Arecibo radar observations help fly NASA spacecraft  

Arecibo Radar Observations of 103P/Hartley

The Arecibo planetary radar system was used to make images of comet 103P/Hartley, the target of the EPOXI spacecraft mission. These observations also measured the precise distance to the comet for use in navigating the spacecraft to the comet. The observations were led by John Harmon.

We observed comet 103P/Hartley 2 on 2010 October 24-27,29-31.

Data	Model

This pair of mosiacs shows twenty radar delay-Doppler images (left) and model images (right) of the nucleus on (from top to bottom) Oct 25-27 and 29-31. Within a row, each of the four frames (two on Oct 30 and 31) is a sum of about 15 minutes of data with a 30 minute gap between the second and third frame. Range from the the observer increases downward. The Doppler (horizontal) axis has been scaled by eye to make the pixels approximately square. The pixel spacing is 75m (0.5 microseconds), with 150m resolution. Absolute range information has been forwarded to the EPOXI nav team.

A simple model of the Hartley-2 nucleus using two prolate spheroidal lobes squashed together fits the data reasonably well. Assuming principal-axis rotation with the spin vector normal to the line-of-sight, John Harmon found that he could reproduce the observations pretty well if he took a primary lobe with 2:1 axial ratio and a smaller lobe with 1.5:1 axial ratio and squashed them together so that the neck width was half the short-axis width of the smaller lobe. The image below shows the model shape in broadside, plane-of-sky aspect. The total length of the nucleus in this model is 2.3 km and the smaller lobe has a major-axis length which is 0.6 that of the larger lobe. The addition of data through Oct 31 allows us to refine the period estimate to 18.15 ± 0.03h. We stress that this is only a crude eyeball model, not a formal fit, and is sensitive to assumed spin-axis aspect. If the spin is NOT normal to the line of sight, then we would be underestimating the long-axis length.

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Submitted to CBAT:

COMET 103P/HARTLEY       
J. K. Harmon, M. C. Nolan, E. S. Howell (Arecibo Observatory)  
and J. D. Giorgini (Jet Propulsion Laboratory) obtained 150-m-resolution  
radar images and radar doppler spectra for Comet 103P/Hartley on  
2010 Oct. 24-27 using the Arecibo Observatory 12.6-cm planetary radar.  
The images show the nucleus to be a highly elongated, bilobate object  
with a long-axis dimension of at least 2.2 km. The images give a  
preliminary rotation period estimate of 18.1 +/- 0.3 hours, although a  
less likely period of 13.2 hours cannot be ruled out. The radar cross  
section of the nucleus is 0.04 km^{2}. The doppler spectra show a broadband  
echo component from large (> cm-size) ejected grains in the inner coma.  
The radar cross section of the large-grain coma is 0.6 km^{2}. This echo  
component is preferentially redshifted, indicating that the bulk of the  
grain ejection is in the anti-earthward direction. The characteristic  
radial velocity dispersion of the grains is 4 m/s.

The Arecibo Observatory is a part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the National Science Foundation with additional funding from NASA for planetary radar studies.

Last modified: Wed Nov 3 14:34:00 AST 2010