The Space Reviewin association with SpaceNews

NEAR at Eros
Some argue that robotic missions, like the NEAR Shoemaker mission to Eros, are sufficient to expand our knowledge of NEOs. (credit: NASA)

Near Earth asteroids: the third option

<< page 1: fear, greed, and curiosity

Planning a mission

While there are a number of good reasons for visiting NEOs, what makes the case for such missions—human in particular—so compelling is the accessibility of these bodies. The proximity of these objects and their small size sharply reduce the delta-v—the change in velocity—and thus the amount of propellant needed to reach them. In many cases, the total delta-v for a NEO mission is less than a mission to the Moon. At a September 2002 conference on mitigating asteroid impact hazards in Arlington, Virginia, Durda described an example of a mission to one NEO, 1991 VG. A round-trip mission lasting just 60 days would require a total delta-v of 6.1 kilometers per second, approximately the same as a one-way mission to the Moon. Extending the mission duration to 90 days decreased the delta-v to 4.9 km/sec.

These factors put manned NEO missions almost entirely within the capacities and experience of human spaceflight today. Durda envisions missions to nearby NEOs lasting 90-120 days, using hardware based on modules developed for the ISS with “a modest investment in new technologies.” The duration of a NEO mission is considerably less than the six months crews are now spending on the ISS, although the station crews have the advantage of regular resupply from Earth. The low surface gravity of asteroids—a one-kilometer body has just a few thousandths the gravity of the Earth—will also remind astronauts of working on the station. “All the EVA experience we have gained on ISS will be applicable here,” Durda said.

Jones estimates a ten-year program consisting of multiple human NEO expeditions would cost less than what’s been spent on the ISS to date.

The new technologies required for human NEO missions are, in large part, items also needed for lunar or Martian missions: improved power systems, including nuclear power; spacesuits that can handle dusty environments, and radiation protection. The extremely low gravity of asteroids would introduce some unique issues, such as the need to anchor ships or even people to the surfaces of these objects. However, Thomas Jones, a former shuttle astronaut with a PhD in planetary science who has worked with Durda on human NEO mission plans, points out in an interview that an asteroid mission would also makes things simpler. “You would not need a beefy lander, just a mobile platform to ferry EVA astronauts over to the asteroid and back,” he said.

The cost of a human NEO mission hasn’t been estimated in any detail, Jones said. He guesses that a NEO program would cost less than the roughly $30 billion that has been spent on the space station program to date. “This amount would be spent over ten years to do not just one, but a series of asteroid round trips,” he said. The same hardware developed for those missions could also be used for lunar missions, he added, with the addition of a lander.


While human missions to NEOs sound compelling and straightforward, they are not without a number of challenges. The asteroid environment will pose a number of challenges, from its near-absence of gravity to rapid rotation rates that create day-night cycles that last just a few hours. “The dynamics of moving around an object will be counterintuitive,” warned Durda. “You can’t just hop from point A to point B.”

Another issue is selling the mission to the public. Virtually everyone is familiar with the Moon and Mars (although the familiarity may be grounded more in fantasy than fact) and thus can grasp, to some degree, the idea of sending people to these worlds. However, outside of a few, largely forgettable movies like Deep Impact and Armageddon, most of the general public is likely unfamiliar with the idea of sending people to asteroids, near Earth or otherwise. In the body of public opinion and knowledge, it would be difficult for a small asteroid, perhaps still carrying an alphanumeric designation like 2000 SG344, to compete with the Moon and Mars.

Durda, speaking at the 2002 asteroid mitigation conference, said that he thinks the public would embrace a human NEO mission, regardless of how obscure the destination, because of the spirit of adventure involved with any human exploration. “I guarantee to you that the first time humans go beyond LEO to a small rock, the public is going to eat it up,” he said.

Perhaps the biggest challenge a human mission faces, though, is selling the need to send humans in the first place. Some advocates of spacecraft missions to NEOs argue that robotic missions can accomplish all the requirements—scientific, threat mitigation, and utilization—for studying these objects for the foreseeable future. Air Force Brigadier General Simon “Pete” Worden, an advocate of asteroid missions in general, was skeptical about the benefits of human missions during a forum on planetary defense last week in Washington organized by the Homeplanet Defense Institute. While calling the concept of human NEO missions “cool”, he warned about the cost and benefit tradeoffs. “My experience with space programs is that whenever you put a person into a mission it is an order of magnitude more cost,” he said. “I can’t imagine that these kinds of things [asteroid missions] would benefit from some kind of human presence.

Durda notes that any human mission would be preceded by robotic reconnaissance, from microsat orbiters to sample return missions. However, human geologists on an asteroid will be far more efficient than their robotic counterparts. “You will never replace the creativity and the intuition of the human mind,” he said. “That is the true benefit of putting humans in the environment.”

Fitting missions into the new plan

For the near term, though, the debate over human expeditions to NEOs appears to be academic. Asteroid missions don’t figure into the president’s new space initiative, at least through 2020. Beyond that, though, advocates believe there is an opening for carrying out these missions as part of preparation for a human Mars expedition. “You can choose either the Moon or asteroids to build up experience for Mars,” said Jones. “Asteroids are natural stepping stones for Mars.”

“You will never replace the creativity and the intuition of the human mind,” Durda said. “That is the true benefit of putting humans in the environment.”

Jones is optimistic that NEO missions can be worked into the overall Bush initiative. “There is nothing in the President’s plan which bypasses the asteroids,” said Jones. “The lunar missions lead naturally to expansion of our experience base to the NEOs. The lunar hardware to be developed can be simply augmented with additional propellant and life support to enable months-long asteroid voyages. One of the major benefits of the President’s plan is that we will be building hardware and capability to take advantage of asteroidal resources and to learn how to work on them.”

Durda, in an interview several weeks before Bush unveiled his new plan, argued that, in the end, it was more important for NASA to establish a new destination-driven program for human exploration. “If we decide to go back to the Moon, I am fully behind it,” he said. Still, he hoped NASA would look for “something new and exciting, something beyond the Moon but not quite as difficult as going to Mars.”

With a new plan in place, but still subject to change by this or a future administration, that opportunity to do “something new and exciting” may still present itself sooner rather than later. “A bonanza of new knowledge, the basis for space commercialization, and planetary protection,” Jones summarized. “NEOs are a convincing combination.”