Jovian Satellites' Simulator V4.0 (c) Gary Nugent, 1989-1994 Introduction In 1610, an Italian astronomer by the name of Galileo Galilei turned his new fangled optical instrument, called a telescope, on the planet Jupiter. It was one of those events in history after which nothing would ever again be the same. Up until that time, the church had proclaimed that the Sun, Moon and planets all orbited the Earth, which was considered to be the centre of the Universe. Scholars who put forward what were deemed to be heretical theories, were quickly forced to recant or face a punishment deemed suitable by the church authorities (usually being burned at the stake). In January 1610, Galileo shocked the world when he announced his discovery of four satellites orbiting Jupiter. It provided proof of Nicolas Copernicus' heliocentric theory - that the Earth and all the planets orbited the Sun, and the Earth was not at the centre of the Universe. Galileo told the story of his discovery: "On the seventh day of January in the present year, 1610, at the first hour of the night, when I was viewing the heavenly bodies with a telescope, Jupiter presented itself to me; and because I had prepared a very excellent instrument for myself, I perceived (as I had not before, on account of the weakness of my previous instrument), that beside the planet there were three starlets, small indeed, but very bright." He thought that these "starlets" were just more of the fixed stars he was discovering with astounding regularity. However, the next night he saw that they had changed position. The following night was cloudy, and then, on January 10, he saw only two "starlets", the third having disappeared behind Jupiter. He wrote on the 11th: "I had now decided beyond all question that there existed in the heavens, three stars wandering about Jupiter, as do Mercury and Venus about the Sun, and this became plainer than daylight from observations on similar occasions that followed." There are, in fact, four bright satellites in orbit around Jupiter, collectively known as the Galilean satellites, in memory of Galileo. Their names, taken from ancient Greek mythology and listed in order of increasing distance from Jupiter are: Io, Europa, Ganymede and Callisto. Jupiter Aptly named in honour of the ruler of Olympus, Jupiter is the largest planet in the Solar System, and is big enough to contain more than 1,300 globes the size of Earth. It is the fifth planet in order of distance from the Sun, and is also the most massive, accounting for more than two thirds of the combined mass of all the planets. It has a total of 16 satellites, four of which are visible in binoculars and telescopes. The planet is currently a brilliant object in the night sky, and is rivalled in brightness only by Venus. Jupiter is believed to have a small rocky core, made up of iron and silicates surrounded by a sea of liquid metallic hydrogen. About 46,000km from the centre of the planet, there is a transition to liquid molecular hydrogen. Outside of this is a gaseous atmosphere consisting mostly of hydrogen and helium in the ratio 4:1. The atmosphere contains a number of minor constituents - hydrogen compounds such as ammonia and methane are responsible for the coloration of the clouds. Jupiter has a fast rate of rotation, of just under ten hours, resulting in the poles of the planet being appreciably flattened. This flattening is easily visible through a moderately sized telescope. The planet has had a dominant effect upon a large area of the Solar System. It is likely that its gravitational force prevented a large planet from forming in the region now occupied by the Asteroid Belt. The Belt, itself, has gaps in it, as asteroids occupying positions with orbital periods that are exact fractions of Jupiter's period, are perturbed by the giant planet, and are gradually moved from those positions. Jupiter has been a favourite object for amateur astronomers, and much of what we know about the behaviour of the planet is due to observations made by amateurs over the last century. On July 10, 1979, Voyager 2 returned a series of images which showed that Jupiter had a single thin ring, with the ring particles extending nearly all the way down to the planet itself. The ring lies well within the classical Roche Limit (where bodies break up due to gravitational forces), and the particles are therefore likely to be relatively high density rocks and dust, rather than the icy material of Saturn's ring system. Theories for the origin of the ring particles include the gravitational break-up of a small inner satellite, material from Io and some of the satellites, or debris from comets and meteors. The Galilean Satellites These satellites move in nearly circular orbits in Jupiter's equatorial plane. All four are in synchronous rotation; in other words, they keep the same face towards Jupiter throughout their orbits. With the exception of Europa, all the satellites are larger than our own Moon. Before the first practical marine chronometer was pressed into service in the 18th century, occultations, transits and eclipses of the satellites, which could be accurately predicted, were regarded as potentially important for the determination of longitude by sailors far from land. In actuality, he method was never used with any great success. The Danish astronomer Ole Romer established that the speed of light was finite by comparing the times of eclipses of the Galilean satellites. Later he obtained a figure for the speed of light which was accurate to within 2 percent. The Galilean satellites