Achieving interstellar travel has been the dream of countless generations, but the challenges remain monumental. Aside from the vast distances involved, there are also the prohibitive energy requirements and the sheer cost of assembling spacecraft that could survive the trip. Right now, the best bet for achieving an interstellar mission within a reasonable timeframe (i.e., a single person’s lifetime) is to build gram-scale spacecraft paired with lightsails. Using high-power laser arrays, these spacecraft could be accelerated to a fraction of the speed of light (relativistic speeds) and reach nearby stars in a few decades.
There are currently two major projects attempting to realize the technology, including Breakthrough Starshot and Project Lyra – which aim to reach Alpha Centauri 20 years after launch and catch up to ‘Oumuamua (respectively). This technology also presents other opportunities, like facilitating communications across interstellar distances. This is the idea recently by a team of researchers led by the Initiative for Interstellar Studies (i4is). In a recent paper, they recommended that a swarm of gram-scale spacecraft could rely on their launch laser to maintain optical communications with Earth.
The study was led by T. Marshall Eubanks, chief scientist at Space Initiatives Inc., chief executive officer at Asteroid Initiatives LLC., and former chief technical officer with the Liftport Group. He was joined by Space Initiatives electrical engineer W. Paul Blase, associate professor Andreas M. Hein of Luxembourg University and former Executive Director of the Initiative for Interstellar Studies (i4is), i4is researcher Adam Hibberd, and i4is Director of Geoengineering & Resource Utilisation Robert G. Kennedy III. A preprint of their paper is available online and will be published in the Breakthrough Starshot Challenge Communications.
Project Starshot, an initiative sponsored by the Breakthrough Foundation, is intended to be humanity’s first interstellar voyage. Credit: breakthroughinitiatives.org
For many years, the i4is has been working through Project Lyra to find optimal ways to explore nearby star systems using fleets (1000 or more) of gram-scale spacecraft. Like Starshot, these efforts began with Project Dragonfly (a feasibility study hosted by i4is in 2015) for small, lightweight, distributed spacecraft propelled primarily by laser sails. Per the study’s specifications, these spacecraft would need to be realizable using technology and space infrastructure available in the coming decades and capable of reaching nearby stars within a century.
Among astrophysicists, gram-scale craft and laser sails are considered the only viable means for mounting interstellar exploration in the foreseeable future. But whereas some mission architectures envision sending a single mission with a large lightsail, Project Lyra envisions using a power laser array to send swarms of spacecraft that could explore distant star systems and exoplanets collectively. As the study team told Universe Today via email:
“Realistic limits (~100-gigawatt power, ~$100 billion dollars cost) in the foreseeable future (this century) to a laser launch system impose an upper limit (a few grams) on what you can push with the laser up to near-relativistic speeds (~10-20% of light). If we want to send spacecraft to the nearest stars in anything like a reasonable time (decades rather than centuries), we will be limited to gram-scale spacecraft. However, that is the limit to the single spacecraft mass. The major fixed cost is in the ‘system,’ not in the marginal cost of the tiny spacecraft themselves or the energy to launch them. If we can send one, we can send many, and we might as well do that.”
Nevertheless, the concept presents many challenges, including the need for shielding against particles in the interstellar medium (ISM) and high-bandwidth communications, both of which become tricky at relativistic speeds. In addition, interstellar distances also pose a significant challenge for communications and tracking, especially where tiny spacecraft are involved. However, as the study team explained, their proposed mission architecture (swarms of spacecraft) presents some possible solutions.
Artist’s impression of the Dragonfly spacecraft concept. Credit and Copyright: David A Hardy (2015)
“A single gram-scale spacecraft with a realistic power supply and communications system will be very hard to detect at interstellar distances,” they said. “The bit rate one tiny spacecraft can support if it can be detected at all, will be very low. But by combining hundreds or thousands of these small craft into a unified system, both the data return and the ability to explore the target system will be greatly enhanced.”
Their plan consists of sequentially launching hundreds of probes for up to a year using time-and velocity-on-target dynamic techniques to create a swarm numbering in the thousands. Contact will be maintained with the swarm via the 100 GW launch laser, which will also be used to synchronize the probes’ internal clocks. The in-flight formation would be maintained by grossly modulating the initial launch velocity between the head and the tail of the string, combined with continuous attitude adjustments (exploiting the drag of the ISM) of selected probes.
The team also developed a simple and cost-effective power source for their probes, a betavoltaic isotopic application. This, they argue, could provide each probe with enough electricity to power optical communications at interstellar ranges for decades. During the 20-year cruise phase, the swarm would dynamically coalesce from its long string formation into a lens-shaped network measuring about 100,000 km (~62,000 mi) in diameter, which would be centered on the destination (Proxima Centauri b) by the time of fly-by. As the team explained:
“It would enable a fairly high data return from Proxima and Alpha Centauri, and a more detailed exploration of planets in that system. Depending on the details of the engineering, the location and power of the drive laser(s), etc., it should be possible to send probe swarms to between five to ten of the nearest stellar systems at the same time we are exploring the Centauri system, possibly reaching out as far as Sirius, the brightest star in the night sky, and about 50 years away with the planned Breakthrough Technology.”
This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. Credit: ESO/M. Kornmesser
Beyond the ability to maintain contact with the probes, the swarm concept also presents solutions to the problem of interstellar dust grains. With over one thousand tiny probes, the swarm could tolerate significant attrition and still be able to provide significant data returns upon arrival. The swarm concept also means that observations of nearby exoplanets can be viewed from multiple viewpoints. Combining up to one thousand observers would also enable data returns orders of magnitude higher than what is possible using a single probe.
This same technology could be used to explore the Solar System, which echoes what many researchers have emphasized about directed energy, lightsails, magnetic sails, and other advanced propulsion concepts. As the team explained, they envision a day when laser launch systems will be considered “utilities” and not a one-off technology:
“Equally exciting is the possibility of using laser-launched spacecraft for solar system exploration. Technically a gram-scale would be called “femtospacecraft” or even “attospacecraft”. But you can push anything with concentrated light – a very tiny spacecraft to relativistic speeds, or something the size of New Horizons to tens of kilometers per second – with a full-size launch laser.
“Even with a much smaller hence cheaper laser launch system, by using picospacecraft we could reach and explore any object we wish in the entire solar system with mission durations of months to ~one year, not decades like it is now. Pushing tiny spacecraft with light would be such a generally useful technique that a laser launch system no matter its size could be employed 100% of the time until it wears out, which vastly reduces the cost-share of the capital expense that any individual mission has to bear.”
Further Reading: Research Gate
