In the 1960s, Kowal observed with the Palomar
48" Schmidt telescope, contributing observations to noted
cosmologist Fritz Zwicky's six-volume
Catalogue of Galaxies and of Clusters of Galaxies. Kowal also began to search for
Type Ia supernovae in other galaxies, in an effort led by Zwicky to calibrate the magnitudes of these exploding stars so that they could be used as
standard candles, reliable measures of the distance of their host galaxies (work which in the present has led to accurate measurements of the
expansion of the universe). In the course of these Palomar supernovae surveys with the 48" Schmidt, Kowal personally discovered 81 supernovae, including
SN 1972E. In 1973, Caltech astronomers
Eleanor Helin and
Gene Shoemaker began an observing program to search out and track previously unknown
near-Earth asteroids, the
Planet-Crossing Asteroid Survey (PCAS), a photographic plate survey that began on the Palomar 18" Schmidt telescope. Although primarily employed by the supernova survey to observe on the 48" Schmidt, Kowal provided "crucial observations" of particularly faint asteroids for the PCAS program with the larger telescope. His asteroid discoveries and co-discoveries include the notable asteroids
Aten asteroid 2340 Hathor; the
Apollo asteroids
1981 Midas,
2063 Bacchus,
2102 Tantalus and
(5660) 1974 MA; the
Amor asteroids
(4596) 1981 QB and
(4688) 1980 WF; and the
Trojan asteroids
2241 Alcathous and
2594 Acamas. PCAS later moved to the 48" Schmidt, and ran in total for nearly 25 years, until June 1995. Kowal provided observations of new Solar System discoveries and reports of new supernovae via the
IAU circular system throughout the 1970s, and searched for new objects. He discovered two
moons of
Jupiter:
Leda in 1974 and
Themisto in 1975, the 13th and 14th moons of Jupiter to be found. Themisto was later lost (i.e. its orbit was not known well enough to reobserve it) and was not rediscovered until 2000. Between December 1976 and February 1985, Kowal searched 6400 square degrees of sky in the
ecliptic plane for distant, slow-moving Solar System objects. Only one object was found beyond Jupiter:
2060 Chiron, discovered in 1977, which had the unusual characteristic of features both like an asteroid and a comet. It became recognised as the first object in the
centaur class after a second one was discovered 15 years later. Centaurs are objects with unstable orbits which orbit between Jupiter and
Neptune. They are probably drawn in from the
Kuiper belt by alignments with larger planets. Chiron remains one of the largest such worlds known, and one of a handful that have a comet-like
coma. Kowal also discovered or co-discovered the periodic comets
99P/Kowal,
104P/Kowal,
134P/Kowal-Vavrova,
143P/Kowal-Mrkos, and
158P/Kowal-LINEAR. In 1980, Kowal's research in astronomical history found a 1613 drawing by
Galileo Galilei showing Neptune near Jupiter, predating the
discovery of Neptune in 1846; Kowal was awarded the inaugural
R. R. Newton Award for Scientific History for this "shockingly outré" finding. Kowal moved to the new
Space Telescope Science Institute in 1985, where he monitored the instruments of the
Hubble Space Telescope as one of the operations astronomers. His book
Asteroids: Their Nature and Utilization was published in 1988, and a second edition in 1996. From 1996 until his retirement in 2006, he worked at the
Johns Hopkins University Applied Physics Laboratory, providing software for the
NEAR Shoemaker spacecraft's mission to land on the asteroid
Eros and mission operations support for the NASA TIMED mission. Kowal died on November 28, 2011, at the age of 71. == Honours and awards ==