Impact Warning Times for the Next Generation of Asteroid Surveys
Paul, Chodas; Chesley, Steve
Jet Propulsion Laboratory

This paper describes a statistical study of the warning times for Earth impact that might result from discoveries of the next generation of asteroid surveys. The study begins with a Chesley-Spahr set of nearly a thousand "typical" impactor orbits based on the Bottke Near-Earth Asteroid population model. The motion of each impactor over the 80-year period leading up to impact is simulated via numerical integration. Each impactor is then analyzed at three different sizes: 700 m (H=18.5), 140 m (H=22), and 70 m (H=23.5). Geocentric optical observations were simulated whenever the object passes within a set of geometrical and magnitude constraints. Observations were excluded at low solar elongations, when the object is near the Moon, or within 3 days of full moon. Discovery opportunities are identified whenever the object magnitude brightens past a discovery magnitude threshold. The magnitude constraints are set to mimic the expected parameters of next-generation surveys. Possible discovery times are determined, and one selected at random, effectively simulating objects impacting randomly within 80 years of discovery. Following discovery, the optical observation rate is varied geometrically according to magnitude. Radar observations are simulated for each object by computing the signal-to-noise ratios (SNR) for both the Goldstone and Arecibo radars. Radar site parameters such as latitude, elevation limits, transmit power, dish radius, and aperture efficiency were factored into this computation, along with asteroid range, size, and radar reflectivity. The orbit prediction process was simulated by repeatedly determining the object's orbit based on the ever-increasing set of observations, while monitoring the impact probability. The warning point is defined to be reached when the impact probability passes a threshold value of 50%, and the "warning time" as defined as the interval between this point and the impact time.

In most cases the warning point is reached during the second observation period ("apparition"), and the warning times are therefore very dependent on object size. Larger objects have frequent apparitions and will typically have a second apparition within 5 years or so. But even some of these objects will have shorter warning times, because of orbits with long synodic periods or with intervening close approaches which make impact predictions difficult. Warning times for smaller objects are shorter because there are fewer observational opportunities for these objects. The role of radar in reducing warning times during the next generation surveys will also be statistically assessed.