UMD Astronomers Celebrate ‘Deep Impact’ 10-Year Anniversary

Deep Impact and its subsequent investigations transformed our understanding of comets.

Ten years ago, University of Maryland astronomers led NASA’s “Deep Impact” mission—when a spacecraft interacted with the surface of a comet for the first time. The mission made history and worldwide headlines.

On July 4, 2005, the Deep Impact Flyby spacecraft released a washing-machine-sized probe that collided spectacularly with comet Tempel 1 at 23,000 mph, while the main craft observed the results. The explosive impact gave scientists their first-ever view of pristine material from inside a comet’s nucleus—the solid central lump of ice and debris that gives a comet its shape. Much to the surprise of scientists and contrary to most theoretical models, Tempel 1 had a fairly uniform composition of ices, with proportions near the surface being similar to those 20 meters deep.

The mission also showed that comets are surprisingly fluffy. Tempel 1’s nucleus has a very low density—only about half that of water. To produce this low density, the nucleus must have a significant amount of empty space distributed throughout to offset the higher density of the dust that the body contains. Measurements at the impact site suggest that the nucleus of Tempel 1 is at least 75 percent empty space, or about as fluffy as freshly fallen snow.

The Deep Impact mission included principal investigator Michael A’Hearn, a UMD Distinguished University Professor Emeritus of Astronomy; co-investigator Jessica Sunshine, a UMD professor of astronomy; and science team members Lori Feaga, an associate research scientist in astronomy at UMD, and Tony Farnham, a senior research scientist in astronomy at UMD. 

After completing the initial mission to Tempel 1, the UMD-led science team convinced NASA to keep the Deep Impact spacecraft operational for continued comet studies, known as Deep Impact eXtended Investigation (DIXI) missions, which included: 

• • A flyby of comet Hartley 2 (2010),
  • Observations of comet Garradd (2012), and
  • Observations of comet ISON (2013).

A'Hearn was DIXI principal investigator. Sunshine served as deputy principal investigator, and Feaga and Farnham were co-investigators for the extended mission to Hartley 2 and its cometary observations. Also, Feaga was a co-investigator for the mission to observe the comet Garradd, and Farnham was a co-investigator for the mission to observe the comet ISON. 

Following up on the successes of Deep Impact, UMD team members have also played key roles in more recent comet investigations, including:

• • The Stardust NExT mission, when NASA’s Stardust spacecraft was redirected to return to comet Tempel 1 in 2011 to document changes in the surface of the nucleus over the one cometary year (5.5 Earth years) that had elapsed since the initial Deep Impact encounter,
  • Studies of comet Siding Spring’s historically close approach to the planet Mars in 2014, and
  • The Rosetta spacecraft’s ongoing study of comet 67P/Churyumov–Gerasimenko.

A'Hearn was a co-investigator for Stardust NExT and is a co-investigator on Rosetta’s Alice ultraviolet spectrograph instrument and Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS) camera teams. Feaga is currently a co-investigator working with the Alice ultraviolet spectrograph instrument on the Rosetta spacecraft. Farnham led one of the teams that performed the analysis of the dust hazards posed by comet Siding Spring during its close approach to Mars. 

Next up for the University of Maryland Deep Impact team: a proposed $450 million NASA Discovery Program mission back to Hartley 2 to investigate curious surface features and variations in composition seen during the 2010 flyby. If approved, the mission—known as CHagall (Comet Hartley Analyses to Gather Ancient Links to Life)—would blast off in 2021 and reach Hartley 2 in 2026. The spacecraft will orbit the comet, documenting its behavior as it approaches the Sun from beyond Jupiter to near the Earth’s orbit. Spending 95 percent of its mission life within 10 kilometers of Hartley 2, CHagall will repeatedly probe the comet’s subsurface compositions, structures and thermal properties before explosively excavating and analyzing more primitive materials from inside the comet. The mission will pave the way for future missions to return cryogenic samples of highly volatile ices that are uniquely preserved on comets. Sunshine is the principal investigator for the proposed mission.

The 2005 Deep Impact mission sparked a decade of comet discovery and innovation at the University of Maryland and elsewhere, significantly advancing our understanding of these “dirty snowballs.”

Related articles:
Rosetta Data Give Closest-ever Look at a Comet (Jan. 21, 2015)
Rosetta's Onboard Camera Spots Philae Lander Drifting Across the Comet (Nov. 17, 2014)
UMD Astronomers to Analyze Surface of Comet as Spacecraft Drops Robotic Probe on It (Nov. 11, 2014)
Closest-ever Comet Orbiter Streams Data From Bi-lobed "Rubber Ducky" (Sept. 4, 2014)
Comet Siding Spring's Brush With Mars Offers Opportunity, Not Danger (June 18, 2014)
UMD-Led Deep Impact Ends, Leaves Bright Comet Tale (Sept. 20, 2013)
Hubble Brings Faraway Comet ISON Into UMD Researchers' View (April 23, 2013)
Astronomers Take a Closer Look at Comet ISON (March 29, 2013)
Comet Debuting in New Deep Impact Movie Expected to Star this Winter (Feb. 5, 2013)
Spacecraft Flew Through 'Snowstorm' on Encounter with Comet Hartley 2 (Nov. 18, 2010)
Primordial Dry Ice Fuels Comet Jets (Nov. 10, 2010)
UMD Leads Deep Impact Spacecraft on Successful Flyby of Comet Hartley 2 (Nov. 4, 2010)
NASA Gives Two Successful Spacecraft New Assignments (July 3, 2007)
NASA's Deep Impact Adds Color to Unfolding Comet Picture (Sept. 6, 2005)
NASA's Deep Impact Tells a Tale of the Comet (July 8, 2005)
NASA's Deep Impact Generates its own Spectacular Photo Flash (July 4, 2005)
Deep Impact Kicks Off Fourth of July with Deep Space Fireworks (July 4, 2005)
Maryland-Led Deep Impact Detects Comet Nucleus (June 21, 2005)

Media Relations Contact: Matthew Wright, 301-405-9267, mewright@umd.edu

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