Herbert Allen Zook (1932–2001)

Herb was 17, above timberline, herding cattle astride a horse. By the look of the sky and by the smell of ozone he knew a mountain thunderstorm was moving toward him. Cowboys on horseback are more likely to get hit by lightning than are sub-par golfers standing on the fairway holding nine irons high over their heads. Herb got the horse to lie down with him in a depression as lightning struck and thunder encircled him and his horse, Herb's kindness and steady calm courage was a hallmark of his colorful life, Herb related that this frightful incident of atmospheric physics and his curiosity about the celestial mystery of bright meteors in cold clear Montana skies had fired his imagination to study physics.

Herbert Allen Zook passed away on 14 March 2001. Born in Laurel, Montana, on 21 April 1932, Herb grew up on a ranch near Red Lodge, Montana With his brother and sister he spent many hard days working the rangeland of this extraordinary part of the United States.

Herb joined the US Army in 1951 and afterwards used the Gl Bill to go to college. He studied physics at Montana State, where he earned BS and MS degrees in physics and then began doctoral studies at the University of Washington. In 1964 his resume attracted the attention of the Meteoroid Technology and Optics Branch at the Manned Spacecraft Center in Houston (later named the Johnson Space Center). The enticement of space flight was so compelling that Herb accepted an offer for employment without finishing his doctorate.

Herb's first assignments were experimental studies of hypervelocity meteoroid impact craters on window surfaces returned from space Herb led teams that examined windows from the Gemini, Apollo and Skylab missions. He was the first to detect orbital debris impacts on manned spacecraft.

In his first major paper, in 1967, he used his knowledge of hypervelocity impacts and the meteoroid environment to define the lunar secondary meteoroid environment. It was during that research that Herb became interested in the distribution of the structure of the solar system dust complex. When this research was interrupted by reorganizations within NASA, Herb worked in Mission Control as a flight controller supporting lunar surface experiments for Apollo and for solar physics observations on Skylab.

After returning to the Solar System Exploration Division, Herb interpreted a strange dust flux from the solar direction recorded by experiments on the Pioneer 8 and 9 space probes. He inferred from these results that fragments from meteoroid collisions near the sun were driven out of the solar system by solar radiation pressure. Herb coined the term 'beta meteoroids' for this type of dust in unbound orbits in the solar system. Herb and O.E. Berg presented this major discovery in 1975.

During the 1980's Herb made many contributions to space science. From cosmic particle tracks in lunar rocks he deduced that large solar flare activity existed over the last 20,000 years. From Apollo 17 sketches and calculations, he predicted the probable existence of a 10 km high dust cloud above the lunar surface. He modeled meteoroid impact vapor and melt production rates on the Moon, later refining and expanding these calculations to the planet Mercury to explain a sodium and potassium atmosphere around both of these bodies. Herb co-authored many journal papers on surface impact and the interplanetary dust complex with E. Grün, H. Fechtig, R.H. Giese, T. Morgan and A. Potter. Herb made significant contributions to the study of the Long Duration Exposure Facility, predicting the directionality of particle impacts observed on LDEF before it was recovered.

Herb was co-investigator for the dust experiments on the Galileo, Ulysses, and Cassini missions. His contributions to these experiments were fundamental during both the instrument development and the data analysis. During the planning phase of Ulysses, he argued strongly for the inclusion of dust measurement instrumentation. The mounting orientation of the dust instruments on the Galileo, Ulysses and Cassini missions were based on his predictions of the meteoroid environment in a previously unmeasured part of the solar system. The success of the measurements and the discoveries made by these instruments is largely due to Herb's vision. Herb was an essential part of the Ulysses experiment team that first discovered interstellar grains flowing through the solar system. He led the effort that identified the characteristics of fast dust streams leaving the Jovian system, first detected by the Ulysses dust detector. Recently Herb was principal investigator for analysis of the zodiacal light and lunar horizon glow photography recorded during the Clementine mission.

In recent times Herb led a program to study the dynamics of interplanetary dust under radiation forces and the perturbing influence of the planets. In 1988, Zook and A.A. Jackson predicted the existence of a ring of dust around the orbit of the Earth. That ring was observed in IRAS and COBE data in the early 1990's. During the 1990's Herb and Jer-Chyi Liou published a number of papers on dust dynamics research topics including interplanetary particles trapped in other types of resonances, the dynamics of dust particles in a three-body problem, and dust from comets and asteroids in the solar system and from the Kuiper Belt. Herb also led an effort to model observation of planets imbedded in extrasolar dust disks.

Herb spent a sabbatical with the Cosmic Dust Group at the Max-Planck-Institute für Kernphysik, Heidelberg and the University of Heidelberg. He again earned enough graduate credit for a doctorate, lacking only a formal thesis to obtain this degree. However degrees, honors and kudos were not of high priority to Herb Zook. Herb authored or co-authored one hundred and eighteen published papers; most are recognized as major space science contributions.

The very fiber of his being was the exploration of mystery in the Universe. He enjoyed all aspects of science, and was a great admirer of Charles Darwin, whose works he read and reread. Herb was a gentle person and extraordinary man; from humble beginnings he became a leading figure in physics of meteorites and interplanetary dust.

Photograph courtesy of NASA's Johnson Space Center, Houston, TX