were first studied in close-up by the Pioneer 10 spaceprobe in December 1973. In December of the following year, images were sent to Earth by its sister probe, Pioneer 11. However, the honours go to the Voyager probes for returning the most spectacular images and detailed information in 1979. The trajectories of the two probes were designed to provide the fullest pictorial coverage of both Jupiter and its satellites, and both were extremely successful. Io Io has proved to be one of the most remarkable objects in the Solar System. It resembles a pizza, its surface being coated in red yellow and black isotopes of sulphur. The satellite is the only other body in the Solar System known to be volcanically active. Images taken by Voyager show some of the eruptions, with material being ejected some 200km above the surface, such is their violence. Some eight active volcanoes were discovered by Voyager 1. Why is it so active? The reason seems to be tidal stresses caused by the gravitational pull of Jupiter and probably Europa. These stresses tend to pull Io out of shape, causing the surface to crack, and the molten interior to well up and fill the cracks. Europa Most of the information on Europa, the second and smallest of the Galilean satellites, was returned by Voyager 2. To everyone's surprise, the satellite turned out to be as smooth as a billiard ball. There are no volcanoes, and practically no craters. The surface, which is undoubtedly ice, is white, and has an almost complete lack of vertical relief. The globe is criss-crossed by dark elongated markings, and shows some darkish patches. A revolutionary theory was proposed recently which suggests that the outer ice sheet is only a few kilometres thick, and below this is an ocean of water some 50km deep. There has been speculation that this ocean may support a very primitive form of life. The Galileo mission is expected to return more information on this satellite. Ganymede Ganymede is the largest known satellite in the Solar System, being larger than the planet Mercury, and marginally smaller than Mars. It is not a very dense body, being only 1.9 times the density of water. It was thought that such a large body may have retained an atmosphere, but Voyager found no trace of one. Ganymede does have a cratered surface. It is thought that the surface, composed of icy material, was originally darkish in colour, but that geologically recent cratering has caused fresh, light coloured ice to be deposited on the surface. Callisto The outermost of the Galilean satellites is slightly smaller, less dense and less massive than Ganymede. It is the least reflective of the four, and the least bright as seen from Earth. Callisto is the most heavily cratered body yet discovered, with the distribution of craters being almost uniform. The satellite is believed to have a silicate core, surrounded by a mantle of water or soft ice, and a crust of ice and rock. The surface is truly ancient, dating from the early Solar System. It is probably the only Galilean satellite likely to be visited by future astronauts, as it lies outside Jupiter's lethal radiation belt. Galileo - The Mission to Jupiter The Galileo spaceprobe was launched on October 18, 1989, and will reach the Jovian system in 1996. The mission will allow scientists to study Jupiter, its satellites and magnetospheric domain, at close range for a period of almost two years. The spacecraft consists of two distinct parts - an orbiter and a probe. When the spacecraft is about 150 days away from Jupiter, the probe will separate from the orbiter, and each will follow a separate path to the planet. A few hours before the probe is due to enter Jupiter's atmosphere, the orbiter will fly within 1,000km of Io to make close observations. Io's gravity will slow the orbiter down so that it can be capture by Jupiter. Near the time of the orbiter's closest approach, the probe will penetrate Jupiter's atmosphere, and send data back to Earth, with the orbiter acting as a relay station 200,000km above the Jovian clouds. The probe is expected to penetrate to a depth of ten Earth atmospheres, all the time sampling the planet's atmosphere. The orbiter will complete 11 orbits of Jupiter during the 20 month life of the mission, and make a close flyby of at least one Galilean satellite on each orbit. The camera aboard the orbiter includes a 1500mm f/8.5 optical system which projects images onto an 800x800 CCD. The system is expected to reveal surface markings of the satellites, as well as Jupiter itself, with a resolution of 20 by 50 metres. In contrast, the best Voyager images revealed surface details with a resolution of 1km. PROGRAM DESCRIPTION The program calculates the positions of the four Galilean satellites (Io, Europa, Ganymede and Callisto) which orbit Jupiter, and displays their positions graphically on the screen. It runs on EGA and VGA machines. The .DTA data file is required for access to the pop-down menus. Startup When the program is run, the current date and time are read from the computer's internal clock. Following the Startup screen, the positions of the satellites, for the time read, are displayed. The view of the satellites defaults to that through binoculars on startup. Exiting the Program Pressing ALT-X while on the main screen returns you to the DOS prompt. (ALT-X won't work while on the Tracks screen). Display The display consists of five distinct sections. Section one displays the top-line menu. Section two displays the current date, time and instrument view. Section three is the graphics window which shows a