Satellite (astronomy)

“ It seems, therefore, quite safe to affirm that no globe of sufficient size to be seen by means of our modern telescopes circulates nearer to the sun than the planet Mercury.
Next in importance to the planets, as permanent members of the solar system, come the relatively small and secondary bodies known by the name of Satellites. The name satellite is derived from a Latin word signifying an attendant; for the bodies so-called move along always in close proximity to their respective "primaries," as the planets which they accompany are technically termed. ”

“ In one respect, however, a system of satellites differs materially from a system of planets. The central body around which planets are in motion is self-luminous, whereas the planetary body around which a satellite revolves is not. True, planets shine, and shine very brightly too; as, for instance, Venus and Jupiter. But they do not give forth any light of their own, as the sun does; they merely reflect the sunlight which they receive from him. Putting this one fact aside, the analogy between the planetary system and a satellite system is remarkable. ”

“ Yet if a planet is fortunate enough to have one satellite or more, the astronomer's method of weighing the planet is exceedingly simple; and all the major planets have satellites except the two interior ones, Mercury and Venus. As the satellite travels round its primary, just as the moon does round the earth, two elements of its orbit need to be ascertained, and only two. ”

“ The next satellite in order, proceeding inwards, is tolerably conspicuous; the three next are very minute, and require powerful telescopes to see them; while the two interior satellites, which just skirt the edge of the ring, and move exactly in its plane, have never been discovered but with the most powerful telescopes which human art has yet constructed, and then only under peculiar circumstances. ”

“ A student wishes to write a report on the impacts of satellite constellations on astronomy and the night sky. They complete the SatHub core training curriculum and participate in the discussion forum to learn both the historical context and the latest developments, and produce a well-researched report. An amateur astronomer highly experienced in visual magnitude estimation has meticulously recorded unaided eye brightnesses of satellites over many nights. ”

“ After working through the curriculum, a learner should be able to:
Appreciate the different kinds of satellites in orbit, and the harmful impact of satellite megaconstellations on astronomy, stargazing, and our night skies; Appreciate the purpose and importance of satellite observations; Access and use existing satellite databases; Efficiently use and contribute to SatHub; Plan satellite observations based on criteria such as location, time, hardware (telescope, camera, etc.) ; Carry out satellite observations using the hardware they have available to them ”

“ To these determinations respecting the orbits of the satellites, Dr. Herschel adds his estimate of the distances to which they must move from the body of the planet before they become visible by his 20-feet telescope. The first requires to be at more than half its greatest elongation. The second also becomes invisible when it is within half its greatest elongation. If there be an interior satellite, as large as the first, he imagines it would be visible through so small a part of its orbit, as not to be seen for two nights together. ”

“ At another time we note how the wanderer vanishes by occultation behind the body of the great globe around which it revolves. Then, too, there are the singularly delicate and beautiful phenomena of the transits of the satellites in front of the planet. In this case not only is the satellite itself often to be traced in the act of crossing, but, as a far more striking manifestation, its dark shadow is thrown on the brilliant globe. ”

“ Planetary satellites are a characteristic of our solar system, and now that the able astronomers at Washington have shewn that Mars has two moons, that mythological deity ceases to be exceptional. Neither in rate of motion nor in distance from the planet is there agreement between the two; for we are informed by Mr Christie of the Greenwich Observatory, that 'the outer satellite revolves once in less than a day and a quarter, and the inner three and a quarter times in one day. ”

“ There is nothing in the motion of either asteroids or comets to invalidate the general proposition which affirms, that the planes of the orbits of the heaviest and most important bodies in the solar system are very nearly coincident.
Many of the planets are accompanied by satellites, and these satellites revolve round the planets, just as the planet accompanied by its satellites revolves round the sun. The orbit of each satellite is contained within a plane, and that plane passes through the centre of the planet to which it is appended. ”

“ The great advantage of the satellite was its ability to provide a continuous flow of information for long periods of time. The first science satellites were the forerunners of later vehicles that would demonstrate the direct benefits that satellites could offer to such varied fields as weather observation and communication. ”

“ The planet would, in fact, always bear the same face towards an observer situated on the satellite, just as the moon always turns the same face to the earth. It is, however, certain that the new satellite is somewhat outside this critical position. ”

“ Fortunately, modern technology—given a boost by the world’s interest in rockets, missiles, and the exploration of space—has shown us one answer to this problem: the communications satellite. The conventional pathways for long-distance communication have led along the earth’s surface, under the oceans, and through the lower atmosphere. ”

“ Each satellite transmits signals on two radio frequencies, superimposed on which are navigation and system data. Included in this data are predicted satellite ephemeris, atmospheric propagation correction data, satellite clock error information, and satellite health data. This segment normally consists of 21 operational satellites with three satellites orbiting as active spares. The satellites orbit at an altitude of 20,200 km, in six separate orbital planes, each plane inclined 55° relative to the equator. The satellites complete an orbit approximately once every 12 hours. ”

“ The rings may be held to be formed of a multitude of tiny satellites, travelling nearly in one plane, each pursuing its own course around Saturn, according to the laws of satellite motion, though of course disturbed by the attraction of its fellow-satellites. ”

“ The satellites are started from those smaller nuclei which happen to be moving with just the right speeds not to escape entirely the attractions of the principal nuclei, nor to fall into them. The planes of the planetary orbits and, in general, the planes of the satellite orbits should agree quite closely with each other, but they could differ and should differ from that of the Sun's equator. ”

