Autonomous orbit maintenance system

Orbit maintenance is a major cost factor for Earth satellites in specialized orbits, such as a repeating ground track, or in formations. While autonomous attitude control is well established, the spacecraft's orbit is usually uncontrolled or maintained by ground station commands. For small, lower cost satellites, operations costs can be a dominant element of both cost and risk. This implies a need for low-cost autonomous orbit maintenance in order to allow such systems to be economically viable, particularly in today's constrained budget environment.

In addition, if the position of the spacecraft is controlled, it is therefore known in advance. Thus, mission planning can be done as far in advance as desired, without the need for replanning and frequent updating due to unpredictable orbit decay. An interesting characteristic of autonomous orbit maintenance is that it typically requires less software, and less complex software, than does orbit control from the ground. In many cases, an onboard orbit propagator is not needed.     

International cooperation in remote sensing for global change research: political and economic considerations

Activities concerning international cooperation in space-based remote sensing for global change research have for the most part focused on technical/functional aspects such as data harmonization, research project coordination, and sensor selection and deployment. Until fairly recently, little attention has been directed towards the various political and economic constraints which may act as ‘stumbling blocks’. A review of the contemporary international remote sensing milieu as it relates to the global change research agenda is presented. Several important political and economic conditions affecting this area are identified: (1) data access and pricing policies; (2) national security concerns; (3) developed/developing nation relations; and (4) inconsistent political/financial commitments. A set of recommendations is offered to enhance international cooperation in the use of space-based remote sensing data for global change research.     

Air motions in the tropical stratosphere deduced from satellite tracking of horizontally floating balloons

Between June and November of 1970, 26 constant level balloons were released from Ascension Island (8 S) for flight at 30 and 50 mb. The balloons were positioned by the Interrogation, Recording and Location System (IRLS) aboard the Nimbus D satellite. In general, balloon positioning appeared to be accurate to within a few kilometers, although occasionally there was doubt as to whether the balloon position was to the right or left of the satellite subtrack. Eight of the flights at 50 mb and three of the flights at 30 mb were tracked for more than one month, and one 50 mb flight was tracked continuously for more than 5 months while making 7 circumnavigations of the Earth. From the satellite-determined 12-hourly balloon positions in the tropics, 223 smoothed 24-hour-average zonal and meridional winds were obtained at 30 mb and 693 such winds were obtained at 50 mb. Near the equator the balloons moved from east to west at a speed of about 23 ms^–1 at 50 mb and 28 ms^–1 at 30 mb, while undergoing a mean northward drift of approximately 0.1 ms^–1. The northward drift was a maximum in the Northern Hemisphere winter, suggesting a weak upward extension of the Hadley Cell to 50 mb. Superimposed on this drift were oscillations in meridional velocity of about 2-month period, with these oscillations also most pronounced in the Northern Hemisphere winter. Small (1–3 ms^–1) short-period fluctuations in meridional velocity were evident directly above the equator at 50 mb. These waves appear to move westward at speeds of 30–40 ms^–1 and to have a wavelength of about 90° longitude. They were responsible for transporting small amounts of westerly momentum into the winter hemisphere. Fluctuations in zonal velocity (Kelvin waves) were also delineated by flights near the equator. These waves appear to move eastward at speeds of 30–40 ms^–1 and to have a wavelength of 360° longitude. Some comparisons are made between these IRLS data and the data obtained from GHOST balloon flights at the same heights in early 1969.     

Coherent radio emission from pulsars

Our knowledge of radio pulsars is reviewed with particular emphasis on properties of radio emission that are relevant to an understanding of the emission mechanism. We discuss the tendency for emission to occur in two orthogonal modes of polarization, observational limits on the location of the emission region, and the issue of whether coherence is established by a broadband or a narrowband mechanism.Proceedings of the NASA/JPL Workshop on the Physics of Planetary and Astrophysical Magnetospheres.     

Estimating irrigation water use and withdrawal of ground water on the High Plains,U.S.A.

In four decades following the Dust Bowl days of the 1930's, extensive areas of dry farming and rangeland on the semi-arid U.S. High Plains were transformed into a vast region of irrigated oases, producing meat and grain for much of the world. The agricultural economy has experienced such rapid growth in part because of the availability of ground water and because of development of new irrigation technology to use that water for agriculture. However, more water is being used than is being replaced. To estimate both the volume of water withdrawn and the regional scope of the problem a technique has been developed that combines multispectral data from Earth-orbiting satellite with known pumpage data for the same growing season. The location and extent of irrigated cropland—some with different crops watered at different times—is inventoried using computer-assisted analysis of the data from Landsat. The amount of water used is estimated by multiplying and summing surface area of irrigated agriculture and the average measured pumpage from sampled sites. Published findings to date are cited in the Selected References. All suggest transferability of a promising technology to the study of land transformation processes elsewhere.     

A reappraisal of the habitability of planets around M dwarf stars

Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute.