Robotic spacecraft

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1. Introduction

The space environment represents one of the most challenging applications of robotics. Indeed, there is a widely-held but contentious viewpoint that space application represents a natural and inevitable arena for the advancement of robotics by imposing the requirement for high autonomy in space robotic systems. A minority extension of this viewpoint is that robotics is a discipline that has been stultified by its association with manufacturing, and space exploration provides an essential application in order to advance robotics as a discipline further towards its goal of developing human-like capabilities in the machine. Regardless of whether this may be so, or not, space application of robotics imposes unique drivers on robotics technology. The metric for success in space systems is the same as that for biological organisms – survival in a hostile and unrelenting environment. In this paper, I introduce some of the concepts of spacecraft engineering and how this impacts the design of robotic systems for space. I end the paper which some specific applications of robotics to space development and exploration to introduce two such applications that will be explored in the subsequent two papers.

2. Robotic Spacecraft

The first port of call in this paper is to put to rest a contentious, and often emotive, argument that plagues the political arena of space exploration. Every few years (the most recent following the Shuttle disaster), the eternally-resurgent question of whether humans or robots should be adopted for space exploration is dusted off for regurgitation (Ellery, A., 2003). This debate is misplaced – there is a well-defined distribution of tasks across the human and the machine, and this distribution is of an evolutionary nature. There are tasks that are suited to robotics, and, likewise, there are tasks suited to humans. Robotics serves to ease the burden of more manual and repetitive tasks from the human astronaut allowing his/her deployment to tasks requiring the beyond the state-of-the-art machine intelligence. There is little doubt that human spaceflight provides a degree of flexibility in space activities that is unattainable in robotic missions.

As the capabilities of robotics become more sophisticated over time, so the role of humans will shift exclusively to tasks of greater complexity (Ellery, A., 2001). However, human exploration missions will always require prior reconnaissance by robotic missions – robots do not suffer the fragility of the human body and can reach further into outer space than human beings. Human space missions and robotic space missions are complementary. The human v robots debate is thus futile at best and vacuous at worst. There is little doubt that robotics and automation has great potential in space activities. It is uncontroversial that no space system in the foreseeable future will be entirely autonomous. However, the space engineering community have a particular aversion to placing their trust in machines, preferring to rely on the human being, be it ground operator or astronaut, to oversee, and often even manually control space activities such as rendezvous and docking. This emplacement of control on the human being can be dangerous – human performance is limited by strength, vigilance, fatigue and reaction speed. Indeed, human error has been the root cause of 65–70% of civil airline accidents. The general lesson is that if a procedure can be automated safely, then it should be. Automation is commonly adopted for fault diagnosis, power management and scheduling, and active thermal control of spacecraft.

The second port of call is to define terminology: “robotic spacecraft” is a generic term used to refer to deep space probes of all types with an emphasis on planetary explorers, but often used also to refer to space telescopes. The term emphasises their unmanned nature with the implication of significant degrees of autonomy, particularly for deep space probes that are characterised by deployment at great distances. In this paper, I shall describe the constraints on spacecraft design that impose stiff requirements on the implementation of robotics for space application.

3. Spacecraft Design

The first constraint imposed on the robotic spacecraft is the necessity of functioning in a hostile, non-terrestrial environment. All spacecraft must survive the stresses of launch, the vacuum and radiation of space, and for planetary deployment, the stresses of landing and the environment of the target planet. The application of robotics to spacecraft engineering imposes its own demands on the spacecraft engineer. All space missions are designed to achieve the mission goals that traditionally have been telecommunications provision, Earth observation (including meteorological) data return, military expediency, navigation functions, or scientific data return. Most spacecraft to date have thus been almost entirely designed as platforms for sensors for the collection of data and its transmission from space to Earth without physical interaction with the space environment (Shaw, G., Miller, D. & Hastings, D., 2000). Most spacecraft actuation mechanisms have been associated with propulsion, attitude control or mechanical deployment of large structures such as communications antennae and solar array panels. However, robotic actuation under closed loop control is becoming increasingly important for future space missions. The addition of robotic actuation imposes an order of magnitude increase in complexity to spacecraft design in terms of the performance of tasks that physically interact with their environments. For robotic space and planetary exploration, these environments may be partially or totally a priori unknown. Indeed, there is a peculiar contradiction between the spacecraft engineer who tends to avoid mechanical actuation systems as potential single point failure modes, and the roboticist for whom actuation provides the mode of interaction with the environment.

