Sky & Telescope - March 2024 - 14

Star's brightness
LEAH TISCIONE / S&T, SPECTRUM SOURCE: J. D. LOTHRINGER ET AL. / NATURE 2022
Mission Update
Observatory. Astronomers recommended HWO as a top
priority in their 2020 decadal survey, and the space agency is
investigating options for its construction. But even if HWO
comes to fruition and has the same (or greater) UV coverage
that Hubble has, there likely will be a gap between the end of
Hubble's effective lifetime and the launch of the new facility.
" If Hubble ends its mission, the whole field is going to be
without a UV-optical telescope in space with these kinds of
capabilities for years, " Brown says. By keeping Hubble alive
and healthy as long as possible, " we'd like to close that gap, "
he says. " People depend on these capabilities in UV-optical,
and once they go away, it's going to be shocking. "
What Are the Worries?
What's likely to make Hubble go away? There's always the
possibility of individual system failures, such as gyroscopes,
batteries, or computer chips.
Each of Hubble's gyroscopes contains a rapidly spinning
wheel that enables precise measurements of changes in
orientation. The gyros allow the huge telescope to turn from
one target to another and then maintain its position with
rock-steady stability. The instrument was designed to use
three gyros - one for each direction of motion (x, y, and z) -
to provide this pointing and stability. It launched with six of
them to provide redundancy, in case of failures.
Four of them did indeed fail within a few years. Astronauts
replaced all six gyros in 1999 and again during the 2009 servicing
mission, this time including three that had an updated
design. These have exceeded expectations, lasting at least
five times as long as the earlier type. Meanwhile, the three
remaining " old-type " gyros have since failed.
Glitches with one of the gyros have triggered multiple safe
modes, twice in August 2023 alone. Once Hubble enters safe
mode, and after the underlying problem has been identified
and corrected, it takes at least 24 to 30 hours to get all
systems back online, says project manager Patrick Crouse
(NASA Goddard), who oversees the telescope's operations
and management.
The team is working on ways of reducing that downtime.
Meanwhile, a new operating process seems to have eliminated
the glitches, at least for the time being, he says.
" We're confident we will have at least some subset of the
three for some years to come, " says Wiseman. And even if one
or two of the new gyros do fail, the telescope can continue
to carry out good science. " We already have a technique that
we've tested and tried, that uses only one gyroscope in combination
with Hubble's fine guidance sensors to continue to be
able to point the telescope and keep stability. "
There would be a significant limitation imposed by the loss
of a gyro, however. " When we lose another gyroscope, we no
longer have what we call the field of regard, covering the whole
sky outside of the Sun, " Brown says. " We'll only have about
half the sky available at any one time, which is still good.
Over the course of a year, the whole sky comes into view, but
it's not on any one day. " Neither the Webb nor Roman telescopes
have that all-sky capability, he says, " so that will be a
loss in some science efficiency for following up events. "
If two gyros fail, productivity will drop by 25%, Crouse
says. Some planned observations would simply become
impossible because of the greater limitations on the telescope's
sky coverage.
Other equipment onboard also continues to soldier on,
more than a decade after the last servicing mission. Repairs
to the Advanced Camera for Surveys and the Space Telescope
Wavelength
ATMOSPHERIC SPECTRUM When a planet with an atmosphere passes in front of its star, different wavelengths of starlight are blocked
to varying degrees by the atmosphere, depending on the atmosphere's composition. (In this illustration, shorter, bluer wavelengths are
absorbed.) Astronomers can use the absorption pattern to infer which elements and compounds are present.
14 MARCH 2024 * SKY & TELESCOPE

Sky & Telescope - March 2024

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