OhioBob

Something else that makes Duna transfers a bit easier is that the planet's orbit has a very low inclination of 0.06 degrees, versus Eve at 2.1 degrees. This means that Kerbin and Duna orbit in almost the same plane, which makes it easier to find a transfer trajectory that intersects both bodies. With practice you'll find that it’s not all that hard to get to Eve either, but being this is your first time, Duna is definitely easier.

I'll look into that. Thankyou.

Ah, thanks. I've edited everything to make it http. I didn't even notice that I used https, which was my mistake.

Well that's disheartening. I've never seen or heard of this before. Sounds like I may need to contact my website host. Can you access my web page, in signature below?

Here's the link, maybe this will work better http://www.braeunig.us/pics/KSP/Composite.png - - - Updated - - - I've made contributions to the Wiki, but in this case I'm not sure it is appropriate. The Wiki seems to limit itself to things that are actually in the game. To say something like "at perihelic oppostion Duna shines at magnitude -2.5 and subtends and angle of 22 arcseconds" is just hypothetical and is not something that is depicted in the game or that can be experienced while playing the game. If enough other people think that it is appropriate to include this type of information in the Wiki, then perhaps I'll consider adding it.

I like it, but that style of writing is something I'd have trouble with. I'm not that creative with language. My style is better suited to a astronomy textbook. Of course that is one of the advantages of possibly turning it into a community project, different people can contribute in different ways. I also like the idea pre and post space age versions. The pre-version would be really lacking in photos (there are only so many fuzzy telescopic pics that could be included), but most of the old astronomy books that I've seen were illustrated with artist impressions. If we don't have any artists, Photoshop has some filters that could possibly take some screenshots and make them look like artist illustrations. The post-version could be chock-full of "photos" (screenshots actually). I might take what I have already done and see if I can turn into more of a book format. There is certain quite a bit of information that I can add to it. If I'm able to come up with a sample, I can make it available as a PDF.

If anything like that were to happen, I would want to write it in a different style. What I wrote in this thread is just an interpretation of given facts, along with a little bit of speculation surrounding some "what if" questions. I purposely tried not to make any creative decisions. If I were to write something more formal, I'd want to put it into the style of a fictionalize astronomy book and exercise some creative freedom. For example, instead of speculating about what Kerbals might know if they have spectroscopy, I would want the creative freedom to say that Kerbals do (or do not) have spectroscopy and this is what they do know (or at least what they think they know). I think it might also be interesting to alter some of the "facts" just a bit to show how uncertain their pre-space age understanding is. For example, if you were to read a 60 year ago astronomy book, you'd see that we were wrong about some things. Of course, writing in this way means that I'd be introducing some new fiction to the story that didn't come from Squad, but I think it would be more entertaining to read.

I'm not familiar with that mod. I'll have to investigate.

I had the same thought. I think it would make the game seem more real if we had to go out and discover these things rather than have them handed to us right from the start. It might make it feel like we were doing real sicence rather than just collecting points. It might also be interesting if some of the properties of the planets/moons were randomized so that each game were a little different with unknowns that would have to be discovered. Perhaps as an option: play with standard planets, or play with randomized planets. - - - Updated - - - I don't know, I've never used imgur. I'll have to check it out.

