A novel propulsion system would hurl hypervelocity pellets at a spacecraft to speed up it

Today, several space agencies are investigating innovative propulsion concepts that allow rapid transit to other bodies in the solar system. These include NASA’s Nuclear-Thermal or Nuclear-Electric Propulsion (NTP/NEP) concepts, which could allow transit times to Mars in 100 days (or even 45), and a nuclear-powered Chinese spacecraft, the Neptune and its largest moon, Triton could explore. While these and other ideas could enable interplanetary exploration, traversing the solar system presents some major challenges.

As we examined in a previous article, it would take a conventionally powered spacecraft between 19,000 and 81,000 years to even reach the nearest star, Proxima Centauri (4.25 light-years from Earth). To that end, engineers have studied proposals for unmanned spacecraft that rely on directed beams of energy (lasers) to accelerate light sails to a fraction of the speed of light. A new idea proposed by researchers at UCLA sees a twist on the beam sail idea: a pellet beam concept that could accelerate a 1-ton spacecraft to the edge of the solar system in less than 20 years.

The concept, titled “Pellet-Beam Propulsion for Breakthrough Space Exploration,” was proposed by Artur Davoyan, an assistant professor of mechanical and aerospace engineering at the University of California, Los Angeles (UCLA). The proposal was one of fourteen selected by NASA’s Innovative Advanced Concepts (NIAC) program as part of its 2023 selection, which awarded a total of $175,000 in grants to advance the technologies. Davoyan’s proposal builds on recent work using Directed Energy Propulsion (DEP) and light sail technology to realize a solar gravity lens.

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Image of a gravitationally lensed galaxy producing an Einstein ring. Source: NASA/ESA/Hubble

As Prof. Davoyan emailed Universe Today, the problem with spacecraft is that they are still bound by the rocket equation:

“All current spacecraft and rockets fly by expanding fuel. The faster the fuel is ejected, the more efficient the rocket is. However, there is a limited amount of fuel that we can carry on board. As a result, the speed to which a spacecraft can be accelerated is limited. This fundamental limit is dictated by the Rocket equation. Rocket Equation’s limitations result in relatively slow and costly space exploration. Missions like Solar Gravitational Lens are not feasible with current spacecraft.”

The Solar Gravitational Lens (SGL) is a revolutionary proposal that would be the most powerful telescope ever conceived. Examples include the Solar Gravity Lens, which was selected for NIAC Phase III development in 2020. The concept relies on a phenomenon predicted by Einstein’s general theory of relativity, known as gravitational lensing, in which massive objects alter the curvature of spacetime and magnify the light from objects in the background. This technique allows astronomers to study distant objects with greater resolution and precision.

By positioning a spacecraft at the heliopause (about 500 AU from the Sun), astronomers have been able to study exoplanets and distant objects with the resolution of a primary mirror about 100 km (62 miles) across. The challenge is to design a propulsion system that could take the spacecraft to that distance in a reasonable amount of time. To date, the only spacecraft to reach interstellar space have been the Voyager 1 and 2 probes, which were launched in 1977 and are currently about 159 and 132 AU from the Sun, respectively.

As it left the solar system, the Voyager 1 probe was moving at a record speed of about 17 km/s (38,028 mph), or 3.6 AU per year. Despite this, it still took this probe 35 years to reach the boundary between the Sun’s solar wind and the interstellar medium (heliopause). At its current speed, it will take Voyager 1 over 40,000 years to fly past another star system – AC+79 3888, an obscure star in the Ursa Minor constellation. For this reason, scientists are studying directed energy (DE) propulsion to accelerate light sails that could reach another star system in a few decades.

Project Starshot, an initiative sponsored by the Breakthrough Foundation, aims to be humankind’s first interstellar journey. Credit: Breakthrough Initiatives.org

As explained by Prof. Davoyan, this method offers some distinct advantages, but it also has its disadvantages:

“Unlike conventional spacecraft and rockets, laser sailing does not require onboard fuel to accelerate. Here the acceleration comes from a laser propelling the spacecraft through radiation pressure. In principle, speeds close to the speed of light can be achieved with this method. However, laser beams diverge at long distances, so there is only a limited range of distances over which a spacecraft can be accelerated. This limitation of laser sailing results in the need for exorbitantly high laser powers in the gigawatt and in some proposals in the terawatt range, or to limit the mass of the spacecraft.”

Examples of the laser beam concept include Project Dragonfly, a feasibility study by the Institute for Interstellar Studies (i4is) for a mission that could reach a nearby star system within a century. Then there’s Breakthrough Starshot, which proposes a 100 gigawatt (Gw) laser array that would accelerate gram-scale (Starchip) nanocrafts. With a top speed of 161 million km (100 million miles), or 20% the speed of light, Starshot will be able to reach Alpha Centauri in about 20 years. Inspired by these concepts, Prof. Davoyan and his colleagues propose a novel twist on the idea: a pellet beam concept.

Like Starshot and Dragonfly, this mission concept could serve as a fast-transit precursor interstellar mission. But for their purposes, Davoyan and his team are studying a pellet beam system that would deliver a payload of ~900 kg (1 US ton) a distance of 500 AU in less than 20 years. said Davoyan:

“In our case, the jet that propels the spacecraft is therefore made up of tiny pellets [we call it] the pellet stream. Each pellet is accelerated to very high speeds by laser ablation, and then the pellets carry their momentum to propel the spacecraft. Unlike a laser beam, pellets don’t diverge as quickly, allowing us to accelerate a heavier spacecraft. The pellets, being much heavier than photons, carry more momentum and can impart greater force to a spacecraft.”

Image of a simulated Earth at the distance of Proxima Centauri as projected by the SGL onto an image plane 650 AU from the Sun. Photo credit: Toth H. & Turyshev, SG

In addition, the small size and low mass of the pellets mean that they can be powered by relatively low power laser beams. Overall, Davoyan and his colleagues estimate that a 10-megawatt (Mw) laser beam could accelerate a 1-ton spacecraft to speeds of up to ~30 AU per year. For phase I, they will demonstrate the feasibility of the pellet beam concept through detailed modeling of the different subsystems and proof-of-concept experiments. They will also explore the utility of the pellet beam system for interstellar missions that could explore neighboring stars in our lifetime.

“The Pellet Beam aims to change the way space is explored by enabling rapid transit missions to distant targets,” Davoyan said. “With the pellet beam, outer planets can be reached in less than a year, 100 AU in about three years, and solar gravitational lenses at 500 AU in about 15 years. Importantly, unlike other concepts, the Pellet Beam can propel heavy (~1 ton) spacecraft, greatly expanding the scope of possible missions.”

If realized, an SGL spacecraft would allow astronomers to directly image neighboring exoplanets (like Proxima b) at multipixel resolution and obtain spectra from their atmospheres. These observations would provide direct evidence for atmospheres, biosignatures, and possibly even technosignatures. In this way, the same technology that allows astronomers to image exoplanets directly and study them in detail would also allow interstellar missions to study them directly.

Further reading: NASA

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