From: Ames Research Center
Posted: Friday, February 27, 2004
Orange County, CA, February 23-26 2004
This edition of NEO News continues reporting from the AIAA Planetary Defense Conference. Focus is on mitigation, and three classes of deflection missions are considered below: changing the asteroid orbit with nuclear explosives, low thrust plasma engines, or ablation with a solar collector. Following are my notes plus a press clipping from Space.com. [Editor's note" the space.com article ahs been omitted for copyright reasons.]
PLANETARY DEFENSE CONFERENCE PART 2: MITIGATION PROPOSALS
Notes by David Morrison
Dennis Byrnes (JPL) reviewed spacecraft visits to comets and asteroids under the title "Asteroids and Comets I Have Loved". The list is quite impressive, with 11 comets and asteroids explored by spacecraft, as follows: ICE flyby of Comet Giacobini-Zinner. Multiple flyby missions to Comet Halley. Giotto (retarget) to Comet Grigg-Skellerup. Galileo flybys of asteroids Gaspra and Ida (and Ida satellite Dactyl). NEAR-Shoemaker flyby of asteroid Mathilde on the way to orbit and land on Eros. DS-1 flybys of asteroid Braille and Comet Borrelly. Stardust flyby of asteroid Annefrank and recent sample collection from Comet Wild 2. For future we can expect: Hayabusa (MUSES-C) to asteroid Itokawa, Rosetta to Comet Churyumov-Gerasmenko, Deep Impact to Comet Tempel 1, and Dawn to orbit asteroids Vesta and Ceres.
Mike Belton (Tucson) described the results of the NASA-sponsored Arlington workshop on mitigation held in September 2002, focusing on what needs to be done now to ensure that an adequate base of scientific knowledge is created to allow efficient development of a reliable mitigation system when needed in the future. We should do early experiments to test mitigation approaches. The prime impediment is the lack of assigned responsibility or authority to any person or organization either to plan for mitigation or to deal with a specific threat if one develops - in other words, nobody is in charge of protecting the planet against impacts. Policy makers should formulate a chain of responsibility for action in the event a threat becomes known, and the organizers of the Arlington workshop recommend that this responsibility be assigned to NASA.
Alan Harris (Space Science Institute) spoke on Deflection Techniques: What Makes Sense? For asteroid deflection, a 10 yr lead time requires delta v of a bit less than 1 cm/s. The upper limit on impulsive delta v is escape v, which is of order the diameter in km (1 km asteroid has escape v about 1 m/s). For disruption, weeks to months are required for debris to disperse so only small fraction hits the earth. With short lead time no technique works well, while with more than a decade lead-time, either disruption or deflection can work. None of above takes resonant returns into account; such passages allow much smaller delta v to be applied before the earth swing-by -- needs deflection of only hundreds of km, and delta v of order a mm/sec. The problem is to understand the NEA orbit well enough before the keyhole is encountered. For intersecting orbits, the delta v to rendezvous is comparable to impact velocity, e.g., 10-20 km/s. (For example, the B612 scenario below requires 15 times more fuel to get to the asteroid than to make 0.2 cm/s velocity change). Dealing with spinning asteroid can be difficult; non-principle axis rotation may be common among small asteroids. What makes sense for us to work on now: (1) heavy lift launch vehicles (2) high efficiency space propulsion systems (3) well documented impact experiments (4) theoretical studies of nuclear options. Harris emphasized that asteroid mitigation is not a job for leftover ICBMs and warheads, that experimental nuclear explosions in space are politically difficult, and of course the cost of risk prevention should not exceed its value.
Several speakers discussed the use of nuclear explosives either for destruction or deflection. David Dearborn (Livermore National Lab) discussed coupling of nuclear blast energy into NEOs. Nuclear explosives provide by far the most efficient packaging of energy (a million times more energy than the same mass of chemical explosive). For deflection, he noted that only by applying force vector along trajectory do we change the total angular momentum of the NEA and provide a shift that is cumulative over many orbits. Forces applied in other directions to change inclination or eccentricity do not produce cumulative effects (except in unusual resonant return cases). With conservation of momentum, we can readily calculate how much reaction mass must be ejected based on mass of asteroid, velocity of material ejected, and required change in asteroid velocity. For deflection, must keep delta v much less than escape velocity. Standoff explosion heats rock and vaporizes material to exert reaction force. The explosion needs to be within 1 radius or we lose most of energy. For a delta-v of 1 cm/sec on kilometer-scale object, we need to vaporize the top 2 cm. X-rays don'to penetrate deep enough, but neutrons are more penetrating and give centimeters of heating. We could deflect a 1 km object by cm/s with a few megatons explosion of a neutron-rich nuclear device.
