A Milli-Newton Propulsion System for the Asteroid Mobile Imager and Geologic Observer (AMIGO)

Abstract

Exploration of small bodies, namely comets and asteroids remain a challenging endeavor due to their low gravity. The risk is so high that missions such as Hayabusa II and OSIRIS-REx will be performing touch and go missions to obtain samples. The next logical step is to perform longer-term mobility on the surface of these asteroid. This can be accomplished by sending small landers of a 1 kg or less with miniature propulsion systems that can just offset the force of asteroid gravity. Such a propulsion system would ideally be used to hop on the surface of the asteroid. Hopping has been found to be most efficient form of mobility on low-gravity. Use of wheels for rolling presents substantial challenges as the wheel can't gain traction to roll. The Asteroid Mobile Imager and Geologic Observer (AMIGO) utilizes 1 kg landers that are stowed in a 1U CubeSat configuration and deployed, releasing an inflatable that is 1-m in diameter. The inflatable is attached to the top of the 1U lander, enabling high speed communications and a means of easily tracking lander from an overhead mothership. Milligravity propulsion is required for the AMIGO landers to perform ballistic hops on the asteroid surface. The propulsion system is used to navigate the lander across the surface of the asteroid under the extremely low gravity while taking care to not exceed escape velocity. Although the concept for AMIGO missions is to use multiple landers, the more surface area evaluated by each lander the better. Without a propulsion system, each AMIGO will have a limited range of observable area. The propulsion system also serves as a rough attitude control system (ACS), as it enables pointing and regulation over where the lander is positioned via an array of MEMS thrusters. Several different techniques have been proposed for hopping nano-landers on low gravity environments including use of reaction wheels, electro-polymers, and rocket thrusters. In this concept, we will be heating sublimate to provide propulsive thrust which is simple and effective. Storing the propulsive gas as a solid provides much better storage density to ensure the longest lifetime possible. The starting point for this propulsion system involves selection of an appropriate, high-performance sublimate that meets the mobility needs of AMIGO. The paper will cover selection of the right sublimate, initial design and prototyping of the thruster using standard off-the-shelf arrays of electro-active MEMs valves. The device will be integrated with the fuel source with thrust profiles measured inside a milli-newton test stand inside a vacuum chamber. MEMS devices are to be used because of the weight and volume saving potential compared to traditional large-sized thrusting mechanisms. Through these efforts we are advancing on a milli-newton thruster that use non-combustibles and can be readily integrated onto nano-landers for asteroid surface exploration.