LunarSail is a 3U Cubesat (10cm x 10cm x 30cm) spacecraft loosely using as a baseline the design of NASA’s NanoSail-D2, NASA’s first-ever solar sail to operate in space and launched in 2010. LunarSail will contain numerous hardware and software improvements as well as upgraded sail design.
NanoSail-D: A Good Starting Point
One of NanoSail-D’s several mission objectives was to demonstrate the capability to deploy a large sail structure from a highly compacted volume without re-contacting the spacecraft. NanoSail-D deployed off the Fast, Affordable, Science and Technology, or FASTSAT. In addition to solar sails, the demonstration can be applied to deploy future communication antennas, sensor arrays or thin film solar arrays to power spacecraft.
Unlike the LunarSail mission, which involves progressively raising the spacecraft’s orbit, a key objective of NanoSail-D2 was to demonstrate the opposite – testing the de-orbiting capabilities of solar sails. NASA hopes to one day use thin membranes to de-orbit satellites and space debris. Finally, NanoSail-D2 successfully demonstrated solar sailing.
“The final rate of descent depended on the nature of solar activity, the density of the atmosphere surrounding NanoSail-D and the angle of the sail to the orbital track,” said Dean Alhorn, principal investigator for NanoSail-D at Marshall Space Flight Center. “It is astounding to see how the satellite reacted to the sun’s solar pressure. The recent solar flares increased the drag and brought the nanosatellite back home quickly.”
NanoSail-D orbited the Earth for 240 days performing well beyond expectations and burned up during re-entry to Earth’s atmosphere on Sept. 17, 2011.
NASA formed a partnership with Spaceweather.com to engage the amateur astronomy community to submit images of the orbiting NanoSail-D solar sail during the flight phase of the mission. NanoSail-D was a very elusive target to spot in the night sky — at times very bright and other times difficult to see at all. Many ground observations were made over the course of the mission. The imaging challenge concluded with NanoSail-D’s deorbit.
A Low-Cost, Open-Source Lunar Spacecraft
A key objective of the LunarSail mission is that it be relatively low-cost, at least in comparison to other deep space missions. This led to the decision to implement the mission as a Cubesat. We believe that a solar sail-carrying cubesat spacecraft can be designed and assembled for less than $50,000, excluding the cost of avionics and the sail/boom assembly.
Since we didn’t want to re-invent the Cubesat, we looked at previous solar sail-powered spacecraft designs for a conceptual reference point. Most of the ideas involve very large spacecraft with massive sail dimensions. Since we wanted LunarSail to be a Cubesat-class mission, our options quickly narrowed down to a small sub-selection, with NanoSail-D becoming the obvious choice since it had already successfully completed an Earth-orbiting mission.
LunarSail is envisioned as a 3-unit cubesat, comprised of 3 1-unit modules. Depending on the final design for the solar sail and boom assembly, the eventual design may grow to 6 units, i.e. 2 3 unit assemblies side-by-side. The sail assembly and deployment mechanism will be housed in the forward two sections while the command, control and communication hardware and payloads will be in the aft third.
LunarSail is being developed in an open manner as much as possible. We are using open-source hardware and software wherever applicable. The main computer system is based on the Raspberry Pi architecture, modified for the space environment. We chose this platform because it is open and uses the Linux operating system but also because it has not been utilized in a spacecraft before. Currently, there are projects focused on the Arduino platform, which would certainly be an acceptable choice. However, one of our goals is to advance technology development and utilize components in ways they haven’t been used before. This consideration drove the choice to base the LunarSail primary computer on the Raspberry Pi. From a technical standpoint, this choice of platform open up flexibility in the design choices of other components because it contains a variety of standard interfaces for external hardware. For example, an off-the-shelf camera module is available as a programmable add-on which could prove useful for taking photos and video during the mission.
The design of the sail and associated boom assembly will present the greatest challenge. The sail material will be an extremely thin metallic film with < 1 micron thickness. The total area covered will be dictated by the final mission trajectory design and spacecraft mass and volume budget but will be between 100 and 400 square feet. The larger the sail, the greater its propulsive capacity and hence shorter travel time to lunar orbit.
Due to the advanced manufacturing techniques involved, it is likely we will have to use an outside supplier, either industrial or through a government partnership, for its development.
Further challenging spacecraft design, the sail must be constructed using four independent sub-sections. Each section needs to be capable of being trimmed, or rotated, independently of the others in order to give the sail the ability to steer the spacecraft and adjust its trajectory en route to the Moon and to execute the desired lunar orbit insertion. This will be the first time the technique is used in an actual mission in order to achieve precision rendezvous and orbit of another body in space.
Payloads And The Public
LunarSail will carry several payloads for both educational and scientific research.
First, the sail will be constructed with sensors to detect micrometeoroid impacts. Since the trip to the vicinity of the Moon will take much longer than spacecraft propelled by traditional chemical propellants, the mission provides a good opportunity to conduct an in-depth characterization of the micrometeoroid and debris environment in cislunar space.
We will be able to directly study the dynamics of the solar wind and potentially take cosmic ray and radiation measurements during the mission.
Another payload will be a high-definition camera that will shoot photos and video of the lunar surface and the Earth from lunar orbit and transmit them to Earth.
A central purpose of the LunarSail mission is to involve students and the public in the excitement of a mission to the Moon. We are inviting people to submit photos, short videos and writings to be included on the spacecraft and transmitted back to Earth while LunarSail is orbiting the Moon.
Additionally, we are looking at possible methods for utilizing near real-time communication via social media (e.g. Facebook and Twitter) to and from the spacecraft to permit the public to submit messages for re-transmission as well as enabling the spacecraft to communicate indirectly with people on the ground. Due to the low-power consumption of the spacecraft and the distance between the lunar region and Earth, communications of this nature may or may not be possible, but the mission is being designed with social media integration at all phases from design and development through testing, launch and transit to the Moon.
In summary, LunarSail is the first space mission to use a solar sail for primary in-space propulsion and navigation to orbit another celestial body, the first deep-space “citizen” space mission and will further advanced in-space technology development.