Principal Investigators: Andreas Hein and Kelvin F. Long
This was a short study completed in March 2016 to design a Gram-scale interstellar probe to be sent to the nearest stars at 0.1c using current or near envisage (up to 20 years) technology. A team of 15 people were assembled to complete the report and subsequent follow-up assessments, which was a delivery for the Breakthrough Initiative Project Starshot. The final report produced was titled "Initial Considerations for the Interstellar (Andromeda) Probe: A Three Day Study". In addition to the principal investigators who led the study, the other contributors were Rob Swinney, Richard Osborne, John Davies, Stefan Zeidler, Angelo Genovese, Bill Cress, Martin Langer, Dan Fries, Nikolas Perakis, Lukas Schrenk, Marc Casson, Sam Harrison, Adrian Mann and Professor Rachel Armstrong.
The field of interstellar studies has taken probably the most important step in its history when on the 12th of April 2016, the physicist and entrepreneur Yuri Milner announced Project Starshot: A $100 million research and development program for a laser-propelled gram-sized probe to Proxima Centauri. However, the spectacular announcement was preceded by the careful evaluation of different options for the Starshot architecture. The Project Starshot team has been informed from various sides regarding the architecture of the mission and had consulted certain groups.
During early 2016, the Initiative for Interstellar Studies had also been asked by the Breakthrough team to provide its own perspective on a laser-propelled interstellar mission. This was especially because we had been running our own Project Dragonfly, an ongoing laser sail project that had been launched in 2013.
During March 2016 Executive Director Kelvin F Long met with the Breakthrough Starshot team whilst on a visit to NASA Ames Research Centre in San Francisco, California. The result of this meeting led to the assembly of a group of experts within the i4is team, who had to design a starship in three days. The briefing was given late on a Friday evening, whilst Kelvin was on a stopover in New York. Andreas Hein then assembled the team which also included Rob Swinney, Richard Osborne, John Davies, Stefan Zeidler, Angelo Genovese, Bill Cress, Martin Langer, Dan Fries, Nikolas Perakis, Lukas Schrenk, Marc Casson, Sam Harrison, Adrian Mann and Professor Rachel Armstrong. Peter Milne also gave some consultancy assistance. This team performed above and beyond the call of duty and were eventually awarded the i4is Alpha Centauri prize and a specially designed Andromeda probe mission patch.
The report was titled “Initial Considerations for the Interstellar (Andromeda) Probe: A Three Day Study” and it was delivered on the desk of the Breakthrough Initiative by the following Tuesday. This represented an astonishing effort by the team, who dropped ‘normal life’ to focus 100% on doing a good job for the Breakthrough Initiative and their innovative Project Starshot.
The main requirements that the team had to design for included (i) laser sail propulsion (ii) 50 year time of flight (iii) 10% speed of light cruise velocity (iv) the target was assumed to be within the Alpha Centauri A/B system at around 4.3 light years away (v) gram-scale mass. Working with the Breakthrough Initiative, some subsequent work and calculations were also conducted by the i4is team to give improved insights into the problems and potential solutions. The results of our teams work, complemented work done by others (particularly Professor Philip Lubin and scientists from Harvard University), to give the Breakthrough Initiative confidence in the concept of a laser sail mission to the stars, and Yuri and his team went live on the 12th April 2016 to announce this incredible and inspiring project.
In the rest of this short report, we will provide an overview of the mission architecture, subsystems of the spacecraft, and the proposed beaming infrastructure that was derived by the i4is Andromeda probe team during our brief 3-day study.
Before we present our mission architecture, we present two key parameters for a laser sail mission. All existing mission architectures take a certain spot within this coordinate system. A laser-propelled interstellar mission comprises two basic elements: the laser infrastructure and the spacecraft. First, the longer that the laser beam from the infrastructure can hit the spacecraft with the laser sail, the longer it accelerates and the higher its final velocity. However, the longer the distance to the spacecraft from the laser, the more difficult it gets to focus the beam on the sail. Hence, either you accelerate the spacecraft extremely quickly, in order to avoid any focusing issues or you accelerate for a long time. But in the latter case, you need a kilometre-sized lens. In the former case, you need a lot of power and you need a sail material that can withstand the extremely high power flux (up to dozens of GW per square metre).
