A NASA working document on ways to find and deflect celestial objects that might threaten Earth is deeply flawed in ways that exaggerate the cost and difficulty of the programme, critics say.
In December 2005, the US Congress gave NASA one year to submit plans for a survey that would catalog 90% of all potentially hazardous near-Earth objects 鈥� spanning at least 140 metres across 鈥� by the end of 2020.
In March 2007, the agency delivered to Congress only a sketchy that lacked any detailed analysis, a budget or an implementation plan. It recommends continuing NASA鈥檚 annual $4.1 million search for asteroids down to 1 kilometre across. But it does not address searches for smaller asteroids, saying flatly, 鈥渄ue to current budget constraints, NASA cannot initiate a new program at this time鈥�.
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Congress was not pleased with the conclusion. Bart Gordon, chair of the House of Representatives鈥� Committee on Science and Technology, said the report did not offer 鈥渁 credible plan鈥� and vowed to push NASA towards 鈥渁 more responsive approach鈥�.
Critics also blasted NASA for failing to publish the longer analysis that led to the report. That analysis, which took the form of a 271-page report, was circulated only to study group members and labelled 鈥渇inal report鈥� 鈥� a mistake a NASA spokesman blamed on 鈥渟tupidity鈥�. The spokesman also said NASA had decided 鈥渁 short, concise report would be more helpful鈥� than a detailed one, and that the agency is waiting for direction 鈥� and a budget 鈥� from Congress and the Administration.
Extreme cases
NASA has now posted the (PDF), with a list of errata, on its Freedom of Information Act (FOIA) site. But a (23 MB PDF) was earlier posted on the web site of the B612 Foundation, a group of asteroid researchers chaired by former astronaut Rusty Schweickart.
Dated 28 December 2006, the long report includes a 鈥渇airly good鈥� analysis of the requirements of hunting down asteroids as small as 140 metres, says Clark Chapman of the Southwest Research Institute in Boulder, Colorado, US. It projects costs of $820 million for a survey using ground-based instruments, or $1.0 to $1.3 billion using a dedicated infrared telescope in an orbit near Venus, which could reach the survey goal three years faster and would be more sensitive.
But Chapman and others are extremely critical of the document鈥檚 analysis of asteroid risk. Chapman says the six potential threats studied in the report 鈥渁re not representative鈥� because they are extreme cases 鈥� including a 1-kilometre-wide asteroid and a long-period comet discovered less than two years before impact.
鈥淭hey didn鈥檛 study the case of a multi-decade warning of a 100-metre object,鈥� he says. Small objects of that scale account for some 95% of potential impacts and could be deflected over long periods of time using gravity tractors 鈥� spacecraft that fly next to the asteroids and nudge them off course using the craft鈥檚 own gravity, or space tugs, which could gently push the space rocks into safer orbits.
Return with a vengeance
That focus on extreme risks tilted the scales toward deflection techniques that use nuclear explosions 鈥� either on the asteroids themselves or nearby to push them away from Earth. The study鈥檚 approach was that 鈥渢he bigger the bang is better鈥�, says Schweickart.
But he says explosions are not as precise as the gentler techniques because they would scatter the asteroids or resulting debris into new orbits around the Sun that could take them through points called 鈥渒eyholes鈥�, narrow zones where the slight changes in the balance of forces could put them on a collision course with the Earth. 鈥淚f you just shove it out there to miss the Earth, you can鈥檛 tell that you haven鈥檛 put it into another return keyhole,鈥� Schweickart told New 杏吧原创.
Chapman points to other potential problems with nuclear weapons: 鈥渆xisting international treaties about bombs in space and the potential dangers of maintaining and launching nuclear weapons.鈥�
The analysis also misunderstands the requirements for a gravity tractor, Schweickart says. He told the study panel that gravity tractors could reach their targets using ion propulsion, such as that used in the successful $147 million Deep Space 1 mission. But the NASA panel instead assumed a gravity tractor would require a $10 billion development of an electric propulsion system based on a space nuclear reactor, similar to NASA鈥檚 now-cancelled Project Prometheus.
NASA says administrator Mike Griffin has been talking with Chapman and Schweickart about their criticisms.
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