Itokawa 2004 Planning

Background

Summary:

In June 2004, both Arecibo and Goldstone will attempt delay-Doppler imaging of near-Earth asteroid 
25143 Itokawa (1998 SF36), the target of Japan's Hayabusa (formerly MUSES-C) sample-return mission.  
The goals are to develop an optimally-refined physical model of the asteroid in preparation for Hayabusa's 
2005 encounter, to use radar ranging to assist detection of Yarkovsky nongravitational (thermal emission) 
alteration of the asteroid's orbit, and to help to detect and measure "YORP" thermal-emission alteration 
of the asteroid's spin state.

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Science Background:

Asteroid 25143 Itokawa (1998 SF36) was discovered on Sep. 26, 1998, by LINEAR (MPEC 1998-S45).  Of the 
roughly 1100 currently known near-Earth asteroids (NEAs) at least as bright (that is, with absolute
magnitude H < 18.8), Itokawa is distinguished by having the lowest delta-V for a spacecraft rendezvous:
4.29 km/s vs. 5.95 for 433 Eros. This attribute is why the Japan's Hayabusa (formerly MUSES-C) mission 
to return a sample from an asteroid selected Itokawa as their target. The Hayabusa spacecraft is en route 
to a June 2005 rendezvous and a June 2007 return of the sample to Earth.

Visible and near-infrared spectroscopy (Binzel et al. 2001) suggested that Itokawa has a surface 
composition like that of ordinary chondrite meteorites and is similar in spectral characteristics 
and modeled olivine/pyroxene content to the LL chondrite class.  Kaasalainen et al. (2003) present 
photometry and an analysis yielding estimates of the asteroid's sidereal spin period, 
P = 12.132 +/- 0.0005 h, and pole direction: ecliptic long., lat. = 355, -84 +/- 5 deg. They estimated 
a three-dimensional shape, concluding that Itokawa is "elongated, with rough global dimension ratios 
a/b = 2.0, b/c = 1.3, but the elongation is not due to a bifurcated shape. The surface is not likely 
to contain major concavities. No significant albedo variegation was detected."

We carried out radar observations of Itokawa during its 2001 close approach, obtained images with 
15-meter resolution, and used those images along with photometric results to construct a preliminary 
estimate of Itokawa's shape (Ostro et al. 2004).  Our model can be described as a slightly asymmetrical, 
slightly flattened ellipsoid with extents along its principal axes of 548 x 312 x 276 m +/- 10%.  
Itokawa's topography is very subdued compared to that of other asteroids for which spacecraft images or 
radar reconstructions are available. The asteroid's radar reflectivity and polarization properties 
indicate a near-surface bulk density within 20% of 2.5 g/cm^3.
 
The orientational coverage of the strongest images from 2001 is more limited than that of the data sets 
used for other radar-derived 3-D models, in part because the asteroid's nearly half-day rotation period 
prevents any appreciable day-to-day migration in Arecibo's rotation-phase window..  The effect of this 
limitation on the reconstruction's accuracy depends strongly on the asteroid's detailed physical 
characteristics and may also extend to our ability to discern subtleties of the asteroid's spin state.

Fortunately, Itokawa's June 2004 close approach (Table 1) will bring it to within about one-third of its 
2001 minimum distance, and we expect that radar echoes during the observations proposed here will be very 
much stronger than those obtained in 2001.  Arecibo images will be an order of magnitude stronger than 
Goldstone images.  Our intention is to observe Itokawa at Arecibo at high signal-to-noise ratios until 
it goes south, and then for several days at Goldstone.

The strong 2001 images were confined to subradar latitudes between +18 and +46 deg and the less than 40% 
of a rotation between west longitudes 135 and 276 deg.  Our Arecibo tracks are at negative latitudes and, 
more importantly, west longitudes not imaged at all by Arecibo in 2001.  

We plan to use Goldstone to observe the asteroid at southern latitudes, which are needed to produce 
a new model that is more accurate, especially in its reconstruction of the shape of the asteroid's 
southern "hemisphere".

The primary goal of our observations is to obtain decameter-resolution delay-Doppler images and to 
construct a refined Itokawa physical model, which is to be used by the Hayabusa Mission to orchestrate 
close-orbit maneuvering and sampling operations next year. Significant improvement in the fidelity of 
our reconstruction of Itokawa's shape, topography and spin state are expected.  Additional, related 
goals are to use our images to help to measure Itokawa's nongravitational "Yarkovsky" acceleration as 
well as its "YORP" thermal-emission rotational slowing, as follows.

