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-size 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.

After completing the initial mission to Tempel 1, the University of Maryland-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).

Following up on the successes of Deep Impact, University of Maryland 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.

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 Analyzes 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.

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.”

Michael A’Hearn is a Distinguished University Professor Emeritus of Astronomy at the University of Maryland. He was the Deep Impact/DIXI principal investigator, 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.

Jessica Sunshine is a Professor of Astronomy at the University of Maryland. She was a co-investigator for the Deep Impact mission to Tempel 1 and deputy principal investigator for the extended mission to Hartley 2 and its cometary observations. Sunshine is also the principal investigator for the proposed CHagall mission to investigate the subsurface of comet Hartley 2 in 2021.

Lori Feaga is an Associate Research Scientist in astronomy at the University of Maryland. She was a science team member for the Deep Impact mission to comet Tempel 1, and a co-investigator on its extended mission to comet Hartley 2 and observations of comet Garradd. Feaga is currently a co-investigator working with the Alice ultraviolet spectrograph instrument on the Rosetta spacecraft.

Tony Farnham is a Senior Research Scientist in astronomy at the University of Maryland. He was a science team member for the Deep Impact and Stardust NExT missions to comet Tempel 1 and a co-investigator for DIXI’s mission to comet Hartley 2 and observations of comet ISON. He also led one of the teams that performed the analysis of the dust hazards posed by comet Siding Spring during its close approach to Mars.

The 10-year anniversary of Deep Impact’s first comet flyby, which saw it crash a probe craft into Tempel 1 and generate worldwide headlines and unprecedented comet science, will be on July 4, 2015. The researchers are available to speak with the media any time.

Matthew Wright
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Abby Robinson
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