TL;DR: A two-volume NASA record of an October 1974 colloquium in Greenbelt, Maryland, preserved the scientific case that later guided U.S. and international comet flybys, impacts and rendezvous.
What the archive preserves
The publication at the center of this retrospective is Background for a Mission: The Study of Comets, the proceedings of a colloquium held in Greenbelt, Maryland, in October 1974, according to Science.
The set was issued by the National Aeronautics and Space Administration in Washington, D.C., in 1975 and was made available from the Superintendent of Documents in Washington, D.C., according to Science.
It appeared in two parts with xxxiv + 1,084 pages and illustrations, priced at $11.25, and designated NASA SP-393, according to Science.
The bibliographic record identifies the editors as B. Donn, M. Mumma, W. Jackson, M. A'Hearn and R. Harrington, according to Science.
A review by Freeman D. Miller of the Department of Astronomy, University of Michigan, Ann Arbor, discussed the volumes in Science, according to Science.
That review appeared in Science on 29 October 1976, in Volume 194, Issue 4264, on pages 515-516, with DOI 10.1126/science.194.4264.515, according to Science.
Who compiled the Greenbelt record
The editor list links the 1974 meeting directly to researchers who later shaped American comet exploration.
Michael Francis A'Hearn was born on November 17, 1940, in Wilmington, Delaware, in the United States, according to Michael A'Hearn.
He received a B.A. in physics at Boston College in 1961, completing the degree in three years, and received a Ph.D. in astronomy at the University of Wisconsin–Madison in 1966 with a thesis titled The Polarization of Venus, advised by Arthur Code, according to Michael A'Hearn.
In 1966 he joined the Department of Physics and Astronomy at the University of Maryland, where he spent 50 years, according to Michael A'Hearn.
In the middle of the 1970s, A'Hearn became interested in comets and began observing them with ground-based telescopes and then space-based telescopes, including Hubble, according to Michael A'Hearn.
He was one of the first researchers who used narrowband filters for measuring comets' gas production rates and for mapping the structure of gas and dust in their comae, according to Michael A'Hearn.
Using that approach, he developed the first chemical classification system based on carbon chain molecules, according to Michael A'Hearn.
He aided in development of systems for surveying abundances in comets as well as techniques for determining the sizes of cometary nuclei using optical and infrared measurements, according to Michael A'Hearn.
From planning volume to first deliberate impact
A'Hearn was the principal investigator for NASA's Deep Impact/EPOXI mission, which performed the first impact of a comet, according to Michael A'Hearn.
Deep Impact was first proposed to NASA in 1996, with Michael J. Belton as principal investigator and A'Hearn as deputy, according to Michael A'Hearn.
Belton left the project, leaving A'Hearn to lead it in the 1998 proposal, according to Michael A'Hearn.
The project was described as one of the first active interplanetary experiments that included an impactor, according to Michael A'Hearn.
During the EPOXI phase, the spacecraft was sent to observe comet Hartley 2, according to Michael A'Hearn.
The distinction between the striking element and the observing element is also emphasized in a later historical summary, which notes that Deep Impact separated the striking impactor from the observing flyby craft, according to Famous Comets That Made History.
That same summary lists Tempel 1 as the comet that received a deliberately targeted spacecraft impactor and notes that the 2005 experiment exposed newly disturbed cometary material, according to Famous Comets That Made History.
Tempel 1 was last glimpsed in 2005 by NASA's Deep Impact mission as the comet was shooting toward the sun on its five-year orbit between Mars and Jupiter, according to Nasa mission spacecraft unravels comet mystery.
Deep Impact pummelled the comet with a special impactor spacecraft, and the material that came out was a surprise to scientists, according to Nasa mission spacecraft unravels comet mystery.
A cloud of fine powdery material emerged, not the water, ice and dirt that was expected, according to Nasa mission spacecraft unravels comet mystery.
Deep Impact also found evidence of ice on the surface of the comet, not just inside it, according to Nasa mission spacecraft unravels comet mystery.
What the Valentine's-night revisit measured
NASA's Stardust-NExT spacecraft later revisited Tempel 1, and its flyby showed erosion on Tempel 1's surface since it skimmed by the sun in 2005, according to Nasa mission spacecraft unravels comet mystery.
The revisit also revealed the first clear pictures of the crater made by a Deep Impact probe, according to Nasa mission spacecraft unravels comet mystery.
The Valentine's-night encounter required the Stardust-NExT mission spacecraft to fight an onslaught of debris from the comet in order to snap dozens of revealing pictures, according to Nasa mission spacecraft unravels comet mystery.
Stardust-NExT co-investigator Don Brownlee said comets in the inner part of the solar system, where the Earth is, are literally coming apart and sending tons of gas, rocks and dust into space, according to Nasa mission spacecraft unravels comet mystery.
Brownlee said they do not spew material in a uniform way but send off clods of dirt, ice and rock that come apart, according to Nasa mission spacecraft unravels comet mystery.
Co-investigator Pete Schultz of Brown University said the team never saw the crater during the first passage because it was obscured, a gap that helped create the follow-up mission, according to Nasa mission spacecraft unravels comet mystery.
Principal investigator Joe Veverka of Cornell University said the approach was planned so that on approach the team would see the Deep Impact area, which meant arriving at precisely the right time and the right place, according to Nasa mission spacecraft unravels comet mystery.