side-on view of the satellites in relation to Jupiter. Compass directions, related to the current view, are printed at the top left of the window. The satellite and Jovian disks are drawn to scale, as are the satellites' distances from Jupiter (within screen resolution limits). Section four displays a plan view of the satellites. Each satellite is represented by a different colour in both graphics windows: (Io - White; Europa - Yellow; Ganymede - Red; Callisto - Blue). Section five displays numerical information for each satellite, namely its distance from Jupiter, as measured in Jupiter radii, and its angle from Inferior Conjunction. Inferior Conjunction occurs at zero (0) degrees, i.e. when a satellite is directly behind Jupiter. The angles are measured westward from this point, going to 90 degrees West, 180 degrees West (when the satellite is immediately in front of Jupiter, to 270 degrees West and finally back to zero. Top Line Menu This menu offers five options as described below. A bar highlights the current option. Newcalc This option allows the user to key in/edit a new date and/or time. If neither the date nor time is to be changed, simply press the ENTER key. The ESC key can be used to return to the top-line menu without retaining any date or time that has been keyed in and the original date and time will be restored. The time is in UT/GMT. There is no provision for different time zones or Daylight Savings time. If there are requests for this, I will include such options. View This allows the user to choose the type of instrument through which he/she would normally observe the satellites. Altering the view causes the side-on graphics display, compass directions and the numerical information to be automatically updated. A pop-down menu offers the four possible views with the most popular types of instrument (Binoculars, Astronomical Telescope, Star Diagonal and Other - the remaining optical configuration). Tracks This option produces a satellite track diagram for a user specified number of days (1 - 32), starting at the date and time most recently specified in the Newcalc option. If no such date or time was entered, the system time and date are used. Each satellite track is drawn in a different colour. (Io - white; Europa - Yellow; Ganymede - Red; Callisto - Blue). The month in which the tracks are being drawn labels the diagram at the top of the screen. The month name displayed depends on the first date printed. So, for example, if a 14 day track was being plotted from October 31 to November 13, the month name displayed would be October, not November. Horizontal lines range from the left to the right of the diagram and are labelled on the top with the date. Animation The user can enter an increment value (in hours, minutes and seconds) which determines the time between each successive calculation. The satellites are plotted in the two graphics windows continuously, thus providing an animation effect. The bigger the increment the faster the satellites will appear to orbit Jupiter. Pressing any key will stop the animation and return the user to the top-line menu. The animation entry box also shows the current setting for satellite Tracking. If ON then the satellites will leave trails behind them on both the side and plan views. Turning tracking off clears the trails from both windows. Tracking is toggled on and off by pressing the 'T' key. The animation begins from the time and date last specified in the Newcalc option. Realtime Choosing this option sets the program into Real Time operation. The graphics display and numerical data are updated every two seconds. In this mode, the program can be left to itself as a demonstration package, showing the position of the satellites to the nearest two seconds. Pressing any key will terminate this option. Movement Within Menus Use the left and right cursor keys to move between options in the top- line menu. Use the up and down keys to move within the pop-down menus. In all cases the ENTER key is pressed to select the required option. Display Zooming With NumLock on, the '8' and '2' keys on the numeric keypad zoom in and out of the side-on graphics window. Display Tracking With NumLock on, the '4' and '6' keys on the numeric keypad move the side-on graphics window to the left an right. This is to allow close passes of two or more satellites, which occur from time to time away from Jupiter, to be seen. The Use Of Colour Coding Colour applies particularly to the satellites as drawn in the graphics windows and their numerical information. Jupiter is always drawn in Cyan. Io is white; Europa, yellow; Ganymede, red and Callisto, blue. Numerical information is normally printed in black on a light grey background. If, when using the zooming option, some satellites move out of the display window, they will not be drawn. The corresponding numerical information of any such satellite will then be printed on a Red background. If a satellite is transiting, (passing in front of Jupiter's disk), then it's numerical information will be printed on a Yellow background. Finally, if a satellite is being occulted by Jupiter, (behind Jupiter's disk), it's corresponding numerical information is printed on a Blue background. Registration If you like this program please pass it on to others who may find it of interest. The registration fee is £5 (Europe), $10 (USA). Please send any suggestions for improvement or descriptions of any bugs you find to Gary Nugent 54a Landscape Park, Churchtown, Dublin 14, Ireland. Comments or suggestions can be emailed to: gnugent@cara.ie.