A series of UK scientific satellites launched by NASA between 1962 and 1979. Ariel 1, the first UK satellite, studied solar X-rays and the Earth’s outer atmosphere. The next three studied the atmosphere and radio astronomy. The longest-lived mission was the X-ray astronomy satellite Ariel 5, which operated from 1974 to 1980. Ariel 6, the last UK satellite, was primarily concerned with cosmic rays but also carried X-ray instruments.

http://heasarc.gsfc.nasa.gov/docs/ariel5/ariel5.html Information page at Goddard Space Flight Center