PLANETARY RESOURCES (PR)
The following information is paraphrased from
Planetary Resources' website, explaining how their spacecraft designs propose to solve many of the problems of sending vehicles to intercept near earth asteroids:
ATTITUDE DETERMINATION & CONTROL SYSTEMS (ADCS)
Planetary Resources (PR) has built a new suite of sensors and actuators for their vehicles. These systems can measure a spacecraft rotating at a rate slower than the hour hand of a clock while pointing a beam within the width of a dime one mile away. By controlling the design of both the components and the system, they aim to balance capabilities and risks where they are appropriately taken rather than depend on historical vendor decisions.
Traditional spacecraft control systems are designed to meet the pointing requirements for a single vehicle with specific mission goals. This narrow scope produces an architecture that is unique and rigid, driving up development costs and effort for the attitude control system of each new vehicle. Planetary Resources is developing a spacecraft control architecture that is both modular and upgradeable. This investment supports the rapid deployment and evolution of PR spacecraft as internal and external demands change.
Controlling spacecraft attitude is inherently a system level function. Thus, PR has moved away from a traditional, centralized approach in which a single compute element is responsible for the ADCS system and have instead adopted the idea of basic, instinctual behaviors. Instincts are a way of commanding and protecting critical spacecraft components locally, using an integrated and distributed network of low-level hardened compute elements. Similar to a person instinctively removing their hand from a hot surface, the ADCS system has built-in instinctual responses that react to protect the system without relying on the central brain.
LOW COST AVIONICS & SOFTWARE
Interplanetary space is a challenging environment for spacecraft avionics. Not only can the thermal, vacuum, and radiation environments damage electronics, the sheer distance from Earth drives designers to take conservative approaches to new technology adoption. Solutions today often rely on redundant architectures using expensive, centralized, heritage components specifically designed for operations in deep space. This philosophy limits the design team’s ability to benefit from the tremendous advancements in the microelectronics industry, leaving them consistently behind the technological curve. The Mars Curiosity rover, for instance, is controlled by a redundant system that uses a CPU originally developed almost two decades ago.
PR is breaking away from this model. Instead of an architecture that relies on a single, centralized and expensive set of avionics hardware, PR takes a tiered and modular approach to spacecraft avionics. In its model, a distributed set of commercially-available, low-level hardened elements each handle local control of a specific spacecraft function. According to PR, this dis-aggregation of functional responsibility allows for a number of advantages:
- It is easier to accommodate modern components and COTS hardware because a single component or sub-system can be replaced without perturbing the rest of the design.
- PR can rapidly iterate on spacecraft design because the exact form and configuration of the system can be modified late in a design flow.
- PR can reduce system inter-dependencies as each component has its own independent compute element that coordinates with other parts of the system.
- PR can decouple hardware and software through virtualization so that each may advance at their own pace.
Even with this change in architecture, the radiation environment of space is still challenging. Instead of making their system completely “hardened”, PR makes their systems resilient to the effects of radiation by designing the system to allow normal operations to continue despite random reset events on various elements of the architecture. A modular de-centralized hardware approach allows for a fault to be contained locally at only the component affected. The ramifications of such an event do not propagate through power, data, and functional interfaces to the rest of the system.
SPACE COMMUNICATIONS
“Talking” is a difficult task for any intrepid robotic explorer on its way to near Earth asteroids (NEA), as the distance back to Earth can exceed 2 Astronomical Units (AUs), or nearly 200 million miles. Traditional spacecraft use radio frequency, or RF, communications to solve this problem. While proven and reliable, RF systems require massive and power-intensive hardware that drive the cost of deep-space probes outside of the constraints of commercial budgets. At the same time, very large, sensitive receivers on Earth are necessary to pick up the incredibly faint signals from the spacecraft. NASA’s Deep Space Network, which includes some of the largest radio telescopes on Earth (as large as 70 meters in diameter), was specifically designed to communicate with interplanetary probes in this way. And it does so every day, providing critical communications for dozens of US government and international spacecraft around the solar system. Unfortunately, this makes for a pretty busy interplanetary network, one that is difficult to rely on for commercial operations in deep space.
PR has found a solution to this problem in the form of optical communications. Due to the shorter wavelength of optical communications when compared to RF, lasers allow for information to be communicated through a more tightly controlled beam using a significantly smaller aperture. This narrower focus greatly reduces the power required for a given communications data rate and distance, allowing a small spacecraft to effectively relay scientific and technical data, even when it is on the other side of the Solar System.
PR is developing a multi-function main instrument for its Arkyd spacecraft platform, one that integrates remote imaging, optical navigation, and optical communications into a single, resource-efficient tool. The system will take advantage of many of the advancements made in free space optical communications here on Earth, as well as previous work performed for NASA, with MIT as a partner, on miniaturized stabilization for optical communications on nano-satellites.
HIGH ΔV SMALL SATELLITE PROPULSION SYSTEMS
Sending a spacecraft into deep space is an energetically expensive proposition. Conventionally, a spacecraft headed out into the Solar System would be placed directly on its outbound trajectory by its own launch vehicle. This launch vehicle alone can be a $100 Million proposition, or more. PR is taking a different path. Its Arkyd prospecting spacecraft are small enough to hitch a ride into space with larger, primary payloads. PR plans to launch one at a time into an orbit based on the needs of the rocket’s primary payload. This presents a challenge, as a rendezvous with a solar-orbiting asteroid requires departing Earth at a very specific time, at a specific speed, and in a very specific direction. Otherwise, you could miss your rendezvous by thousands, or even millions, of kilometers.
PR solves this problem by being able to make its own way to near Earth asteroids directly from the low Earth orbit where it is placed as a secondary payload. Once in orbit, the Arkyd spacecraft uses its onboard propulsion system and an advantage of the Earth’s gravitational influence called the Oberth effect to escape Earth’s gravity well and head towards a future rendezvous with the NEA of interest.
The Arkyd spacecraft also employs two key technologies to enable this scale of propulsive capability on such a small platform. First, the system uses one of a new family of green, non-toxic mono-propellants. This allows the spacecraft, as a secondary payload, to be successfully integrated for launch without significant schedule impact or safety risk to the rocket’s primary satellite customer. Second, this propellant is stored and managed within a propulsion system that is directly integrated into the spacecraft’s primary structure. Working with its strategic investor and partner 3D Systems, PR is using additive manufacturing techniques to directly integrate the system’s manifold, plenum, and routing geometries directly into structural elements that support the spacecraft’s elements during the rigors of launch. By doing so, a system that conventionally consists of hundreds of parts and countless workmanship-sensitive assembly operations is now simplified down to just a handful of components, resulting in a system that is at once lighter, cheaper, safer, and much easier to build again and again.
