On this day in 1990 - May 20th - the Hubble Space Telescope sent back its first image, that of several stars, captured with its Wide Field/Planetary Camera.
Unfortunately, they then discovered there was a flaw in the main mirror which meant fuzzy images for three years until the first shuttle servicing mission. Interestingly, the images were still much better than the ones from ground-based telescopes of the time.
On this day in 2008 - May 25th - NASA's Phoenix lander landed on Mars.
Phoenix was expected to spend 90 Martian sols searching for environments suitable for microbial life. It carried out its mission for 157 Martian sols. [A sol is a Martian day - it's about 40 minutes longer than an Earth day.]
On this day in 1961 - May 25th - President John F. Kennedy challenged the USA to "commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to the Earth."