The Yarkovsky effect, a very subtle nongravitational phenomenon involving acceleration of a rotating 
object due to its anisotropic thermal emission of absorbed sunlight, can cause an object's orbit to 
undergo semimajor axis drift (Rubincam 1995, Farinella et al. 1998, Bottke et al.  2000).  Vokrouhlicky' 
et al. (2000) predicted that radar-refined orbits with sufficiently long astrometric time bases could 
permit detection of nongravitational acceleration of NEAs due to the Yarkovsky effect, and Chesley et al. 
(2003) recently achieved such a detection for 6489 Golevka.  The available radar+optical astrometry may 
not allow detection of the Yarkovsky effect during Itokawa's close approach in June 2004, but if our 
proposed observations are successful, then range-Doppler tracking of the Hayabusa spacecraft during its 
2005 Itokawa encounter should detect the effect.

The Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect is the alteration of an asteroid's spin state by 
torques from the reflection and thermal re-emission of sunlight from the asteroid's surface (Rubincam 2000, 
Voukrouhlicky' and Capek 2002, Vokrouhlicky' et al. 2003).  Vokrouhlicky' et al. (2004) predict that YORP 
rotational slowing of Itokawa should be detectable using optical lightcurves and radar images with a 
2001-2004 time base.  As with the Yarkovsky effect, the YORP effect depends on the asteroid's physical 
properties, including its shape, rotation state, mass, and surface properties.  Therefore, the proposed 
observations can help to constrain not just Itokawa's shape and spin state, but its mass and thermal 
properties as well.


REFERENCES 

Binzel R. P., A. S. Rivkin, S. J. Bus, J. M. Sunshine and T. H. Burbine 2001.  MUSES-C target asteroid 
(25143) 1998 SF36: A reddened ordinary chondrite.  Meteorit. Planet. Sci. 36, 1167-1172.

Bottke, W. F. Jr., D. P. Rubincam, and J. A. Burns 2000. Yarkovsky thermal forces. Icarus 145, 301-331.

Chesley, S. R., S. J. Ostro, D. Vokrouhlicky', D. Capek, J. D. Giorgini, M. C. Nolan, J. L. Margot, 
A. A. Hine, L. A. M. Benner, and A. B. Chamberlin 2003.  Direct detection of the Yarkovsky effect via 
radar ranging to asteroid 6489 Golevka.  Science 302, 1739-1742.

Farinella D., D. Vokrouhlicky', and W. K. Hartmann W. K. 1998. Meteorite delivery via Yarkovsky orbital 
drift.  Icarus 132,378-387.

Kaasalainen, M., T. Kwiatkowski, M. Abe, J. Piironen, T. Nakamura, Y. Ohba, B. Dermawan, T. Farnham, 
F. Colas, S. Lowry, P. Weissman, R. J. Whiteley, D. J. Tholen, S. M. Larson, M. Yoshikawa, I. Toth, and 
F. P. Velichko 2003.  CCD photometry and model of MUSES-C target (25143) 1998 SF36.  Astron. Astrophys. 
405, L29-L32.

Ostro, S. J., L. A. M. Benner, M. C. Nolan, C. Magri, J. D. Giorgini, D. J. Scheeres, S. B. Broschart, 
M. Kaasalainen, D. Vokrouhlicky', S. R. Chesley, J. L. Margot, R. F. Jurgens, R. Rose, D. K. Yeomans, 
S. Suzuki, and E. M. De Jong 2004.  Radar observations of asteroid 25143 Itokawa (1998 SF36).  
Meteorit. Planet. Sci. 39, 407-424.

Rubincam, D. P. 1995.  Asteroid orbit evolution due to thermal drag. J. Geophys. Res. 100, 1585-1594.

Rubincam, D. P. 2000.  Radiative spin-up and spin-down of small asteroids.  Icarus  148, 2-11.

Vokrouhlicky', D., A. Milani and S. R. Chesley 2000.  Yarkovsky effect on small near-Earth asteroids: 
Mathematical formulation and examples. Icarus 148, 118-138.

Voukrouhlicky', D. and D. Capek 2002.  YORP-Induced long-term evolution of the spin state of small 
asteroids and meteoroids: Rubincam's approximation.  Icarus 159, 449-467.

Vokrouhlicky', D., D. Nesvorny, and W. F. Bottke, Jr. 2003. The vector alignments of asteroid spins 
by thermal torques. Nature 425, 147-151.
 
Vokrouhlicky', D., D. Capek, M. Kaasalainen, and S. J. Ostro 2004. Detectability of YORP rotational 
slowing of asteroid 25143 Itokawa. Astron. Astrophys. 414, L21-L24.

Last updated: 2004 June 24

Ephemeris, SNRs, Assigned Tracks, Uncertainties

Goldstone Track Assignments

Setups

Instructions

ITOKAWA POINTING: