Astronomical engineering: A strategy for modifying planetary orbits

The Sun's gradual brightening will seriously compromise the Earth's biosphe re within similar to 10(9) years. If Earth's orbit migrates outward, howeve r, the biosphere could remain intact over the entire main-sequence lifetime of the Sun. In this paper, we explore the feasibility of engineering such a migration over a long time period. The basic mechanism uses gravitational assists to (in effect) transfer orbital energy from Jupiter to the Earth, and thereby enlarges the orbital radius of Earth. This transfer is accompli shed by a suitable intermediate body, either a Kuiper Belt object or a main belt asteroid. The object first encounters Earth during an inward pass on its initial highly elliptical orbit of large (similar to 300 AU) semimajor axis. The encounter transfers energy from the object to the Earth in standa rd gravity-assist fashion by passing close to the leading limb of the plane t. The resulting outbound trajectory of the object must cross the orbit of Jupiter; with proper timing, the outbound object encounters Jupiter and pic ks up the energy it lost to Earth. With small corrections to the trajectory , or additional planetary encounters (e.g., with Saturn), the object can re peat this process over many encounters. To maintain its present flux of sol ar energy, the Earth must experience roughly one encounter every 6000 years (for an object mass of 10(22) g). We develop the details of this scheme an d discuss its ramifications.