Everything seemed to go according to plan. "The navigation was perfect — better than I expected," said NASA scientist Donald K. Yeomans as the first images came in. "I can't imagine how this could go any better." Scientists, engineers, and administrators at the Jet Propulsion Laboratory in Pasadena, California, celebrated loudly as the first images came back confirming a hit.
Despite less-than-perfect focus in the flyby spacecraft's high-resolution camera, the images were nothing short of breathtaking. The projectile, which also relayed a steady stream of detail-rich images until just seconds before its demise, struck a sunlit portion of the comet’s elongated nucleus near one end. A bright incandescent flash appeared briefly, then a broad fan of debris could be seen splashing out into the surrounding space.
Now scientists involved with the $333-million mission will be looking at what the impact left behind. The size and shape of the resulting crater should speak volumes about the structure of the comet’s nucleus. If the gouge is small — say, the size of a house — it means the nucleus has a solid internal structure, like that of a frozen-solid ice cube.
As more images and infrared specta are sent to Earth throughout the day, mission scientists will be examining them intently. For the moment they can only speculate about the comet's structure. However, the huge amount of debris seen in the initial photographs strongly suggests that a stadium-size divot was left behind and that Tempel 1’s interior is likely porous material bound together weakly by gravity.
Comets are likely unaltered collections of volatile ices and rocky material left over from the creation of the solar system 4.5 billion years ago. Short-period comets like Tempel 1, while relatively easy to reach by spacecraft, are exposed to strong sunlight each time they enter the inner solar system. So Deep Impact’s main objective was to excavate pristine materials from far below the surface of the nucleus. According to principal investigator Michael A'Hearn (University of Maryland), infrared spectra acquired during the collision have already revealed the presence of yet-to-be-identified compounds.
As the Deep Impact team in Pasadena focused on the mission itself, amateur and professional astronomers worldwide watched the event through the eyepiece. Right now Tempel 1 is 0.89 astronomical unit (134 million kilometers) from Earth, and prior to the collision it had been a diffuse, elusive 10th-magnitude telescopic target. Observing reports suggest that in the moments after impact the comet's overall brightness increased about 0.5 magnitude, while the brightness of the nucleus itself increased more than a full magnitude.
Just how bright it gets will depend on how much dust was excavated in the first place, the rate at which the dust cloud expands, how long it stays concentrated and highly reflective, and whether the impact created an active region on the comet from which more material can spew out. For now, says A’Hearn, we do know that it was "a very big impact.”
Tempel 1 was discovered in France in 1867 by Ernst Wilhelm Leberecht Tempel. Measuring 9 km long and 3 wide (about half the size of Manhattan), the comet orbits between Mars and Jupiter and takes 5.5 years to loop around the Sun.