THE ROLE OF GROUNDBASED RADAR
IN NEAR-EARTH OBJECT TRACKING,CHARACTERIZATION, AND
THREAT MITIGATION

Steven J. Ostro and Jon D. Giorgini,
JPL/Caltech

For the NASA Workshop on Near-Earth
Object Detection, Characterization, andThreat Mitigation

reconnaissance could almost immediately
allow one to recognize this by distinguishing
between an impact trajectory and a near miss,and would dramatically reduce the difficulty
and cost of any effort to prevent the collision.
Financial support for the Arecibo andGoldstone radars has declined during the past
several years, weakening their reliability androbustness. Both sites need adequate, reliablefinancial support if their capabilities are to bepreserved during the coming decade.

Introduction
Vail, Colorado
June 2006

NOTE: This White Paper consists largely of
excerpts from Ostro and Giorgini (2004).

Summary
Ground-based radar is uniquely able toreduce uncertainty in trajectories and physicalproperties of NEOs. Radar can prevent theloss of a newly discovered object caused by
the normal decay of orbit knowledge prior to
the next optical observing opportunity, canadd decades or centuries to the interval overwhich close Earth approaches can accurately
be predicted, can significantly refine collision
probability estimates that are based on opticalastrometry alone, can reveal whether an object
is single or binary, and can produce detailed
information about the sizes, shapes, spinstates, and surface characteristics ofpotentially hazardous asteroids (PHAs). If asmall body is on course for a collision with
Earth in this century, delay-Doppler radar

Ground-based radar is a knowledgegathering tool that is uniquely able to shrink
uncertainty in NEO trajectories and physicalproperties. The power of radar stems largely
from the precision of its measurements. The
resolution of echoes in time delay (range) andDoppler frequency (radial velocity) is often oforder 1/100 the extent of a large NEA, so
several thousand radar image pixels can beplaced on the target. Delay-Doppler
positional measurements often have afractional precision finer than 1/10,000,000,
comparable to sub-milliarcsecond opticalastrometry.
The world's only effective NEO radars areat Arecibo and Goldstone, whose effective
declination windows are -1 to 38 deg and > -40
deg, respectively, and whose sensitivites aredescribed by Ostro (2006a). In this white
paper we outline how groundbased radar could
help at each stage of detecting and mitigating
an impact hazard encountered during thiscentury.

Post-discovery astrometric follow-up
Once an asteroid is discovered, its orbital
motion must be followed well enough topermit reliable prediction and recovery at thenext favorable apparition. A single radar
detection of a newly discovered NEA shrinks
the instantaneous positional uncertainty at theobject's next close approach by orders ofmagnitude with respect to an optical-onlyorbit, thereby preventing “loss” of the object.
Comparison of radar+optical with opticalonly positional predictions for recoveries of
NEAs during the past decade shows thatradar-based predictions have had pointing
errors that average 310 times smaller than their
optical-only counterparts, dramaticallyfacilitating recovery. Furthermore, radar
astrometry (Yeomans et al. 1987, Ostro et al
1991; see Giorgini 2006 for a tabulation) cansignificantly reduce ephemeris uncertainties
even for an object whose optical astrometryspans many decades.
A goal of optical searches is to provide as
much warning as possible of any possibly
dangerous approach of NEAs as large as 140
m. However, since an orbit estimate is basedon a least-squares fit to measurements of anasteroid's position over a small portion of its
orbit, knowledge of the future trajectory
generally is limited by statistical uncertainties
that increase with the length of time from theinterval spanned by astrometry. Trajectory
uncertainties are greatest and grow most
rapidly during close planetary encounters, as
the steeper gravity field gradient differentiallyaffects the volume of space centered on thenominal orbit solution within which theasteroid is statistically located. Eventually theuncertainty region grows so large, generally
within the orbit plane and along the directionof motion, that the prediction becomes
meaningless.

