[meteorite-list] Targets Scarce for NASA's Asteroid-Capture Mission

[Ron Baalke]

http://www.space.com/21608-nasa-asteroid-capture-mission-targets.html?cmpid=514648

Targets Scarce for NASA's Asteroid-Capture Mission
by Mike Wall
space.com
18 June 2013

NASA's audacious plan to grab an asteroid and park it near the moon is 
short on candidate space rocks at the moment, one researcher says.

It's possible that not a single known object meets the current criteria 
of NASA's asteroid redirect mission (ARM), astrodynamical consultant Dan 
Adamo said during a presentation  with NASA's Future In-Space Operations 
working group on Wednesday (June 12).

To pull the mission off in a timely fashion, Adamo added, it may be necessary 
to mount a dedicated, space-based asteroid survey in the near future or 
consider snagging a chunk of a larger asteroid rather than returning an 
entire (relatively small) space rock. 

Capturing an asteroid

NASA unveiled the asteroid-retrieval mission in April. The current plan 
is to drag a roughly 500-ton, 23-foot-wide (7 meters) asteroid to a stable 
orbit near the moon using an unmanned probe.

Astronauts would then visit the object using NASA's Orion capsule and 
Space Launch System rocket, which are slated to fly together for the first 
time in 2021.

The ARM concept fits within President Barack Obama's vision for NASA's 
manned exploration program, which calls for the agency to send astronauts 
to a near-Earth asteroid (NEA) by 2025, then on to the vicinity of Mars 
by the mid-2030s.

ARM is similar to an idea proposed last year by scientists based at Caltech's 
Keck Institute for Space Studies in Pasadena, Calif. The Keck study estimated 
that a robotic spacecraft could drag a 23-foot NEA into a high lunar orbit 
for $2.6 billion.

Such a mission would help develop asteroid-mining technology and advance 
scientists' understanding of the early solar system, advocates say. Capturing 
an asteroid could also have big returns in the manned exploration arena.

"Experience gained via human expeditions to the small, returned NEA would 
transfer directly to follow-on international expeditions beyond the Earth-moon 
system - to other near-Earth asteroids, [the Mars moons] Phobos and Deimos, 
Mars and potentially, someday to the main asteroid belt," the Keck team 
wrote in a feasibility study of their plan. 

Scarce targets

Scientists think at least 1 million asteroids zip through Earth's neighborhood, 
but only 10,000 or so of these close-flying space rocks have been identified 
and catalogued to date.

Adamo scrutinized the known objects, looking for potential ARM targets. 
He first zeroed in on bodies with "sufficiently Earthlike" orbits - nearly 
circular paths in much the same plane as Earth, with an average separation 
from the sun within 20 percent of the Earth-sun distance (which is about 
93 million miles, or 150 million kilometers, and is known as an astronomical 
unit, or AU).
Adamo then excluded any asteroids in this group that would zoom past Earth 
at too great a speed - more than 4,475 mph (7,200 km/h) relative to Earth 
- and came up with a list of 18 "highly accessible" space rocks.

But some of these 18 are definitely too big for ARM as it's currently 
envisioned, and many others may not fit the size criterion (estimates 
of the space rocks' diameters are generally imprecise).

Further, most of these rocks won't approach Earth closely enough in the 
near future to be viable targets. In fact, just seven of the 18 asteroids 
will fly within 0.1 AU of Earth before 2030, and only four will do so 
before 2021, Adamo said. (This 0.1 AU threshold provides a good rule of 
thumb to help narrow down the candidate field, he explained.)

In short, the cupboard is looking pretty bare at the moment, with no slam-dunk 
asteroid targets just begging to be captured.

Two strategies could help give ARM the best chance to succeed in the near 
future, Adamo said.

The first is to find more potential targets, by launching an asteroid-hunting 
space telescope as soon as possible. The second idea involves modifying 
ARM to retrieve a chunk of a big asteroid - a 100-ton piece of a 330-foot 
(100 m) object, perhaps - rather than an intact small space rock.

There are several advantages to targeting larger near-Earth objects (NEOs), 
Adamo said. They're easier to find and track, for example, and are more 
likely to exhibit stable and slow rotation.

"Larger NEOs are just going to be easier to approach and get close to," 
Adamo said.

Bigger asteroids are also more likely to be compositionally diverse, offering 
a probe several different sampling targets, he added.

"You're like a kid in a candy store instead of just out there with one 
object, as the current concept would advocate," Adamo said.

However NASA decides to proceed with ARM, the agency will need to work 
quickly if it hopes to execute the bold mission in the next decade or 
so, Adamo said. That's especially true if NASA aims to launch a precursor 
mission to verify the suitability of an intended target.

"This is a compressed schedule any way you slice it, particularly if you 
want some kind of mission assurance," Adamo said.