Asteroid Dreams, Part 7: keyword=uncertainty


The week before last was a near-marathon of meetings in Washington, D.C., about near-Earth objects (NEOs) and the Obama administration’s proposal for an “Asteroid Redirect Mission” (ARM) and its “Asteroid Redirect Robotic Mission” (ARRM). Today I’ll try to sum up what I heard at the first two of these meetings. (I’ll cover the third meeting in a subsequent post.)

The primary message I took home by the end of that week is that many, many different types, levels, and ranges of uncertainty are involved in most if not all of the predictions, projections, and estimates of NEO populations and characteristics, NEO target candidates for robotic retrieval and human exploration, ARM spacecraft design, etc., etc.

On Monday July 8, the American Association for the Advancement of Science and the Secure World Foundation provided a lunchtime briefing for congressional staffers on “NEOs: Addressing the Current Threat Level.”

First, a comment: the “threat/response” rhetoric so prevalent in the NEO community, and which I heard at this briefing, is bothersome, as “threat” implies agency. A nation or an army or a person can, by choosing a target (identifying an “enemy”), pose a threat to another nation or army or person. An asteroid has no agency. It is a natural object, and it cannot choose a target. And the pinball-machine or “cosmic shooting gallery” analogy is not adequate to describe the space environment, as no agency is involved (that is, no one’s operating the flippers or pulling the trigger).

At the Capitol Hill event, NEO experts told a standing-room-only crowd (offer them free lunch, and they will come) about current NEO tracking and characterization efforts and ideas for defending Earth from NEO impacts. Experts discussed probabilistic NEO impact risk assessments, deaths-per-year estimates, and “significant uncertainty” in estimates of the NEO population.

Keyword: uncertainty.

At Tuesday July 9’s “Target NEO 2” workshop, organized by the Johns Hopkins Applied Physics Laboratory (APL), sponsored by Ball Aerospace, and held at the National Academy of Sciences in Washington, D.C., NASA, industry, and academic experts aired their plans, assumptions, questions, and concerns about the ARM.

NASA associate administrator for human exploration and operations Bill Gerstenmeier said NASA’s ARM plan calls for an asteroid robotic retrieval mission (ARRM) to capture a small NEO (7-10 meters diameter, ca. 500 tons) and redirect it into an orbit in the vicinity of the Moon, for future human exploration. On NASA’s proposed ARM schedule, the ARRM spacecraft must be built before its NEO target is identified, in 2016, with ARRM launch planned for 2017 and human exploration of the asteroid planned for as early as 2021.

Southwest Research Institute’s (SwRI’s) Bill Bottke asked why NASA is proposing such an aggressive schedule for the ARM. Gersteinmeier said it’s because NASA’s already working on the components of the ARM (NEO identification and characterization, solar electric propulsion, autonomous guidance and control, new extravehicular activity technology, and the Orion/SLS human space flight system).

APL NEO expert Cheryl Reed asked if the ARRM spacecraft will have to be “overdesigned” due to the need to build it before its target is identified. Gerstenmeier said it “probably” will be, noting that NASA is still considering whether the ARRM target should be a small asteroid or a “boulder” to be grabbed off the surface of a larger asteroid.

Many NEO experts expressed reservations about the viability of the latter option.

NEO expert Al Harris cautioned that “in choosing a very low [velocity] target, you need to have very good physical characterization of the object if you want to be sure you aren’t bringing a piece of the moon back to its home, or even an old rocket body.”

APL’s Andy Rivkin reported that among factors that need to be pinned down precisely before a NEO can be identified as a suitable ARM target are composition, albedo, density, size, and rotation rate. Thus far these factors for small NEOS tend to be extrapolations from observations of large NEOs (Doctor Linda would call them guesstimates.)

SwRI’s Bill Bottke said the accuracy of current NEO-orbit models for calculating the orbits of small NEOs “is unknown – I would say very suspect.” Bottke also reported that recent observations have detected “mini-moons” orbiting some near-Earth asteroids that are temporarily captured in Earth orbit (for years or decades). Among these mini-moons are a few meter-size objects and a dozen half-meter size objects. Perhaps these “minimoons” should be considered as possible ARM targets, he suggested.

Bottke and others noted that electrostatic forces are important in forming small NEOs, and consequently many of these objects could be as loosely bound as “sandcastles” or “sandbars.” If a spacecraft were to try to bag, grab, capture or probe such an object, it could disintegrate.

Paul Chodas of the Jet Propulsion Laboratory’s NEO Program Office said 14 known asteroids satisfy ARM candidate requirements…. Four candidates on this list have been or will be partially characterized.” In the ARM candidate characterization process, he noted, rapid response after discovery “is essential.”

Tim Spahr, director of the Minor Planet Center at the Harvard Smithsonian Astrophysical Observatory, warned NEO researchers “to be very cautious about identifying uncertainties” in their observations, estimations, models, and predictions.

Steve Larson of the University of Arizona’s Catalina [NEO] Sky Survey noted that, compared to NEO searching in general, searching for ARM targets is problematic, because smaller NEOs are usually faint and move fast and have short observing windows.

JPL’s Amy Mainzer, principal investigator for NASA’s now-completed NEOWISE mission, reported that NEOWISE discovered 750 NEOs, including 146 new objects, in its one-year survey, the smallest of which is 8 meters in diameter. If NASA were to restart NEOWISE – which apparently is likely – it could discover 50-60 new NEOs per year, 25 percent of which would likely be potentially hazardous asteroids. (Mainzer and her colleagues have also proposed a “NEOCam” mission – a NEO observing camera that would be launched as a piggy-back payload with a larger satellite to geostationary orbit. NEOCam could discover and characterize up to 1,000 NEOs per year, she said.)

Tim Spahr commented that NEO discovery by amateur observers, using small-ground-based telescopes, “is basically over,” as most of the larger, brighter objects have been found. They’d need bigger, better hardware to find the smaller, fainter objects, he said. JPL’s NEO radar expert Lance Benner noted that amateurs can now make other contributions to the NEO survey effort.

Okay, this post is plenty long enough. I’ll finish up my notes on the Target NEO 2 Workshop in a later post. Stay tuned.


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