Current ground-based optical astrometricmeasurements typically have angular
uncertainties of between 0.2 and 1.0 arcsec (astandard deviation of 0.5 to 0.8 arcsec iscommon), corresponding to tens or hundreds
or thousands of km of uncertainty for any
given measurement, depending on theasteroid's distance. Radar can provide
astrometry with uncertainties as small as ~10
m in range and ~1 mm s-1 in range rate. Since
radar measurements are orthogonal to planeof-sky angular measurements and haverelatively high fractional precision, they offer
substantial leverage on an orbit solution andnormally extend NEO trajectory predictability
intervals far beyond what is possible with
optical data alone.
Radar and collision probability prediction
For newly discovered NEOs, a collision
probability is now routinely estimated (Milani
et al. 2002) for close Earth approaches, and iscombined with the object’s estimated diameterand the time until the approach to rate thehazard using the Palermo Technical Scale(Chesley et al. 2002). The JPL Sentry
program’s risk page (Chesley, 2003) lists
objects found to have a potential for impact
within the next 100 years. However, fornewly discovered objects, the limited initialastrometry typically does not permit accurate
trajectory prediction. When an object's opticalastrometric arc is only days or weeks long, theorbit is so uncertain that a potentiallyhazardous close approach cannot be
distinguished from a harmless one or even a
non-existent one. The object is placed on the
Sentry page, then typically removed later,when additional optical astrometry is obtained
and the span of observations is extended. It isextremely rare for a radar-observed object tobe on the Sentry page.

Negative predictions, positive predictions,and warning time
To a great extent, the dominance of NEA
trajectory uncertainties is a temporary artifactof the current discovery phase. Predictions are
made for single-apparition objects having a
few days or weeks of measurements. The
uncertainty region in such cases canencompass a large portion of the inner solar
system, thereby generating small but finite
impact probabilities that change rapidly as thedata arc lengthens, or if high-precision radar
delay and Doppler measurements can be made.
Impact probabilities in such cases are
effectively a statement that the motion of theasteroid is so poorly known that the Earthcannot avoid passing through the asteroid's
large uncertainty region -- hence the apparent
impact “risk”. As optical measurements are
made, the region shrinks. The resulting change
in impact probability, up or down, is
effectively a statement about where theasteroid won't be -- a “negative prediction” -rather than a “positive prediction” of where it
will be. This is due to the modest positional
precision of optical measurements.
In contrast, radar measurements provide
strong constraints on the motion and hence“positive predictions” about where an asteroid
will be decades and often centuries into the
future. Thus radar substantially opens thetime-window of positive predictability.
99942 Apophis (2004 MN4)This several-hundred-meter asteroid was
discovered in June 2004 and lost until it wasrediscovered in December 2004. Integration of
the orbit calculated from the half-year-long setof optical astrometry revealed an extremely
close approach to Earth on April 13, 2029,

and possibly hazardous subsequent
approaches. Arecibo delay-Doppler radar
astrometry obtained during late January 2005
showed the object to be several hundred
kilometers closer than had been predicted by
the optical measurements (Benner et al. 2005,
IAU Circ. 8477). Radar observations inAugust 2005 (Giorgini et al. 2005, IAU Circ.
8593) and May 2006 (Benner et al. 2006, IAU
Circ. 8711) further refined the orbit, moving
the predicted 2036 Earth encounter to a lowerprobability region within the distribution of
possible orbits. The current radar+opticalcollision 2036 collision probability is about
one-third of the optical-only value (S. R.
Chesley, pers. comm.)

29075 (1950 DA)
Integrations of this object’s radar-refined orbit(Giorgini et al. 2002) revealed that in 2880
there could be a hazardous approach notindicated in the half-century arc of pre-radar
optical data. During the observations, a radar
time-delay measurement corrected the opticalephemeris's prediction by 7.9 km, changing an
optical-only prediction of a nominal missdistance of 20 Earth radii in 2880 into a radarrefined prediction of a 0.9-Earth-radiusapproach. The uncertainty in the collision
probability (which could be as low as zero oras high as 1/300) is dominated by the
Yarkovsky acceleration, which is due to thethermal reradiation of absorbed sunlight anddepends on the object's mass, size, shape, spinstate, and global distribution of optical andthermal properties. This example epitomizes
the fundamental inseparability of NEA
physical properties and long-term predictionof their trajectories.

