Today, multiple space agencies are investigating cutting-edge propulsion ideas that will allow for rapid transits to other bodies in the Solar System.
These include NASA's Nuclear-Thermal or Nuclear-Electric Propulsion (NTP/NEP) concepts that could enable transit times to Mars in 100 days (or even 45) and a nuclear-powered Chinese spacecraft that could explore Neptune and its largest moon, Triton.
While these and other ideas could allow for interplanetary exploration, getting beyond the Solar System presents some major challenges.
As we explored in a previous article, it would take spacecraft using conventional propulsion anywhere from 19,000 to 81,000 years to reach even the nearest star, Proxima Centauri (4.25 light-years from Earth). To this end, engineers have been researching proposals for uncrewed spacecraft that rely on beams of directed energy (lasers) to accelerate light sails to a fraction of the speed of light.
A new idea proposed by researchers from UCLA envisions 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 proposals chosen by the NASA Innovative Advanced Concepts (NIAC) program as part of their 2023 selections, which awarded a total of US$175,000 in grants to develop the technologies further. Davoyan's proposal builds on recent work with directed-energy propulsion (DEP) and light sail technology to realize a Solar Gravitational Lens.
As Prof. Davoyan told Universe Today via email, the problem with spacecraft is that they are still beholden to the Rocket Equation:
"All current spacecraft and rockets fly by expanding fuel. The faster the fuel is thrown away, the more efficient is the rocket. However, there is a limited amount of fuel that we can carry on board. As a result, the velocity a spacecraft can be accelerated to is limited. This fundamental limit is dictated by the Rocket Equation. The limitations of Rocket Equation translate into a relatively slow and costly space exploration. Such missions as 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 in 2020 for NIAC Phase III development.
The concept relies on a phenomenon predicted by Einstein's Theory of General Relativity known as Gravitational Lensing, where massive objects alter the curvature of spacetime, amplifying 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 (~500 AU from the Sun), astronomers could study exoplanets and distant objects with the resolution of a primary mirror measuring around 100 km (62 miles) in diameter. The challenge is developing a propulsion system that could get the spacecraft to this 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 launched in 1977 and are currently about 159 and 132 AUs from the Sun (respectively).
When it left the Solar System, the Voyager 1 probe was traveling at a record-breaking velocity of about 17 km/s (38,028 mph), or 3.6 AU a year. Nevertheless, this probe still took 35 years to reach the boundary between the Sun's solar wind and the interstellar medium (the heliopause).
At its current speed, it will take over 40,000 years for Voyager 1 to fly past another star system – AC+79 3888, an obscure star in the constellation Ursa Minor. For this reason, scientists are investigating directed energy (DE) propulsion to accelerate light sails, which could reach another star system in a matter of decades.
As Prof. Davoyan explained, this method offers some distinct advantages but also has its share of drawbacks:
"Laser sailing, unlike conventional spacecraft and rockets, does not require fuel on board to accelerate. Here acceleration comes from a laser pushing the spacecraft by radiation pressure. In principle, near-speed-of-light velocities can be reached with this method. However, laser beams diverge at long distances, meaning that there is only a limited distance range over which a spacecraft can be accelerated. This limitation of laser sailing leads to a need of having exorbitantly high laser powers, gigawatts, and in some proposals, terawatts, or puts a constraint on spacecraft mass."
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 nanocraft (Starchip).
At a maximum velocity of 161 million km (100 million miles) or 20 percent of 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.
This mission concept could serve as a fast-transit interstellar precursor mission, like Starshot and Dragonfly.
But for their purposes, Davoyan and his team examined a pellet-beam system that would propel a ~900 kg (1 U.S. ton) payload to a distance of 500 AU in less than 20 years. Said Davoyan:
"In our case, the beam pushing the spacecraft is made of tiny pellets, hence [we call it] the pellet beam. Each pellet is accelerated to very high velocities by laser ablation, and then the pellets carry their momentum to push the spacecraft.
Unlike a laser beam, pellets do not diverge as quickly, allowing us to accelerate a heavier spacecraft. The pellets, being much heavier than photons, carry more momentum and can transfer a higher force to a spacecraft."
In addition, the small size and low mass of the pellets mean that they can be propelled by relatively low-power laser beams. Overall, Davoyan and his colleagues estimate that a 1-ton spacecraft could be accelerated to velocities of up to ~30 AU a year using a 10-megawatt (Mw) laser beam.
For the Phase I effort, 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 lifetimes.
"The pellet beam aims to transform the way deep space is explored by enabling fast transit missions to far-away destinations," said Davoyan. "With the pellet beam, outer planets can be reached in less than a year, 100 AU in about three years, and solar gravity lens at 500 AU in about 15 years. Importantly, unlike other concepts, the pellet-beam can propel heavy spacecraft (~1 ton), which substantially increases the scope of possible missions."
If realized, an SGL spacecraft would allow astronomers to directly image neighboring exoplanets (like Proxima b) with multi pixel resolution and obtain spectra from their atmospheres. These observations would offer direct evidence of atmospheres, biosignatures, and possibly even technosignatures.
In this way, the same technology that lets astronomers directly image exoplanets and study them in extensive detail would also enable interstellar missions to explore them directly.