Space exploration is dangerous business. Even six decades after the first human orbited the Earth, launching men and women into space is far from a routine exercise. We rely on the most technologically complex machinery ever devised by man. There are countless variables and unforeseen challenges that sometimes have to be solved on the fly, so to speak. We also have to face the harsh and unforgiving laws of physics and nature that cannot be bent to accommodate our lofty whims and fragile bodies.

As with any major discovery of merit, we pay a high price to unlock the mysteries of our universe. In the American space program, 23 men and women have made the ultimate sacrifice, giving their lives so that the rest of us can gain a better understanding of space.

Every year since 2004, on the last Thursday in January, NASA holds a Day of Remembrance at Arlington National Cemetery honoring these brave souls. Family, friends, and colleagues of NASA’s fallen gather to remember those who gave their lives.

Included in the tribute are:

• The crew of Apollo I – Virgil Grissom, Edward White II, Roger Chaffee – who died in a fire during a pre-launch test on January 27, 1967
• The crew of Space Shuttle Challenger – Commander Francis Scobee, Pilot Michael Smith, Mission Specialist Ellison Onizuka, Mission Specialist Judith Resnik, Mission Specialist Ronald McNair, Payload Specialist Christa McAuliffe, Payload Specialist Gregory Jarvis – who died on January 28, 1986 when the shuttle blew up shortly after liftoff
• The crew of Space Shuttle Columbia – Commander Rick Husband, Pilot William McCool, Mission Specialist David Brown, Mission Specialist Laurel Salton Clark, Mission Specialist Kalpana Chawla, Payload Commander Michael Anderson, Payload Specialist Ilan Ramon – who died on February 1, 2003 when the shuttle broke apart during reentry.

The tribute also includes seven members of the astronaut corps who died during training or test flights. The timing of the tribute, though, is directly influenced by the eerie fact that these major accidents all occurred so close to one another on the calendar, even though they were years apart.

NASA’s Day of Remembrance celebrates the heroism and bravery of America’s space explorers. It is also a reminder of the dangers that lay ahead as we plan our return to the Moon and to send people to Mars. As the conquest of space continues, we must never shy away from these dangers. Instead, we should remain curious, eager, and inspired to learn more, do more, and keep pushing the boundaries.

Because that’s what humans do.