The first time I played Kerbal Space Program, one of the things that I though was a little odd is that we can go to the Tracking Station and immediately zoom in on all the planets and moons and see what they look like. We can also read many precise facts and figures about them. We can do all this before we launch our first rocket. This got me thinking about what the solar system would look like if we were restricted to Kerbin based observations. How big and bright would the moons and planets be, how much detail could we see through a telescope, what phenomenon could we observe? In other words, what would be the state of pre-space age astronomical knowledge? If we assume that Kerbal and human technology developed along the same or similar path, then Kerbal understanding of their solar system at the start of Year 1 would be much like human understanding of our solar system in the 1950s. Below is my interpretation of the information that would likely be known to Kerbin-bound observers. Much of the information, such as apparent magnitudes and sizes, is computed from given data. Some of the other information is speculation, but I try to provide the justification for such speculation. Although all the phenomenon that I describe, such as eclipses and transits, would exist in a real solar system, they are not necessarily depicted graphically in the game. Many facts and figures, such as orbital parameters, would be well known, but other physical characteristics, such as the size and mass of a small and distant body, would be known to much less certainty. I've tried to imply the degree of confidence in a particular value by the number of significant figures use. There are likely some things that Kerbals would be completely wrong about, but I'm not willing to speculate that far. Sun (Kerbol) Diameter: 523,000 km Mass: 1.757×1024 kg Rotation period: 20 days Luminosity: 3.161×1024 watts Surface temperature: 5,840 K (from game files) Atmosphere: Yes, hydrogen? The Sun is the dominate body at the center of the solar system. It contains 99.97% of all the mass in the solar system. It has the color and temperature of a G-type main sequence star. However, because of the scaled down size of the Kerbal universe, its luminosity is only 1/18th of what would be expected given its size and temperature. By measuring the value of the solar constant at Kerbin, astronomers can compute the Sun's luminosity. The solar constant is defined in the game's configuration files as 1360 W/m2, from which a luminosity of 3.161×1024 watts is calculated. This would normally imply a surface temperature much cooler than is presumed to be true. It is also presumed that the Sun's radiation peaks in the visible spectrum, as would be the case for a G-type star, rather than in the infrared, as would be the case for a low luminosity star. The Sun is an average of 13,599,840 km away from Kerbin, a distance that Kerbals define as their astronomical unit, or 1 AU. From this distance the Sun shines at an apparent magnitude of -26.7, the same as Earth's sun. The solar disk in Kerbin's sky subtends an angle of 132 arcminutes, more than four times the apparent angular diameter of Earth's sun. If Kerbals have come to discover the science of spectroscopy, then they likely have some knowledge of the chemical composition of the Sun. If so, then Kerbals have knowledge that we don't. Chemical composition is something that is undefined in KSP, thus we a left to only speculate about what might exist. Of course it is reasonable to assume Kerbals would figure out that the Sun is composed mostly of hydrogen, just as all stars are. On the Sun's surface are areas of solar activity that appear relatively dark when seen in white light. These sunspots appear, disappear, and change in shape over time. By following their motion it is determined that the Sun's rotation period is 20 days. (Sunspots may be illustrated as static in the game, but we know in real life they change.) Moho Mean distance from Sun: 5,263,000 km Orbital period: 102.6 days Diameter: 500 km Mass: 2.53×1021 kg Rotation period: 56 days Albedo: 0.10 Effective temperature: 163 oC Atmosphere: none Satellites: none Although Moho is one of the brighter planets in Kerbin's sky, its closeness to the Sun can make it difficult to observe. At its greatest elongation, Moho is never more than 28o from the Sun. Like the real life planets Mercury and Venus, Moho is seen to pass through phases, appearing full when on the far side of the Sun and a crescent as it nears Kerbin. The problem with observing Moho is that, at the times when the sky is dark enough to effectively observe it, it is never high above the horizon. This means that it must be observed through a thick and turbulent atmosphere, which degrades a telescope's ability to resolve small details. Moho's disk subtends an angle that varies between 5.2 and 14 