Shoemaker and Hayabusa have provided detailed information on two vastly dissimilar NEOs. NASA’s NEAR Shoemaker spacecraft visited one of the largest NEOs, Eros, in February 2000; the Japan Aerospace Exploration Agency’s Hayabusa probe rendezvoused with the subkilometer-sized asteroid Itokawa in September 2005. Both of these robotic missions generated much scientific interest in NEOs and revealed many intriguing surprises and new paradigms for asteroid scientists to consider. It is now apparent from just these two missions, and the suite of ground-based optical and radar observations of NEOs, that NEOs have a much wider range of internal structures, more diverse physical conditions, and more complex surfaces than had previously been realized.
Essential physical properties relevant for the mitigation of NEOs are best determined from dedicated spacecraft missions. Although ground-based observations can provide significant information about the physical properties of NEOs (e.g., rotation rates, size estimates, and composition), dedicated spacecraft missions to NEOs providing extended periods for observations and investigation close to NEOs obtain detailed characterizations of their rotational motions, masses, sizes, shapes, surface morphology, internal structure, mineral composition, and collisional history. The data collected from NEO characterization missions would also help to calibrate the ground- and space-based remote sensing data and may permit increased confidence in the remote classification of NEOs and their associated physical characteristics, which could inform future mitigation decisions.
Flyby missions are not well suited for these detailed types of investigations because of the limited time for performing observations during the spacecraft encounter. To attain the required details of an NEO’s physical characteristics for hazard mitigation, much more time must be spent near the NEO than is possible in a flyby in order to operate instruments making gamma-ray, x-ray, and other compositional measurements. Constraints on some surface characteristics and on the object’s mass can be obtained, but the uncertainties on the NEO’s physical properties obtained from a flyby encounter are far too large to be useful for hazard mitigation purposes. Such missions may be suitable for basic reconnaissance of the NEO population, but overall the data return relevant to mitigation is low relative to cost.
Continued efforts to obtain characterization data from ground-based studies are desirable, and spacecraft observations of representative NEOs are very important. Spacecraft characterization of any NEO for which orbit change is to be attempted is essential (see Chapter 5).
Finding: Dedicated flyby spacecraft missions to NEOs provide only limited information relevant for hazard mitigation issues.
Finding: Rendezvous spacecraft missions can provide detailed characterization of NEOs that could aid in the design and development of hazard mitigation techniques. Such in situ characterization also allows the calibration of ground- and space-based remote sensing data and may permit increased confidence in the use of the remote classification of NEOs to inform future mitigation decisions.
During its deliberations, the committee was briefed on the possibilities of human missions to near-Earth objects. This subject also received attention during meetings of the Human Space Flight Review Committee and was mentioned as part of its “Flexible Path” option in its final report.
In the future, NASA’s Exploration Systems Mission Directorate may conduct human missions to one or more near-Earth objects. The committee identified no cost-effective role for human spaceflight in addressing the hazards posed by NEOs. However, if human missions to NEOs are conducted in the future, the committee recommends that their scientific aspects be maximized to provide data useful for their characterization.
Recommendation: If NASA conducts human missions to NEOs, these missions should maximize the data obtained for NEO characterization.