‘LightCraft’ is the term used by Slava Turyshev’s team at JPL and elsewhere to identify the current design of the ambitious mission we looked at briefly in the previous post. This is a Technology Demonstrator Mission (TDM), which can be considered a precursor to what may become a mission to the solar gravitational lens. The mission concept is under active investigation, partly via a Phase III grant from NASA’s Innovative Advanced Concepts office. Reaching the focal region (for practical purposes, beyond 600 AU) in less than 25 years requires changes to our thinking in propulsion, not to mention payload size and the potential of robotic self-assembly en-route.

Hence the paper the researchers have just released, “Science opportunities with solar sailing smallsats,” which makes the case for leveraging our growing expertise in solar sail design and the highly successful move toward miniaturization in space systems, which the authors believe can be extended to include smallsats operating in the outer Solar System.

The TDM mission is conceived as a series of preparatory flights that allow the testing and validation of the technology and operational concepts involved in a mission to the focal region. The implications are hardly limited to the outer Solar System, for the smallsat/sail paradigm should be applicable to a wide range of missions in the inner system as well.

Let’s pause for a moment on the term ‘smallsat,’ which generally refers to a spacecraft that is both small and lightweight, usually less than 500 kilograms, and sometimes much less, as when we get into the realm of CubeSats. Frequently in the news as we explore their capabilities, CubeSats can get down to less than 2 kilograms. What the authors have in mind is a demonstrator design that is scalable, the initial payload in the 1-2 kilogram range, but capable of moving up to between 36 and 50 kilograms.

The goal is a demonstrator mission that will perform a one to two-year test flight using a solar sail and a sundiver maneuver to achieve speeds greater than 5 AU per year. The figure works out to something on the order of 23.6 kilometers per second, an impressive feat given that Voyager 1, our current record holder, is moving at 17.1 kps. With the TDM demonstrating the capabilities of the sail’s vane structure and the needed control for perihelion passage, the full solar gravitational lens mission contemplates still higher velocities, reaching 20 AU per year (roughly 95 kilometers per second).

The SGL mission concept is being built around in-flight cruise assembly of the full spacecraft through modules separately delivered as 20 kilogram or less smallsats. Given that overall design, you can see the need for the demonstrator mission to shake out both sail and sundiver concepts. Thus, while the TDM payload includes science instruments, the real focus here is on demonstrating the method: Use smallsat technologies with a highly maneuverable sailcraft to enable the fast travel times that will make reaching the focal region feasible. This is not the place to get into exoplanet imaging; we’ve discussed what a full-scale SGL mission could accomplish in these pages before. See, for example, A Mission Architecture for the Solar Gravity Lens.

So let’s focus on the sail and the sundiver maneuver. In the last post I mentioned the unusual design of the sail, which grows out of work at JPL in conjunction with L’Garde, further refined by space services company Xplore. The sail design, pictured below, draws on square panels aligned along a truss to provide the cumulative sail area needed for the mission. It’s a striking object, not the conventional image of a solar sail – I did a double take when I first encountered it in 2020. L’Garde has put together an eye-catching 1:3 scale model that hangs at the Xplore facility in Washington state.

Image: This is Figure 3 from the paper. Caption: TDM vehicle configurations (PDR: July 18, 2022). Credit: Turyshev at al.

The LightCraft TDM is envisioned as a 3-axis controlled spacecraft capable of the attitude control crucial for the Sundiver maneuver it will perform to reach cruise speed. Here are a few relevant details from the paper. Note the remark at paragraph close:

Each sail element, or vane, can also be articulated to provide fine control to both the resultant thrust from solar radiation pressure and the vehicle’s attitude. Each dynamic vane element is also a multifunctional structure hosting photovoltaics and communication elements with the requisite degrees of freedom to meet competing operational and mission requirements. The current TDM design total vane area is 120 m2 and the mass of the integrated TDM vehicle is 5.45 kg, resulting in an area-to-mass ratio of A/m = 22 m2/kg, or nearly 3 times the performance of other existing and planned sailcraft.

