Scientists’ ability to detect NEOs is dependent on how bright each individual object appears in the sky—which depends primarily on its distance from Earth, its size, its albedo (how well light reflects from its surface), and its location relative to the Sun. The observation of NEOs that appear very close to the Sun when viewed from Earth is difficult or even impossible. The brightness of each NEO also changes as it moves through its orbit, coming closer to and going farther away from Earth. As a result, it is very difficult to detect all NEOs, particularly smaller (fainter) asteroids, in the entire population. Figure 2.3 shows the distribution (in January 2010) of known asteroids in the inner solar system. (Note that the asteroids represented in Figure 2.3 are not all in the same orbital plane, and so it is more accurate to envision some of the objects above the page and some below it. The image is also very misleading in the sense that on this scale, the asteroids would be invisible. The vast majority of the solar system is empty space, but there are nonetheless many objects present.) Of course, while many NEOs have been located, there are many yet to be discovered, some of which may represent a significant threat of impact on Earth. Using estimates of the distribution and orbits of these undiscovered NEOs, the committee can statistically address the hazard posed by NEOs, particularly those that are large enough to cause significant damage should they impact Earth.
To determine what fraction of the entire NEO population has been detected, it is necessary to compute the total expected number of objects from knowledge of the properties of known NEOs and how objects are expected to get brighter and fainter as they and Earth move around their orbits. Using computer models one can determine the fraction of all NEOs of different sizes that will be detected for a particular survey strategy. As surveys approach completion and the knowledge of the NEO population increases, refinements are possible to the computer simulations that allow greater confidence in the predicted numbers of NEOs in each size range. Current estimates (Harris,