23 December 2010

Experts Push for a NASA Asteroid-Hunting Spacecraft


Experts Push for a NASA Asteroid-Hunting Spacecraft
By Leonard David
SPACE.com’s Space Insider Columnist
posted: 21 December 2010
04:12 pm ET
NASA needs an asteroid-hunting spacecraft to finally get serious about the potential threat of nearby space rocks that could slam into Earth, experts say. Lately, support is building to finally develop such a mission for both safety and scientific reasons.

An asteroid hunter might take the form of an infrared imaging telescope placed in a Venus-like orbit around the sun. This high-tech spotter scope could view a much larger portion of the sky for possible asteroid threats than could observatories from the Earth.

Such a mission could also provide a rapid means of compiling an inventory of viable Near-Earth Object (NEO) targets for potential human exploration – now on NASA's to-do list as called for by President Barack Obama.

Moreover, a dedicated NEO-studying spacecraft could help humanity finally come up with a plan for how to thwart ominous NEOs on track to smack our fragile world.

Census mandate

NASA already has a congressional mandate to catalogue nearby space rocks.

Named after the late congressman, the George E. Brown, Jr., Near-Earth Object Survey section of the 2005 NASA Authorization Act called upon the space agency to detect, track, catalogue and characterize the physical characteristics of at least 90 percent of potentially hazardous NEOs larger than roughly 460 feet (140 meters) in diameter by the end of the year 2020.

But blue-ribbon panels of experts looking into the matter for the National Research Council reported in back-to-back reports in 2009 and 2010 that a lack of cash and political muscle make it "infeasible" that such a NEO census can be accomplished by 2020.

"If we seriously want to find all the asteroids which could be an impact hazard to the Earth, as well as find the asteroids which would be good destinations for human spaceflight, then a space-based survey telescope in solar orbit interior to Earth's would be the most rapid way to do that," NASA's Lindley Johnson told SPACE.com. Johnson is the space agency's NEO Observations Program Executive in the planetary science division of NASA's Science Mission Directorate in Washington, D.C.

Last October, the final report of the Ad-Hoc Task Force on Planetary Defense of the NASA Advisory Council (NAC) was released.

It reported, among a number of findings, that to achieve NASA's NEO search goals in a timely manner as directed by the George E. Brown NEO Survey legislation, the nation will likely need a new space probe in addition to ground-based systems.

"A spacecraft operating with sensors in the infrared band from an orbit sunward of Earth's (such as a Venus-like orbit) offers great advantages in rapid search and repeat observation frequency," the NAC task force wrote. Essentially, the observatory would be able to monitor space rocks over time to determine their hazard potential.

Detailed appraisal

One concept that has already been fleshed out is dubbed the NEO Survey mission, a detailed appraisal done by Ball Aerospace & Technologies Corporation in Boulder, Colo.

Results of a study by Ball Aerospace highlighted how best to meet the George E. Brown objectives for detecting NEOs. [5 reasons to care about asteroids.]

As explained in a Ball Aerospace white paper review provided to SPACE.com, in only 1.6 years, a spacecraft could locate all of the roughly 165 feet (50 meter) diameter, and larger, nearby space rocks that are potentially accessible for human spaceflight, and within 7.5 years could catalogue 90 percent of all NEOs greater than 459 feet (140 meters) in diameter.

"We have more work to do, but what we've created is a very high-quality existence proof. We have a point design based on real engineering with real parts," said Robert Arentz, a Ball Aerospace Advanced Systems Manager.

Arentz told SPACE.com that the NEO survey spacecraft draws upon the firm's heritage of working on NASA's space-based observatories – from the Hubble Space Telescope and the Kepler exo-planet hunter to the Spitzer infrared telescope and the Wide-field Infrared Survey Explorer, along with the company's comet-smacking Deep Impact spacecraft.

Tractable problem

The internally funded Ball Aerospace concept has not yet been given a green light by NASA, noted Kevin Miller, a Ball Advanced Systems Manager, but the point design does showcase proven capabilities and an affordable approach, he said. The work uses a recipe "to establish confidence that, yes, this really is a very tractable problem," Miller said.