Images and physical models
With adequate orientational coverage,
delay-Doppler images can be used to constructthree-dimensional models (e.g., Hudson et al.
2000, 2003), to define the rotation state, andto constrain the internal density distribution.
Even a single echo spectrum jointly constrains
the target's size, rotation period, and sub-radar
latitude. A series of Doppler- only echo
spectra as a function of rotation phase canconstrain the location of the center of mass
with respect to a pole-on projection of theasteroid's convex envelope (e.g., Benner et al.
1999). For objects in a non-principal-axis spinstate, the hypothesis of uniform internal
density can be tested directly (Hudson and
Ostro, 1995). Given a radar-derived modeland the associated constraints on an object's
internal density distribution, one can use ashape model to estimate the object's gravity
field and hence its dynamical environment, as
well as the distribution of gravitational slopeson the surface, which can constrain regolith
depth and interior configuration.
For most NEAs, radar is the only Earthbased technique that can make images with
useful spatial resolution. Therefore, although asufficiently long, multi-apparition opticalastrometric time base might provide about asmuch advance warning of a possibly dangerous
close approach as a radar+optical data set, theonly way to compensate for a lack of radar
images is with a space mission.
Extreme diversity
Radar has detected 193 NEAs, including
108 PHAs (Ostro 2006b). As reviewed by
Ostro et al. (2002), NEA radar has revealed
both stony and metallic objects, principal-axis
and complex rotators, very smooth andextraordinarily rough surfaces, objects thatmust be monolithic and objects that almost
certainly are not, spheroids and highly
elongated shapes, objects with complex
topography and convex objects virtually
devoid of topography. It is meaningless to talkabout the physical characteristics of a“typical” NEA.
Surface roughness and bulk density
Porous, low-strength materials are very
effective at absorbing energy (Asphaug et al.
1998). The apparently considerable
macroporosity of many asteroids (Britt et al.,2002) has led Holsapple (2004) to claim that
impact or explosive deflection methods may
be ineffective, even for a non-porous asteroid
if it has a low-porosity regolith only a few cm
deep.
The severity of surface roughness would
be of concern to any reconnaissance mission
designed to land or gather samples. The
wavelengths used for NEAs at Arecibo (13cm) and Goldstone (3.5 cm), along with theobserver's control of the transmitted andreceived polarizations, make radar experiments
sensitive to the surface's bulk density and to
its roughness at cm-to-m scales (e.g., Magri etal., 2001). Bulk density is a function ofregolith porosity and grain density, so if anasteroid can confidently be associated with ameteorite type, then the average porosity ofthe surface can be estimated. Values ofporosity estimated by Magri et al. (2001) fornine NEAs range from 0.28 to 0.78, with amean and standard deviation of 0.53 + 0.15.
The current results suggest that most NEAs
are covered by at least several centimeters of
porous regolith, so the above warning by
Holsapple may be valid for virtually any
object likely to threaten collision with Earth.

The fact that NEAs' circular polarization
ratios (SC/OC) range from near zero to near
unity means that the cm-to-m structure onthese objects ranges from negligible to much
more complex than any seen by the spacecraftthat have landed on Eros (whose SC/OC isabout 0.3, near the NEA average), the Moon,
Venus, or Mars.
Binary NEAs: mass and density
The most basic physical properties of anasteroid are its mass, its size and shape, its
spin state, and whether it is one object or two(or more; Shepard et al. 2006). The current
Arecibo and Goldstone systems are uniquely
able to identify binary NEAs and at thiswriting have observed 20 (Margot et al. 2002,
and references therein; Nolan et al. 2002),
most of which are designated PHAs. Current
detection statistics, including evidence fromoptical lightcurves (Pravec 2003) suggest thatbetween 10% and 20% of PHAs are binary
systems.
Analysis of echoes from these binaries isyielding our first measurements of PHA
densities. Delay-Doppler images of 2000
DP107 (Margot et al. 2002) reveal a 800-m
primary and a 300-m secondary. The orbital
period of 1.767 d and semimajor axis of
2620 + 160 m yield a bulk density of
1.7 + 1.1 g cm-3 for the primary. DP107 andother radar binaries have spheroidal primaries
spinning near the breakup point forstrengthless bodies. Whether binaries'
components were mutually captured following
a highly dispersive impact into a much larger
body (Richardson et al. 2002 and references
therein) or formed by tidal disruption of an
object passing too close to an inner planet(Margot et al. 2002), it seems likely that mostof the primaries are unconsolidated,

