Solar Sailing Kinetic Energy Interceptor (KEI) Mission for Impacting and Deflecting Near-Earth Asteroids
Prof. Bong Wie
Dept. of Mechanical and Aerospace Engineering
Arizona State University
Tempe, AZ 85287
(480) 965-8674, Fax (480) 965-1384
The spectacular collision of comet Shoemaker-Levy 9 with Jupiter in July 1994 was a clear evidence of the fact that the risk of impacts upon Earth by near-Earth asteroids (NEAs) is very real. It is now widely accepted by scientists and geologists that an impact by a large asteroid of greater than 10 km in diameter caused the extinction of the dinosaurs. A 2-km object is known to be capable of causing catastrophic alteration of the global ecosystem, which may lead to the end of civilization. Ocean impacts of even smaller objects are of some concern because the destructive potential caused by the resulting tsunamis may be above that from a same-size object's land impact. There is also a growing concern that such a devastating impact in the wrong area at the wrong time could be mistaken as a nuclear attack, possibly leading to a nuclear war. The probability of a major impact to cause the extinction of humanity is extremely low, but it is not zero. Unlike many other natural disasters, such as earthquakes, tsunamis, hurricanes, and tornadoes, which cannot be prevented, the threat posed by NEAs can be mitigated given adequate warning time.
A solar sailing mission architecture, which requires at least ten 160-m, 300-kg solar sail spacecraft with a characteristic acceleration of 0.5 mm/s^2, is proposed as a realistic near-term option for mitigating the threat posed by NEAs. Its mission feasibility is demonstrated for a fictional asteroid mitigation problem created by AIAA. This problem assumes that a 200-m asteroid, designated 2004WR, was detected on July 4, 2004, and that the expected impact will occur on January 14, 2015. The solar sailing phase of the proposed mission for the AIAA asteroid mitigation problem is comprised of the initial cruise phase from 1 AU to 0.25 AU (1.5 years), the cranking orbit phase (3.5 years), and the retrograde orbit phase (1 year) prior to impacting the target asteroid at its perihelion (0.75 AU from the sun) on January 1, 2012. The proposed mission will require at least ten kinetic energy interceptor (KEI) solar sail spacecraft. Each KEI sailcraft consists of a 160-m, 150-kg solar sail and a 150-kg microsatellite impactor. The impactor is to be separated from a large solar sail prior to impacting the 200-m target asteroid at its perihelion. Each 150-kg microsatellite impactor, with a relative impact velocity of at least 70 km/s, will cause a conservatively estimated delta-V of 0.3 cm/s in the trajectory of the 200-m target asteroid, due largely to the impulsive effect of material ejected from the newly-formed crater. The deflection caused by a single impactor will increase the Earth-miss-distance by 0.45 Earth radii. Therefore, at least ten KEI sailcraft will be required for consecutive impacts, but probably without causing fragmentation, to increase the total Earth-miss-distance by 4.5 Earth radii. This miss-distance increase (29,000 km) is outside of a typical uncertainty/error of about 10,000 km in predicting the Earth-miss-distance. A conventional Delta II 2925 launch vehicle is capable of injecting at least two KEI sailcraft into an Earth escaping orbit. NASA and industries are currently developing a 40-m solar sail in 10 years for a possible flight validation experiment, and a 160-m solar sail is thus expected to be available in 20 to 30 years.
Furthermore, the proposed mission concept will be significantly enhanced by NASA's Deep Impact mission, which will explore the internal structure and composition of the nucleus of comet Tempel 1 before, during, and after impacts, and return the observations to Earth. The Deep Impact spacecraft was launched on January 12, 2005 and will release a 370-kg impactor spacecraft which will, on July 4, 2005, create a crater approximately 20-m deep and 100-m wide on the surface of the 6-km target comet. The impact velocity will be 10 km/s. The European Space Agency (ESA) is also currently planning a kinetic energy impactor mission (named the Don Quijote mission), targeted for the 500-m asteroid. These kinetic energy impactor missions by NASA and ESA will thus provide significant technological advances required for the proposed solar sailing KEI mission of mitigating the threat of NEAs.