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Appendix B: Small Spacecraft Applications Small spacecraft can play an important role in Earth observations, remote sensing, and communications missions through: . . . . . . . . use of the Global Positioning System (GPS), with which senses! data from several different spacecraft can be temporally ant! positionally connected, thereby enabling some missions that previously required a large spacecraft to be performed by several small spacecraft; deployment of one or more sensors in non-sun-synchronous orbits to avoid diurnal effects (e.g., tides); quick-response, rapid-repeat-cycle applications to serve civil and defense operational or quasi-operational needs, such as disaster response planning; specialized measurements in a relatively small number of spectral bands, with good radiometric accuracy, high spatial resolution, en c! a narrow swathwidth; deployment of all-electronic, long-lived small-satellite constellations for data collection, search and rescue, or global communications; use of autonomous teleoperated spacecraft for the inspection, repair, ant! maintenance of critical space systems; use of planetary microrovers with small, calibrates! science instruments and small robotic manipulators capable of extender! operations and improved analytical capability on the planets; use of small spacecraft for life science studies; and augmentation of larger research missions through the flight of complementary sensors. As the capability of small spacecraft is improved through research and development, their ability to support a broader mission spectrum will be enhanced. Each of the above missions is discusser} briefly below. ~6
Appendix B Small Spacecraft; Surrogates for Large Spacecraft With position, velocity, and time signals from the GPS, the data from sensors on several spacecraft can be temporally and positionally connected. This can be used to substitute constellations of smaller spacecraft for one larger spacecraft in those applications where simultaneity of data is a major requirement. Further refinements include control systems based on differential GPS schemes, which accurately establish position to hold con stelIations of small spacecraft in desirable formations. Non-Sun-Synchronous or Other Less Common Orbits Small spacecraft have notable value in missions requiring the deployment of a few sensors to non-sun-synchronous orbits, as in the case of microwave instruments that do not rely upon the illumination of the sun for their operation ant! that do not contribute to or require simultaneity of measurement with other sensors. Such orbits are also used for the conduct of measurements where results would be obscurer} by tidal or other diurnal effects. As a further example, a sensor may require an equator crossing time that differs from that of the majority of sensors to be flown. A small spacecraft might be used to orbit that sensor, rather than forcing overall performance to be compromiser} in a suboptimum orbit. Quick-Response Missions for Operational Uses Small spacecraft can also play an important role in operational or quasi-operational Earth-observation missions in which quick response is at a premium and where the instrument to be carried can be designed well in advance ant! held in reacliness to meet very specific, well-understood needs. For example, in contrast to the Earth sensors of Mission to Planet Earth, with their wide swathwiciths ant] extensive spectral bands, a high-spatial-resolution, panchromatic sensor with off-nadir) pointing capability might be deployed in an orbit that is chosen to provide rapid repeat coverage of a disaster site or to serve national security needs. Off-nadir pointing at angles that would! make the atmospheric correction of multispectral data difficult or impossible may be entirely satisfactory for disaster evaluation or (defense applications. ~7 ~ Nadir is the point of a celestial sphere that is directly opposite the zenith ant! vertically downward from the observer.
