A key element of any comprehensive mitigation strategy is the ability to deliver a payload to a hazardous NEO, either by means of a rendezvous (e.g., for characterization, for attaching an accurate tracking device, or for applying a slow-push-pull technique to the NEO) or a high-speed approach (e.g., to deliver a kinetic impactor or to deliver a nuclear explosive package to change the orbit). Once an NEO has been identified as hazardous and the time to impact has been determined, the question becomes: Is it technologically possible to act and succeed in preventing an impact on Earth within the time available? The committee notes that the time to design, build, and launch a mission is typically a large fraction (more than half) of a decade, but this time could be shortened with a necessarily expensive “crash program.” The part that is harder to control is the time from launch to arrival at the NEO, which depends on the NEO’s orbit. A second key element, equally important for mitigation either by a gravity tractor or by a kinetic impactor, is the amount of mass that can be delivered to the NEO. This section addresses the issues of mass deliverable to an NEO and the time to reach the NEO after launch. The discussion of developing crash programs is left to the arena of public policy.
NEOs as a group have a very wide range of orbital properties, from nearly circular orbits with orbital periods of about a year, to very elongated orbits with periods from less than a year to decades if the discussion ignores the long-period comets, and to much longer periods if they are included. A complete statistical description of the time to reach an NEO with an orbit anywhere within this distribution is beyond the scope of this study, so only a very small number of examples is considered here. The statistical distribution of the orbits of the NEOs has been studied by Chesley and Spahr (2004), while Perozzi et al. (2002) have considered trajectories to NEOs as well as the deliverable mass. Any optimization of the trajectory to a given NEO would depend on the goal, as well as on the details of the individual orbit. Prior statistical studies will provide a start on this problem, but a detailed study of possible trajectories to any specific NEO will be needed.
The warning time—the length of time from the decision to prevent an impact until the predicted time of impact—is a key parameter. For short warning times, of say a decade, high-speed intercepts may be the only possible choice. For longer warning times, of many decades, one could choose between a high-speed intercept and a rendezvous, depending on the size and physical nature of the NEO.
The key parameters of a launch are the mass that can be launched to escape Earth’s gravity and then the additional velocity that must be provided to put the spacecraft on a trajectory to the NEO of interest. The former is determined entirely by the available launch vehicles, whereas the latter is determined by the details of the orbit of the NEO. (Note, too, that the mass of the fuel required to provide the Earth-escape velocity and this additional velocity will come at the expense of payload mass.) The additional velocity that must be provided is usually characterized by a parameter called C3, which is a measure of this extra propulsion energy needed to change the spacecraft’s trajectory. This quantity can range from almost zero to very many tens of kilometers per second squared for realistic missions. Values of hundreds of kilometers per second squared may be required for some trajectories, but for traditional scientific missions these are not considered feasible. The use of in-space propulsion, such as the engines commonly called solar-electric propulsion or nuclear-electric propulsion, can significantly reduce the mass of fuel that the spacecraft needs at launch but with a cost in time for using in-space propulsion.
Table 5.4 lists the maximum payload in tons that can be carried by various launch vehicles currently available, as well as an estimate of the corresponding capability of the Ares V launcher, which is being developed and might be available for use in the near future. The capability of these launch vehicles is well above the capability assumed nearly a decade ago by Perozzi et al. (2002). The table includes in the first two rows data taken from published literature that provide a starting point, but which in themselves are not directly relevant. The values in the table are for the maximum payloads that can be delivered to a low-Earth orbit (LEO, such as the orbit of the International Space Station) and to a higher orbit that is commonly used as an intermediate step before going to interplanetary space, the geostationary transfer orbit (GTO). The third row lists the mass that can be launched to escape Earth’s gravity, and in the last row shows the mass that can be launched to a relatively easy-to-achieve but realistic orbit that intercepts an NEO.
The differences in Table 5.4 between the corresponding entries in the last two rows—a factor of two—show that even for the NEOs in orbits easiest to reach, the penalty on payload mass is severe. For orbits harder to reach,