In order to meet the George E. Brown requirements to find 90 percent of all NEOs larger than some 460 feet (140 meters) within 7 years, the NEO Survey mission would cost roughly $638 million. The catalog it would yield is a superset of the targets that NASA human spaceflight planners would find of interest for piloted excursions to selected space rocks.

Given a go, the NEO hunter from start to launch should take around 42 months to develop, Arentz added.

But there are technological challenges in building the NEO survey spacecraft.

Dealing with solar radiation is one. The heat load from a location so near the sun means the spacecraft would need a large thermal shield and cryocooler hardware. Also, the telescope's photon-gathering array requires highly advanced engineering.

The key is to prevent the intense solar radiation at Venus from reaching the telescope. This is done by careful design of the spacecraft's solar array and use of two thermal shields between the main array and the telescope.

The spacecraft design, Arentz said, is based largely on the Kepler planet-hunting spacecraft design to reduce cost and risk.

And, if two NEO-hunting spacecraft were placed in roughly opposite locales in a Venus-like orbit, this would allow a binocular view of space rocks, and scientists could chart them with an even greater degree of tracking accuracy.

Moral imperative

Needless to say, space-based searches for NEOs come with technical challenges. But finding the funding for the concept within an already over-subscribed NASA presents another challenge altogether.

"There's also a compelling need to generate momentum and show some positive progress towards this general class of mission in the near-term," Miller said.

Scientists say there would be a significant payoff, eventually.

By the end of the proposed spacecraft's lifetime – in the range of seven to eight years after launch – the NEO catalog would contain somewhere between half a million and a million new objects.

Beyond feeding the scientific community a wealth of new information, the telescopic capability also supports NASA's desire to fly both robotic and human expeditions to select asteroids. Furthermore, the spacecraft would enable a planetary defense response if any Earth-threatening NEOs are found, proponents say.

"It's a moral imperative," Arentz said, whether there's no planetary defense problem at all or our planet has a problem child headed our way.

"So it's exactly like cancer. The sooner you know, the better it is. You either know you're safe or you've got the necessary lead time to fix it," Arentz added.

Potential cost benefit

Former astronaut Rusty Schweickart, who served as co-chair of the NASA Advisory Council Ad-Hoc Task Force on Planetary Defense told SPACE.com: "If perfect tracking were possible using [ground-based] telescopes we'd never have to launch a mission to verify whether or not a NEO were going to impact ... we'd know and launch only if we were definitely going to be hit."

Schweickart said that with current, and even future ground-based telescope tracking, missions would have to be launched from time to time to rendezvous with a threatening NEO to verify whether or not the NEO was in fact headed for a bruising impact with Earth.

This would be necessary, he said, both due to the limits inherent in any telescopic image and to the intermittent nature of tracking, which comes with long data drop-outs.

"With a telescope in a Venus-like orbit, the episodic tracking component of the tracking challenge will all but disappear, the result of which will be a reduction in the need for transponder rendezvous missions," Schweickart said.

The potential cost benefit of improving tracking by deploying a telescope in a Venus-like orbit, and thereby eliminating transponder missions, can only be determined by doing a rigorous analysis, Schweickart said – a step recommended by the NAC task force.

This logic, while critical to planetary defense and cost-saving, only comes into play when one goes beyond finding NEOs into deflecting them, Schweickart suggested. Indeed, a recent White House policy letter for the first time assigns NASA responsibility beyond simply finding, characterizing and cataloging NEOs.

That's welcome news, Schweickart said.

"The interesting thing is that a telescope in a Venus-like orbit contributes to so many different interests... discovery, tracking and building up the catalog for exploration. But planetary defense is the primary rationale for its deployment," Schweickart concluded.

Leonard David has been reporting on the space industry for more than five decades. He is past editor-in-chief of the National Space Society's Ad Astra and Space World magazines and has written for SPACE.com since 1999


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