Optimizing search strategies for near Earth Objects: Lessons learned from Pan-STARRS1

Abstract

The Pan-STARRS1 telescope has been spending most of its time for the last 3 years searching the sky for Near Earth Objects (NEOs). The Pan-STARRS telescopes are located in Hawaii, relatively close to the equator, and the telescopes can survey from the north celestial pole to −49° declination in the south. Pan-STARRS1 has a large field-of-view, enabling it to survey large areas of sky. This enables us to examine NEO discovery rates relative to ecliptic latitude, and this insight can be used to improve the NEO discovery rate. The NEO searches are most sensitive when the moon is set, or only a thin crescent is illuminated. At these times, a broad filter can be used, and the sky is darkest. The full moon requires use of an infrared filter to suppress the sky background — this decreases the sensitivity by approximately 1 magnitude (a factor of approximately 2.5). Most contemporary searches, including Pan-STARRS1, have been spending large amounts of their observing time during the dark moon period searching for NEOs close to the ecliptic. The rationale for this is that many objects have low inclination, and that all objects in orbit around the Sun must cross the ecliptic. New search capabilities are now available, including Pan-STARRS2, and the upgraded camera in Catalina Sky Survey's G96 telescope. These allow NEO searches to be conducted over wider areas of the sky, and to extend to higher ecliptic latitudes. We have examined the discovery rates relative to location on the sky for new NEOs from Pan-STARRS1, and find that the new NEO discoveries are less concentrated on the ecliptic than might be expected. This finding also holds for larger objects. The southern sky has proven to be very productive in new NEO discoveries — this is a direct consequence of the major NEO surveys being located in the northern hemisphere. Our preliminary findings suggest that NEO searches should extend to at least 30° from the ecliptic during the more sensitive dark moon period. At least 6,000 square degrees should therefore be searched each lunation. This is possible with the newly augmented NEO search assets, and repeat coverage will be needed in order to recover most of the NEO candidates found. However, weather challenges will likely make full and repeated coverage of such a large area of sky difficult to achieve. Some simple coordination between observing sites will likely lead to improvement in efficiency.