“ Of course, no amateur’s telescope can show the faintest glimpse of it.
Satellite I, situated at a mean distance of 261,000 miles from Jupiter’s center—about 22,000 miles farther than the moon is from the earth—is urged by its master’s overpowering attraction to a speed of 320 miles per minute, so that it performs a complete revolution in about forty-two hours and a half. The others, of course, move more slowly, but even the most distant performs its revolution in several hours less than sixteen days. ”

“ But of course, if the planet has an atmosphere thousands of miles deep, laden more or less heavily with cloud-masses, it might quite readily happen that a satellite should be seen apparently through the planet, — not, of course, through the middle of the planet, but through parts lying thousands of miles within the apparent outline. ”

“ Of the many astronomical observatories in the United States and abroad, none has ever photographed an object that remotely resembled a spaceship. Since 1957, hundreds of members of Moonwatch teams throughout the world have watched the skies to record passages of the many artificial satellites, but no Moonwatch team has yet reported the presence of a spaceship. Radar stations on all continents keep track of every artificial satellite and fragment of satellite orbiting the earth. ”

“ If you threw a satellite away from its path around Earth and directly into Earth's wake, but at a speed less than Earth's forward speed in its orbit, the satellite would break free, but would continue in Earth's orbital direction. And then with a brief side-blast, you could warp into an orbit around the Sun all your own, or—into that of the planet Venus, if you chose....
And beneath that eternal shell of cloud—who knew?
He had his computers. He had his reference-tapes. He had his refractor and his scanners, and his star-charts and his precious store of fuel. ”

“ The advent of the earth satellite provided scientists with a new and valuable research tool. Science satellites have been used for such tasks as solar and astronomical observations, biology experiments, or atmospheric investigation. ”

“ Television cannot practically be beamed more than a few hundred miles because the wavelengths needed to carry it will not bend around the earth and will not bounce off the region of the atmosphere known as the ionosphere. To solve this knotty problem, satellites may be the thing, for they can serve as high-flying radio relay stations. Several suitably-equipped and properly-spaced satellites would be able to receive TV signals from any point on the globe and to relay them directly—or perhaps via a second satellite—to any other point. ”

“ The use of sun-synchronous satellites simplifies the situation considerably, a rational initial constraint which could be removed at some later time when more sophisticated modeling becomes available. It appears reasonable to have a set of sun-synchronous satellites operating continuously so that each Earth ground point is covered at the equator every 2 days by at least one satellite of the set. ”

“ Launching and Tracking Problems How big a rocket booster would be needed? From what part of the world should a satellite be launched? What kind of orbit should it go into? (See table below) How far up should the satellite go? How can a satellite be tracked once it is in orbit? How do we predict future orbits of a satellite? Orientation and Control Problems How can orders be given to a satellite while it is in orbit? If it is to stay in a fixed attitude, how can it be kept there? What can be done to keep a satellite properly stabilized? Can optical measurements be made on a satellite? ”

“ We can assume that the planes are evenly spaced and that the satellites in a single plane are on average evenly spaced along the orbit (possibly with some rule for adding some randomness to the phases along the orbit) . This allows us to instantiate a suitable set of orbital elements for each satellite in the constellation. For this purpose (to assess the impact of a particular new constellation design) , perfect circular Keplerian orbits are likely a sufficiently accurate representation; detailed propagation models are not needed. ”

“ Communications satellites can be grouped into two broad categories. Passive vehicles reflect signals from one ground station to another. Active satellites accept ground signals and either amplify and rebroadcast them immediately or record messages for later transmission.
The Echo satellite balloons typified the passive category of communications spacecraft. These satellites “bounced” radio signals from one ground station to another. Uninflated Echo payloads were carried into orbit packed in special storage containers. ”

“ Such observations require facilities spread over latitude and longitude to capture Sun-angle-dependent effects.In the longer term, support a comprehensive satellite constellation observing network with uniform observing and data reduction protocols for feedback to operators and astronomical programs. Mature constellations will have the added complexity of deorbiting of the units and on-orbit aging, requiring ongoing monitoring. ”

“ Satellites must first be lifted beyond the reach of atmospheric resistance. It is absence of atmospheric resistance plus speed that makes the satellite possible. It may seem odd that weight or mass has nothing to do with a satellite’s orbit. If a feather were released from a 10-ton satellite, the two would stay together, following the same path in the airless void. There is, however, a slight vestige of atmosphere even a few hundred miles above the earth, and its resistance will cause the feather to spiral inward toward the earth sooner than the satellite. ”

“ Satellites are much closer to the observer than the more traditional targets and will leave a (bright) trail on the image. Tracking satellites is possible, but is quite challenging and will inevitably result in star trails.
As described in Section 1, SatHub will serve as a hub for astronomers and satellite operators to work together towards quantifying and cataloging various observational parameters (timing, satellite brightness, location, velocity, etc.) of non-classified satellite constellations. ”

“ We should, however, say that though what we have here indicated appears to explain fully the evolution of some of the systems, such, for instance, as that of Jupiter and his four moons, or Saturn and his eight or nine, the circumstances with regard to the earth and the moon are such as to require a very different explanation of the origin of our satellite. In the first place we may notice that the great mass of the moon, in comparison with the earth, is a wholly exceptional feature in the relations between the planets and their satellites in the other parts of the system. ”