Spacecraft are designed according five main design budgets:

(i)cost budget which imposes a ceiling on the costs of the design, development, construction, validation, and launch of a spacecraft;
(ii)mass budget which imposes a ceiling on the total mass of the spacecraft to be integrated into the launcher; severe limitation in mass – this favours lightweight designs using composite materials with consequent imposition of structural flexibility;
(iii)propellant budget which imposes limits on the manoeuvring capability of the spacecraft once it is in orbit (this is a sensitive function of the total mass of the spacecraft);
(iv)power budget which imposes a limitation of the power and energy available to each spacecraft subsystem and the payload – this favours the use of low power electronics, high efficiency motors, with high efficiency power scheduling with degraded computational resources;
(v)data budget which imposes severe limits on the capacity of the communications link to the ground and on the processing and storage capacity of onboard computing systems – shared human-machine autonomy is essential with sophisticated operator interfaces with predictive graphics.

All spacecraft are comprised of eight major subsystems:

(i)propulsion system which includes propellant, tankage, pumps, etc for manoeuvring in orbit;
(ii)attitude control subsystem which control the orientation of the spacecraft to ensure that all components point in the correct direction, eg. solar panels point to the sun, thermal radiators to deep space, communications antennae to the Earth, and payload sensors to the targets of interest;
(iii)structural and mechanical subsystem which mounts all other components onto the spacecraft in their required orientations, deploys all packaged components, and provides structural integrity to the spacecraft;
(iv)power subsystem which generates, stores, distributes and controls all electric power to all the spacecraft components including the payload;
(iv)thermal control subsystem which ensures that all components are maintained within their operational and/or survivable temperature ranges;
(vi)communications subsystem which maintains the radio frequency communications link between the spacecraft and Earth to ensure uploading of commands and downloading of telemetry and payload data;
(vii)onboard data handling (computer) subsystem which acquires, processes and stores all command, telemetry and payload data onboard the spacecraft;
(viii)payload subsystem which provides the business end of the spacecraft, typically sensors and scientific experiments but may comprise a robotic system.

An additional constraint is the need for high reliability to ensure that the spacecraft survives until the end of its mission. This constraint places high emphasis on validation and testing of components and assemblies under space-like conditions. Of particular significance is the requirement for high payload capacity as a fraction of total mass as this defines the raison d’etre of the mission.

The payload may include a robotic system comprising:

(i)human-computer interfaces including telerobotic control and virtual reality interfaces;
(ii)real-time control system including trajectory planning and generation, feedback servo-control laws, etc;
(iii)onboard computer (probably a dedicated control computer) with fault tolerance;
(iv)sensors and sensor processing, including stereo-vision, etc;
(v)actuation systems including electromechanical motors, etc.

All these subsystems must be integrated into a complete, functional, reliable spacecraft yet achieve this within the design budget constraints:

(i)complete system integration of large number of complex subsystems – this requires extensive testing and validation (under simulated thermal vacuum and launch conditions in particular) and tight project management;
(ii)long lifetime (>10 y typically) during which the spacecraft must operate reliably without frequent maintenance with self-checking diagnostics but have the capacity for upgrade and repair-by-replacement of modules;
(iii)extremely high reliability/safety (>90% typically) with fault tolerant (reliability) and triple fail-safe (safety) design protection – product assurance requires deterministic software approaches, disfavouring soft computing methods, and limitations in mechanical complexity, particularly moving parts which require special lubrication and/or hermetic sealing methods for operation in vacuum.

