Jon D. Giorgini

Radar Imaging of Binary Near-Earth Asteroid (66391) 1999 KW4

High-resolution radar images reveal near-Earth asteroid (66391) 1999 KW4 to be a binary system. The ∼1.5-kilometer-diameter primary (Alpha) is an unconsolidated gravitational aggregate with a spin period ∼2.8 hours, bulk density ∼2 grams per cubic centimeter, porosity ∼50%, and an oblate shape dominated by an equatorial ridge at the objects potential-energy minimum. The ∼0.5-kilometer secondary (Beta) is elongated and probably is denser than Alpha. Its average orbit about Alpha is circular with a radius ∼2.5 kilometers and period ∼17.4 hours, and its average rotation is synchronous with the long axis pointed toward Alpha, but librational departures from that orientation are evident. Exotic physical and dynamical properties may be common among near-Earth binaries.

Mercury's moment of inertia from spin and gravity data

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, E00L09, doi:10.1029/2012JE004161, 2012 Mercury’s moment of inertia from spin and gravity data Jean-Luc Margot, 1,2 Stanton J. Peale, 3 Sean C. Solomon, 4,5 Steven A. Hauck II, 6 Frank D. Ghigo, 7 Raymond F. Jurgens, 8 Marie Yseboodt, 9 Jon D. Giorgini, 8 Sebastiano Padovan, 1 and Donald B. Campbell 10 Received 15 June 2012; revised 31 August 2012; accepted 5 September 2012; published 27 October 2012. [ 1 ] Earth-based radar observations of the spin state of Mercury at 35 epochs between 2002 and 2012 reveal that its spin axis is tilted by (2.04 AE 0.08) arc min with respect to the orbit normal. The direction of the tilt suggests that Mercury is in or near a Cassini state. Observed rotation rate variations clearly exhibit an 88-day libration pattern which is due to solar gravitational torques acting on the asymmetrically shaped planet. The amplitude of the forced libration, (38.5 AE 1.6) arc sec, corresponds to a longitudinal displacement of

Radar Images of Asteroid 4179 Toutatis

450 m at the equator. Combining these measurements of the spin properties with second-degree gravitational harmonics (Smith et al., 2012) provides an estimate of the polar moment of inertia of Mercury C/MR 2 = 0.346 AE 0.014, where M and R are Mercury’s mass and radius. The fraction of the moment that corresponds to the outer librating shell, which can be used to estimate the size of the core, is C m /C = 0.431 AE 0.025. Citation: Margot, J.-L., S. J. Peale, S. C. Solomon, S. A. Hauck II, F. D. Ghigo, R. F. Jurgens, M. Yseboodt, J. D. Giorgini, S. Padovan, and D. B. Campbell (2012), Mercury’s moment of inertia from spin and gravity data, J. Geophys. Res., 117, E00L09, doi:10.1029/2012JE004161. 1. Introduction [ 2 ] Bulk mass density r = M/V is the primary indicator of the interior composition of a planetary body of mass M and volume V. To quantify the structure of the interior, the most useful quantity is the polar moment of inertia Z r ð x; y; z Þ x 2 þ y 2 dV : C ¼ V In this volume integral expressed in a cartesian coordinate system with principal axes {x, y, z}, the local density is multiplied by the square of the distance to the axis of rotation, which is assumed to be aligned with the z axis. Moments of Department of Earth and Space Sciences, University of California, Los Angeles, California, USA. Department of Physics and Astronomy, University of California, Los Angeles, California, USA. Department of Physics, University of California, Santa Barbara, California, USA. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D. C., USA. Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA. Department of Earth, Environmental, and Planetary Sciences, Case Western Reserve University, Cleveland, Ohio, USA. National Radio Astronomy Laboratory, Green Bank, West Virginia, USA. Jet Propulsion Laboratory, Pasadena, California, USA. Royal Observatory of Belgium, Uccle, Belgium. Department of Astronomy, Cornell University, Ithaca, New York, USA. Corresponding author: J.-L. Margot, Department of Earth and Space Sciences, University of California, 595 Charles Young Dr. E., Los Angeles, CA 90095, USA. ( jlm@ess.ucla.edu) ©2012. American Geophysical Union. All Rights Reserved. 0148-0227/12/2012JE004161 inertia computed about the equatorial axes x and y are denoted by A and B, with A < B < C. The moment of inertia (MoI) of a sphere of uniform density and radius R is 0.4 MR 2 . Earth’s polar MoI value is 0.3307 MR 2 [Yoder, 1995], indicating a concentration of denser material toward the center, which is recognized on the basis of seismological and geochemical evidence to be a primarily iron-nickel core extending

Dynamical Configuration of Binary Near-Earth Asteroid (66391) 1999 KW4

55% of the planetary radius. The value for Mars is 0.3644 MR 2 , suggesting a core radius of

