The comet that received a rare dual visit from two different spacecraft is 9P/Tempel 1. This unique exploration involved the Deep Impact probe in 2005, followed years later by the repurposed Stardust spacecraft, renamed Stardust-NExT. The decision to send two missions to the same comet was rooted in a scientific strategy: to study the interior of a comet and then observe how the resulting impact site and the comet’s surface evolved over time. This paired approach provided a dynamic view of cometary structure and behavior that a single flyby could never achieve.
Identifying Comet 9P/Tempel 1
Comet 9P/Tempel 1 is a relatively small, short-period comet belonging to the Jupiter-family, whose orbits are shaped by Jupiter’s gravity. It was discovered by Ernst Wilhelm Leberecht Tempel on April 3, 1867. Its designation, 9P, signifies that it was the ninth periodic comet.
The comet completes one orbit around the Sun approximately every 5.5 years, placing it mostly between the orbits of Mars and Jupiter. Gravitational encounters with Jupiter have changed its orbit over centuries, making it an accessible target for space missions. Its nucleus is shaped like a flattened sphere, measuring roughly 7.6 by 4.9 kilometers.
Like all comets, 9P/Tempel 1 is composed of a mixture of rock, dust, and frozen volatile materials, earning it the description of a “dirty snowball.” Scientists estimated the comet’s density to be low, likely between 300 and 900 kilograms per cubic meter, suggesting it is a highly porous structure. This low density and composition made it an ideal candidate for an experiment designed to probe the subsurface.
Deep Impact’s Mission: Studying the Interior
The first mission to encounter the comet was NASA’s Deep Impact, launched in January 2005. Its objective was looking beneath the comet’s surface. Scientists hypothesized that continuous exposure to solar radiation created a crust of dust, concealing the more pristine ices within. The only way to sample this interior was through a controlled excavation.
On July 4, 2005, Deep Impact separated into two components: a flyby spacecraft and a 370-kilogram copper impactor. The impactor slammed into the nucleus of 9P/Tempel 1 at a high relative speed of about 10.3 kilometers per second. Copper was chosen because its distinct spectral signature would not contaminate the spectroscopic analysis of the comet’s material.
The immediate result was a brilliant flash and a large plume of material ejected from the surface, which obscured the view of the newly formed crater. Spectroscopic instruments on the flyby spacecraft analyzed this vaporized ejecta. This analysis confirmed the presence of water ice, silicates, and various organic molecules, providing the first direct look at the composition of a cometary interior.
The data showed that comets are not uniformly composed, with the exposed subsurface containing more primitive materials than the surface layer. The large volume of ejected material, which appeared more dusty than icy, suggested the nucleus was far more porous and fragile than predicted. This initial impact left the precise dimensions and morphology of the crater a mystery, setting the stage for the second visit.
Stardust-NExT: Examining the Aftermath
The second mission, Stardust-NExT, repurposed the veteran Stardust spacecraft, which had already returned samples from Comet Wild 2. The mission was approved in 2007 to investigate the crater left by Deep Impact and track changes on the comet’s surface. The “NExT” stood for New Exploration of Tempel 1.
On February 14, 2011, five and a half years after the first impact, Stardust-NExT flew past 9P/Tempel 1. The primary objective was to photograph the impact site, which had been hidden by the dust cloud during the Deep Impact event. The resulting images revealed the crater, measuring about 150 meters in diameter, a size consistent with predictions.
The Stardust-NExT images provided crucial details about the crater’s morphology, including a distinct mound of material near its center. This suggested that much of the blasted material had fallen back into the depression. Stardust observations also documented how the comet’s surface had changed after its passage around the Sun, showing erosion in some smooth regions estimated at 20 to 30 meters deep.
New Insights from Dual Exploration
The combined data provided an unprecedented “before and after” picture of a single comet, transforming our understanding of these icy remnants. Deep Impact offered a snapshot of the comet’s internal chemistry, while Stardust-NExT mapped its physical response to both the impact and natural solar heating. The dual exploration demonstrated the highly layered and heterogeneous nature of the cometary nucleus.
The shallow depth and partially refilled shape of the 150-meter crater confirmed that the cometary material is extremely weak and loosely packed, acting more like a fluffy pile of dust than a solid block of ice. This low structural integrity suggests that the nucleus is a fragile aggregate. The two missions also revealed that the process of surface evolution, driven by the sublimation of ices as the comet nears the Sun, is not uniform.
Instead of steady, even erosion, Stardust-NExT showed that substantial changes occurred in specific regions, such as the deepening and merging of pre-existing pits. This localized activity, coupled with Deep Impact’s findings of distinct chemical compositions, supports the view that comets are dynamic, complex bodies with varied internal structures and localized sources of outgassing. The paired missions established a new paradigm for cometary science, showing that sequential observations are powerful.