Deep Surveys to Discover Sub-km NEAs

In anticipation of achieving the "Spaceguard Goal" of discovering 90% of NEAs larger than one km in diameter by 2008, it is natural to consider "what next?" Other papers in this session outline the residual hazard and identify a next-generation goal of discovering ~90% of NEAs down to about 150 m diameter. This is possible from ground-based telescopes, but requires much larger systems and more sophisticated survey strategies. Two systems are under serious design and development. One is Pan-STARRS, a system of four 1.8-m telescopes planned by the University of Hawaii and funded through the USAF. The other is LSST, a single 8.4 meter telescope, planned by the National Optical Astronomy Observatories (NOAO), intended to be funded by some combination of NSF, NASA, and private foundations. A few other 4-m class instruments are under design (e.g. Next-Generation Lowell Telescope) that can contribute to the next generation survey. Unlike current survey systems, these much larger instruments will share priorities with various other astrophysical programs (supernova searches, weak and strong lensing surveys, multi-color sky surveys, etc.), so we must design a survey strategy that can accomplish the NEO survey task with only a fraction of the total time, or with images not optimized for the NEO survey. Four factors promise substantial improvements in survey performance: (1) optimal design and location of telescopes and detectors to obtain higher resolution and larger fields of view; (2) sky subtraction of current images against a catalog of summed past images rather than comparing one image with another taken the same night (this is used successfully by supernova surveys); (3) more advanced linkage strategy along with better quality astrometry to obtain orbits with fewer observations over shorter time spans; and (4) optimizing the search area to cover less than "all sky" while retaining most of the discovery rate. Preliminary evaluations of these improvements suggest that a successful NEO survey can be accomplished using half or less of the total observing time of a single wide-field survey telescope, as opposed to about two such telescopes, as would be required by simply scaling up from present instruments and strategies. Thus it appears possible to design an NEO survey that is compatible with other uses of the same instrument. INTRODUCTION Current surveys to discover NEAs (e.g., LINEAR, LONEOS) reach a threshold limiting V magnitude of about 19.5-20.0, attempt to cover as much sky area as is visible from one site two or three times per month, and make detections of moving objects by comparing images taken ~20 minutes apart. In order to eliminate false detections and image flaws, as well as to get a rate of motion, the various surveys take from three to five images of the same patch of sky during a night. As an example, consider the LINEAR system. The 1-m aperture camera has a field of view (FOV) of 2.0 square degrees. To reach magnitude 19.5 or so requires an exposure time of six to ten seconds, so assume a mean duty-cycle between exposures of ten seconds, including move and settle time. A single telescope can thus cover ~750 sq. deg. per hour once. The observing protocol used is to re-cover the same area five times in the night, so the rate of coverage of unique sky areas is about 150 sq. deg. per hour. There are about 150 "dark" hours per month (without moon interference), and typically ~100 hours net allowing for weather and equipment problems. Thus a single telescope can cover about 15,000 square degrees of sky area per month. This happens to be just about how much sky is observable from a single location, so a single telescope can do this approximately once per month. In order to link objects and derive orbits, objects must be re-observed on at least three nights. Since the LINEAR project has two telescopes currently operating, and there are other surveys as well, the requisite number of observations can normally be made in the course of routine surveying. Presently targeted follow-up observations are obtained as needed by other observers, many of them amateurs, so the current survey is successful in maintaining an "all sky" survey in this mode, using a few fully dedicated telescopes. The next generation of survey under consideration will go much deeper, to about V magnitude 24, employing telescopes in the multi-meter size class. At this level, other observers using smaller telescopes will not be able to contribute to the follow-up requirements, so the survey will have to perform its own follow-up. Furthermore, the next generation instruments contemplated will be used for other programs as well, such as week lensing surveys, supernova surveys and 2004 Planetary Defense Conference: Protecting Earth from Asteroids 23 26 February 2004, Orange County, California AIAA 2004-1404 Copyright © 2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.