gravitationally bound aggregates, so
Holsapple's warning applies to them.
Mission design and spacecraft navigation
Whether a PHA is single or binary,
mitigation will involve spacecraft operations
close to the object. Maneuvering near a small
object is a nontrivial challenge, because of the
weakness and complexity of the gravitational
environment (Scheeres et al. 2000).
Maneuvering close to either component of abinary system would be especially harrowing.
The instability of close orbits looms as
such a serious unknown that unless we havedetailed information about the object's shapeand spin state, it would be virtually
impossible to design a mission capable ofautonomous navigation close to the object.
Control of a spacecraft operating close to an
asteroid requires knowledge of the asteroid's
location, spin state, gravity field, size, shapeand mass, as well as knowledge of any satellitebodies that could pose a risk to the spacecraft.
Radar can provide information on all theseparameters. Knowledge of the target's spinstate as well as its shape (and hence nominal
gravity harmonics under the assumption of
uniform density; Miller et al. 1999) would
permit design of stable orbits immune toescape or unintended surface impact. (Upon
its arrival at Eros, the NEAR Shoemakerspacecraft required almost two months torefine its estimate of the gravity field enoughto ensure reliable close-approach operations.)
If it turns out to be necessary to have asequence of missions beginning with physicalreconnaissance and ending with a deflection,then a radar-derived physical model would
speed up this process, reduce its cost,decrease complexity in the design andconstruction of the spacecraft, and improvethe odds of successful mitigation. [Radarderived shape models of small NEAs havemade it possible to explore the evolution andstability of close orbits (e.g., Scheeres et al.
1996, 1998). This experience and radar
imaging results for Itokawa (Ostro et al. 2001,
2005) were used by the Japanese Institute ofSpace and Astronautical Science in planning
Hayabusa’s encounter.] A reduced need forcontingency fuel could be significant enough toallow a smaller launch vehicle for the mission.
For example, the result might save $100
million via a switch from a Titan III launchvehicle to a Titan IIS, or $200 million for aswitch from a Titan IV to a Titan III. The
ability of prior radar reconnaissance to reduce
mission cost, complexity and risk wasembraced by the Department of Defense in
their proposed Clementine II multiple-flyby
mission (Hope et al., 1997), all of whose
candidate targets either had already been
observed with radar (Toutatis, Golevka) or
were radar observable prior to encounter (1987
OA, 1989 UR).
Radar refinement of physical properties
and radar refinement of orbits are very tightly
coupled: shape modeling necessarily involves
refinement of the delay-Doppler trajectory of
the center of mass. With very precise radar
astrometry determining the target body orbit, aspacecraft could be guided to rendevous withfewer or smaller maneuvers.

Comets
The risk of a civilization-ending impact during
this century is about the same as the risk of acivilization-ending impact by a long-period
comet (LPC) during this millennium. At
present, the maximum possible warning time
for an LPC impact probably is between a few

months and a few years. Comet trajectory
prediction is hampered by optical obscurationof the nucleus and by uncertainties due totime-varying, non-gravitational forces.
Comets are likely to be very porous
aggregates, so concern about the
ineffectiveness of explosive deflection isunderscored in the case of comets. Radarreconnaissance of an incoming comet would be
the most reliable way to estimate the size ofthe nucleus (Harmon et al., 2004), could reveal
the prevalence of centimeter-and-larger
particles in the coma (Harmon et al., 1989,
1997; Nolan et al. 2006), and would bevaluable for determining the likelihood of acollision.
Uniqueness of radar opportunities
How much effort should be made to makeradar observations of NEAs? For newly
discovered objects, it is desirable to guarantee
recovery to ensure accurate prediction of close
approaches at least throughout this century.
Moreover, a target's discovery apparition
often provides the most favorable radar
opportunity for decades and hence a uniquechance for physical characterization that
otherwise would require a space mission.
Similarly, even for NEAs that have alreadybeen detected, any opportunity offering asignificant increment in echo strength andhence imaging resolution should be exploited.
Binaries and non-principal-axis rotators, forwhich determination of dynamical andgeophysical properties requires a long,
preferably multi-apparition time base, should
be observed extensively during any radar
opportunity.

Current State of Arecibo and Goldstone
Both Arecibo and Goldstone are alreadyheavily oversubscribed, with only several
percent of their time available for asteroid
radar. More to the point of this workshop,
for each system, the financial support has
declined during the past several years and is
not adequate to sustain the radar capabilities.
The result has been increasing failures ofobservations due to system (usuallytransmitter) problems, cancellation of NEA
observations due to insufficient support
personnel, and difficulty in maintaining
support personnel at critical mass. Both sites
need adequate, reliable financial support.

Acknowledgment. This research was
conducted at the Jet Propulsion Laboratory,
California Institute of Technology, undercontract with the National Aeronautics andSpace Administration (NASA).
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