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DEFLECTION OF LARGE NEAR-EARTH OBJECTS

Gregory H. Canavan, P-DO

Date: 11 January 1999

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DEFLECTION OF LARGE NEAR-EARTH OBJECTS

Gregory H. Canavan

The requirements to deflect NEOs with significant material strength are known reasonably well, but the strength of actual NEOs is not known. Meteor impacts give some information, which implies that large, weak objects could also be deflected efficiently.

The Earth is periodically hit by near Earth objects (NEOS) ranging in size from dust to mountains. The small ones are a useful source of information, but those larger than about 1 km can cause global damage. The requirements for the deflection of NEOs with significant material strength are known reasonably well; however, the strength of large NEOs is not known, so those requirements may not apply. Meteor impacts on the Earth’s atmosphere give some information on strength as a function of object size and composition. This information is used below to show that large, weak objects could also be deflected efficiently, if addressed properly.

Detection of NEOs is conceptually a solved problem. In the past, limited sky searches with modest electronics produced low discovery rates, which would have required a century to detect the NEOs larger than 1 km that represent the major hazard. Recent advances in focal plane technology and improved all-sky search strategies have increased discovery rates by about an order of magnitude. The major NEOs could thus be found in a few decades-given adequate dedicated facilities and modest manpower.’

only hits the Earth about once every million years. However, if one did, the damage and loss of life would be catastrophic. It is estimated that it is worth - $10B/yr to insure against this possibility?

The detection of all NEOs does not eliminate all possible threats. Long period comets (LPCs) are a particularly bothersome threat in that they would probably be undetected before final approach by ground-based telescopic searches, approach from unexpected quarters, and have higher mass and velocity than NEOs. Concepts for their detection are in a formative phase?

Requirements for deflection of objects with significant mechanical strength are reasonably well understood. Figure 2 shows the predictions developed in the Congressionally mandated Workshop on “NE0 Interception’” and refined in subsequent technical meeting^.^ The NE0 diameters that can be deflected increase monotonically with the amount of reaction time available. With the -100 year warning expected from telescopic surveys, objects - 2-3 km across could be deflected with nonnuclear technologies such as rockets, rail guns, or sails due to the long time available for displacement and high efficiency of energy transfer at perigee.

It is likely that this would not discover any threats, as Figure 1 shows that a large NE0

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For time scales of decades, nonnuclear approaches are restricted to - 100 m objects, unless coupled with advanced or nuclear propulsion. Nuclear deflection could still address objects several km across with conventional propulsion.

For time scales of years, only nuclear approaches are effective for globally threatening objects. Those 1-10 km in diameter could be deflected, provided they have enough strength to maintain their integrity during deflection. However, the impulses required increase rapidly. If the NE0 has velocity V, is detected at range R, and the interceptor has speed - c (specific impulse), then because c << V, they meet a distance - ct - cRN from the Earth. For the NE0 to miss the Earth, it must be deflected an angle - vet, where Re is the Earth’s radius. That means the interceptor must impart a transverse velocity

which illustrates the difficulty of intercepting a fast object detected close to impact. To produce a velocity v in an object of mass M, the interceptor must deliver an impulse Mv. Its delivery time is determined by the time for impact for kinetic deflection or for explosion for nuclear deflection. Thus, the force and pressure on-and stresses in-the object increase as V2/R. For very fast objects or close detection, they can exceed the tensile strength of the object. That could propagate a strong shock through it, reduce it to rubble, and spall and equal mass off the far surface without transmitting any net transverse momentum to the NEO. That would replace a single threat with a swarm of impactors, which could do even more damage.

