With the impending departure of Lori Garver as NASA’s deputy administrator, the Obama administration has lost the most effective advocate for its latest human spaceflight initiative: a mission to capture an asteroid and haul it into lunar orbit for closer inspection by astronauts.

Proposed in April, the Asteroid Redirect Mission (ARM) was greeted with skepticism and has been losing ground ever since. Legislation drafted this summer in the House would bar NASA from spending any money on the concept next year, and while that measure might be chalked up to a Republican majority reflexively opposed to all things Obama, support in the Democrat-controlled Senate appears lukewarm at best. 

The scientific community — which is not entirely impartial when it comes to human spaceflight but probably is as close as it gets — seems to have little use for the idea. Of course, scientists aren’t very enthusiastic about returning to the Moon either, which is where the ARM’s political opponents think NASA should be headed.

NASA, for its part, has tried different tacks to build support for the idea. One was that the ARM would equip the nation to detect and, if necessary, divert a killer asteroid. In July, Ms. Garver, apparently prompted by criticisms that the 7- to 10-meter-diameter rock targeted by the ARM would not pose a huge threat to Earth, floated the idea of visiting a larger asteroid, breaking off a piece and bringing that in for closer inspection. 

That was a desperate-sounding stretch, and might well have been the last gasp for the planetary defense argument. NASA Administrator Charles Bolden in recent weeks has backed away from the idea. 

Mr. Bolden has tried to make the case that the ARM could fulfill the president’s goal, spelled out in an April 2010 speech, of sending humans to an asteroid by 2025. It’s a reverse take on the reality-based proverb that says if the mountain won’t come to Mohammed then Mohammed must come to the mountain. At this point, however, it’s by no means clear that any human-asteroid rendezvous is achievable on the schedule laid out by the president.

Perhaps the most logical reason to do the ARM, articulated recently by Ms. Garver, is that it’s the only plausibly affordable concept for deep-space exploration that can leverage the heavy-lift Space Launch System and Orion capsule that Congress has directed NASA to build. These vehicles, whose development is costing nearly $3 billion annually, have no other place to go under any realistic budget scenarios for NASA. 

Of course, the ARM-by-default argument rests on the assumptions that NASA can reliably identify a suitable target asteroid and then pull off the mission for $2 billion to $3 billion, both of which are questionable. Either way, it leaves a lot to be desired in the inspiration department.

One has to wonder whether the task of defending the ARM played a role in Ms. Garver’s decision to leave the space agency, or at least its timing. Even for someone who spearheaded the privatization of NASA crew transport to and from the international space station — overcoming fierce resistance in the process — selling the asteroid capture mission was bound to be a monumentally difficult, if not impossible, task.

NASA’s human spaceflight program, meanwhile, remains caught between two opposing visions, neither of which appears grounded in reality. As this sad theater of the absurd plays on, U.S. political support for human spaceflight outside the states that directly benefit from the activity will erode, perhaps even to the point of collapse. The issue then will be less about where to send human explorers than whether to have such a program in the first place.