Complex sentence: The spacecraft's speed increases as it approaches the planet and decreases while escaping its gravitational pull (which is approximately the same), but because the planet orbits the Sun the spacecraft is affected by this motion during the maneuver.
Split into simpler sentences: The spacecraft's speed increases as it approaches the planet and decreases while escaping its gravitational pull (which is approximately the same). Because the planet orbits the sun, the spacecraft is affected by this motion during the maneuver.