Symmetry and efficiency. In addition to the - thousand to milion-fold greater energy density of nuclear explosives, some prefer nuclear explosives simply because they can be released at a distance from the object, which averages the energy deposition and reduces sensitivity to object shape. However, that advantage in symmetry is purchased at a price in reduced efficiency. The efficiency is greatest for subsurface kinetic or nuclear releases, which share their energy with a great amount of material. It is less for surface releases, which share their energy only with the material in the crater they form. It is least for standoff releases, which heat only the f is t few centimeters of the surface. This is characterized by the typical velocity v, of the ejecta, which is 0.1 km/s for subsurface, 1 km/s for surface, and 10 km/s for standoff releases.

release of energy E is - mv; -, so that the impulse transferred is

The impulse transferred varies inversely with the ejecta velocity; thus, surface and standoff releases suffer penalties of 10 and 100 relative to buried releases. Despite the advantages of standoff in symmetry, surface or buried releases may have to be used for fast or close engagements. Doing so will further increase the strength of the shocks launched as well as their

v - w y c t - V~R,/CR, (1)

With good coupling, the kinetic energy imparted to the blow off material of mass m by a

Mv - v,m - veE/v? - E/v, (2)

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geometry. The fundamental problem is that the mechanical strength of large NEOs is not known, so the limiting pressures that can be applied without fracture are not known. There is some information from meteor impacts.

we know about the composition of NEOs is from their finds, supplemented by telescopic spectroscopic observations. Much of the information is anecdotal, but in recent decades several nets have been organized for systematic observations and some metric satellite data has been released. The observations typically involve radiance time histories, but some have spectral information as well. A subset has direct measurements of brightness as a function of altitude,

Conceptually, it is simple to invert the light curves to determine an object's size and speed, as the power radiated by an object of diameter D and speed V at altitude h scales as P - e' mV3D2. As the object decelerates, P has a maximum at h - H, which determines V3D2. However, D also changes due to erosion; thus, absent altitude-time information, a detailed understanding of radiation efficiency, and fracture criteria, it is difficult to uniquely determine the size and strength of the object.

A complication is that the large objects generally fracture before they reach peak radiation. Figure 3 shows the radiation from Sandia Network object 3 (SN3 or PN91277), a - 2.3 m object that breaks up at - 33 km. The two peaks represent two separate pieces. The pressure on the front of the object increases as p - e-h/HV2, so it fragments at an altitude where p - S, its tensile strength. As V has generally changed little by that altitude, measurements of V and h determine S - e-mV2, independent of uncertainties and changes in D.

These observations have led to the development of a rough categorization of objects into type I Stony, 11 Carbonaceous, IIIA Cometary, and IIIJ3 Soft Cometary objects, which are characterized by strengths of 1,000, 100, 10, and -1 bar, respectively. Figure 4 uses these categories to organize the strengths and diameters inferred for the largest objects for which breakup altitudes are known. A few of the objects are cometary, but the majority are carbonaceous with strengths of - 100 bar. One object is at the lower end of the Stony category. Iron objects would lie about an order of magnitude higher.

still. Thus, real NEOs appear to be about a factor of lOO-l,OOO weaker than theoretical ones. There is no evidence that their strength increases with size, although the largest object has the largest strength by a factor of - five. It would be useful to measure the strengths of larger objects, but they hit the atmosphere infrequently, so it is necessary to do experiments in space.

NEAR mission underway to survey, rendezvous with, and explore a large NE0 that has been observed extensively. The DoD has proposed the Clementine II mission to impact a small

Meteor impacts were a source of both inspiration and iron for the ancients. Much of what

Earlier estimates summarized in Figure 2 assume strengths an order of magnitude higher

Space experiments can be performed on large NEOs in situ. NASA currently has a

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payload on a NE0 and measure the visible signature and lidar return from the plume. The temperature of the plume can be related through similarity solutions to the outflow to the compressive strength and density of the material. The amount of cold material ejected can be related to its tensile strength. The former is important for predictions of the coupling efficiency of kinetic or nuclear energy release; the latter for the rubbelization threshold and the amount of material expected to be spalled off the back side in deflection attempts. Such experiments are not inexpensive, costing - $100M apiece, but they do provide direct experiments of the main uncertainties in deflection and fragmentation.

