Thousands of near-Earth asteroids (NEAs) circle us from almost every angle. But null
Because a portion of these whirling planetary dervishes represent the most primitive and un-evolved bodies in our solar system’s 4.56 billion year history, they can still tell us loads about our solar system’s earliest formation history.
But to be fair, from the ground, they are some of the most difficult objects in the solar system to study. And to date, there’s only been one sample return mission to a nearby asteroid.
That should change over the next two decades, as both NASA and Japan prepare new NEA sample return missions — NASA’s OSIRIS-REx due for launch in 2016 and the Japanese Space Agency’s (JAXA) Hayabusa-2 due for launch later this year. Meanwhile, the painstaking work of characterizing suitable future NEA targets for such robotic missions continues.
To that end, a forthcoming paper in the journal Astronomy & Astrophysics, details observations that for the first time taxonomically characterizes nine near-Earth asteroids, two of which appear to be of the most primitive carbonaceous type and arguably worthy of a visit by a robotic spacecraft.
As part of an ongoing NEA survey, using visible and near infrared spectroscopy taken with telescopes in Chile, Hawaii and the Canary Islands, the team was particularly interested in classifying asteroids with delta-Vs of less than 10.5 km/s. (The delta-V is simply the amount of effort needed to change a spacecraft’s trajectory from one orbit to another; in this case from low-Earth Orbit to a rendez-vous trajectory with an asteroid).
Thus, low delta-V targets can help save on mission costs, says Simone Ieva, the paper’s lead author and an astronomer at both the Paris and Rome Observatories. Ieva notes that the lower the delta-V, the less the fuel consumption and the lower the cost of the mission.
Ieva says that the two most scientifically interesting objects characterized in the survey were both carbonaceous — (13553) Masaakikoyama and 203471 (2002 AU4), simply because they have been less altered since their formation early in the history of our solar system.
But observing them from Earth isn’t good enough anymore.
“To get good ground-based observations is really pretty hard,” said Kevin Walsh, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who wasn’t involved with the reported observations. “There aren’t many objects that are big enough to study in great detail that would actually be good space mission targets.”
And as Ieva notes, robotic missions to such objects remain the next step. For the most part, he says, these near-Earth asteroids are easy to reach on budgets that are palatable to most of the world’s major space agencies.
What’s needed is a sample return mission from a C-type asteroid. To date, the one and only sample returned from a near-Earth asteroid was done by the Japanese Space Agency’s Hayabusa spacecraft.
“But that was a more evolved, silicate asteroid,” said Ieva.
However, in 2023, OSIRIS-REx will bring back a 2.1 ounce sample from Bennu (formerly 1999 RQ36); a primitive carbonaceous type asteroid of the sort that may have been important in seeding earth with water and organics.
By studying these NEAs, Walsh says, in some ways they are seeing a reflection of leftover building blocks [from the reservoir of objects] that built the terrestrial planets.
“By doing sample return missions from these objects, we can look back in time at how solids first formed and came together to build asteroids and then the planet that we live on,” said Walsh, who is a collaborator on the OSIRIS-REx science team.
With current rocket technology, it’s unlikely we would be able to first send a robotic survey mission and then follow up with a human mission says Hal Levison, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who also wasn’t involved with the observations.
In other words, the objects now being surveyed robotically are not likely to be accessible to a follow-on manned mission simply because spacecraft capable of carrying humans for such a rendez-vous can’t reach the necessary velocities. Thus, the best NEA candidates for manned missions require a very low delta-V.
As for eventually commercially mining such objects?
As Levison notes, some rare elements and minerals are more common on asteroids than at a ratio of less than 1000 to one in Earth’s crust.
“So, if you’re bringing the stuff back to earth, I’ve never understood the argument for why mining asteroids could be economically advantageous,” said Levison. “It seems to me a hell of a lot easier to mine a thousand tons of material here on earth than one ton of material in space.”
Even so, more than 85 percent of the estimated 11,000 near-Earth objects remain uncharacterized. However their compositions are thought to run the gamut and include everything from melted basalts to metals.
“We’re just beginning our reconnaissance of these populations of objects,” said Levison.