Thursday marks the 30th anniversary of the loss of the space shuttle Challenger and its crew of seven. Since then, the United States lost another space shuttle, Columbia, in 2003, and terminated the shuttle program altogether in 2011.
It is easy to forget that the shuttle was created as the next logical step toward a manned Mars mission. The shuttle was to fly safely, reliably and economically to a space station, from which a human mission would then depart for the Red Planet.
The end of the space station is also approaching, but still NASA and many of its supporters remain focused on Mars.
This fascination with sending humans to Mars began in the early 1960s, when the musicals "Camelot" and "Man of La Mancha" stoked the American imagination. In 1961, President John F. Kennedy, our King Arthur, challenged the wizards at NASA to land men on the moon. When those wizards completed that quest, they wanted to reprise the achievement by sending humans to Mars.
That was almost 50 years ago, and the impossible dream seems just as remote now as it did when first proposed. Why have we been trying for so long and making so little progress -- Matt Damon's recent Hollywood foray notwithstanding?
There are two main reasons. First is cost. Estimates for sending humans to Mars range from $100 billion to $1 trillion.
Space enthusiasts pine for the halcyon days of Apollo, when peak funding for NASA reached 4.4 percent of the federal budget. But they fail to note that this would now amount to about $180 billion a year, or almost 10 times NASA's current budget. And NASA already spends more on space than the rest of the world combined.
Presidents have occasionally paid lip service to a manned Mars mission as a long-term objective. Not since Kennedy, however, has a president been willing to pay for it. In fact, even Kennedy tried to back away from his commitment when he realized the impact on more pressing national needs.
In his re-election year of 2004, George W. Bush launched the Constellation Program to land humans on Mars, but he never gave it a fraction of the funding it needed to succeed.
The second great reason why no humans are on Mars or headed there any time soon is the lack of a compelling vision of what they might do when they arrive. The most futuristic visionaries imagine colonies on Mars, but the cost of such an expedition dwarfs the cost of just getting there. When we first put people on the moon, we could find no justification for colonies there, or even for return visits. Mars is vastly more difficult and expensive to reach than the moon, more hostile to human habitation and less useful to humans back on Earth.
Mars exploration can be done better by automated machines controlled from Earth. They can stay longer, roam farther, and do more work than humans at a fraction of the cost and risk. The impact on the earthbound human psyche of a manned Mars mission will be no greater than that of the moon landings, which we now take for granted.
Yet, there is one argument for a human mission to Mars that blends the romantic vision with a practical payoff. Instead of simply getting to Mars, the United States might well lead an international consortium of public and private entities to collaborate on a Mars mission.
Some cooperation is already underway. The European Space Agency, for example, has delivered an initial module to support NASA's Mars spacecraft. And several companies -- most notably Elon Musk's SpaceX -- are demonstrating how the private sector can develop innovative new technologies to lower the cost of spaceflight.
Drawing the world's spacefaring nations and aerospace industries into a collaborative enterprise would be a heroic undertaking in its own right -- well worth doing no matter when or even if it succeeds. After all, the moon landings of the 1960s and 1970s never paid off technically or scientifically, but they paid off in the Cold War demonstration of America's technical prowess. Getting there was more important than being there.
The same could be true of Mars. Such a collaboration, unprecedented in human history, is really an impossible dream for which the world would be better by far.
Alex Roland is an emeritus professor of history at Duke University and a former NASA historian. He wrote this for The Philadelphia Inquirer.