Previous laser sail missions have positioned themselves in the long acceleration spot of the trade space due to material temperature limitations (Forward, 1984). If the power flux from the laser is too high, the material will absorb too much heat and the sail will simply melt away. Geoffrey Landis was the first to propose a concept that uses dielectric materials that have very high reflectivity values, which allows for a high power flux on the sail, as the sail does not absorb as much heat as a material with low reflectivity values (Landis, 1989).
Finally, Phil Lubin’s proposal for a laser-propelled interstellar mission went to the extreme by proposing an acceleration within minutes to velocities of 20% of the speed of light, only possible by using materials with a 99.9999% reflectivity that still need to be developed (Lubin, 2016). The key parameter is the size of the sail and correspondingly its mass. The smaller the sail, the more difficult it is to hit it with the laser beam. But a smaller sail is also lighter. The larger the sail, the easier to hit, but also heavier. Furthermore, the larger the sail, the lower the power density, as the beam is spread out on a larger surface. Again, past concepts have positioned themselves differently with respect to this trade-off. The concepts of Robert Forward and Geoffrey Landis proposed large sails, mainly to reduce the power flux by distributing it on a larger surface area of several square kilometres. Conversely, Professor Lubin proposed to use a sail of just a few square metres.
The i4is Andromeda architecture positions itself similar to past laser sail architectures: We use a long duration of acceleration and use a large sail. However, we leverage on recent innovations that could have a disruptive effect on how we think about laser sail missions. First, we use a segmented lens, meaning that the lens that focuses the beam on the sail consists of several lenses that are positioned sequentially. Each individual lens has a size of a few hundred metres. This is still larger than anything that has been put into space. However, recent advances in 3D-printing have now lead to a stage where in just a few years, large truss structures of up to 100 metres can actually be manufactured in space.
The lenses are put at different locations along the path the spacecraft intends to travel. Each time the spacecraft passes a lens, the laser beam is transmitted from lens to lens and each subsequent lens refocuses the beam on the spacecraft. With this approach, kilometre-sized lenses can be avoided. Ten sequential Fresnel lenses, each with a radius of 95 m, are used. Circular structures of this diameter are currently conceived by Tethers Unlimited for in-orbit manufacturing. A potential Fresnel lens material is Graphene sandwich. Graphene lenses have been demonstrated in the lab in 2016. A graphene lens is expected to be extremely light. The lens infrastructure is shown in Figure 1.
The parameters for the laser infrastructure are given in the tables in the report. It shows that the laser beam power is orders of magnitudes lower than for the architecture proposed by Lubin. The acceleration distance with about 2 astronomical units (300 Mkm) is also much shorter than previous architectures.
The second innovation is carbon nanotube sails. They have been proposed by Greg Matloff (Matloff, 2012). Although carbon nanotubes have a very low reflectivity value, they have the advantage that they are extremely light and can withstand very high temperatures.
Further innovation has been inserted in the subsystems of the spacecraft. We propose an inflatable camera aperture that can be extended, once the probe nears Proxima Centauri and its recently discovered exoplanet. The camera is at the same time used for taking pictures but also as a star tracker for navigation in interstellar space. For power supply, an advanced beta-voltaic battery is used.
Beta-voltaic batteries directly convert the impact of radioactive particles from a radioactive material into electricity, in contrast to radioisotopic batteries which use heat to generate electricity. The advantage is a higher efficiency and smaller size. The battery serves two purposes. First, it powers the spacecraft subsystems. Second, it heats critical components of the spacecraft that need to be kept at temperatures above 3 Kelvin, the cosmic background temperature. As the power output of such a battery is below 1 W, the energy needs to be stored and accumulated, in order to enable short bursts of communication with Earth over interstellar distances. For storage, recently developed graphene capacitors are used that have an extremely high power density and can be rapidly discharged.