Vadim Simonenko (Russian Federal Nuclear Center, Snezhinsk) spoke of the use of nuclear detonation, for either dispersion or deflection. Sub-km objects can be dispersed (blown apart) by nuclear explosions without detailed knowledge of their material properties, but larger objects are more challenging. For deflection we would probably use multiple explosions. Oleg Shubin (also of the Russian Federal Nuclear Center) discussed legal and political issues. He argues that before we implement a nuclear defense system, we need the study the efficiency of nuclear explosion effects on NEAs. We will also need experimental modeling, culminating in full-scale space testing. Are such tests consistent with Nuclear Test Ban treaties? Although these are not weapons of mass destruction (not bombs), the current international treaties ban all tests in space, whether for weapons or peaceful purposes. Most restrictive is the Comprehensive Test Ban Treaty, which completely prohibits nuclear explosions in space. Shubin concluded that the chances to change the CTBT are insignificant until the world community changes its attitude so that peaceful explosions are decoupled from concerns about a new generation of nuclear weapons.
Mark Barrera (Aerospace Corp), presented results of a study of using nuclear explosions for Deflecting a NEO with Today's Space Technology. One of the example scenarios was a 200 m asteroid with only 11 yr warning, discovery in 2005, requiring use of current technology. Assume the USAF has task to deflect it using standoff nuclear (neutron-rich) explosions and existing launch vehicles. The goal is to reduce probability of impact below 1 in 100,000. To achieve this goal we must deflect the error ellipsoid, which requires more than deflecting centerline of predicted orbit. Requires delta v of several cm/s depending on how early we can reach the asteroid and apply impulse. For nominal coupling of blast to object this plan requires a 1500 kg explosive package. He assumes 2-3 years to develop the spacecraft. Several launch windows (2008-2012) all require heavy-lift launch vehicle, with multiple interceptors to improve system reliability. In this study, mission reliability is the key factor that drives us to multiple designs and launches.
Ed Lu (NASA JSC) discussed deflection using high-efficiency plasma engines, noting that we can only learn how to build and operate spacecraft by doing it. Thus the B612 Foundation's demo mission (described in their Scientific American article 4 months ago) proposes to change the orbit of a small NEA in a measurable way by 2015. The nominal approach is to dock and push or pull with a tether. Emphasis is on controllable deflection that does not risk breakup. What we will learn will be applicable to most any methods of controlled deflection. To rendezvous with the asteroid we need to plan for spacecraft delta v of at least 10 km/s - beyond the range of chemical rockets. Since we need a high ISP plasma system to get there, we might as well use the same system to move it. He argues for a single spacecraft launch, with no space assembly required, and a test target near 200 m (10 million tons mass). Delta v of 0.2 cm/s (enough to measure) requires several months of thrusting; this same approach would change velocity enough to prevent a collision given 50 yr warning (thrust of order 1N, with ISP of several thousand). Plasma drive power requirement is 200 kw (reasonable for nuclear electric power, consistent with NASA Prometheus Program.)
Continuing with various aspects of the B612 proposal, Mike Houts (Los Alamos National Lab) discussed Near-term Fission Power Systems for spacecraft. As an illustration, he noted that fissioning 12 fl oz (a soda can) of uranium releases 50 times the energy contained in a Shuttle external tank. Current designs for Prometheus use U-235 fueled, 1200 kWt electric power, Beryllium neutron reflector, but with different cooling (heat transport) modes under study. The reactor will be launched cold so that there is no danger of radioactive contamination even if there is a launch accident. Dan Scheeres (University of Michigan) presented a paper on Close Proximity Operations at Small Bodies, which will be a necessary part of all mitigation approaches. The situation of orbiting or operating near a very small body is complex, depending on size, shape, internal structure, spin state (relative to orbit plane), and satellites. Scheeres also discussed The Mechanics of Moving an Asteroid (the Asteroid Tug). The requirement is to alter the asteroid trajectory in a controlled way. Must control asteroid and thrusting spacecraft as a unit in order to apply thrust along orbit trajectory. Delta v of 1 cm/s velocity change shifts asteroid by about 1000 km/yr at encounter. Dan Durda (SWRI) further discussed the B612 Mission, including the new idea of Lu gravitational "tractor beam" to link spacecraft with asteroid, so that no landing is required and we don't need to worry about asteroid physical properties or spin state.
Jay Melosh (U. Arizona) presented a paper on the Mirror Ablation Mission. He told how a decade age, he and I. Nemchinov began to work on non-nuclear options in reaction to suggestions by Edward Teller and others to build bigger nuclear bombs to defend against asteroids. His case study focused on a 1 km asteroid: requires 5000 N applied for one year. Needs mass flow rate of few kg/s to mimic comet jets. Evaporation of solids into near-vacuum is well understood. Requires temperature as high as 3000 K. Entrained solids don't matter as long as the mass flows away from the asteroid; solids give more mass but lower ejection velocity. Temperature rise time is of order 1 min, so we may not need to worry about rotation unless they rotation rate is high. Large inflatable mirrors have been designed, but the biggest unsolved problem is degradation of the optical system by outflowing hot gas and dust. Theoretical performance is very high, but there are many practical problems to be overcome.
more to come later.....
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