The Science of Orbit@Home

We present here the research carried out at the orbit@home project, that focuses on producing an optimized search strategy for dedicated astronomical surveys to search for near-Earth asteroids. This work is lead by Pasquale Tricarico at the Planetary Science Institute (PSI), in collaboration with Ed Beshore, Steve Larson, Andrea Boattini at the Catalina Sky Survey (CSS), and Gareth Williams at the Minor Planet Center (MPC).

The starting point in this research is the large set of astronomical observations of asteroids accumulated over the last decade. During this period, the activity in this field has grown exponentially, thanks to the introduction of electronic devices (CCD) to replace photographic films in acquiring detailed images of the night sky. As a result, it is not uncommon today for a dedicated telescope to observe thousands of asteroid in a single night, most of which are usually already known, and discover a few dozen new ones, of which only a handful are near-Earth asteroids.

When a telescope takes an image of the sky, only a small fraction of all the asteroids in that field are visible and thus detected. In the figure below we show an example, where the red dots are the asteroids that are observed, while the green dots represent all the known asteroids within the region of the sky has been imaged.

The main factor which determines whether or not an asteroid is observed is its brightness. Asteroids that are large and close are brighter and easier to observe than those small and far. But there are also other factors, such as the apparent speed of the asteroid on the sky, it's altitude above the horizon, and the density of stars in the background, which can have important effects. In the figure below, we show an example of the relative weight of each factor.

Determining these efficiency curves is necessary in order to characterize which asteroids can be observed and which cannot on a given night. It is at this point that the power of orbit@home comes into play. Once we model the performance of a telescope, we want to know, for any possible near-Earth asteroid, the probability to be observed during that single night. The orbits that are tested in this process cover the full range that is allocated to the near-Earth asteroids, on a finely grained grid that allows to precisely track the asteroids over the decade-long period analyzed. The final result for a single night is displayed in the figure below, where the red regions are for asteroids with an average probability up to 1/1,000 to be observed in that night, and the green regions for a probability of 1/10,000.

These probabilities are relatively low for a single night, but the cumulative probability over a long period can grow quite rapidly and will asymptotically reach unity. What happens typically is that, within a given small orbital region, real near-Earth asteroids are discovered, while the cumulative probability also grows accordingly. The ratio between discovered asteroids and cumulative probability produces an important estimate of the total number of near-Earth asteroids within that orbital region. Two examples are displayed in the figure below.

On the left, the N_known curve shows that only about 80 of the total 100 asteroids are discovered over the decade 2000-2010, while the "Completeness" curve in the lower plot tracks the cumulative probability to observe each one of the 100 asteroids over the same period. The ratio of the two curves provides the estimate for N_pop, the total population, that is always close to the 100 asteroids we started with. The population estimate gets more and more accurate as time passes, and the gray area indicates the uncertainty on the population estimate.

One important consequence of this work, which enables us to develop a search strategy, is the possibility to estimate the difference between the total population of near-Earth asteroids and the number of known ones. In other words, we can get an estimate of how many near-Earth asteroids are still out there unknown, never observed before, with a very detailed map of their orbital distribution. This means that for any future observing night, we can prepare beforehand a sky map of the distribution of these missing asteroids, and use it to plan the search. An example is displayed below, where the asteroids from one particular orbital region are displayed. The grey area represents the orbital region, while the black dots represent the fraction that can be effectively observed on that night.

The sky map of the distribution of missing near-Earth asteroids represents the main tool of the search strategy developed at orbit@home. By running orbit@home on their computers, the volunteers contribute directly to the realization of this important tool and to the search for near-Earth asteroids.

The orbit@home project is hosted at the Planetary Science Institute in Tucson, Arizona. PSI does research and provides educational outreach on a wide variety of topics in planetary science. To learn more about our programs go to www.psi.edu and please consider getting more involved by becoming a "Friend of PSI".