Observing 2013 NJ: A study case


4 December 2013

If an asteroid is discovered which could come very close to Earth, it is important to coordinate observational activities quickly to better know its precise flyby distance.This is illustrated by the following case of near-Earth asteroid (NEA) 2013 NJ.
2013 NJ (on the left- Discovery images of 2013 NJ, obtained on 1 July 2013 by the NASA-funded Pan-STARRS telescope in Hawaii.) is a 100-meter class object (absolute magnitude H = 22) discovered in early July 2013. It had a close encounter with the Earth on 26 November 2013, 1 million kilometers away from our planet. Even if the object was carefully tracked by observers around the time of discovery, its observational geometry then kept it close to the Sun for the past few months, making it unobservable from the ground until after its current close encounter with the Earth.
As a result, when the object became observable a few days ago from the Southern hemisphere, its position in the sky was poorly known. Without immediate additional astrometric observations the asteroid position in the sky would not have been constrained enough to allow pointing narrow-field instruments devoted to physical characterization (i.e. spectrometers, polarimeters) to the asteroid. Luckily, with a peak visual magnitude of 14 this object was bright enough to be easily reobserved even with small telescopes.
It was actually recovered from Cerro Tololo in Chile, less than 24 hours after its close approach, providing the astrometric information needed to significantly improve its orbit and predict its path in the sky for the following days. But what would have happened if 2013 NJ were a much smaller object?
Things would not have proceeded as smoothly as they did. Such an asteroid would have been at risk of being lost, unless the object would have been recovered when still on the southern hemisphere - difficult, as there are not so many telescopes there. Physical characterization needed also to be planned in a timely manner and attempted quickly. The asteroid would be going through a much fainter magnitude range, thus implying more demanding observational requirements in terms of time scale and telescope performances.
This "study case" scenario demonstrates that a quick coordinated response, able to ensure challenging astrometric and physical follow-up of small objects, is essential for the future developments of NEA research and mitigation. Even more since the US and European NEO space programmes are increasingly focused on small accessible asteroids, which are likely to be discovered in circumstances resembling that of 2013 NJ, forcing astronomers to react on short timescales and organize their observations during very short visibility windows.