The mission concept relies on placing the sailcraft in a trajectory that takes it to solar perihelion – head first for the Sun, then leave it at high velocity, using the momentum of solar photons to push the craft, and again using the precise attitude control available through the SunVane design to adjust subsequent trajectory as needed. What this trajectory demands, then, is sail materials that can withstand a perihelion in the range of 15 to 20 solar radii, which the Phase III study research indicates will be available within the present decade.

This proof-of-concept demonstrator mission would aim at deployment through a rideshare launch, sharply reducing the cost in comparison with larger payloads, with checkout in a ‘super-synchronous’ orbit (meaning higher than geostationary orbit and moving faster than Earth’s rotation). The paper describes an ‘outspiral’ into interplanetary space following the checkout phase, with a pivot at perihelion (listed here as 0.24 AU) to harvest the solar momentum needed to reach cruise velocity. The SunVane design allows the necessary maneuvering, as follows:

The trajectory is achieved with three simple control laws to maneuver the vehicle from geosynchronous orbit to perihelion and then egress: 1) maximum acceleration: align vanes perpendicular to the Sun to increase velocity; 2) no acceleration: align vanes edge-on to the Sun; and 3) maximum deceleration: align vanes so that the resultant force is opposite to the heliocentric velocity vector, to decrease orbital kinetic energy.

Image: This is Figure 2 from the paper. Caption: Common TDM mission phases and systems engineering objectives. Trajectory plot shown is for the SGL mission. Credit: Turyshev et al.

You would think the diciest part of the mission would be at perihelion (and of course it’s crucial), but I was interested to see that the authors consider the most dynamic phase for the sailcraft is during the exit from Earth, where the vehicle alternates between acceleration and no-acceleration (factoring in eclipse periods). Reaching interplanetary space, the sail decelerates inward toward the Sun. The sail vanes are re-oriented at perihelion, with six degrees of freedom to ensure responsiveness to error.

All of this, the authors report, is well within the capabilities of the kind of onboard inertial sensors we already use in space operations. With the vanes used for propulsion, attitude determination and control are handled by reaction wheels, gyro, star tracker, sun sensors and accelerometers for yaw, pitch and roll. The preliminary studies reported in this paper show a sail area on the order of 100–144 m2, with the overall spacecraft mass coming in between 4.2 and 6.4 kg. Note that the demonstrator would use photovoltaic elements on the sail vanes for power. Future missions to the outer system will also demand radioisotope power.

I’ll turn you to the paper for further details about how the smallsat/sail concept can scale the TDM into future missions, such as sail material (currently Kapton but with other choices emerging), insulation for perihelion, and the various investigations re communications, batteries and the development of small radioisotope power sources.

So how likely is a Technology Demonstrator Mission to fly? The next steps are cited in the paper:

The 2020 NIAC Phase III study concluded with a TDM Preliminary Design Review (PDR) on July 18, 2022 [7]. Next is pre-project mission development, which includes final design, hardware development, full-scale prototype construction, as well as hardware and software testing… Should funding be available, the TDM Critical Design Review (CDR) may be conducted in November 2023, when flight project commitment is expected, including a firm costing of the TDM. The total project cost will depend on the selected mission objectives, science payload, and experiments, and is expected to be in the range of $17–20M.

It’s compelling to learn that a lightweight sundiver mission may be built at a cost of tens of millions (the authors cite $30-75 million), which is quite a contrast to the $2 to $5 billion cost of the typical flagship mission to deep space. Developing such technologies pushes us forward on the miniaturization of scientific sensors that will benefit all classes of future missions to deep space. But numerous opportunities would also open up for targets closer to home in the Solar System. We’ll look at some of those next time.

The paper is Turyshev et al., “Science opportunities with solar sailing smallsats,” available as a preprint.