Proposals have been made to ?sail? spacecraft to the outer solar system using the pressure of sunlight, or even to propel interstellar spacecraft with high-powered, Earth-based lasers. Sailing spacecraft would need no fuel a great advantage because fuel constitutes much of the initial weight of any space mission. NASA?s Sunjammer, launched in 2015, is the first sailing spacecraft deployed beyond Earth orbit. Figure 29.15 shows what a similar solar sail might look like in space.

A sail capable of propelling a spacecraft to the outer solar system must be able to overcome the Sun?s gravity. Suppose a spacecraft is designed so the force of sunlight on its sail is 20 times that of solar gravity in the vicinity of Earth?s orbit. If the spacecraft reaches Jupiter, some 5 times as far from the Sun as Earth,a. the sail force will still exceed solar gravity, now by a factor of 4.b. the sail force will be slightly less than solar gravity.c. the sail force will now be 25 times solar gravity.d. the sail force will still be 20 times solar gravity.