As we set our sights on traveling to Mars, it's important to remember the legacies of those astronauts who lost their lives in the service of discovery. Their sacrifices ultimately make future astronauts' journeys safer, and even though these disasters occurred years ago, the losses are no less poignant today.

Perhaps President Obama expressed this sentiment best when he spoke during the 2013 Day of Remembrance, which marked the 10-year anniversary of the loss of space shuttle Columbia, "As we undertake the next generation of discovery, today we pause to remember those who paid the ultimate sacrifice on the journey of exploration. Right now we are working to fulfill their highest aspirations by pursuing a path in space never seen before, one that will eventually put Americans on Mars."

To keep the fallen astronauts in mind, NASA commemorates all lost astronauts every year. This year, NASA's Day of Remembrance is marked today (Jan. 28), the 30th anniversary of the Challenger tragedy. That explosion took the lives of Christa McAuliffe, Gregory B. Jarvis, Judith A. Resnik, Francis R. Scobee, Ronald E. McNair, Michael J. Smith and Ellison S. Onizuka.

NASA's Day of Remembrance always falls at the end of January or the beginning of February because all three disasters occurred in this window. Apollo 1 was lost on Jan. 27, 1967, claiming the lives of Virgil Grissom, Edward White and Roger Chaffee. Columbia broke apart on Feb. 1, 2003, killing Rick D. Husband, William C. McCool, Michael P. Anderson, Kalpana Chawla, David M. Brown, Laurel Clark, and Ilan Ramon.

The sense of determination that drives space exploration is a recurring theme when presidents speak about them. When then-President George W. Bush addressed the nation on the day of the Columbia tragedy, he said, "The cause in which they died will continue. Mankind is led into the darkness beyond our world by the inspiration of discovery and the longing to understand. Our journey into space will go on."

Ronald Reagan's address to the nation on the day of the Challenger tragedy, punctuated by his famous quotation from the poem "High Flight," reinforced these sentiments.

The idea of the continued journey in spite of tragedy speaks to the kind of person willing to put his or her life at risk. This is what makes astronauts such important figures. We have a lot to thank astronauts for. Their work in space influences our life on Earth. The risks they take make them role models for children and inspirational figures for the rest of us. Astronauts are made of "the right stuff" our culture craves. Just think of the young girls who may see themselves in female astronauts and enter into a STEM field to achieve that goal.

Space travel unites us. When you watch a control room during a mission, you can see the communal sense of anticipation followed by sheer joy when the mission is a success. The euphoria of the 2012 landing of the Curiosity Rover on Mars was a great example — and we celebrated with them.

The shared emotion applies to tragedies, too. Space exploration sums up what it is to be human: to wonder and to dream. NASA's Day of Remembrance reminds us to acknowledge the sacrifices of those who risked their lives getting us where we are now — planning for a manned journey to Mars.

This year, NASA will mark the Day of Remembrance with wreath-laying ceremony at the Space Mirror Memorial at the Kennedy Space Center. The Marshall Space Flight Center will also commemorate the date with a candle-lighting ceremony. At the Johnson Space Center, flowers will be placed at the Apollo, Challenger and Columbia Trees. NASA will be providing live coverage of the events, and the agency has also set up a webpage that will be updated as the day plays out.

Let's join NASA in saluting those fallen astronauts who have "slipped the surly bonds of Earth."