Obsolescence
MIDAS was at best a qualified success — early problems included mistaking sunlight reflected from clouds as an enemy missile launch. The W-17 infrared sensor proved unable to detect the initial heat plume of a missile through the Earth's atmosphere, and only with the introduction of the W-37 sensor was a launch detected from orbit. Even with this success, the MiDAS system was hampered by unsuccessful launches that destroyed satellites and killed any hope of round-the-clock coverage of the Soviet Union. In addition, the lack of a continuous power source such as a nuclear reactor or solar panels meant that the satellites' batteries were exhausted after a few short weeks in orbit.
Though the MiDAS program itself failed to meet expectations, it paved the way for the eventual introduction of the Defense Support Program system of satellites that were first launched in the 1970s and provide early warning of missile launches today.
Read more about this topic: Missile Defense Alarm System