The implications to the roboticist to these constraints are thus: (Ellery, A., 2000Putz 1998):

(i)the robotic spacecraft and its components must have lightweight construction to minimise its launch mass yet survive launch/impact loads (eg. up to 20 g axial acceleration and 145 dB acoustic noise for launch);
(ii)the launch configuration is limited by the volume available in the payload fairing of the launcher necessitating that large area constructions be folded for launch and deployed reliably once in orbit (typically single-shot mechanisms);
(iii)0operate in vacuum environment (of 10-3 Pa at LEO and 10-15 Pa at GEO) – this requires the use of construction materials which are resistant to outgassing in vacuum, the use of dry lubrication in mechanisms, brushless electronic commutation in motors, and elimination of ultrasound as sensory modality;
(iv)microgravity conditions has particularly ramifications for robotic dynamic control algorithms such that there is no ground reaction in space and non-linear dynamics effects become important, favouring low speed motion (∼0.01m/s) as space lacks a damping medium for the dissipation of vibrations generated by the movement of boom-type configurations, and smooth motion profiles with high gearing ratios;
(v)the robotic spacecraft must endure severe temperature extremes and thermal cycling (−120°C to +60°C typically) – multilayer insulation, electric heaters, heat pipe and thermal radiators are required to maintain thermal limits;
(vi)sensitive components such as electronics, computers, sensors and instrumentation must function under exposure to a high charged particle radiation environment (around 106 rad/y) – electronics and computing equipment require some shielding or hardening against SEUs; the limited capacity for onboard computational resources available (traditionally, radiation-hard processors are used, though there have been recent implementations of using COTS (commercial off-the-shelf) processors for small satellites in low Earth orbits) imposes severe restrictions on control and navigation algorithm complexity that can be implemented for real-time control;
(vii)the lack of grounding of the robotic spacecraft may induce spacecraft electrostatic charging and discharges;
(viii)the space environment provides a highly variable illumination environment with extreme constrasts and shadowing effects due to a lack of atmospheric scattering effects – this makes image processing difficult;
(ix)the robotic spacecraft must operate for significant periods of time without direct human intervention (except for telecommanding and software uploading through the radio communications channel of limited bandwidth but these are subject to high signal transmission delays due to time-of-flight distance between the ground and space, and limited communication windows due to eclipsing disruption to the line-of-sight channel) – this imposes a need for significant levels of sensor-based autonomy with high fidelity ground operator interfaces.

4. Space Applications of Robotics

Although we have considered general robotic spacecraft issues here which are of critical importance to the space roboticist, space robotics as a discipline is focussed on more specific issues and reflects more closely the subject-area covered by terrestrial robotics. Indeed, space robotics, like its terrestrial counterpart, is generally divided into two subject-areas (though there is significant overlap):

  1. robotic manipulators – such devices are proposed for deployment in space or on planetary surfaces to emulate human manipulation capabilities; they may be deployed on free-flyer spacecraft or on-orbit servicing of other spacecraft, within space vehicles for payload tending, or on planetary landers or rovers for the acquisition of samples;
  2. robotic rovers – such devices are proposed for deployment on planetary surfaces to emulate human mobility capabilities; they are typically deployed on the surfaces of terrestrial planets, small bodies of the solar system, planetary atmospheres (aerobots), or for penetration of ice layers (cryobots) or liquid layers (hydrobots).

In the following two papers, I shall consider two case studies, one from each of these two topics: the use of manipulators mounted onto free-flying spacecraft to provide on-orbit servicing tasks, and planetary surface rovers for providing terrain-crossing mobility. I have specifically selected these two case studies to illustrate two issues – in the first, I consider the modification of traditional robotics techniques to the space environment; and in the second, I consider how new techniques may be borrowed from other disciplines (namely, vehicle terrainability) and applied to robotic planetary rovers.

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