My first Magazine Sky & Telescope - 03.2019 | Page 23

TWIRLING TELESCOPES
by Benjamin Skuse
ough Space
had a “sail” section that always pointed toward the Sun and
a “wheel” part that swiveled full circle every 2–10 seconds,
like an Oreo twirling on a fl ag skewer. Both sections carried
instruments, but the larger, more complex ones were housed
in the stationary sail part of the telescope — after all, the
main goal was to stare at a fi xed target, the Sun.
AGILE’s Happy Accident
But spinning can provide much more than stability, as the
experience of the Italian AGILE satellite team so vividly
shows. Launched in 2007, Astrorivelatore Gamma a Immag-
ini Leggero (AGILE) is a wide-fi eld gamma-ray instrument
that explores cosmic high-energy physics. The initial aim
was for the telescope to point in one direction for a couple
of weeks and then move on to another area until it covered
a large swath of sky. This worked well until the end of 2009,
when the team hit a snag: The telescope’s inertia wheel keep-
ing the platform stable malfunctioned.
Automatically AGILE switched into a safe mode: The
spacecraft swiveled so that its solar panels constantly faced
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