Complex sentence: Their aim was to reach a height of thirty km, beating the world record for high-altitude flight, and they were equipped with a Geiger-Mueller tube telescope, intending to measure the variation in intensity of cosmic rays at different altitudes.
Split into simpler sentences: Their aim was to reach a height of 30 kilometres, beating the world record for high-altitude flight. They were equipped with a Geiger-Mueller tube telescope, intending to measure the variation in intensity of cosmic rays at different altitudes.