118 Technology for Small Spacecraft Specialized, Narrow-Swathwidth Measurements Other applications (e.g., long-term mapping) may also be well server! by multispectral sensors employing a relatively small number of spectral bands and narrow swathwidths. Narrow swathwidths reduce geometric registration errors. In some cases, narrow-swathwidth measurements could be used for quick response-type missions, which were previously discussed. "All-Electronic" Spacecraft Constellations Multiple small spacecraft in different orbits may provicle rapid repeat coverage for data collection, commercial communications, and search-and-rescue transceivers. These devices are readily adaptable to small spacecraft. Such all-electronic payloads on small spacecraft may have an inherently long lifetime that could produce a much lower system cost than other alternatives. This is contingent upon a decision being made to seek more-rapid repeat coverage than is currently available. Data collection and search-and-rescue payloads are currently carried on two to four U.S. and Russian polar-orbiting spacecraft. The number and orbits of the spacecraft produce waiting times of up to six hours before distress signals can be detected or data relayed. Constellations of six to twelve spacecraft would produce profound recluctions in delay time, especially at the midIatitudes. Post-accident survival is a direct function of waiting time, so the reduction in delay would also increase the number of lives saved. Both voice and messaging, as well as data-only spacecraft constellations have been proposed. On the commercial communications side, several companies have recently announced their intentions of employing a large number of tow-Earth-orbiting small spacecraft for worldwide communications (Seitz, 1993b; Seitz en c! de Selcling, 19931. Servicing Spacecraft for Space Systems The Space Station is a major investment in space infrastructure, where a limited number of humans must be provided with systems that will help improve their efficiency. The proper application of automation and robotics can improve the return on this investment by freeing the crew from repetitious tasks and allowing for more direct involvement of ground-based researchers in mission execution via teleoperations. Within the research environment of the Space Shuttle and the Space Station, small intravehicular activity robots, such as the German ROTEX on the 1993 Space Lab mission can turn a limited flight opportunity into a productive research project. The automation of human- tended teleoperated investigations that are based in space is within the technical capability of university and industry investigators. External to the manned system, small robotics can be developed that can reduce, and in some cases eliminate, the need for extravehicular activity and ShuttTe-related operations. In the vicinity of the Space Station, small free-flying robots can be
Appendix B programmed for autonomous or teleoperated inspection of critical Space Station systems as an integral part of repair and maintenance. The concepts and technology base to develop automated systems that are efficient and fault tolerant to human safety needs exist within NASA, the universities, and industry. The new, small space robots can become unique, relatively low-cost tools for the crews of the Space Station and can help bring research productivity more in line with earlier (1980) expectations involving larger crew complements. Deployment of Small, Robotic Planetary Explorers In the area of planetary exploration, small spacecraft systems are already being developed. NASA's Mars Pathfinder project employs an innovative microrover operating in the vicinity of a landing craft that serves as a science base and as a communications center with the Earth. The major challenge for this microrover is to apply mobility to a lander experiment, thus providing automated operations that can significantly increase the total returned knowledge. Major enhancements in the capability of such Microsystems can be made by investing in the development of small, calibrates} science instruments and small robotic manipulators capable of extended operations and improved analytical capability on the planets. Life Science Studies for Small Spacecraft Small spacecraft can be used for life science studies that cover topics ranging from exobiology to the effects of microgravity and radiation during spaceflight on cell cultures, plants, and animals. The feasibility of using small spacecraft for life science experiments depends on the complexity of the studies to be carried out. While there is no great difficulty in providing the appropriate life support systems for plants, fungi, and cell cultures, studies of whole animals on small spacecraft raise special problems because of the complexity of providing adequate life support and a safe return. Considerable progress could be made using interactive, expert systems where a number of variables could be monitored and appropriate responses made. The animals could be observer! with video cameras, and the temperature anti humidity of the cages; movement of the animals; mass of the animals; water anti foot] consumption; and temperature, heart rate, and electrocardiogram of the animals could be monitored. For lone-duration microaravitv and radiation studies. a small animal centrifuge. various levels ~ ~ ~ , , ~ ~ r ~ · ~ '. ~ · · , · ~ '. r . , ~ ~ ~ ~ ~ .. a. 01 shielding, and a m~n~atunzed bite support system would be needed on the spacecraft. There would also be a need for access to the payload just prior to launch and rapid access to the payload after spacecraft recovery, so that the animals could be given proper care. To decrease the spacecraft-landing deceleration shock, a paraglider type of parachute might be employed. A few years ago, NASA initiated a study of a small spacecraft for life science experiments called Lifesat. The satellite included a life support system for small animals ~9
120 Technology for Small Spacecraft and was designed to be recovered by coming down on land. Two types of orbit were envisaged: low Earth orbit, and an elliptical orbit to subject the payload to high radiation levels. Although this program was subsequently cancelled because of its cost, the subject of radiation hazards and proper protection remains as a central issue for long-term human space interplanetary flight. Examination of radiation effects on small animals in small spacecraft offers an excellent opportunity for further exploration of radiation effects and countermeasures. Mission Augmentation Small spacecraft can play a valuable role in augmenting larger Earth-observations research spacecraft that are planner! as a part of the Mission to Planet Earth. The aggregate lifetime of the instruments on a large spacecraft may be only a few years, while a single instrument may have a considerably longer lifetime. In such instruments, use of a dedicated, long-life small spacecraft could be the most economical means of carrying out the measurement and ensuring continuity of data.
