2005

Deep Impact: Tempel-1

Missions like the Giotto flyby of Halley’s Comet in 1986 or the Stardust flyby of Wild-2 in 2004 revealed comet nuclei to be small, icy bodies. They are also very dark bodies, typically reflecting only about 3–5 percent of the sunlight that strikes them (about as dark as charcoal). It seems odd that icy bodies are so dark—but the reason is that ice is evaporated by the Sun’s heat, leaving behind a lag of rocky and organic grains that darken the surface. Impacts or tidal forces can crack this surface lag, letting fresh ice escape from cracks into the interior.

If this model of a comet’s surface is correct, reasoned some scientists in a mission proposal to NASA is 1999, then it should be possible—with a powerful enough impact—to design a mission to punch a hole in the surface lag and expose a comet’s “pristine” icy materials for study. NASA accepted the bold mission idea, and the Deep Impact mission, carrying a 815-pound (370-kilogram) copper projectile probe, was launched toward comet Tempel-1 in early 2005.

As Deep Impact got close to Tempel-1, imaging of the comet’s nucleus revealed it to indeed be a dark, irregular body about 5 × 3 miles, or 8 × 5 kilometers, in size, with surprisingly complex geologic landforms, including possible impact craters and lobes of strangely layered terrain. The spacecraft released the impactor for its July 4, 2005, crash landing, and the resulting impact generated some impressive cosmic fireworks. The projectile penetrated below the lag crust and into the comet’s subsurface, releasing a huge spray of ice and dust (including clays, carbonates, and silicates). Images from the Deep Impact flyby spacecraft showed a spectacular flash and a giant debris cloud that hid the resulting impact crater from view. Later analysis of the flyby data showed Tempel-1 to have a low density (0.6 grams per cubic centimeter), indicating a rather porous, icy interior composition.

NASA’s Stardust spacecraft, which had completed its sample return mission from comet Wild-2 in 2006, was redirected to a close flyby past Tempel-1 in February 2011. Stardust successfully imaged the 500-foot-diameter (150 meters) impact crater made by the Deep Impact probe’s encounter six years earlier.

A blinding shock wave of light emerges from the nucleus of comet Tempel-1 about 67 seconds after the Deep Impact spacecraft’s impact probe smashed into the comet at 23,000 miles per hour (10 kilometers per second). The collision excavated ice and dusty silicate minerals from the comet’s subsurface.