Near-Earth objects (NEOs) are asteroids and comets with orbits that intersect or pass near that of our planet. About 400 NEOs are currently known, but the entire population contains perhaps 3000 objects with diameters larger than 1 km. These objects, thought to be similar in many ways to the ancient planetesimal swarms that accreted to form the planets, are interesting and highly accessible targets for scientific research. They carry records of the solar system's birth and the geologic evolution of small bodies in the interplanetary region. Because collisions of NEOs with Earth pose a finite hazard to life, the exploration of these objects is particularly urgent. Devising appropriate risk-avoidance strategies requires quantitative characterization of NEOs. They may also serve as resources for use by future human exploration missions. The scientific goals of a focused NEO exploration program are to determine their orbital distribution, physical characteristics, composition, and origin.
Physical characteristics, such as size, shape, and spin properties, have been measured for approximately 80 NEOs using observations at infrared, radar, and visible wavelengths. Mineralogical compositions of a comparable number of NEOs have been inferred from visible and near-infrared spectroscopy. The formation and geologic histories of NEOs and related main-belt asteroids are currently inferred from studies of meteorites and from Galileo and Near-Earth Asteroid Rendezvous spacecraft flybys of three main-belt asteroids. Some progress has also been made in associating specific types of meteorites with main-belt asteroids, which probably are the parent bodies of most NEOs. The levels of discovery of NEOs in the future will certainly increase because of the application of new detection systems. The rate of discovery may increase by an order of magnitude, allowing the majority of Earth-crossing asteroids and comets with diameters greater than 1 km to be discovered in the next decade.
A small fraction of NEOs are particularly accessible for exploration by spacecraft. To identify the exploration targets of highest scientific interest, the orbits and classification of a large number of NEOs should be determined by telescopic observations. Desired characterization would also include measurements of size, mass, shape, surface composition and heterogeneity, gas and dust emission, and rotation. Laboratory studies of meteorites can focus NEO exploration objectives and quantify the information obtained from telescopes. Once high-priority targets have been identified, various kinds of spacecraft missions (flyby, rendezvous, and sample return) can be designed. Some currently operational (Near-Earth Asteroid Rendezvous [NEAR]) or planned (Deep Space 1) U.S. missions are of the first two types, and other planned U.S. (Stardust) and Japanese (Muses-C) spacecraft missions will return samples. Rendezvous missions with sample return are particularly desirable from a scientific perspec-
tive because of the very great differences in the analytical capabilities that can be brought to bear in orbit and in the laboratory setting.
Although it would be difficult to justify human exploration of NEOs on the basis of cost-benefit analysis of scientific results alone, a strong case can be made for starting with NEOs if the decision to carry out human exploration beyond low Earth orbit is made for other reasons. Some NEOs are especially attractive targets for astronaut missions because of their orbital accessibility and short flight duration. Because they represent deep-space exploration at an intermediate level of technical challenge, these missions would also serve as stepping stones for human missions to Mars. Human exploration of NEOs would provide significant advances in observational and sampling capabilities.
The Committee on Planetary and Lunar Exploration (COMPLEX) has considered appropriate baseline research efforts, as well as a number of augmentations to existing programs for the discovery and characterization of NEOs. With respect to ground-based telescopic studies, the recommended baseline is that NASA and other appropriate agencies support research programs for interpreting the spectra of near-Earth objects (NEOs), continue and coordinate currently supported surveys to discover and determine the orbits of NEOs, and develop policies for the public disclosure of results relating to potential hazards. Augmentations to this baseline program include, in priority order, that relevant organizations do the following:
- Provide routine or priority access to existing ground-based optical and infrared telescopes and radar facilities for characterization of NEOs during favorable encounters, or
- Provide expanded, dedicated telescope access for characterization of NEOs.
The baseline recommendation with respect to laboratory studies and instrumentation is that NASA and other appropriate agencies should support continued research on extraterrestrial materials to understand the controls on spectra of NEOs and the physical processes that alter asteroid and comet surface materials. An appropriate augmentation to this baseline is to support the acquisition and development of new analytical instruments needed for further studies of extraterrestrial materials and for characterization of returned NEO samples.
Spacecraft missions and the development of the associated technology and instrumentation are essential components of any program for the study of NEOs. The baseline recommendation in this area is to support NEO flyby and rendezvous missions. Appropriate augmentations include, in priority order, the following:
- Develop technological advances in spacecraft capabilities, including nonchemical propulsion and autonomous navigation systems, low-power and low-mass analytical instrumentation for remote and in situ studies, and multiple penetrators and other sampling and sample-handling systems to allow low-cost rendezvous and sample-return missions.
- Study technical requirements for human expeditions to NEOs.
Although studies evaluating the risk of asteroid collisions with Earth and the means of averting them are desirable, they are beyond the scope of this report.