Scientists proposed the idea of a laser thermal propulsion system, which arrived on Mars in 45 days

Can a laser send a spacecraft to Mars? A team of scientists at McGill University in Canada replied yes.

They proposed a design that used a 10-meter-wide laser array on Earth to heat a hydrogen plasma in a chamber at the tail of the spacecraft and use hydrogen to generate thrust to send the spacecraft to Mars in 45 days. Once on Mars, it will make aerial brakes in the Martian atmosphere, deliver supplies to human colonists, and even one day send astronauts to Mars.

In 2018, NASA challenged engineers to design a Mars mission that could send at least 1,000 kilograms of payload to Mars in no more than 45 days. NASA hopes to transport cargo and future astronauts to Mars in less time while minimizing their exposure to the devastating effects of cosmic rays and solar storms. For comparison, elon Musk's SpaceX envisioned a human journey to Mars using its chemical-fueled rockets would take six months.

McGill University's concept design, called laser thermal propulsion, relies on an array of infrared lasers from Earth's base, 10 meters in diameter, combined with many invisible infrared beams, each with a wavelength of about 1 micron and a total power of 100 megawatts, equivalent to power supplying 80,000 homes. The payload will operate in an elliptical mid-Earth orbit with a reflector on board to direct a laser beam from Earth into a heated chamber containing hydrogen plasma. Its core is then heated to 40,000 degrees Kelvin (about 39,727 degrees Celsius), and the hydrogen flowing around the core will reach 10,000 degrees Kelvin and eject from the nozzle, creating a thrust to push the spacecraft away from above the Earth. Each ground laser acceleration will be spaced at intervals of 58 min. (The side thrusters will align the vehicle with the laser beam as the Earth rotates.) ）

When the laser beam stopped, the payload sped away at a speed of about 17 km/s relative to Earth, fast enough to allow the vehicle to fly over the moon's orbital distance in as little as 8 hours. When it reaches the Martian atmosphere a month and a half later, it will still be flying at 16 km/s; however, once there, how to place the payload in an orbit of 150 km around Mars is a difficult problem for the engineering team.

The difficulty is that the payload cannot carry chemical propellant to ignite the rocket giving a reverse thrust to slow itself down. Until humans on Mars were able to build an equivalent laser array for a flying spacecraft to provide reverse thrust using its reflectors and plasma chambers, aerial capture was the only way to slow down the payload on Mars.

Even so, aerial capture or air braking in the Martian atmosphere could be a risky move, as the spacecraft's deceleration is as high as 8 g (g is the gravitational acceleration on the Earth's surface, 9.8 m/s2), which is basically the limit of human endurance. And due to atmospheric friction, the large heat flux on the aircraft will exceed the range that traditional thermal protection system materials can withstand.

Researchers on the project say that one of the big advantages of the laser thermal propulsion mission concept is its extremely low mass-to-power ratio, "laser thermal propulsion is able to achieve a 1-ton rapid transport mission using a volleyball court-sized laser array", and even far lower than the mass-to-power ratio of advanced nuclear propulsion technology, because the power source comes from the earth and is processed by low-mass reflectors. Researchers expect that around 2040, laser thermal propulsion technology may become a reality. The study was published in the Acta Astronautica.

Concept diagram of a laser thermal propulsion system