One long-standing question is: how many asteroids – especially near-Earth asteroids (NEAs) – were comets in the past? In the classical conception, comets have surface ices and form tails and comae in the vicinity of the Sun; asteroids are simply rocks in space. In the recent years, the two classifications became less clear than that.
Comets can end up on orbits that bring them close to the Earth. We can observe them. But what happens to them after spending a while there? For some comets, like ISON, being close to the Sun is fatal; they simply disintegrate and disappear. For other comets in near-Earth space, the strong degree of insolation forces them to lose their surface ices quickly and eventually their activity stops – they are dead or dormant comets. Since comet surfaces have very low albedos, these dead comets simply look like low-albedo asteroids and are really hard to identify.
However, not all dead comets are really dead. Don Quixote is a good example, since we discovered activity in this objects that has not been detected in the last three decades. For this reason, dead comets should really be called dormant comets, as it is not clear if they ceased activity for good, or if they only sleep. The real question is: do these objects still harbor ices in their interiors? If so, this can have significant implications for the transport of water and other volatiles to Earth in the past and for the potential resource utilization in space in the future.
In order to identify dead comets that hide in the NEA population, we performed a statistical analysis in two steps. In the first step, we compared the orbits of currently active comets with those of asteroids and identified asteroids on comet-like orbits. In the second step, we identified those asteroids on comet-like orbits with comet-like (i.e., low) albedos. Those are most likely candidates for dead comets. We found a total of 23 dead comet candidates in albedo data provided by ExploreNEOs, NEOWISE, and Akari. Interestingly, only about 50% of all asteroids on comet-like orbits also have comet-like albedos.
In a second analysis, we estimated how many asteroids are likely to be of cometary origin. This question sounds easier than it is. The problem is that most asteroids are discovered with optical telescopes, which are more likely to discover asteroids with high albedos than those with low – and especially comet-like – albedos. Also, dead comets move on orbits that take them far away from the Sun, which makes them even less likely to be discovered by asteroid surveys. Hence, we have probably discovered proportionally fewer dead comets than other asteroids.
In order to resolve this discrepancy and account for this so-called ‘discovery bias’, we used a well-characterized survey that operates in the thermal-infrared, which is less prone to albedo-based discovery bias. NEOWISE is a program that uses the WISE all-sky survey data to search of all kinds of asteroids. Assuming a realistic but synthetic NEA population, we checked, how many of these synthetic NEAs would have been discovered by NEOWISE. With that information and those NEAs actually observed by NEOWISE, we can estimate how many more dead comets there are. We checked two different samples: dead comets larger than 1 km in diameter and dead comets with absolute magnitudes brighter than or equal to 21. We find that 0.3-3.3% of the NEAs with H <= 21 and 9 (+2/-5)% of the size-limited NEA population are dead comets. These numbers are slightly lower than previously assumed, implying that fewer NEAs than previously assumed have a cometary origin.
All the details of this analysis are available in a paper that is currently in press at AJ, but already available on arxiv.