a change to the body's velocity of as little as one centimetre per second will allow it to avoid hitting the Earth. Proposed and experimental deflection methods include ion beam shepherds, focused solar energy and the use of mass drivers or solar sails. Initiating a nuclear explosive device above, on, or slightly beneath, the surface of a threatening NEO is a potential deflection option, with the optimal detonation height dependent upon the NEO's composition and size. In the case of a threatening "rubble pile", the stand off, or detonation height above the surface configuration has been put forth as a means to prevent the potential fracturing of the rubble pile. However, given sufficient advance warning of an asteroid's impact, most scientists avoid endorsing explosive deflection due to the number of potential issues involved. Other methods that can accomplish NEO deflections include: Gravity tractor An alternative to an explosive deflection is to move a dangerous asteroid slowly and consistently over time. The effect of a tiny constant thrust can accumulate to deviate an object sufficiently from its predicted course. In 2005 Drs. Ed Lu and Stanley G. Love proposed using a large, heavy unmanned spacecraft hovering over an asteroid to gravitationally pull the latter into a non-threatening orbit. The method will function due to the spacecraft's and asteroid's mutually gravitational attraction. When the spacecraft counters the gravitational attraction towards the asteroid by the use of, for example, an ion thruster engine, the net effect is that the asteroid is accelerated, or moved, towards the spacecraft and thus slowly deflected from the orbital path that will lead it to a collision with Earth. While slow, this method has the advantage of working irrespective of an asteroid's composition. It would even be effective on a comet, loose rubble pile, or an object spinning at a high rate. However, a gravity tractor would likely have to spend several years stationed beside and tugging on the body to be effective. The Sentinel Space Telescope's mission is designed to provide the required advance lead time. According to Rusty Schweickart, the gravitational tractor method also has a controversial aspect because during the process of changing an asteroid's trajectory, the point on Earth where it would most likely hit would slowly be shifted temporarily across the face of the planet. It means the threat for the entire planet might be minimized at a temporary cost of some specific