Asteroids are found throughout the entire solar system. The most 
accessible objects for rendezvous are located in the vicinity of 
Earth and have low eccentricities and inclinations. 
In other words, these are objects that require a minimum amount of 
fuel to travel to and are within the capabilities of the Discovery 
and New Frontier programs. Power and thermal constraints require the 
target asteroids to approach no closer than 0.8 AU from the Sun and 
travel no further than 1.6 AU from the Sun. 

Size provides an additional constraint. A large fraction of asteroids 
with diameters less than 200 meters rotate as quickly as once every 1 
to 60 minutes. Not only does such rapid rotation greatly increase the 
risk to near-surface spacecraft operations, but also centrifugal 
forces can eject regolith particles from the surface. As a result, 
small asteroids may not possess any loosely bound material for sample 
return.

There are only 27 objects that have low delta-V orbits and are large 
enough to be sample return targets. Of these, a dozen have been 
taxonomically classified, of which five objects are known to be 
carbonaceous. 

It is expected that additional sample return targets will be identified 
in the future. Last year over 800 near-Earth asteroids were discovered 
and the discovery rate should increase by an order of magnitude as new 
surveys such as PanSTARRS and the LSST become operational. The large 
discovery rate of known near-Earth asteroids is not producing a similar 
increase in the number of objects suitable for sample return. The total 
number of known near-Earth asteroids has been steadily increasing since 
the advent of wide-field CCD surveys in the late 1990s. The number of 
accessible asteroids greatly increased between 1996 and 2002. Since 2002 
the increase in accessible asteroids slowed markedly even though the 
capabilities of the surveys have improved (increased sky coverage, deeper 
limiting magnitude, coverage extended to the Southern Hemisphere). 

The cause for the slowdown in the discovery of accessible asteroids is 
due to the great efficiency for finding objects on such orbits. 
Accessible asteroids are located on orbits that do not stray very far 
from the Earth's orbit and once every 3-10 years each object experiences 
a low delta-V encounter with Earth. During the course of these encounters, 
these asteroids are discoverable by existing surveys for many months. As a 
result, most accessible asteroids have been found. Future surveys will 
uncover many fainter objects on accessible orbits but they will be too 
small to consider for easy sample return.

Of the 27 currently known objects on accessible orbits, 16 remain 
unclassified. A program to physically characterize these asteroids has
been established. As a 
result of this program, the number of characterized sample return 
targets will increase by a factor of 2 or 3. The increase in knowledge
will greatly aid the planning of future sample return missions.