What’s the Best Way to Communicate With an Interstellar Probe When it’s Light-Years Away From Earth?

It’s no secret that humanity is poised to embark on a renewed era of space exploration. In addition to new frontiers in astronomical and cosmological research, crewed missions are also planned for the coming decades that will send astronauts back to the Moon and to Mars for the first time. Looking even further, there are also ideas for interstellar missions like Breakthrough Starshot and Project Dragonfly and NASA’s Starlight.

These mission concepts entail pairing a nanocraft with a lightsail, which would then accelerated by a directed-energy array (lasers) to achieve a fraction of the speed of light (aka. relativistic velocity). Naturally, this raises a number of technical and engineering challenges, not the least of which is communications. In a recent study, a team of scientists sought to address that very issue and considered various methods that might be used.

The study, titled “Challenges in Scientific Data Communication from Low-Mass Interstellar Probes,” recently appeared in The Astrophysical Journal Letters Supplemental Series. The study was led by David G. Messerschmitt, the Roger A. Strauch Professor Emeritus of electrical engineering and computer science at UC Berkeley.

He was joined by co-authors Prof. Philip Lubin, the leader of the Experimental Cosmology Group at UC Santa Barbara – who is also a member of Breakthrough Starshot’s Management and Advisory Committee – and Dr. Ian Morrison, a research fellow with Curtin University’s International Centre for Radio Astronomy Research (ICAR) (formerly a researcher with the Swinburne University of Technology.)

To recap, modern concepts like Breakthrough Starshot are based on research pioneered by UC Santa Barbara and the NASA Spacegrant Consortium. In 2009, the Experimental Cosmology Group at UCSB (which is led by co-author Philip Lubin) launched the Directed Energy Propulsion for Interstellar Exploration (aka. DEEP-IN) program to research the possibility of sending lightsails at relativistic speeds on interstellar missions.

In 2013, this was followed by the Initiative for Interstellar Studies (i4iS) hosted a design competition called Project Dragonfly. The competition called for spacecraft designs that would rely on lasers to accelerate a light sail and spacecraft to 5% the speed of light, thus allowing for missions that could reach the nearest star (Alpha Centauri) in about a century.

By 2016, the research evolved into the basic ideas adopted for Breakthrough Starshot whose goal is to accelerate a probe to 20% the speed of light – roughly 60,000 km/s (215.85 million km/h; 134 million mph) – to reach Alpha Centauri in just 20 years.

Combining their expertise, Messerschmitt, Lubin, and Morrison set out to find the most effective way to communicate with spacecraft as they explore nearby star systems and tell us what’s out there. At present, this method remains the only feasible method for interstellar travel, barring massive technological breakthroughs. As Dr. Messerschmit told Universe Today via email:

“Low-mass probes propelled by directed energy with laser optical communication of data back to earth is the only method known today that may yield feasible interstellar probes at relativistic speeds with 21st-century technology. Of course, there are a lot of smart scientists and engineers considering other options such as antimatter annihilation and nuclear fusion, and we can expect progress toward feasible solutions in those technological domains to occur as the century progresses.”

For over fifty years, NASA has relied on the Deep Space Network (DSN) to communicate with missions operating in deep-space. This international network of giant radio antennas – which are located outside of Goldstone California), Madrid (Spain), and Canberra (Australia) – has supported all of NASA’s interplanetary missions and some missions to Low-Earth Orbit (LEO).

Already, NASA is looking to create a more robust communications infrastructure to handle the massive amounts of radio traffic it is expecting from future missions to the Moon, Mars, and beyond. But for missions traveling at relativistic speeds to distance light-years away, an especially powerful and efficient system will be needed to handle communications.

For the sake of their study, the team considered how a DE array might also provide sufficient amounts of data transmission to a probe. Building on a previous study conducted by Lubin and his colleagues at UCSB (titled “Roadmap to Interstellar Flight“) they proposed using the Directed Energy System for Targeting of Asteroids and exploRation (DE-STAR). As Lubin told Universe Today via email:

For our NASA Starlight and the Breakthrough Starshot programs (Directed Energy relativistic propulsion) the communications problem of getting data back from any interstellar mission remains an issue of getting sufficient photon rate back from the mission at the Earth given the extreme distance and relatively low spacecraft power budget on the currently envisioned missions.“

This time around, Messerschmitt, Lubin, and Morrison considered the same challenges but focused on the fundamental physical and statistical limitations of a communication system rather than implementation. They considering data latency vs. volume tradeoff, using a single receiver to target multiple probes, and potential interference from Earth’s atmosphere, as well as cosmic and background radiation.

Lubin also pointed out that these calculations can be performed on the Experimental Cosmology Group’s website, where users have the ability to adjust the weight of their spacecraft, the size of the sail, the power of the DE array, the distance to the target, etc. What they found from their own calculations was that communicating with wafer-sized spacecraft in the gram regime makes things more complicated. As Lubin explained:

“In the beginning, we will focus on these very small low mass probes as a means to achieving relativistic flight (lower spacecraft mass = higher speed) but while these are easiest probes to GET to relativistic speeds they are the MOST challenging to receive data back from due to their low power available for communications (transmit power) AND their limited ability to carry transmit optics.”

Nevertheless, the team found that it was possible to send and receive lots of data (even with small probes) over interstellar distances. For starters, they determined that a burst pulse-position modulation (BPPM) system – where bits are encoded by transmitting single pulses at intervals – would help. On the receiving end, they found that it would be necessary to build a large number of optical telescopes to increase the receiver area back on Earth.

In other words, their research highlights the need not only for a large scale DE array for the sake of propulsion, but also a large scale Earth-side data receiver that is optimized for optical communications (laser) rather than radio communications – what Lubin refers to as an “Interstellar Deep Space Network (IDSN). The development of these transmission and receiving concepts ultimately go hand in hand.

Of course, both of these concepts will require significant technological innovation and invention, especially when it comes to compensating for things like Doppler Shift, atmospheric interference, interference from stellar or background sources, and the distances involved. But as Messerschmitt explained, these challenges were true of all previous space programs:

“Flying a single probe and receiving back scientific data is a serious technical challenge. Some technological capabilities needed like optical detectors with very low dark counts and low-mass optical sources on the probe with very high peak powers are not available today but will benefit from general technological progress as well as specific development efforts for this project. Once the technology is available, actually constructing the Earthbound infrastructure will be very expensive; however, this is true of existing chemical rocket programs as well.”

And as Lubin stressed, this study and those that came before it can serve as a “road map” to further investigation and research. “Like getting to the Moon with the Apollo program we went through the Mercury, Gemini, and then early Apollo efforts before our final objective of landing on the Moon,” he said. “The roadmap is critical to [building] both confidence and the required technical infrastructure.”

As always, it’s all about the small steps. To space, to the Moon, to Mars, to beyond! Hopefully, within our lifetimes, we will be able to see distant planets up close for the first time and gather data on them directly. Combined with missions that explore the Solar System in-depth, we might finally determine if there’s life beyond Earth.

Further Reading: arXiv