Fragmentation breaks the NE0 down into pieces too small to survive reentry. It would be prohibitively expensive for very strong objects, but is the preferred mode of negation for very weak ones, which might be negated for the kinetic energy required to push their pieces apart. It involves placing the explosives optimally near the center of the object, which produces lower ve and hence higher efficiency. This assumes that it is possible to penetrate to the center of very large objects, but that issue has been studied extensively and appears to be within the scope of current penetration weapon technology.6 It is not needed for distant detection, for which the optimal forces and stresses are modest, but it could be required for short notice engagements.

M(VRJct)2. Thus, the diameter that can be negated by an energy release E is

Which is shown on Figure 5 for E = 10 Megaton and p = 3 t0n/m3. D falls as tm from 100 km at t - 30 years to 10 km at t = 1 to - 2 km at t = 1 month, all of which are useful. The needed deflection velocity increases from - 0.1 to 30 m/s, and the angular deflection from 4 to I ,OOO, micro radians. The distance of engagement decreases from 8 to 0.03 AU (Astronomical Unit - 180,000,000 km, the mean distance from the Sun to the Earth). A LPC might only provide a few months warning, which corresponds to - 5 km if intercepted at 0.125 AU.

For larger objects, larger releases would be required, particularly as D only scales as D - E In. Figure 5 is drawn for roughly the largest yields currently available. Larger releases could be produced, and they scale favorably on mass, but those assumed are at the limit for current, prompt deep space missions. Assembling larger payloads in space and using higher specific energy fuels are possible, but ~ndeveloped.~

deflection of NEOs of various sizes with a wide range of technologies. There are many options for warning times of centuries, fewer for decades, and only nuclear for years or months. Their analysis is complicated by the fact that the material strengths of actual NEOs is not known. Some impulsive approaches could fail catastrophically or make matters worse for very weak objects such as LPCs. Data from meteor impacts on the Earth's atmosphere provides a partial data base

From Eq. (l), the deflection velocity needed is v - VRJct, so the energy required is E - D - @/p) ' " (C~NR,)~, (3)

Summary.There is reasonably good agreement on estimates for the requirements for

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that suggests most NEOs have very little strength-a result space experiments are intended to test. However, very weak objects can be pushed apart for little more than the kinetic energy required. Doing so through kinetic or nuclear energy release deep in the interior of the object could fragment - 5-10 km objects with a years warning into fragments too small to survive entry into the atmosphere and too diffuse to deposit critical heat fluxes. Thus, the technological elements for NE0 and LPC negation appear to be roughly as mature as those for detection and consistent with the warning they should provide.

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References

G. Canavan, “Optimal Detection of Near-Earth Object Threats,” J. Rem0 ed, Near-Earth Objects: the United Nations International Conference (Annals of the New York Academy of Science, Vol. 822, 1997), pp. 539-543.

* G. Canavan, “Cost and Benefit of Near-Earth Object Detection and Interception,” T. Gehrels ed, Hazards Due to Comets and Asteroids (University of Arizona Press, Tucson & London,

G. Canavan, “Asteroid and Comet Defenses,” Testimony before the Committee on Science

1994), pp. 1157-1 189.

Subcommittee on Space and Aeronautics of the US House of Representatives (Los Alamos Report LA-UR-98-2171,21 May 1998); G. Canavan letter to Chairman Rohrabacher (Los Alamos P/AC:98-140,28 May 1998).

G. Canavan, J. Solem, and J. Rather, Proceedings ofthe Near-Earth Object Interception Workshop (Los Alamos LA-12476-C, February 1993).

G. Canavan, J. Solem, and J. Rather, “Near-Earth Object Interception Workshop, T. Gehrels ed, Hazards Due to Comets and Asteroids , op. cit., pp. 93-124.

L. Wood, R. Hyde, M. Ishikawa, and E. Teller, “Cosmic Bombardment V: Threat Object- Dispersing Approaches to Active Planetary Defense, J. Nuckolls ed, PZunetary Defense Workshop (Lawrence Livermore National Laboratory report COW-9505266,22-26 May 1995).

Defense Workshop, op cit., pp. 17-20. R. Barker and V. Simonenko, “Interdiction Summary Report,” J. Nuckolls ed, Planetary

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