THE CASE FOR A DEDICATED
NEAR-EARTH-OBJECT
RADAR OBSERVATORY

Steven J. Ostro, JPL/Caltech

For the NASA Workshop on Near-Earth
Object Detection, Characterization, andThreat Mitigation
Vail, Colorado
June 2006

The role of radar in dealing with the impact
hazard and the relationship between radarreconnaissance and optical surveys wasdescribed by Ostro (1994; see also Ostro andGiorgini 2004):"One can expect a progression from dedicatedsearch programs to identification of objects
that will make very close Earth approaches
during, say, the next century, to refinement ofthose objects' trajectories, eventually resulting
in either classification of the object asnonthreatening or decisions to takeincreasingly serious forms of action, beginningwith groundbased observing campaigns andperhaps proceeding to spacecraft
reconnaissance. The pervasive issue will be
the state of our uncertainty about possibly
threatening objects, what can be done to
reduce that uncertainty, and associated costs.
The response of NASA (and other agencies) to
the NEO hazard attacks three realms ofignorance. First, although we know the grosscharacter of the population and averagecollision rates, we have discovered only aninsignificant fraction of the potentialimpactors. Optical search programs aim todispel this kind of ignorance. The second kindof uncertainty concerns known objects' orbits
and the circumstances of future close
approaches, and the third concerns theoutcome of the impact of a specific object on a
specific collision course, and hence the object's
physical properties, including mass,dimensions, composition, internal structure,
and whether the object is one object or two. If
spacecraft inspection or defensive action are to
be undertaken, then spin state, detailed surface
properties, and the presence of accompanying
swarms of macroscopic particles could be ofconcern. Groundbased radar is uniquely suitedfor cost-effective trajectory refinement andphysical characterization, that is, for reducing
both kinds of post-discovery uncertainty
about potential NEO hazards."
Search programs aiming for achievement of
90% completeness for NEAs as large as 140 mwithin the next 15 years will produce anenormous surge in the NEA discovery rate. In
coming years, it will become increasingly clearthat most of the NEO radar reconnaissancethat is technically achievable with theoptimally supported Arecibo and Goldstoneantennas is precluded by their limitedaccessibility, and that a dedicated NEO radarobservatory is desirable.
An ideal NEO radar system (Ostro, 1997)
might consist of two antennas like the 100-m
NRAO Greenbank Telescope (GBT, in West
Virginia), operating at a wavelength of about
1 cm (Ka band), one antenna for receiving only
and one equipped with a megawatt transmitterand far enough from the receive-only antennato ensure its isolation from leakage of thetransmitted signal. Each antenna's gain could
be ~88 dB, compared to 72.4 dB for Arecibo.

A two-antenna (bistatic) configuration
would eliminate the frequent transmit/receive
alternation and klystron power cycling
required in single-antenna observations ofNEOs -- the roundtrip travel time (RTT) of
the signal between the radar and the target inseconds is numerically nearly equal to thetarget's distance in mAU, e.g., 10 seconds for atarget 0.01 AU away. Advantages of a twoantenna telescope for NEOs include:(1) no on/off switching of the transmitter,
which ages klystron amplifier tubes;(2) no interruption of data acquisition or ofcoherent integration;(3) no interruption of orientational coverage;(4) no switching of antenna pointing between
transmit and receive directions, which
can differ by more than a beamwidth fora rapidly moving target;(5) doubled data integration time; and(6) accessibility of arbitrarily close targets,
including those within a few EarthMoon distances (RTT < 10 s), using thesame configuration as for distanttargets.
Since this instrument would be dedicatedto NEOs, it would compile more NEO
observation time in one year than Arecibo andGoldstone could in a decade. Given thecurrent state of the art of antennas, receivers
and transmitters, it could be an order ofmagnitude more sensitive than the upgraded
Arecibo telescope. The telescope's lifetime
would probably be at least several decades,
during which it would do flyby-level science(delay-Doppler image sequences placing
thousands of pixels on the target, over enoughorientations to allow detailed reconstruction of
the 3D shape) of over a thousand of theoptically discoverable NEAs, as well as orbitrefining follow-up astrometry on many
thousands of newly discovered NEOs.

The capital cost of this system, ascalibrated by the experience with the GBT,would be comparable to the cost of a
Discovery mission. (It may be cost-effective
to substitute an antenna array for at least thereceive-only dish.) Three such bistatic
systems suitably spaced in longitude andlatitude would ensure constant visibility ofany part of the sky.

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

References
Ostro, S. J. (1994). The role of groundbased
radar in near-Earth object hazard
identification and mitigation. In Hazards
Due to Comets and Asteroids, T. Gehrels,ed., Univ. of Arizona, Tucson, pp. 259282.
Ostro, S. J. (1997). Radar reconnaissance ofnear-Earth objects at the dawn of the next
millennium. In Near-Earth Objects: TheUnited Nations International Conference(J. Remo, ed.), Annals of the NewYork
Academy of Sciences 822, pp. 118-139.
Ostro, S. J., and J. D. Giorgini (2004). Therole of radar in predicting and preventing
asteroid and comet collisions with Earth.
In Mitigation of Hazardous Comets andAsteroids (M. J. S. Belton, D. K. Yeomansand T. H. Morgan, eds.), Cambridge Univ.,
pp. 38-65.