arcseconds. When highest in the sky and best positioned for observation, Moho's apparent diameter is about 8". Given this small size and the poor astronomical seeing, it is unlikely that anything more than brightness variations are visible on its surface. Whether or not visual observations of Moho are of adequate quality to deduce the planet's rotation period is unknown. However, if Kerbal radio astronomy is advanced enough, measuring the Doppler effect of radio waves bounced off its surface can provide this data. It is likely Kerbals know that Moho's rotation is very slow, making less than two rotations every Moho year. It's possible for astronomers to measure a planet's albedo, the fraction of sunlight that it reflects, from the known amount of sunlight that hits it and the amount they see reflected off of it. We can't do that, but we can compute a planet's brightness if we know its albedo (we'll just pretend that Kerbal astronomers did it the other way around). The albedo of each body is given in the game's configuration files. It's uncertain if the value given is geometric albedo or bond albedo. Geometric albedo is used in brightness calculations and bond albedo in temperature calculations. Since we don't know, we'll assume the same value for both. In Moho's case, its albedo is 0.1. Moho is at its brightest when full, at which time it can reach an apparent magnitude of -2.4. At greatest elongation it dims to about magnitude -1.3. At this time Moho is bright and conspicuous enough in the evening or morning sky that it was likely known in ancient times. Knowing the amount of solar radiation Moho receives and its albedo, astronomers can compute its effective temperature, which roughly approximates the global average. Performing these calculations show that Moho is hot, with an effective temperature of 163 oC. Further calculations show that the maximum temperature at the subsolar point should be 360 oC. Planetary atmospheres can be detected by the dimming of starlight that occurs when a planet's limb occults a star, or by the silhouette the atmosphere produces when the planet transits another body. These techniques would reveal to Kerbal astronomers that Moho has no atmosphere. Furthermore, the ability of a planet to retain an atmosphere can be computed from its escape velocity and temperature. In KSP, the celestial bodies have been scaled down in size to facilitate better game play, with an apparent scale factor of 0.1 on diameter and 0.01 on mass. To determine the fictionalized ability of a KSP planet to hold onto an atmosphere, we should scale them back up to the size of their real world analog, and then perform the computation on the analog. Doing this for Moho shows that it is too small and too hot to retain an atmosphere of any significance. The mass of a planet is computed from its gravitational effects. The handiest method is to measure the distance and orbital period of a satellite, then insert these two figures into the proper equation derived from the law of gravitation, which gives the mass of the parent body. Since Moho has no natural satellite, its mass is calculated from the effect of its gravitation on other bodies. Every so often Moho passes in front of the Sun as seen from Kerbin. During these transits, Moho is visible as a black dot silhouetted against the face of the Sun. These events occur at varying intervals, with a normal of 7 transits occurring every 13-year period. Eve Mean distance from Sun: 9,833,000 km Orbital period: 261.9 days Diameter: 1,400 km Mass: 1.224×1023 kg Rotation period: 22.4 hours Albedo: 0.45 Effective temperature: 9 oC (atmosphere) Surface temperature: 135 oC ? Atmosphere: Yes, carbon dioxide? Satellites: 1 Eve is the brightest planet in Kerbin's sky. Through a telescope, Eve is seen to pass through phases because its orbit between Kerbin and the Sun makes different amounts of its illuminated hemisphere visible at different times. When full, Eve shines at an apparent magnitude of -3.9. It is brightest when at about a 40% crescent phase, shinning at magnitude -4.9 (the same as Venus at its brightest). Eve comes closer to Kerbin than any other planet. Its disk subtends an angle that varies between 12 and 79 arcseconds. At greatest elongation, Eve is 46o from the Sun and has an apparent diameter of 31". Eve is the second largest planet by both diameter and mass, with only Jool being larger. Its density is nearly 1.5 times that of Kerbin, easily the highest density among all planets. This suggests that Eve's interior must contain a large core of heavy elements. Telescopic observations of Eve reveal the presence of a thick atmosphere. However, the question that remains is whether or not there is a thick cloud deck. Eve's albedo is 0.45, which is much higher than expected from its darkly colored surface. This suggests that much of the planet is covered by clouds, even