The solar wind is a continuous stream of particles, mostly protons and electrons, moving away from the Sun. When the particles encounter Earth's upper atmosphere, they are moving at speeds of several hundred km/s. If such a particle reaches Earth's upper atmosphere with a speed of 92 km/s, how fast was it moving when it crossed Mercury's orbit at a distance of 5.8 ´ 107 km from the Sun? (HINT: Assume that the particles are slowed by the Sun's gravitational field as they move away from the Sun and ignore any increase the particles experience from the Earth's gravitational force.)