Tempel 1 is not a bright comet; its brightest apparent magnitude since discovery has been 11, far below naked-eye visibility. Its nucleus measures 7.6 km × 4.9 km (4.7 mi × 3.0 mi). Measurements taken by the Hubble Space Telescope in visible light and the Spitzer Space Telescope in infrared light suggest a low albedo of only 4%. A two-day rotation rate was also determined.
Deep Impact space mission
The head-on collision of comet 9P/Tempel and the Deep Impact impactor
On 4 July 2005 at 05:52 UTC (01:52 EDT), Tempel 1 was deliberately struck by one component of the NASA Deep Impact probe, one day before perihelion. The impact was photographed by the other component of the probe, which recorded a bright spray from the impact site. The impact was also observed by earthbound and space telescopes, which recorded a brightening of several magnitudes.
The crater that formed was not visible to Deep Impact due to the cloud of dust raised by the impact, but was estimated to be between 100 and 250 meters in diameter and 30 meters deep. The probe's spectrometer instrument detected dust particles finer than human hair, and discovered the presence of silicates, carbonates, smectite, metal sulfides (like fool's gold), amorphous carbon and polycyclic aromatic hydrocarbons. Water ice was detected in the ejecta. The water ice came from 1 meter below the surface crust (the devolatized layer around the nucleus).
In part because the crater formed during the Deep Impact collision could not be imaged during the initial flyby, on 3 July 2007, NASA approved the New Exploration of Tempel 1 (or NExT) mission. The low-cost mission utilized the already existing Stardust spacecraft, which had studied Comet Wild 2 in 2004. Stardust was placed into a new orbit so that it approached Tempel 1. It passed at a distance of approximately 181 km (112 mi) on February 15, 2011, 04:42 UTC. This was the first time that a comet was visited twice.
On February 15, NASA scientists identified the crater formed by Deep Impact in images from Stardust. The crater is estimated to be 150 m (490 ft) in diameter, and has a bright mound in the center likely created when material from the impact fell back into the crater. The geometry of the flyby allowed investigators to obtain considerably more three-dimensional information about the nucleus from stereo pairs of images than during Deep Impact's encounter. Scientists were able to quickly spot locations where an elevated flow-like formation of icy material on the comet's surface receded due to sublimation between encounters.
Comets are in unstable orbits that evolve over time due to perturbations and outgassing. Tempel 1 passed within 0.04 AU - or 5.9 million km (3.7 million mi) - of the dwarf planet Ceres on November 11, 2011. Then, as a Jupiter-family comet, it will spend years interacting with the giant planet Jupiter, finally passing within 0.02 AU - or 3.0 million km (1.9 million mi) - of Mars on October 17, 2183.