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T.L. Farnham (University of Maryland)
We obtained images of Comet 81P/Wild~2 from Lowell Observatory on 10 nights in 1997, using both broad and narrowband filters. The data include observations at +63 and +148 days from perihelion, which brackets the time of the Stardust encounter at +98 days. Coma features are visible throughout our observations. A primary fan-shaped jet is visible from February through April, and its unchanging nature during this time frame indicates that the source must lie within 10--20\circ of the spin axis. In July, a secondary source is visible and must be located at a mid-latitude on the opposite hemisphere from the primary. Photometry and dust production measurements suggest that the primary is larger than the secondary by a factor of two or more. Using the apparent position angles of the primary jet, we found that the spin axis is oriented at \alpha=218\circ, \delta=+13\circ. Although the primary jet will be in shadow at the time of the Stardust encounter, the sun will be nearly directly over the secondary at local noon, so there is a strong possibility that the spacecraft will pass through material being emitted by the smaller secondary source.
A sharply defined parabolic envelope is visible in the 9~July images and by adopting a fountain model, with sunward-emitted dust being turned by radiation pressure, we estimate the dust emission velocity to be 100--200~m/s for 1--2~\mum particles (Sanzovo et al. 2001 found the optically important grain size to be 1.7~\mum.) We measured A(\theta)f\rho, a proxy for the dust production, on the two dates bracketing the spacecraft encounter time frame. On both dates, we found a value of ~240~cm in a 15~arcsec radius aperture, which suggests that it will remain around this value during the time of the Stardust encounter. We also used the surface brightness profile of the coma to obtain a more physical representation of the dust production rate. Adopting simple assumptions about the grain properties (a=1.7~\mum, v=140~m/s, \rho=1~g/cm3, A=5%) we found the dust production in the primary jet on 5~March to be ~20~kg/sec, and in the secondary on 9~July to be ~10~kg/sec. On both dates, the coma region outside the jet had a production rate about half that in the jet.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.