Which satellite first detected ripples in the cosmic microwave background?

COBE, in 1992, which found variations of about one part in 100,000 in the temperature of the microwave sky - the seeds from which every galaxy grew. Stephen Hawking called it the discovery of the century, if not of all time, which was overstating it, and the press ran with a physicist's remark about seeing the face of God.

The measurement is harder than the number suggests. The background is at 2.725 kelvin, and the signal is variation in the fifth decimal place, against foregrounds from our own galaxy, the instrument's own heat, and a much larger apparent variation caused simply by the solar system's motion through the radiation. COBE also showed the spectrum was a near-perfect blackbody, matching prediction so exactly that the plotted error bars were smaller than the width of the curve.

The background itself was found by accident in 1965 by two engineers who spent a year trying to eliminate a hiss in a radio antenna, including evicting a pair of pigeons and scrubbing out their droppings, before learning that a group nearby had predicted exactly this. WMAP and Planck later mapped the ripples far more finely, which is how the universe's age, composition and geometry are now measured to a few per cent.