Perseverance is one of a few Mars spacecraft carrying laser retroreflectors. The devices could provide new science and safer Mars landings in the future.
When the Apollo astronauts landed on the Moon, they
brought devices with them called retroreflectors, which are essentially small
arrays of mirrors. The plan was for scientists on Earth to aim lasers at them
and calculate the time it took for the beams to return. This provided exceptionally
precise measurements of the Moon’s orbit and shape, including how it changed
slightly based on Earth’s gravitational pull.
Research with these Apollo-era lunar retroreflectors
continues to this day, and scientists want to perform similar experiments on
Mars. NASA’s Perseverance rover –
scheduled to land on the Red Planet on Feb. 18, 2021 – carries the palm-size
Laser Retroreflector Array (LaRA). There’s also small one aboard the agency’s InSight lander, called Laser
Retroreflector for InSight (LaRRI). And a retroreflector will be aboard the ESA
(European Space Agency) ExoMars rover that launches in 2022.
While there is currently no laser in the works for this
sort of Mars research, the devices are geared toward the future: Reflectors
like these could one day enable scientists conducting what is called
laser-ranging research to measure the position of a rover on the Martian
surface, test Einstein’s theory of general relativity, and help make future landings
on the Red Planet more precise.
retroreflectors are shiny, pointlike position markers,” said Simone
Dell’Agnello, who led development of all three retroreflectors at Italy’s
National Institute for Nuclear Physics, which built the devices on behalf of
the Italian Space Agency. “Because they’re simple and maintenance-free, they can work for decades.”
A Box of Mirrors
The devices work a lot like a bike
reflector, bouncing light back in the direction of its source. Perseverance’s LaRA,
for example, is a 2-inch-wide (5-centimeter-wide) dome speckled with half-inch
holes containing glass cells. In each cell, three mirrored faces are positioned
at 90-degree angles from one another so that light entering the holes is directed
back out at exactly the same direction it came from.
LaRA is much
smaller than the retroreflectors on the Moon. The earliest ones, delivered by the Apollo
11 and 14 missions, are about the size of typical computer monitor and embedded
with 100 reflectors; the ones delivered by Apollo 15 are even larger and
embedded with 300 reflectors. That’s because the lasers have to travel as much
as 478,000 miles (770,000 kilometers) to the Moon and back. By the return trip,
the beams are so faint, they can’t be detected by the human eye.
The beams that Perseverance’s LaRA and InSight’s LaRRI were
built to reflect would actually have a far shorter journey, despite Mars being
some 249 million miles (401 million kilometers) away at its farthest
point from Earth. Rather than traveling back and forth from Earth, which
would require enormous retroreflectors, the laser beams would just need to
travel back and forth from a future Mars orbiter equipped with an appropriate
Such an orbiter could determine the precise position of a retroreflector
on the Martian surface. And since Perseverance will be mobile, it could provide
multiple points of reference. Meanwhile, the orbiter’s position would also be
tracked from Earth. This would allow scientists to test Einstein’s theory of general
relativity, as they have with retroreflectors on the Moon. Each planet’s orbit
is greatly influenced by the bend in space-time created by the Sun’s large
“This kind of science is important for understanding
how gravity shapes our solar system, the whole universe, and ultimately the
roles of dark matter and dark energy,” Dell’Agnello noted.
In the case of the InSight lander, which touched down on
Nov. 26, 2018, laser-ranging science could also aid the spacecraft’s core
mission of studying Mars’ deep interior. InSight relies on a radio
instrument to detect subtle differences in the planet’s rotation. In
learning from the instrument how the planet wobbles over time, scientists may finally
determine whether Mars’ core is liquid or solid.
And if the science team were able to use the lander’s retroreflector,
they could get even more precise positioning data than InSight’s radio provides.
LaRRI could also detect how the terrain InSight sits on shifts over time and in
what direction, revealing how the Martian crust expands or contracts.
Mars landings are
hard. To help get Perseverance safely to the surface, the mission will rely on Terrain-Relative Navigation, a new technology that compares images taken
during descent to an onboard map. If the spacecraft sees itself getting too
close to danger (like a cliffside or sand dunes), it can veer away.
But in such a
mission-critical event, you can never have too many backups. Future missions
barreling toward the surface of the Red Planet could use the series of
reference points from laser retroreflectors as a check on the performance of
their Terrain Relative Navigation systems – and perhaps even boost their
accuracy down to a few centimeters. When the difference between successfully
landing near an enticing geological formation or slipping down the steep slope
of a crater wall can be measured in mere feet, retroreflectors might be critical.
could open up new kinds of Mars exploration,” Dell’Agnello said.
More About the Mission
A key objective for
Perseverance’s mission on Mars is astrobiology, including the search for signs of ancient microbial
life. The rover will also characterize the planet’s climate and geology, pave
the way for human exploration of the Red Planet, and be the first planetary
mission to collect and cache Martian rock and regolith (broken rock and dust).
Subsequent missions, currently under consideration by NASA in cooperation with
the European Space Agency, would send spacecraft to Mars to collect these
cached samples from the surface and return them to Earth for in-depth analysis.
Managed for NASA by JPL, a division of Caltech in Pasadena,
California, the Mars 2020 Perseverance rover is part of a larger program that
includes missions to the Moon as a way to prepare for human exploration of the
Red Planet. Charged with returning astronauts to the Moon by 2024, NASA will
establish a sustained human presence on and around the Moon by 2028 through
NASA’s Artemis lunar exploration
For more information about NASA’s Perseverance, go to:
For more about NASA’s Moon to Mars plans, visit:
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Source: Jet Propulsion Laboratory