In the first story of our NASA series The Washington Post detailed the genesis of the controversial Asteroid Retrieval Mission, later renamed the Asteroid Redirect Mission, or ARM. We called that first article “NASA’s Mission Improbable,” because, for a number of reasons, it is difficult to imagine that this asteroid mission as described by NASA will ever actually happen.

It has just gotten more difficult, as I’ll explain.

Some background: The ARM, announced last spring as part of President Obama’s 2014 budget request, came as a big surprise to much of the space community. The notion is that NASA will build a robotic spaceship that will travel to a small asteroid 7 to 10 meters in diameter, swallow it in a bag-like contraption and haul it back to lunar orbit. Then two astronauts will journey in the new Orion capsule, launched by the new Space Launch System rocket, to rendezvous with the captured asteroid and take samples. NASA said this could happen in 2021 on the first crewed flight of Orion.

This is an elaborate and ambitious mission, and the robotic-capture portion alone would cost $1 billion, never mind the human space flight portion. NASA had several motives in putting this together. First, the president had instructed NASA to send humans to an asteroid. He obviously meant an asteroid in its natural orbit, but NASA lacks the hardware to do that (it would take hundreds of days minimum, and NASA would need to build a new habitat module for the astronauts — something for which there’s no money). NASA administrator Charles Bolden told The Post that a visit to a captured asteroid would meet the president’s goal: “He said humans to an asteroid. There are a lot of different ways to do that. There are probably thousands of ways. I think we have come up with the most practical way, given budgetary constraints today. We’re bringing the asteroid to us.”

The mission had other attractive elements: It could be described as part of the effort to identify potentially hazardous asteroids and develop means for redirecting them. The robotic portion would offer a demonstration of solar-electric propulsion, a NASA priority. Perhaps most important, the bagged asteroid would also provide the new SLS rocket and Orion capsule a destination in space. These are programs that, if carried out, would funnel tens of billions of dollars to aerospace contractors. Right now it’s not entirely clear what NASA would do with the new rocket and capsule (reformers argue that NASA should just go with cheaper, commercial spacecraft being developed by companies such as SpaceX). The default plan for Orion, absent a visit to a captured asteroid, is to go into orbit around the moon, something NASA first accomplished with Apollo 8 in 1968. (The astronauts can’t land on the moon because NASA is not building a lunar lander.)

NASA conducted two feasibility studies — at the Jet Propulsion Laboratory and at Johnson Space Center — and concluded that the mission was technically doable. But there was an important element missing in all this: a target rock. That’s why we began our first story with the sentence, “NASA is looking for a rock.” And it’s why, soon after floating the concept, NASA started saying it was also considering a Plan B, in which it would send a probe to break a chunk off of a much larger, well-characterized asteroid.

There are enormous numbers of rocks out there, and there are some already listed as “candidates” for the mission. But for a candidate to become a true target it has to be characterized and studied. There are lot of requirements. It can’t be spinning too rapidly. It can’t be too big, or else it couldn’t be bagged. And it can’t be too small, because it would look silly to have astronauts climbing around an asteroid the size of a sofa. (You don’t want to bring to mind the miniature-Stonehenge scene in “This Is Spinal Tap.”)

When NASA did the feasibility studies, it had a “reference” rock, an asteroid named 2009 BD that fit the mission requirements in terms of its basic orbital dynamics. No one could be sure that 2009 BD was the right size, though. The plan was to take a closer look this fall with the Spitzer Space Telescope, an infrared telescope that is in an Earth-trailing orbit of the sun and that would be in position in October to examine 2009 BD as it passed nearby.

While at the AGU meeting in San Francisco I learned what happened.

A team of scientists headed by Michael Mommert of Northern Arizona University (Flagstaff) aimed the Spitzer telescope at 2009 BD over the course of 25 hours of observation time.

But saw nothing.

The Spitzer never detected 2009 BD.

The asteroid exists (it didn’t suddenly evaporate, presumably), but it is must be quite small. The scientists [who have submitted to The Astrophysical Journal a paper titled “Constraining the Physical Properties of Near-Earth Object 2009 BD"] think it’s less than five meters in diameter, and perhaps only half that size. That would seem to suggest that it’s too small for the ARM. Nor would you want to build a mission around capturing an object that’s not  carefully characterized (it could be a rubble-pile asteroid, or a smaller, more compact object).

There are other candidate rocks out there, and in February the Spitzer will take a look at one named 2011 MD.

Maybe that one would work. Or maybe NASA will go to its Plan B. But the NASA 2015 budget is currently being put together by OMB, and it’s possible that the ARM could become a casualty of the mission’s uncertainties.

Here’s the official statement provided to me by NASA:

NASA continues ARM concept studies along two major approaches – return of an entire small asteroid or a boulder picked off a larger asteroid.  The studies are exploring the advantages and challenges involved in these two approaches to arrive at a decision on which approach to pursue by Fall 2014.

The viable asteroid targets for the mission will then of course depend on which approach is chosen, but there have already been several potential candidates identified for each approach as proof of existence of a viable target once the decision is made.

One of those potential candidates, a small asteroid designated 2009 BD, was the subject of Spitzer observations last October but not detected, which implies that it is smaller than expected. Meanwhile the NEO Program will continue to search for even better potential targets for the ARM as part of its effort to find asteroids hazardous to the Earth and destinations for future robotic and human exploration.