Sky & Telescope - August 2021 - 35

ARECIBO OBSERVATORY / NASA / NSF
among the background sounds of countless other creatures.
What was it like to observe with this awesome structure?
Consider the scene in late July 2020, when, following a precautionary
shutdown due to Tropical Storm Isaias, the planetary
radar team fi red up the radio transmitter and nursed
their receiver and computers back to life, just in time to catch
the passage of the potentially hazardous asteroid known as
2020 NK1. Within hours the researchers calculated its orbit
and concluded that at its closest approach this century, in
2043, this asteroid will pass harmlessly at a distance of 2.5
million miles. That meant scientists could remove it from the
list of potential future impactors managed by NASA's Center
for Near-Earth Object studies.
" It is very exciting to have a successful
observing run like this, " says observatory
scientist Flaviane Venditti as she recalls her
experience. " We felt that what we did was
meaningful. "
But now the meaning has gone. After she
woke up and heard the news on that fateful
day in December, her brain didn't register it
for several hours. " I was in a mix of denial
and shock. It was only hours later when a
minute of silence was requested during a
virtual call with more than 100 Arecibo users
and friends that it really hit me. The telescope
had died. That was when I went from denial
to mourning. "
From Pulsars to Asteroids
Engineers completed construction of Arecibo's great dish in
1963. Originally, defense agencies funded the observatory,
but after only a few years the National Science Foundation
(NSF) became its steward, joining with NASA in the 1970s to
undertake a massive upgrade. A second overhaul in the 1990s
smoothed the dish's nearly 40,000 aluminum surface panels
to 2-mm accuracy, enabling it to observe at frequencies as
high as 10 GHz. Although the dish itself was immobile,
astronomers could point at different parts of the sky by moving
the instruments on the overhead platform into different
positions, thereby catching radio waves refl ected off the dish
from the target.
The 30-meter-wide Gregorian dome was
NO THREAT Arecibo observations
at the end of July 2020
enabled astronomers to confi rm
that the asteroid 2020 NK1 will
not strike Earth for at least the
next 200 years. The object is
several hundred meters across.
added to the instrument platform in the
second upgrade. It essentially converted the
dish from its spherical shape into a paraboloid,
focusing the radio waves from space to
a single point instead of them coming to a
focus along a sequence of points in a line. The
upgrade also enabled astronomers to smoothly
change between receivers at different frequencies:
Mounted on a turntable inside the
dome, the desired receiver could be moved to
the focal point and brought into action at the
fl ick of a switch. This instrument platform is
what plunged into the dish, ending more than
a half century of science.
sk yandtelescope.org * AUGUST 2021 35
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Sky & Telescope - August 2021

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