The exosphere is the outermost layer of Earth's atmosphere, extending from about 600 kilometers (370 miles) above the Earth's surface to around 10,000 kilometers (6,200 miles). In this region, the atmosphere is extremely thin, with particles, primarily hydrogen and helium, being so sparse that they can travel hundreds of kilometers without colliding with one another. The exosphere gradually transitions into outer space and plays a crucial role in satellite orbits and the transmission of radio signals.