The orbit of asteroids like APOPHIS is difficult to extrapolate on the long term mainly due to the uncertainties upon the effect of non gravitational forces. The Yarkovsky Effect (YE), which is the main unknown, is a tiny but permanent thrust, the intensity and direction of which are directly related to the nature of the soil, the rotation characteristics and the physical properties of the asteroid. The SHADOW mission we propose would be phased in the following way: 1. as soon as possible, send a probe to rendezvous with APOPHIS and provide the requested data to assess the YE. The probe itself being tracked from the ground, the characterization of the resulting thrust can be achieved after a few months flyby. 2. after assessing the risks of Earth impacts for the coming decades, a decision can be made about the possible mitigation techniques. 3. if the YE is proved to be important, cancelling it will be sufficient to avoid a collision with the Earth. This can be achieved by shadowing and cooling the asteroid with a flotilla of solar shields. We propose to name this strategy YES: Yarkovsky Effect Shadowing.
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