the Earth orbital radius. The position of the Earth and of the asteroid corresponds to the 2037 encounter, formerly an impact solution.
Over the following days new observations were collected and when the impact predictions were recomputed some of the possible dates were discarded, but others became associated to a higher hazard. In particular, a possible close approach in June 2065 was now showing an impact probability of about 1 in 8000, enough to classify an object of this size at level 1 of the Torino scale (out of 10 levels).
Usually additional follow-up observations are likely to remove the possible threat, but for 2016 WJ1 this was not the case. Those collected during the subsequent week, including some from our team with ESA's OGS telescope in Tenerife, did not significantly change the impact scenario. However, they did improve our knowledge of the asteroid's orbit enough to start searching for precoveries.
The left panel shows the behaviour of the impact probability over time for two possible impact dates and its cumulative value (blue area). Note the steady increase and the almost sudden drop to a value difficult to appreciate in the scale of the plot, after the 2003 precovery observation was found. The right panel shows how the accuracy with which the 2016 WJ1 semimajor axis was known, increased with the length of the observed arc, significantly extended by precoveries.
The archives of the survey telescope Pan-STARRS revealed a promising set of images taken on 13 October 2016. An extremely faint source was indeed spotted at the expected position, but it was so barely visible to prevent deciding whether it was the footprint of 2016 WJ1 or just a fortunate alignment of noise features. Yet the information was not useless: pretending that the Pan-STARRS detection was true, the asteroid orbit was refined to the point of allowing us to push the search further back in the past - until a couple of decades ago. Were another object to be found following the new predictions, this would confirm that the Pan-STARRS faint detection was indeed correct.
The Canadian SSOIS image search system, one of the most efficient on-line precovery services available, with which the NEO Coordination Centre is collaborating, was used to this end. Two sets of images taken on 4 and 5 July 2003 with the Canada-France-Hawaii Telescope, where the object should have been visible, were retrieved. After careful inspection we were able to locate the object only 4 arcminutes away from the prediction based on the Pan-STARRS detection.Accurate astrometry was promptly extracted from these images and the outcome was lowering the probability associated to possible impact scenarios to safer values.
Following asteroid footsteps backward in time is an exciting game, to which a future development of the ESA SSA-NEO Segment is likely to provide a significant contribution: the deployment of the so-called "Fly-Eye" telescopes. Aimed at NEO discovery they will cover the whole visible sky every night thus making available a huge database which in the long run will greatly increase the chances of carrying out successful precoveries.
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.