Schultz said the crater is subdued, about 150 metres across, with a small central mound in the centre, and described material going up and coming back down, according to Nasa mission spacecraft unravels comet mystery.
He said the surface where the impact occurred is very weak and fragile, so the crater partly healed itself, according to Nasa mission spacecraft unravels comet mystery.
The historical summary adds that ejected dust can complicate immediate views of a deliberately disturbed surface and that later observations can add context to an earlier encounter's results, according to Famous Comets That Made History.
Halley, Jupiter impacts and an extended rendezvous
A'Hearn initiated the International Halley Watch in 1984, described as a worldwide undertaking designed to collect and preserve observations of comet Halley during its 1986 return, according to Michael A'Hearn.
He then organized an observation campaign for the Comet Shoemaker-Levy 9 collision with Jupiter in 1994, according to Michael A'Hearn.
The later historical summary lists Shoemaker-Levy 9 as the case that supplied a scheduled sequence of impacts on Jupiter and notes that the 1994 impact campaign combined different instruments and vantage points, according to Famous Comets That Made History.
That summary also notes that the Galileo spacecraft supplied a viewpoint on the Jupiter impacts unavailable from Earth alone, according to Famous Comets That Made History.
A'Hearn was also a co-investigator on the Stardust-NExT mission that revisited Tempel 1 after Deep Impact's impact, according to Michael A'Hearn.
He was a co-investigator for two instruments on ESA's Rosetta mission to comet 67P/Churyumov-Gerasimenko: the OSIRIS camera and Alice spectrograph, according to Michael A'Hearn.
The historical summary identifies 67P as the destination of an extended accompanying mission and says the 2014 rendezvous turned changing activity into a sustained observing program, according to Famous Comets That Made History.
It adds that Philae's surface work complemented Rosetta's observations from nearby space and that a companion mission can follow rotational and seasonal changes in activity, according to Famous Comets That Made History.
A'Hearn later became principal investigator of the Small Bodies Node of NASA's Planetary Data System, according to Michael A'Hearn.
He served as head of the AAS Publications Board in 2005-2008 and was also president of the solar-system division of the International Astronomical Union, according to Michael A'Hearn.
Measuring comets as primitive bodies
A'Hearn's research showed that comets are consolidated, primitive, loosely packed icy mud balls from the dawn of the solar system and building blocks of the planets, according to Michael A'Hearn.
Astronomer Carey M. Lisse is cited as describing A'Hearn's best-known contributions as helping to discover the presence of water ice and diatomic sulfur emission in cometary comae, according to Michael A'Hearn.
The same attribution credits introducing the Afrho geometry-independent measure of coma dust density, determining rotation rates of comets by observing cyanogen jets, and resolving the dark, low-albedo nature of cometary nuclei, according to Michael A'Hearn.
A'Hearn supervised 18 PhD students, according to Michael A'Hearn.
By many counts, he trained, collaborated with and employed more than 80 percent of the pre-Rosetta generation of cometary astronomers, according to Lisse as cited in Michael A'Hearn.
He authored over one hundred papers, including more than 30 on Comet Churyumov-Gerasimenko, according to Michael A'Hearn.
In June 1986, the main-belt asteroid 3192 A'Hearn, discovered by American astronomer Edward Bowell at Lowell's Anderson Mesa Station in Flagstaff, Arizona, was named after him in honor of his contributions to cometary science, according to Michael A'Hearn.
His honors included fellow of the American Association for the Advancement of Science in 1989, the NASA Exceptional Scientific Achievement Medal in 2006 and 2012, and the Gerard P. Kuiper Prize in 2008, according to Michael A'Hearn.
Why the 1974-1975 set still matters
The Greenbelt proceedings captured comet science before the spacecraft era tested its models at close range.
Halley's predicted return had linked recorded apparitions to one periodic body, with the 1758 return testing a prediction made decades earlier, according to Famous Comets That Made History.
Halley's Comet orbits the Sun in about 75 years, passing as far as the orbit of Neptune, according to the List of Halley-type comets.
Hyakutake later produced an unexpectedly strong X-ray signal detected by the ROSAT observatory in 1996, while Hale-Bopp enabled extensive observations through changing solar distances, according to Famous Comets That Made History.
The International Cometary Explorer's September 11, 1985 flight through the tail of Comet Giacobini-Zinner became the first comet flyby in history for NASA and Europe, according to a 2026 account citing Space.com, in International Cometary Explorer.
By July 2025, the field had expanded beyond solar-system comets when a large, bright cosmic object from beyond the solar system was found, named 3I/ATLAS, only the third interstellar object ever discovered, according to Live Science.
That object swooped through the inner solar system for about four months, according to Live Science.
Its composition work involved more than a dozen instruments on Earth and around the solar system, recording light intensity across frequencies after the sun warmed the comet and created a fuzzy bright halo, or coma, according to Live Science.
Observations of the coma turned up hydrogen cyanide detected by the Atacama Large Millimeter/submillimeter Array, according to Live Science.
For readers tracing how the United States moved from ground-based comet photometry to active impact experiments and long-duration escort missions, the NASA SP-393 set remains the dated starting point: Greenbelt in October 1974, NASA publication in 1975, and Science review in 1976.