Physics
Scientific paper
Aug 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006iaujd..10e..19i&link_type=abstract
Progress in Planetary Exploration Missions, 26th meeting of the IAU, Joint Discussion 10, 21-22 August 2006, Prague, Czech Repub
Physics
Scientific paper
Velocities of dust particles ejected from comet 9P Tempel 1 after the impactor module collided with the comet were estimated. The brightness of a cloud of ejected material on calibrated MRI (Medium Resolution Instrument) images made several seconds after the impact was studied. In order to make more accurate conclusions on brightness, on some images we didn't consider pixels with brightness less than some limit [1]. This allowed us to use higher-contrast images. The velocities of a level of brightness (the edge of the region of pixels with calibrated physical surface brightness CPSB>lim, where lim is some constant) give us a lower limit of velocities of ejected particles. Actual velocities are greater than velocities of the level of brightness for several reasons: (1) we see only a projection of velocity on the plane perpendicular to a line of sight; (2) if the same amount of material moves from distance r[1] from the place of impact to distance r[2], then the number of particles on a line of sight (and so the brightness) decreases by a factor of r[2]/r[1] . We assumed that it was a continuous ejection of material during several minutes. The projection of the velocity of the center of the brightest spot on an image perpendicular to a line of sight (considered as the ratio of the distance from the place of impact to the time t elapsed after the impact) was about 70 m/s both at t=7.67 s and t=15.6 s if we consider that material of this spot was ejected immediately at the impact. As the velocity is the same for different t, then probably the above assumption is correct. Some particles got much greater velocities, up to several km/s. We concluded that the brightest material began to move approximately at the beginning of the impact and moved with velocity of not more than a few hundred m/s during first 10 seconds. The ratio of semi-major axes of the ellipse corresponding to a level of brightness of bright ejected material was about 4/3. The ellipse doesn't have an ideal form, as we see different sides of the cone from different angles. Inside the bordering ellipse, besides the brightest region, there is a less bright region, which corresponds to a later ejection of material. [1] Ipatov S.I., A'Hearn M.F., 37^ th LPSC, #1462, 2006.
A'Hearn Michael F.
Deep Impact Team
Ipatov Sergei I.
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