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SSkyward ( having formatting issues, sorry)
March 2023
D avid H. Levy
Of comets, more comets, and Fritz Zwicky
Since October 1965, when I spotted
my first comet, Comet Ikeya-Seki, I have seen 227 different comets. Near the dawn of my passion for the night
sky, watching that mighty comet rise, apparently right out the
This project has a rich
history. It is loosely named for
astronomer Fritz Zwicky, one of the founding astronomers at Palomar and one of
the foremost scientists of the last century.
He developed not the big Schmidt but the original smaller 18-inch
Schmidt camera, the very first telescope atop that mountain. Since this project is named after Zwicky,
why are its comets called “ZTF” instead of just Zwicky? It is because the comets are named for the
project, not the man.
The historical Zwicky actually had little interest in
comets. His career leaned towards the
big questions of cosmology, the study of the large-scale issues of the
Universe. But he was the first regular
user of Palomar’s 18-inch Schmidt camera, the telescope Gene and Carolyn
Shoemaker and I used to discover our comets, including the one that collided
with Jupiter in 1994. That in itself was
a tribute to Zwicky, for it offered insights into how comet impacts contributed
to the origin of life on different worlds.
Zwicky was not into comets, but he was deeply concerned with the distant
explosions of massive stars that he and colleague Walter Baade called
supernovae. When he began using the
18-inch there were 12 known supernovae.
He discovered 121 supernovae with the 18-inch, 120 by himself and one
with Paul Wild.
Even though I never met Zwicky, I
can share three aspects of him, not including the most famous one in which he
called anyone he did not like a
“spherical bastard.” The expression was
intended to mean that no matter from which angle you look, that person is (or
was) a bastard. One story I heard from
Walter Hass, founder of the Association of Lunar and Planetary Observers, who
said that when Zwicky was having a quiet chat in a corridor at Caltech with another astronomer,
one could hear him two blocks away. The
other involved Zwicky’s observing coat, which he left in a closet at the
18-inch observatory building. One night
as I was about to observe alone there, as Gene Shoemaker left the building he
said “If you get too cold, you can wear Zwicky’s coat! The thought of that coat haunted me all
night. Third, my friend David Rossetter
named his large 25-inch diameter reflector Fritz, after Zwicky’s first
name. It is a wonderful telescope named
for a brilliant man.
In January, the ion or gas tail of
Comet ZTF showed a sort of disconnection in which the part of the tail closest
to the comet was a thin line which suddenly broadened to a larger fan further
out. This “disconnection event” was
closely tied to a sudden increase in sunspot activity. This ZTF comet teaches us how comets interact
with the solar wind.
As this article goes to press, there
is not one ZTF comet, but two. David
Rossetter and I saw the other one at our club’s dark observing site. The second one is much fainter, visible as an amorphous smudge of small slowly moving haze. As I looked at this second comet, I tried to
understand and appreciate the seminal role that Zwicky played in his time. And in our time, that role has expanded to
explore in still greater detail the night sky that he loved.
