We report on the Hubble Space Telescope program to observe periodic Comet 9P/Tempel 1 in conjunction
with NASA’s Deep Impact Mission. Our objectives were to study the generation and evolution of the
coma resulting from the impact and to obtain wide-band images of the visual outburst generated
by the impact. Two observing campaigns utilizing a total of 17 HST orbits were carried out:
the first occurred on 2005 June 13–14 and fortuitously recorded the appearance of a new,
short-lived fan in the sunward direction on June 14. The principal campaign began two
days before impact and was followed by contiguous orbits through impact plus several hours
and then snapshots one, seven, and twelve days later. All of the observations were made using
the Advanced Camera for Surveys (ACS). For imaging, the ACS High Resolution Channel (HRC)
provides a spatial resolution of 36 km (16 km pixel−1) at the comet at the time of impact.
Baseline images of the comet, made prior to impact,photometrically resolved the comet’s
nucleus. The derived diameter, 6.1 km, is in excellent agreement with the 6.0 ± 0.2 km
diameter derived from the spacecraft imagers. Following the impact, the HRC images illustrate
the temporal and spatial evolution of the ejecta cloud and allow for a determination of its
expansion velocity distribution. One day after impact the ejecta cloud had passed out of the
field-of-view of the HRC.