In the following section, we will provide an overview of the spacecraft’s configuration. Figure 2 above shows the configuration of the spacecraft with its subsystems. The large cylinder is the camera with its lens aperture. The Whipple shield protecting against interstellar particles is located at the front of the spacecraft. This part is facing the direction of flight. Figure 3 shows an orthographic view of the spacecraft without the sail. The size of the depicted spacecraft is about twice the size of a smartphone: about 12 cm by 10 cm. The grey area is the antenna transmitting data using a laser beam. In order to decrease the cross-section of the probe during flight through interstellar space, the antenna is folded during flight. The lens aperture for the camera is also folded, as shown in Figure 4.
Figure 5 shows the spacecraft with the unfolded antenna. The antenna is unfolded and pointed at Earth each time the spacecraft communicates. The power for the communication system is supplied by the graphene supercapacitors which are slowly charged by electromagnetic tethers.
The laser array that is located in space has a total power output of 1.12GW. This solution is preferred to an Earth-based system; there are no atmospheric losses and the laser beam can be continuously pointed at the spacecraft sail. 10 intermediate lenses keep the beam collimated. Figure 6 shows the mission architecture. The spacecraft is accelerated over a distance of 1.8 astronomical units (270 million km) by a space-based laser array. After its acceleration phase, the graphene sandwich sail is detached and the spacecraft continues its flight into interstellar space. Using the FEEP thrusters and momentum wheels, the spacecraft keeps its orientation into flight direction in order to minimise the impact of interstellar dust. Once arriving at the target star system, measurements and pictures are taken by using the onboard sensors and camera with telescope.
Summary
The work performed by the i4is Andromeda probe team was astonishing given the short time we had to complete the work. We also contributed positively to the inspiring Breakthrough Initiative Project Starshot. The architecture chosen by the Breakthrough Initiative team is different to that recommended by the i4is team, (i.e. ground-based beaming versus space-based beaming). But there are clear benefits in a ground-based beaming approach in the interim, including nearer term maturation of the required architecture.
The Breakthrough Initiative has also gone for a 20% speed of light mission, which is a lot more challenging than a 10% speed of light mission, but where would physics be without challenges? The main implications of this is a different laser power requirement that moves from around 1 GW up to something around 100 GW. There are many physics and engineering challenges to be solved on the programme, and these were listed during the April 12th 2016 announcement by Yuri Milner. With effort, determined commitment and the will to succeed we are confident that the stars can be won.
References
Forward, R., 1984. Roundtrip interstellar travel using laser-pushed lightsails. J. Spacecr. Rockets.
Landis, G., 1989. Optics and materials considerations for a laser-propelled lightsail.
Lubin, P., 2016. A Roadmap to Interstellar Flight, JBIS, 69, pp.40-72.
Matloff, G.L., 2012. Graphene, the Ultimate Interstellar Solar Sail Material?, JBIS, 65, pp.378-381, .
Publications
Hein, A. M., Long, K. F., Fries, D., Perakis, N., Genovese, A., Zeidler, S., Langer, M., Osborne, R., Swinney, R., Davies, J., Cress, B., Casson, M., Mann, A., & Armstrong, R., (2017). The Andromeda Study: A Femto-Spacecraft Mission to Alpha Centauri. arXiv:1708.03556 [astro-ph.IM]
Media Coverage
The Times, "Tiny armada to explore space", 8th September 2017
MIT Technology Review, "Femto-Spacecraft Could Travel to Alpha Centauri", 31st August 2017
MIT Technology Review (Japanese edition), "わずか23グラムの極小宇宙探査機は兄弟惑星にたどり着けるか", 4th September 2017