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Laser-thermal
propelled spacecraft in Earth orbit awaiting its departure. Credit: Creative
Commons Attribution 4.0 International License |
Is it possible to launch a spaceship to Mars using a laser? That's the name of a mission proposed by a group from McGill University in response to a NASA request. The laser, a 10-meter-wide array on Earth, would heat hydrogen plasma in a chamber behind the spacecraft, producing thrust from hydrogen gas and allowing the spacecraft to reach Mars in 45 days. It would then aerobrake in Mars' atmosphere, transporting supplies to human colonists or, possibly one day, even humans.
NASA
issued a challenge to engineers in 2018 to construct a mission to Mars that
could transport a payload of at least 1,000 kg in less than 45 days, as well as
lengthier journeys deep into and out of the solar system. The aim to convey
supplies and, perhaps, astronauts to Mars while reducing their exposure to the
destructive effects of galactic cosmic rays and solar storms drove the short
delivery time. With its chemical-based rockets, Elon Musk's SpaceX estimates
that a human voyage to Mars will take six months.
McGill's
concept, called laser-thermal propulsion, relies on an array of infrared lasers
based on Earth, 10 meters in diameter, combining many invisible infrared beams,
each with a wavelength of about one micron, for a powerful total of 100
megawatts—the electric power required for about 80,000 U.S. households. The
payload, orbiting in an elliptical medium Earth orbit, would have a reflector
that directs the laser beam coming from Earth into a heating chamber containing
a hydrogen plasma. With its core then heated as high as 40,000 degrees Kelvin
(72,000 degrees Fahrenheit), hydrogen gas flowing around the core would reach
10,000 K (18,000 degrees Fahrenheit) and be expelled out a nozzle, creating
thrust to propel the ship away from Earth over an interval of 58 minutes. (Side
thrusters would keep the craft aligned with the laser's beam as Earth rotates.)
When the
beaming ceases, the payload accelerates to about 17 kilometres per second in
relation to Earth, which is fast enough to get the payload past the moon's
orbital distance in less than eight hours. It will still be going at 16 km/s
when it reaches the Martian atmosphere in a month and a half; however, once
there, placing the payload in a 150-km orbit around Mars is a difficult task
for the engineering team to overcome.
Because
the payload cannot contain a chemical propellant to launch a rocket to slow
itself down, it is difficult—the fuel required would reduce the payload mass to
less than 6% of the initial 1,000 kilogrammes. Aerocapture is the sole means to
slow the payload at Mars until people on the red planet can build an analogous
laser array for the approaching vehicle to use its reflector and plasma chamber
to create reverse propulsion.
Even so,
the aerocapture, or aerobraking, in Mars' atmosphere could be a risky
manoeuvre, with the spacecraft experiencing decelerations of up to 8 g (where g
is the acceleration due to gravity at Earth's surface, 9.8 m/s2), which is
about the human limit, for only a few minutes as it is captured within a single
pass around Mars. The huge heat fluxes on the vessel caused by atmospheric
friction would be higher than standard thermal protection system materials, but
not those in current development.
Other previously proposed methods of conveyance, such as laser-electric propulsion, in which a laser beam impinges on photovoltaic (PV) cells behind the payload; solar-electric propulsion, in which sunlight on the PV cells creates the propulsive thrust; nuclear-electric propulsion, in which a nuclear reactor creates electricity that produces ions propelled out a thruster; and nuclear-thermal propulsion, in which a nuclear reactor creates electricity that produces ions propelled.
Due to the fact that the power source remains on Earth and the delivered flux can be processed by a low-mass inflatable reflector, the laser-thermal propulsion mission concept presented by Duplay et al. has an extremely low mass-to-power ratio, in the range of 0.001–0.010 kg/kW—"unparalleled," they write, "far below even those cited for advanced nuclear propulsion technologies."
In the
1970s, laser-thermal propulsion was first investigated using 10.6 micron CO2
lasers, which were the most powerful at the time. At one micron, today's
fiber-optic lasers can be stacked in massively parallel, phased arrays with a
wide, effective diameter, resulting in a focal length of power delivery almost
two orders of magnitude greater—50,000 kilometres in Duplay's laser-thermal
propulsion idea.
A group
lead by scientist Philip Lubin of the University of California in Santa Barbara
is developing an architecture for phased-array lasers, according to Duplay.
Individual laser amplifiers of roughly 100 watts apiece are used in Lubin's
array—each amplifier is a basic loop of fibre optics with an LED light as a
pump that can be mass produced cheaply—so the Mars mission envisioned here
would require on the order of 1 million individual amplifiers.
The first
humans to reach Mars are unlikely to do it using laser-thermal propulsion.
"However, if more humans undertake the journey to establish a long-term
colony, we will require propulsion systems that bring us there faster—if only
to escape radiation threats," says Duplay. He speculates that a
laser-thermal expedition to Mars may launch ten years after the first human
missions, in 2040.
References:
Emmanuel
Duplay et al, Design of a rapid transit to Mars mission using laser-thermal
propulsion, Acta Astronautica (2021). DOI: 10.1016/j.actaastro.2021.11.032
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