though these clouds are not depicted in the game's graphics. The amount of cloud cover is unknown, but an albedo of 0.45 is on the low end for most cloud types, suggesting that the cloud cover may not be total. It's possible that portions of the surface can be seen through gaps in the cloud layer. Furthermore, radar studies, if sensitive enough, might also reveal some large scale structure. Perhaps Kerbal astronomers have been able to patiently piece together a crude map of Eve's surface. We know from the game that the molecular mass of Eve's atmosphere is 43 g/mol, suggesting that carbon dioxide is likely the major constituent. It is probable that spectroscopic observation of Eve's atmosphere would find evidence of this carbon dioxide. Eve's effective temperature is 9 oC, which is a theoretical estimate of the cloud layer temperature, but this tells us nothing about the surface temperature. It's possible that some combination of infrared and/or microwave observations has revealed that Eve has a much hotter surface, resulting from the greenhouse effect. Note that most infrared wavelengths are absorbed by water vapor in the atmosphere. Kerbals can get around this by having their infrared telescopes carried aloft by balloons. Like Moho, every so often Eve passes in front of the Sun as seen from Kerbin. Transits of Eve are rarer than transits of Moho, with only 6 or 7 events occurring every 75-year period. Gilly Mean distance from Eve: 31,500 km Orbital period: 18 days Diameter: 26 km Mass: 2×1017 kg Rotation period: unknown Albedo: 0.15 Effective temperature: 40 oC Atmosphere: none Eve's moon Gilly is the smallest known moon in the solar system. At its brightest it shines at magnitude +5.0. At most times Gilly is below the threshold at which telescopes can resolve its disk, though at its closest to Kerbin, it can be seen to subtend an angle of 1.5". Estimates place its diameter at about 26 km. Because Gilly's gravitational effects are small, its mass is not well known. The rotation period of Gilly has not been determined. It is theorized that Gilly may be a captured asteroid. Kerbin Mean distance from Sun: 13,599,840 km Orbital period: 426.090 days Diameter: 1,200 km Mass: 5.2917×1022 kg Rotation period: 5h 59m 9.425s Albedo: 0.35 Effective temperature: -23 oC (atmosphere) Surface temperature: 13.5 oC average Atmosphere: Yes, nitrogen/oxygen Satellites: 2 Kerbin is the third largest planet in the solar system, and the third in distance from the Sun. Its location places it well within the Sun's habitable zone, where temperatures are warm enough for liquid water to exist on its surface. Global temperature extremes vary from 41 oC at the equator, to -35 oC at the poles. The globally averaged temperature is about 13.5 oC. Kerbin has a thick atmosphere that contains oxygen. The average molecular mass of Kerbin air is approximately 29 g/mol, suggesting that it is likely an earthlike nitrogen-oxygen mixture. Kerbin's albedo is 0.35, which is much higher than expected from the reflectance of its surface. This suggests that cloud cover factors significantly into Kerbin's albedo. Kerbin's effective temperature is -23 oC, indicating that there is a substantial greenhouse effect warming its surface. Kerbin has no axial tilt and its orbital eccentricity is zero, thus the planet experiences no seasons of any kind. From any specific location on Kerbin, the Sun is seen to follow the same daily path across sky. The entire globe experiences equal periods of daylight and darkness throughout the year. Kerbin's solar day is exactly 6 hours long. The equators of Kerbin and the Sun lie in the same plane, therefore making it possible to use a single celestial coordinate system for bodies orbiting Kerbin and those orbiting the Sun. Kerbin has two natural satellites, Mun and Minmus. The gravitational interaction of Kerbin and its moons allow the masses of all three bodies to be precisely determined. Mun Mean distance from Kerbin: 12,000 km Orbital period: 6.4345 days Diameter: 400 km Mass: 9.760×1020 kg Rotation period: 6.4345 days Albedo: 0.10 Effective temperature: -2 oC Atmosphere: none Mun is the second largest and brightest body visible from Kerbin, second only to Sun. Its disk subtends an angle of 115 arcminutes, which is nearly four times the apparent diameter of Earth's moon. Like Earth's moon, Mun is seen to pass through phases. At full phase its apparent magnitude is -15.2, or about 10 times brighter than a full moon on Earth. Assuming Kerbal visual acuity is similar to humans, significant detail can be observed with the naked eye. Through a telescope Mun is observed in stunning detail. It is so close by that, assuming an angular resolution of 1 arcsecond, surface features as small as 55 meters can be resolved. Mun is tidally locked to Kerbin and always keeps the same face pointed toward its parent planet, thus only 50% of