A lunar landing makes field science possible

The satellites recorded the first known cosmic gamma-ray burst; its extraterrestrial source was recognized later.

The gamma-ray signal was initially detected by instruments built to monitor nuclear explosions.

The detection later helped establish the study of transient cosmic gamma-ray sources.

The recorded burst had no precise sky position at detection; its meaning became clear only with later analysis.

Armstrong and Aldrin landed in the Sea of Tranquility while Collins remained in lunar orbit.

Michael Collins piloted the command module while the lunar module descended to the surface.

The landing was a milestone in human exploration of the solar system; the first steps took place later in UTC on 21 July.

The spacecraft became the first human-made object to orbit another planet.

Engine firing slowed Mariner 9 enough for capture by Martian gravity.

The new orbit enabled prolonged planetary study, although a global dust storm initially limited surface views.