Solar Sail Missions for Planetary Protection
- Paper number
IAC-07-A3.4.07
- Author
Dr. Bernd Dachwald, German Aerospace Center (DLR), Germany
- Coauthor
Dr. Ralph Kahle, German Aerospace Center (DLR), Germany
- Coauthor
Prof. Bong Wie, United States
- Year
2007
- Abstract
Two solar sail concepts are proposed as feasible options to deflect a near-Earth asteroid that is on collision course with Earth. Solar sails are large lightweight reflectors in space that transform the solar radiation pressure into a propulsive force. This propellantless propulsion system enables missions that are prohibitive for other propulsions systems due to their large Δ V-requirement. Our first mission concept uses a solar sail Kinetic Energy Impactor (KEI) that impacts the asteroid with a very high relative velocity of about 75-80 km/s from a retrograde orbit. Our second mission concept is a solar sail Gravity Tractor (GT) that uses the mutual gravitational attraction of the sailcraft and the asteroid as a towline. Our exemplary target is 99942 Apophis, which provides a typical example for the evolution of asteroid orbits that lead to Earth-impacts after close Earth-encounters that result in resonant returns. Apophis will have a close Earth-encounter in 2029 with potential very close subsequent Earth-encounters (or even an impact) in 2036 or later, depending on whether it passes through one of several less than 1-km sized gravitational keyholes during its 2029-encounter. The current (Feb 2007) probability for an Earth-impact in 2036 is 1/45,000. For our KEI-concept, a pre-2029 kinetic impact to nudge the asteroid out of a keyhole is the most favourable deflection option. The highest impact velocity and thus deflection can be achieved from a trajectory that is retrograde to Apophis’ orbit. With a chemical or electric propulsion system, however, many gravity assists (e.g. EVVEEVVEVEJSJ) and thus a very long time (about 30 years) is required to achieve this. As we show in this paper, the solar sail might be the better propulsion system for such a mission. The spacecraft consists of a 160-m x 160-m, 168-kg solar sail assembly and a 150-kg impactor ( a c = 0.5 mm/ s 2). 4.4 years would be necessary to make the orbit retrograde and another 1.6 years to impact Apophis during its 2026 perihelion passage. This would yield a deflection of more than 100 km in 2029. Although conventional spacecraft can also achieve the required minimum deflection of 1 km for this approx. 270-m sized object from a prograde trajectory, our solar sail KEI concept also allows the deflection of larger objects and objects that do not pass through a keyhole before Earth-impact. In this case, however, many KEIs would be required for consecutive impacts to increase the total Earth-miss distance to a safe value. An asteroid impact is most effective when the asteroid is composed of solid rock. As the Hayabusa mission has shown, however, probably many asteroids are rather rubble-piles than solid bodies. For this type of asteroids, a GT, as it was proposed by Lu and Love in the 10 Nov 2005 issue of Nature, might be the better option. Our proposed solar sail GT is based on their general concept but employs a solar sail instead of ion engines, which has the advantage of longer mission lifetimes and no concern of rocket plume impingement on the asteroid surface. A 5-year towing of Apophis using a 2500-kg solar sail GT spacecraft (equipped with a modest 90 m x 90 m, 50-kg solar sail) and an additional 3-year coasting time will be more than sufficient to move Apophis out of a keyhole.
- Abstract document