its surface is visible from Kerbin. The features of Mun's far side are a mystery. Because Mun's orbital plane lies within the plane of Kerbin's orbit around the Sun, eclipses are a regular occurrence. Solar eclipses occur every new phase and lunar eclipses every full phase. Because Mun's apparent diameter is smaller than the Sun's, solar eclipses are annular (they can also be called transits). Lunar eclipses, however, are total. One consequence of the unusually perfect orbital alignment is that the central path of solar eclipses is always along Kerbin's equator. Partial solar eclipses are never visible outside of about ±45o latitude of the equator. Mun's effective temperature is -2 oC, though actual surface temperatures should vary considerably depending on location. The computed temperature of the subsolar point is 120 oC. Observations show that Mun has no atmosphere, as expected based on its temperature and small size. Minmus Mean distance from Kerbin: 47,000 km Orbital period: 49.875 days Diameter: 120 km Mass: 2.646×1019 kg Rotation period: 1.870 days Albedo: 0.50 Effective temperature: -39 oC Atmosphere: none Minmus is the third largest and brightest body visible from Kerbin. Its disk subtends an angle of 8.8 arcminutes, which is about 1/4th the apparent diameter of Earth's moon. Like Mun, Minmus passes through phases. At full phase, its apparent magnitude is -11.4, which is about the brightness of a gibbous moon on Earth. Minmus' disk is easily resolved with the naked eye. Without optical aid it is likely that a keen-eyed observer can detect albedo variations on Minmus, but probably little else. Through a telescope, Minmus is seen in a resolution similar to viewing its full disk on your computer screen. This is adequate to resolve surface features as small as 225 meters across. Unlike the tidally locked Mun, Minmus has a rapid rotation, thus allowing its entire surface to be seen from Kerbin. Because Minmus' orbit is inclined to that of Kerbin, solar transits and lunar eclipses are less common than they are for Mun. Minmus lies too far away from Kerbin to experience total lunar eclipses, but Minmus does occasionally pass through Kerbin's penumbra and antumbra shadows. Lunar eclipses and solar transits occur at a frequency that produces an average of 1.5 events of each type per year. Although the frequency is the same, lunar eclipses are routinely visible to a larger population than are transits. This is because they can be seen from anywhere on the nighttime side of Kerbin. Solar transits, on the other hand, are visible only from within a specific path across the globe. Observing a transit may require transportation to a faraway and remote site. Since Minmus reflects more sunlight than Mun, it has a lower effective temperature of -39 oC. The computed temperature of the subsolar point is 89 oC. Observations show that Minmus has no atmosphere. Duna Mean distance from Sun: 20,726,000 km Orbital period: 1.881 years Diameter: 640 km Mass: 4.515×1021 kg Rotation period: 18.2 hours Albedo: 0.17 Effective temperature: -58 oC (atmosphere) Surface temperature: -12 oC daytime ? Atmosphere: Yes Satellites: 1 Because Duna is a superior planet, i.e. its orbit lies outside that of Kerbin, it does not pass through phases in the same way as Moho and Eve. Duna is never seen in less than a 76% gibbous phase. Duna is full when it and the Sun are on opposite sides of Kerbin, an alignment called opposition. This is also the time that Duna is closest to Kerbin, reaches its maximum brightness, and can be seen high in the midnight sky. Duna's apparent brightness at opposition varies between magnitude -1.7 and -2.5, depending on Duna's distance from the Sun. Duna's brightness decreases dramatically when not near opposition, dimming to as little as magnitude +1.5 when farthest from Kerbin. When at its greatest distance from Kerbin, Duna's disk subtends an angle of just 3.7 arcseconds. At opposition, however, Duna's disk grows to a size that subtends an angle of between 16" and 22". The larger apparent size is seen during the more favorable perihelic oppositions, which are those that occur when Duna is near its perihelion. Perihelic oppositions occur about once every 15 years. Through a telescope, many light and dark markings can be seen on Duna's surface, as well as white polar ice caps. With astronomical seeing limiting resolution to 1" features as small as 30 km can be seen. By following features on its surface, Duna's rotation period of 18.2 hours is precisely measured. Since this period is approximately the length of three Kerbin days, each night Duna will show a face that is about 1/3rd of a rotation different than the night before. Duna retains a substantial atmosphere. It has an effective temperature of -58 oC, though its surface is likely warmer. We can probably assume that Duna's daytime surface temperatures