Between the exponential growth of the commercial space industry (aka. NewSpace) and missions planned for the Moon in this decade, it’s generally agreed that we are living in the “Space Age 2.0.” Even more ambitious are the proposals to send crewed missions to Mars in the next decade, which would see astronauts traveling beyond the Earth-Moon system for the first time. The challenge this represents has inspired many innovative new ideas for spacecraft, life-support systems, and propulsion.
In particular, missions planners and engineers are investigating Directed Energy (DE) propulsion, where laser arrays are used to accelerate light sails to relativistic speeds (a fraction of the speed of light). In a recent study, a team from UCLA explained how a fleet of tiny probes with light sails could be used to explore the Solar System. These probes would rely on a low-power laser array, thereby being more cost-effective than similar concepts but would be much faster than conventional rockets.
The study was conducted by Ho-Ting Tung, an aerospace engineering grad student from UCLA, and assistant professor Artur R. Davoyan, both of whom are members of the Davoyan Research Group (DRG), of which Prof. Davoyan is the founder. This group is dedicated to the study of directed energy and light-material interactions for the purpose of developing “space photonics.” The paper that describes their findings recently appeared in the journal Nano Letters, an publication overseen by the American Chemistry Society (ACS).
For decades, scientists have investigated light sails as a possible means of space exploration. These spacecraft offer many advantages over conventional concepts, foremost of which is how they forego the need for propellant. For most designs, propellant constitutes a big chunk of a spacecraft’s mass, which necessitates large storage tanks, resulting in additional mass, and so on. Where interstellar space travel is concerned, it becomes a terrible burden.
Using conventional propulsion, getting to even the nearest star system – Proxima Centauri, located about 4.25 light-years away – could take several thousand years. For this reason, multiple organizations are exploring light sail mission concepts as a means of interstellar travel. This includes Breakthrough Starshot, Project Dragonfly, and Project Lyra, which involve using large arrays up to 100 GigaWatts (GW) in power to propel spacecraft to relativistic speeds and achieve interstellar travel.
But as Prof. Davoyan told Universe Today via email, these approaches have applications for exploring the Solar System as well:
“Getting to other star systems is very hard due to astronomical distances. For example, the closest system is about 4 light years away from us. Reaching it with any conventional way of propulsion would require thousands of years. There are several different approaches that are considered to accelerate spaceflight: mainly fusion propulsion and directed energy, such as with the use of lasers.
“At the same time, even getting to the outer reaches of our solar system, such as to outer planets, the Kuiper belt, and entering the interstellar medium is very very challenging. It takes years of flight time and mission development. We discuss a new way of using beamed laser propulsion to send probes to outer planets.”
Artist’s impression of the Dragonfly spacecraft concept. Credit and Copyright: David A Hardy (2015)
For the sake of their study, Ho-Ting and Davoyan considered various spacecraft profiles with varying degrees of size and laser wattage. These included an array ranging from 100 kiloWatts (kW) to 1 megaWatt (MW), which is low-power compared to interstellar concepts. Like Starshot and Dragonfly, they calculated for gram-scale probes ranging from 10 to 100 grams in mass. From this, they envisioned a wafer probe about 45 cm (18 inches) in diameter with integrated electronics on one side and a nanoscale structure on the other.
Beyond directed energy, this concept incorporates another of the Davoyan Research Group’s areas of expertise. This is the field known as nanophotonics, the science of how materials that are a few nanometers in scale interact with light, with applications ranging from broadband communications and photovoltaics to spacecraft propulsion. In the end, they found that 100 kW arrays and sails of silicon or boron nitride would allow for cost-effective and rapid interplanetary missions. Said Davoyan:
“We show that our approach can be much faster than any other conventional propulsion system, such as electric and chemical propulsion. Voyager 1 is the fastest interplanetary spacecraft that was ever built. Traveling at about 17 km/s of cruise velocity it took ~45 years to reach 100 AU. Our system can be 4 times faster than that. Some conceptual approaches with nuclear propulsion with several gravity assists can be similarly fast.
“However, the probes we discuss are low cost and are not constrained by development time or launch window, which makes them more agile. In general, very low cost and the ability to leverage mass manufacturing allow a new way of space exploration, in which everyone can get easy access to deep space missions. We believe this will be transformative to space science.”
In this illustration, NASA’s Hubble Space Telescope is looking along the paths of NASA’s Voyager 1 and 2 spacecraft as they journey through the Solar System and into interstellar space. Credit: NASA, ESA, and Z. Levy (STScI).
The ability to conduct low-cost, rapid-deployment missions presents many advantages. The New Horizons mission holds the record for the fastest object ever launched from Earth, with an escape trajectory of about 16.26 km/s (58,500 km/h; 36,400 mph). Nevertheless, it took the probe nine and a half years to reach Pluto and capture the most detailed images ever taken of its surface. The same is true of the Voyager 1 and 2 missions, which launched from Earth in 1977 and reached the edge of the Solar System in 2004 and 2007, respectively.
While the scientific returns from all of these missions were immeasurable, a low-cost option that could reach their destinations in a fraction of the time would yield these types of returns on a much more regular basis. There will be no shortage of opportunities with missions planned for Europa, Titan, Triton, and the Kuiper Belt in the coming years. In addition to making such missions faster and cheaper, light sail probes could also allow for more missions for the same cost. As Davoyan summarized:
“We believe that our approach could allow a new way for space missions, when the time it takes from an idea to getting science data back would take less than a year. This is not possible today. We foresee that many probes can be sent to different destinations, including Mars to collect science data and therefore accelerate discoveries.
“Today we have to choose between going to Enceladus, Europa or Titan. And then it takes decades and billions to develop a flagship mission. With probes that cost less than $1000 and can be developed in less than a month the space exploration can be changed dramatically.”
Right now, light sail proposals are being considered for rendezvousing with ‘Oumuamua, exploring the Solar System, and mounting interstellar missions to Alpha Centauri in a matter of decades instead of centuries. Variations on the concept, such as directed energy thermal propulsion, are even being considered for sending crewed missions to Mars in a matter of weeks instead of months. With no chemical propellants weighing them down, these missions could be developed at a fraction of the cost.
Over time, the creation of laser arrays throughout the Solar System could lead to a transportation infrastructure that spans the Solar System. Who knows? Combined with next-generation spacecraft that rely on various types of nuclear propulsion, this vision could lead to humans becoming “interplanetary” in terms of their habitation but interstellar in terms of their exploration.
Further Reading: ACS, Nano Letters
