Nuclear Thermal Propulsion for Human Explorationand Potential Threat Mitigation of Near Earth Objects

摘要

High thrust / high specific impulse (Isp) nuclear thermal propulsion (NTP) has been identified as a key technology that can enhance or enable a variety of future NASA missions that include outer planet robotic science and crewed missions to the Moon, near Earth asteroids (NEAs), and eventually Mars. Candidate "1-year" round trip human NEA missions have been identified (1991 JW in 2027) that can provide valuable scientific data on the chemical composition of these near Earth objects (NEOs) important for determining the viability of extraterrestrial resource utilization and for designing NEO intercept / diversion missions should such objects pose a future threat to Earth. High velocity (~20-60 km/s) impacts of kilometer size NEAs, short period (SPCs) and long period comets (LPCs) with Earth can deliver tremendous kinetic energies (measured in 1000's of megatons (MT) of TNT) that can destroy land areas the size of small-to-moderate states. Using the heavy lift launch vehicle capability (~130 t) being proposed by NASA for human lunar return missions, ~5-20 t nuclear payloads (with yield-to-weight ratio of ~1 kiloton per kg) can be delivered at high intercept velocities (-12.3-9.3 km/s) for close approach NEO deflection using NTP. With 5, 10 and 20 t payloads, a 1-km diameter stony NEA (with p~3000 kg/m~3) traveling at ~20 km/s can be deflected by an Earth radii (R_E = 6378 km) if detected at ~2.82, 1.49 and 0.84 AU from Earth, respectively. With the same NTP-injected 20 t payload, 20 km/s NEAs with diameters of ~840 and ~750 m can be deflected even when detected at distances from Earth as small as 0.5 and 0.358 AU, respectively. Long period comets (p-2000 kg/m~3) can impact Earth with significantly higher velocities and require detection at greater distances for deflection to be successful. For a 1-km LPC traveling at 45-60 km/s, the corresponding detection range from Earth is ~2.34-3.98 AU using the same 20 t payload. NTP systems allow a viable response / NEO intercept capability even when the detection range is small (~1 AU or less) and response times are short. NTP may also be the only option available to deflect high velocity LPCs if their detection range is limited to ~4 AU from Earth! Smaller size NEOs (~150 m) might use the "burn-out" mass of the NTP intercept stage itself for kinetic energy deflection at detection distances < 1 AU.