Today, several space agencies are investigating cutting-edge propulsion ideas that will enable rapid transits to other Solar System bodies.
These include NASA’s nuclear-thermal or nuclear-electric propulsion (NTP/NEP) concepts that could allow 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 enable interplanetary exploration, going beyond the Solar System presents some significant challenges.
As we explored in a previous article, it would take spacecraft using conventional propulsion between 19,000 and 81,000 years to reach even the nearest star, Proxima Centauri (4.25 light-years from Earth). To that end, engineers have been investigating proposals for unmanned 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 UCLA researchers 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, assistant professor of Mechanical and Aerospace Engineering at the University of California, Los Angeles (UCLA).
The proposal was one of fourteen proposals chosen by NASA’s Innovative Advanced Concepts (NIAC) program as part of its 2023 selections, which awarded a total of $175,000 in grants to further develop the technologies. Davoyan’s proposal builds on recent work with directed energy propulsion (DEP) and light sail technology to realize a solar gravitational lens.
As Professor Davoyan told Universe Today via email, the problem with spacecraft is that they are still beholden to the rocket equation:
“All current spaceships and rockets fly by expanding fuel. The faster the fuel is ejected, the more efficient the rocket. However, there is a limited amount of fuel we can carry on board. As a result, the speed of a spacecraft It can accelerate up to is limited This fundamental limit is dictated by the rocket equation The limitations of the rocket equation result in relatively slow and expensive space exploration Missions such as the Solar Gravitational Lens are not feasible with today’s 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 is based on a phenomenon predicted by Einstein’s Theory of General Relativity known as gravitational lensing, where massive objects alter the curvature of space-time, amplifying the light of background objects. This technique allows astronomers to study distant objects with greater resolution and precision.
By placing a spacecraft at the heliopause (~500 AU from the Sun), astronomers could study exoplanets and distant objects at the resolution of a primary mirror about 100 km (62 mi) in diameter. The challenge is to develop a propulsion system that can carry the spacecraft that distance in a reasonable amount of time.
So far, 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).
When it left the Solar System, Voyager 1 was traveling at a record speed of about 17 km/s (38,028 mph), or 3.6 AU per year. However, 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 Voyager 1 more than 40,000 years to pass another star system: AC+79 3888, a dim 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 Professor Davoyan explained, this method offers some distinct advantages, but also has its share of drawbacks:
“Laser navigation, unlike conventional spacecraft and rockets, does not require fuel on board to accelerate. Here the acceleration comes from a laser pushing the spacecraft by radiation pressure. In principle, with speeds close to the speed of light can be achieved with this method. However, the laser beams diverge over long distances, which means that there is only a limited distance range over which a spacecraft can be accelerated. This limitation of laser navigation entails the need for exorbitantly high laser powers, gigawatts and, in some proposals, terawatts. , or limits 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 nanocraft (Starchip).
At a maximum speed 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, Professor Davoyan and his colleagues propose a new twist on the idea: a pellet beam concept.
This mission concept could serve as an interstellar rapid transit precursor mission, like Starshot and Dragonfly.
But for their purposes, Davoyan and his team looked at a pellet beam system that would propel a payload of ~900 kg (1 US ton) to a distance of 500 AU in less than 20 years. Davoyan said:
“In our case, the beam that pushes the spacecraft is made of tiny pellets, so [we call it] the pellet bundle. Each pellet is accelerated to very high speeds by laser ablation, and the pellets then carry their momentum to propel the spacecraft.
Unlike a laser beam, the pellets don’t diverge as quickly, allowing us to accelerate a heavier spacecraft. Pellets, being much heavier than photons, carry more momentum and can transfer greater force to a spacecraft.”
In addition, the small size and low mass of the pellets mean that they can be propelled by laser beams of relatively low power. Overall, Davoyan and colleagues estimate that a 1-ton spacecraft could be accelerated to speeds of up to ~30 AU per 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 various 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 transform the way deep space is explored by enabling rapid transit missions to distant destinations,” said Davoyan. “With the pellet beam, the outer planets can be reached in less than a year, 100 AU in about three years, and solar gravity lensing at 500 AU in about 15 years. Importantly, unlike other concepts, the pellet beam can propel heavy spacecraft (~1 ton), which substantially increases the range of possible missions.”
If realized, an SGL spacecraft would allow astronomers to directly image neighboring exoplanets (such as Proxima b) at multi-pixel resolution and obtain spectra of their atmospheres. These observations will provide direct evidence of atmospheres, biosignatures, and possibly even technosignatures.
In this way, the same technology that allows astronomers to directly image exoplanets and study them in great detail would also allow interstellar missions to explore them directly.
This article was originally published by Universe Today. Read the original article.