Radar Observations of Comet 103P/Hartley 2

50% of the planetary radius [Konopliv et al., 2011]. The value for Venus has never been measured. Here we describe our determina- tion of the MoI of Mercury and that of its outer rigid shell (C m ), both of which can be used to constrain models of the interior [Hauck et al., 2007; Riner et al., 2008; Rivoldini et al., 2009]. [ 3 ] Both the Earth and Mars polar MoI values were secured by combining measurements of the precession of the spin axis due to external torques (Sun and/or Moon), which depends on [C A (A + B)/2]/C, and of the second-degree harmonic coefficient of the gravity field C 20 = A [C A (A + B)/2]/(MR 2 ). Although this technique is not applicable at Mercury, Peale [1976] proposed an ingenious procedure to estimate the MoI of Mercury and that of its core based on only four quantities. The two quantities related to the gravity field, C 20 and C 22 = (B A A)/(4MR 2 ), have been determined to better than 1% precision by tracking of the MESSENGER spacecraft [Smith et al., 2012]. The two quantities related to the spin state are the obliquity q (tilt of the spin axis with respect to the orbit normal) and amplitude of forced libration in longitude g (small oscillation in the orientation of the long axis of Mercury relative to uniform spin). They have been measured by Earth-based radar observations at 18 epochs between 2002 and 2006. These data provided strong obser- vational evidence that the core of Mercury is molten, and that E00L09 1 of 11

The History and Dynamics of Comet 9P/Tempel 1

Delay-Doppler images of the Earth-crossing asteroid 4179 Toutatis achieve resolutions as fine as 125 nanoseconds (19 meters in range) and 8.3 millihertz (0.15 millimeter per second in radial velocity) and place hundreds to thousands of pixels on the asteroid, which appears to be several kilometers long, topographically bifurcated, and heavily cratered. The image sequence reveals Toutatis to be in an extremely slow, non-principal axis rotation state.

Mitigation of Hazardous Comets and Asteroids: The role of radar in predicting and preventing asteroid and comet collisions with Earth

Dynamical simulations of the coupled rotational and orbital dynamics of binary near-Earth asteroid 66391 (1999 KW4) suggest that it is excited as a result of perturbations from the Sun during perihelion passages. Excitation of the mutual orbit will stimulate complex fluctuations in the orbit and rotation of both components, inducing the attitude of the smaller component to have large variation within some orbits and to hardly vary within others. The primarys proximity to its rotational stability limit suggests an origin from spin-up and disruption of a loosely bound precursor within the past million years.

Radar imaging and characterization of the binary near-Earth asteroid (185851) 2000 DP107

Comets rarely come close enough to be studied intensively with Earth-based radar. The most recent such occurrence was when Comet 103P/Hartley 2 passed within 0.12 AU in late 2010 October, less than two weeks before the EPOXI flyby. This offered a unique opportunity to improve pre-encounter trajectory knowledge and obtain complementary physical data for a spacecraft-targeted comet. 103P/Hartley 2 is only the fourth comet nucleus to be imaged with radar and already the second to be identified as an elongated, bilobate object based on its delay-Doppler signature. The images show the dominant spin mode to be a rotation about the short axis with a period of 18.2 hr. The nucleus has a low radar albedo consistent with a surface density of 0.5-1.0 g cm{sup -3}. A separate echo component was detected from large (>cm) grains ejected anisotropically with velocities of several to tens of meters per second. Radar shows that, in terms of large-grain production, 103P/Hartley 2 is an unusually active comet for its size.

Radar observations of Asteroids 64 Angelina and 69 Hesperia

Since its discovery in 1867, periodic comet 9P/Tempel 1 has been observed at 10 returns to perihelion, including all its returns since 1967. The observations for the seven apparitions beginning in 1967 have been fit with an orbit that includes only radial and transverse nongravitational accelerations that model the rocket-like thrusting introduced by the outgassing of the cometary nucleus. The successful nongravitational acceleration model did not assume any change in the comet’s ability to outgas from one apparition to the next and the outgassing was assumed to reach a maximum at perihelion. The success of this model over the 1967–2003 interval suggests that the comet’s spin axis is currently stable. Rough calculations suggest that the collision of the impactor released by the Deep Impact spacecraft will not provide a noticeable perturbation on the comet’s orbit nor will any new vent that is opened as a result of the impact provide a noticeable change in the comet’s nongravitational acceleration history. The observing geometries prior to, and during, the impact will allow extensive Earth based observations to complement the in situ observations from the impactor and flyby spacecraft.

Recent radar observations of asteroid 1566 Icarus

Thus reduction in uncertainty is tantamount to ensuring that unnecessary costs are avoided and that necessary actions are undertaken with adequate warning. Ground-based radar is a knowledge-gathering tool that is uniquely able to shrink uncertainty in NEO trajectories and physical properties. The power of radar stems largely from the precision of its measurements (Table 3.1). The resolution of echoes in time delay (range) and Doppler frequency (radial velocity) is often of order 1/100 the extent of a kilometer-sized target, so several thousand radar image pixels can be placed on the target. Delay-Doppler positional measurements often have a fractional precision finer than 1/10 000 000, comparable to sub-milliarcsecond optical astrometry. The single-date signal-to-noise ratio (SNR) of echoes, a measure of the number of useful imaging pixels placed on a target by a given radar data set, depends primarily on the object’s distance and size. Figure 3.1 shows nominal values of