Sky & Telescope - March 2024 - 16

NASA & ESA
Mission Update
mission, the selected partner would fund the effort, with no
cost to NASA or the public.
One submission came from a collaboration between two
companies, Astroscale and Momentus. Their proposal is an
uncrewed mission, using a robotic arm to grasp the telescope
and carry it to a higher orbit.
Such a mission " would buy another five or 10 years of lifetime,
maybe even longer, depending on how active the present
solar cycle is, " says Rob Schwarz, chief technology officer
of Momentus. The level of solar activity matters because it
increases the turbulence of the tenuous upper atmosphere
and thus increases the drag on the spacecraft.
Momentus, which builds what are essentially space tugboats
for relocating satellites, has previously demonstrated its
abilities by propelling a variety of small satellites into their
intended orbits. Meanwhile, its partner Astroscale, which
would provide a rendezvous vehicle, has successfully rendezvoused
and docked with a satellite already in orbit.
" So the pieces of [a Hubble boost mission] have all been
PLANET HUGGER Hubble orbits roughly 530 km above Earth's
surface, as shown in this illustration. Our planet's atmosphere drags on
the space telescope, inexorably robbing the craft of orbital energy. As the
telescope drops lower, an uncontrolled reentry becomes more likely.
changes. Hopefully they'll have completed the critical design
reviews by the end of 2023.
" I think it's going to be a year-and-a-half to two-year
effort to implement this as a fully functional, highly efficient
mode, " he says. " But it's something that we're working on as
a risk mitigator, so that if we experience a failure on the current
side of the system, we still have a recourse to fall back on
to continue operations. "
" That's really one of the great things about Hubble, "
Brown says. " It's got enough hardware redundancy up there,
and a lot of smart engineers working on the operations staff,
so when we do have a failure, most of the time you can figure
out a way around it. "
Inevitable Orbital Decay
But the big looming issue is the telescope's orbit. Launched
in 1990 into a 615-kilometer-high orbit (380 miles high),
Hubble has sunk back toward Earth over time due to our
planet's tenuous upper atmosphere, which drags on the craft
and forces its orbit to decay. The telescope now sits about
530 km up. Once it drops below about 500 km, the risk of
uncontrolled reentry rises. At that point, it would need to
either be propelled into a controlled reentry and burn up in
the atmosphere or be boosted to a higher orbit to preserve it.
In December 2022, NASA issued a request for information
from companies to suggest plans for boosting Hubble by
about 2025. These aren't formal mission proposals; rather,
they're a way for the agency to consider its next steps. Eight
teams responded to the request. NASA is currently evaluating
the submissions and may decide to issue a formal request for
proposals as a follow-up. Should the agency proceed with a
16 MARCH 2024 * SKY & TELESCOPE
demonstrated on orbit, just not all together as one integrated
system, " Schwarz says. Momentus itself plans a mission in
February 2024 that would demonstrate its ability to capture a
satellite in space using a robotic arm, he says.
In addition to the eight proposals is one from SpaceX.
SpaceX's initial study, which led to NASA's wider request for
information, would use its crewed Dragon capsule launched
aboard a Falcon 9 booster. The company would fund the mission,
perhaps in combination with a tourist mission or other
paying customer. Since astronauts would be aboard, they
might be able to do some repairs, too, such as replacing the
three failed gyros.
There might be some risk, however, since the mission
would involve actually having Dragon dock with the telescope,
using a docking port installed during the last servicing
mission. The telescope was not designed to withstand the
impact and vibrations caused by such docking.
If such a mission does get approved and carried out, " we
think even just a modest boost could get us into the 2040s,
and maybe even into the 2050s, " Crouse says. " I think it's
a tradeoff that'll have to be identified between the risk
and complexity of executing a mission like that, versus the
expected lifetime of all the aging systems on Hubble, and if it
makes sense to do that. "
An Ongoing Role to Play
For users of the telescope, any extension of its life would be a
very welcome possibility.
" Hubble's exquisite spatial resolution and ultravioletoptical-near-infrared
sensitivity have been critical in localizing
and characterizing transient phenomena, " says Suvi
Gezari (Space Telescope Science Institute). Its ability to obtain
ultraviolet spectroscopy of supernovae within days of their
explosions is unparalleled, she explains, and it also gives
astronomers key insight into the deaths of stars shredded by
supermassive black holes.
Sky & Telescope - March 2024

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