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Near Earth Objects

Prepared by E. Dotto

The LBT, with a wide field camera and its high sensitivity, could play a major role in the investigation of small bodies of Solar System.

At present we know the orbit of more than 8300 asteroids and we have less detailed information about more than 30000 objects. Most of them orbit the Sun in the asteroid main belt, located between 2.1 and 3.3 AU, but a great number of very interesting small bodies are located out of this region, beyond the orbit of Neptune and in the inner part of the solar system.

The knowledge of the physical and dynamical properties of these objects should be very useful to investigate i) the physical structure of the protoplanetary nebula, ii) the processes which governed the formation of solar system bodies and iii) the possible relations among small bodies of solar system (asteroids, comets and meteorites).

In this context LBT will be a very useful tool to contribute to the discovery of very small and faint objects and to the determination of their physical properties. This very powerful telescope should be addressed to the study of Near Earth Objects, which orbiting the Sun cross the orbits of the inner planets.

About 500 NEAs [5.8% belongs to the Aten group (a<1 AU and Q>0.983 AU), 4.8% belongs to the Apollo group ($a\geq1$ AU and $q\leq 1.017$AU) and 45% belongs to the Amor group (1.017 AU $< q\leq1.3$ AU)] are known and their discovery rate is continuously increasing. The study of the dynamical and physical properties of these objects is necessary for understanding their histories and relations with comets, main belt asteroids and meteorites. Understanding the dynamical evolution of NEAs is important because: i) they can have close approaches with Earth, representing a long-term danger for the biosphere and the human species; ii) they can represent the larger members of meteoroids and small interplanetary objects; iii) physical remote-sensing observations of them have recently revealed a number of surprising features, such as high metal abundance, binary structures and tumbling rotational states which could be related to their asteroidal or cometary origin.

The general idea on the NEA origin is that these objects are efficiently removed from other regions of the Solar System by collisions and subsequent gravitational interactions with the planets on time scale of 106-108 years. Since the NEAs have unstable orbits, a continuous resupply of new objects is needed. Two main mechanisms have been commonly proposed for supplying the Earth-approaching population: the first assumes that NEAs are asteroidal fragments coming from the main belt, while the second supposes that a possible source for NEAs are dormant or extinct comet nuclei.

NEAs of both asteroidal and cometary origin are widely believed to be continuously injected into Earth-crossing orbits through a few different resonant channels, which collect fragments randomly ejected from main-belt asteroids as a consequence of energetic mutual asteroid collisions. These objects undergo a fairly complex orbital evolution process, driven by mean motion and secular resonances, by non-resonant secular perturbations and by a sequence of close encounters with planets. Eventually, they mostly fall into the Sun or are ejected from the solar system after a jovian encounter; only a few percent collide with terrestrial planets. To maintain the balance between the mean rate of asteroids loss in Earth-approaching orbits and the rate of their replenishment, source rates must yield a few hundreds of new objects larger than 1 km in diameter per 106 years.

Several efforts have been spent in surveys to increase the number of known Near Earth Objects and good results have been obtained, but till now too large is the number of objects lost after the first identification and too strict is the limit magnitude. It has been shown (Morrison, 1992, Report of the NASA International Near-Earth-Objects Detection Workshop) that a prevalence of small (faint) Earth crossing asteroids is in the opposition and conjunction directions (that is, toward the Sun and away from the Sun). Also a concentration toward the ecliptic, the central plane of the solar system, is expected. Near opposition, and ignoring detection losses other than trailing produced by the apparent motion of the object, about 160 square degrees must be searched to V = 18 to have a 50 percent chance of detecting an Earth crossing asteroid. To detect one Earth crossing asteroid at V = 20 we must search 25 square degrees, and 7 square degrees at V = 22 (Morrison, 1992). Atens pose a special problem because some of them make very infrequent appearances that may occur far from opposition in celestial longitude. The discovery rate could be increased to nearly 60 percent by biasing the search away from opposition.

Using a wide field camera at the first focus on LBT, we could contribute to the discovery (or the re-discovery of lost objects), the list and the follow-up of bodies up to magnitude 24. Moreover it should be possible to have information about their physical properties (and consequently about their cometary or asteroidal origin) from the colours.

next up previous contents
Next: Minor Planets Up: The Scientific Cases for Previous: White Dwarfs
Guido Buscema