为火星探测寻找最佳着陆地点

行星科学家们正在为下一代火星车寻找既安全又有科学价值的着陆地点，但是资料显示恐怕鱼和熊掌不可得兼。     

Elegant propositions of the boundary-value problem

The fundamental ellipse of the two-body, two-point orbital boundary-value problem of orbital mechanics is introduced and its key role in the basic geometry of this famous problem is developed. Analytic formulas for the orbital parameters of conjugate orbits are derived together with further interesting properties of the mean point-locus.     

Inflection points in the development of energy in space

Basing his comments around a report on the Space Power Systems 1997 conference, ‘Energy and Space for Humanity’, held in Montreal, 24-28 August 1997, Richard Boudreault describes the progress made in getting the concept of power from space accepted by the terrestrial utilities. Increasing worries about the sustainability of current power sources together with deregulation of power distribution make space power more attractive. However, the high cost of access to space is a barrier, though this is starting to fall.     

Collision Avoidance System Synchronization

The collision avoidance synchronization system provides a method by which over 1000 aircraft can be accommodated in a 250-mile radius. Information is exchanged at a data rate of once every 3 seconds. Master-time synchronization permits one-way ranging between aircraft with an rms range error of less than 120 feet (36.6 meters) when both use the same master. A maximum error of less than 1000 feet (304.8 meters) rms between two aircraft occurs when each is tied back to a different master and each is at the limits of a hierarchy air-to-air synchronization extension system. An asynchronous backup mode of operation is provided to permit system operation in low-density areas beyond coverage of the master system time.     

Observations in the thermal IR and visible of a retired satellite in the graveyard orbit,and comparisons to active satellites in GEO

There exists a population of defunct satellites in the geo-stationary arc that potentially pose a hazard to current and future operational satellites. These drifting, non-station-kept objects have a variety of ages and sizes, and many are trapped in libration orbits around the Earth?s two gravitational potential wells (the non-spherical nature of the Earth gives rise to two geo-potential wells or “stable points” that affect objects in geostationary and geosynchronous orbits), whereas others were boosted to higher altitudes into so-called “graveyard” orbits.     

Broadcasting of a Digitally Coded List of Transponder Equipped Aircraft Scanned by FAA Ground Radars as a Means of Collision Avoidance, Navigation, Air Traffic Structuring, and Airfield Approach

An air traffic control system is proposed in which 1) all aircraft are equipped wit transponders, 2) FAA radar sites broadcast a digitally coded list of the identity, altitude, and coordinates of aircraft scanned, 3) each aircraft on receiving this list converts it into a PPI display with the blip of that aircraft branded and blips of off-altitude aircraft blanked, and 4) superimposed on this display are projected maps of airways appropriate to the particular radar and the aircraft altitude. This system could provide a fairly complete capability for air traffic control, structuring, navigation, and collision avoidance at a cost of about 3000 dollars per aircraft. It would replace most navigational and communication equipment now in use and could be operational in several years.     

Acidification of Europa's subsurface ocean as a consequence of oxidant delivery

Oxidants are formed at the surface of Europa and may be delivered to the subsurface ocean, possibly in great quantities. Whether these substances would be available for biological metabolism is uncertain, because they may react with sulfides and other compounds to generate sulfuric and other acids. If this process has been active on Europa for much of its age, then not only would it rob the ocean of life-supporting oxidants but the subsurface ocean could have a pH of ~2.6, which is so acidic as to present an environmental challenge for life, unless organisms consume or sequester the oxidants fast enough to ameliorate the acidification.     

The Lunar Reconnaissance Orbiter Laser Ranging Investigation

The objective of the Lunar Reconnaissance Orbiter (LRO) Laser Ranging (LR) system is to collect precise measurements of range that allow the spacecraft to achieve its requirement for precision orbit determination. The LR will make one-way range measurements via laser pulse time-of-flight from Earth to LRO, and will determine the position of the spacecraft at a sub-meter level with respect to ground stations on Earth and the center of mass of the Moon. Ranging will occur whenever LRO is visible in the line of sight from participating Earth ground tracking stations. The LR consists of two primary components, a flight system and ground system. The flight system consists of a small receiver telescope mounted on the LRO high-gain antenna that captures the uplinked laser signal, and a fiber optic cable that routes the signal to the Lunar Orbiter Laser Altimeter (LOLA) instrument on LRO. The LOLA instrument receiver records the time of the laser signal based on an ultrastable crystal oscillator, and provides the information to the onboard LRO data system for storage and/or transmittal to the ground through the spacecraft radio frequency link. The LR ground system consists of a network of satellite laser ranging stations, a data reception and distribution facility, and the LOLA Science Operations Center. LR measurements will enable the determination of a three-dimensional geodetic grid for the Moon based on the precise seleno-location of ground spots from LOLA.     

The photochemical stability of carbonates on Mars

Carbonates, predominately MgCO3, have been spectroscopically identified at a level of 2-5% in martian dust. However, in spite of this observation, and a large number of climate studies that suggest 1 to several bars of CO2 should be sequestered in carbonate rocks, no outcrop-scale exposures of carbonate have been detected anywhere on Mars to date. To address one hypothesis for this long-standing puzzle, the effect of ultraviolet (UV) light on the stability of calcium carbonate in a simulated martian atmosphere was experimentally investigated. Using 13C-labeled calcite, we found no experimental evidence of the UV photodecomposition of calcium carbonate in a simulated martian atmosphere. Extrapolating the lower limit of detection of our experimental system to an upper limit of carbonate decomposition on Mars yields a quantum efficiency of 3.5 x 10(-8) molecules/photon over the wavelength interval of 190-390 nm and a maximum UV photodecomposition rate of 1.2 x 10(-13) kg m(-2) s(-1) from a calcite surface. The maximum loss of bulk calcite due to this process would be 2.5 nm year(-1) (Mars year). However, calcite is expected to be thermodynamically stable on the surface of Mars, and potential UV photodecomposition reaction mechanisms indicate that, though calcium carbonate may decompose under vacuum, it would be stable in a CO2 atmosphere. Given the expected stability of carbonate on Mars and our inability to detect carbonate decomposition, we conclude that it is unlikely that the apparent absence of extensive carbonate deposits on the martian surface is due to UV photodecomposition in the current environment.