would be revealed by infrared observations. Duna's albedo is 0.17, which suggests it has little cloud cover. Ike Mean distance from Duna: 3,200 km Orbital period: 18.2 hours Diameter: 260 km Mass: 2.78×1020 kg Rotation period: 18.2 hours Albedo: 0.14 Effective temperature: -56 oC Atmosphere: none Ike is the fifth largest moon in the solar system, but the largest in both diameter and mass when compared to its parent planet. Ike is about 40% of Duna's diameter and 6% of its mass. Ike orbits closer to its parent than any other moon. Duna and Ike are tidally locked to each other, with each keeping the same hemisphere facing the other. Ike is the only moon beyond Minmus that is commonly visible to the naked eye. At perihelic opposition, Ike shines at magnitude -0.4, which is about 1/7th the brightness of Duna. With Ike and Duna separated by as much as 3.7 arcminutes, it's possible to spot them as separate bodies. One has to wonder if ancient Kerbals would have correctly interpreted the movement of these lights as one body orbiting the other, and what influence this may have had on their early models of the solar system. Humans had to wait for Galileo's telescope before a similar observation could be made. Through a telescope Ike's disk is seen to subtend an angle that ranges from 1.5" to 8.8". This is large enough to observe light and dark patches on its surface. The regular movements of Ike result in some interesting events that are visible from Kerbin. A transit occurs when Ike moves across the disk of Duna. A shadow transit occurs when Ike's shadow moves across Duna. An occultation occurs when Duna covers the disk of Ike. And an eclipse occurs when Ike moves into the shadow of Duna. Dres Mean distance from Sun: 40,839,000 km Orbital period: 5.204 years Diameter: 280 km Mass: 3.2×1020 kg Rotation period: 1.6 days ? Albedo: 0.12 Effective temperature: -118 oC Atmosphere: none Satellites: none Dres is a small planet that varies in apparent brightest from magnitude +6.4 to +3.5. It can be seen with the naked eye during oppositions but is unspectacular. Its disk subtends an angle that ranges from 0.9" to 2.7". Through a telescope it is large enough to be seen as a tiny disk, but too small to observe any surface features. A small brightness fluctuation occurs every 1.6 days, indicating that there may be a patch of lightly color material on its surface that regularly passes into view as the planet rotates. Because Dres' gravitational effects are so small, its mass is not well known. Jool Mean distance from Sun: 68,774,000 km Orbital period: 11.37 years Diameter: 12,000 km Mass: 4.233×1024 kg Rotation period: 1.67 days Albedo: 0.52 Effective temperature: -170 oC Cloud temperature: -100 oC ? Atmosphere: Yes, hydrogen? Satellites: 5 Jool is by far the solar system's largest planet. Its mass is more than 16 times the mass of all other planets and moons combined. Its diameter is ten times Kerbin's diameter. Despite its great size, Jool is significantly less dense than the other planets. This suggests that it has an entirely different internal structure, consisting not of rock and metals, but of highly compressed lighter elements. The term gas giant has been coined to describe Jool-type planets. Despite its great distance from Kerbin, Jool is so large that its disk subtends an angle of 29" when farthest away and 48" when closest. Jool's size and high albedo makes it the second brightest planet in Kerbin's sky. Jool's apparent magnitude ranges from -1.6 to -3.0. When viewed through a telescope, all that can be seen of Jool is an impenetrable veil of clouds. This cloud surface is green in color with varying light and dark shades forming horizontal bands and swirls. If Jool has a solid surface, it is presumed to be deep beneath the thick atmosphere. Jool's effective temperature is -170 oC, though astronomers may learn through infrared studies that its cloud layer is considerably warmer. The higher than expected temperature would suggest that Jool emits heat of its own. Of considerable interest when observing Jool is its family of moons. There are five in total, listed in order of distance from Jool: Laythe, Vall, Tylo, Bop and Pol. The three innermost are large enough to be planets in their own right. All three are bright enough to be seen with the naked eye, though their closeness to the much brighter Jool makes this very difficult. When observing these moons through a telescope, one can frequently see transits, shadow transits, occultations, and eclipses. Laythe Mean distance from Jool: 27,000 km Orbital period: 2.45 days Diameter: 1,000 km Mass: 2.94×1022 kg Rotation period: 2.45 days Albedo: 0.3 Effective temperature: -160 oC Surface temperature: >0 oC ? Atmosphere: Yes Laythe is Jool's closet and second largest satellite. It is comparable in size to Kerbin, having 83% of Kerbin's diameter and 56% of its mass. As seen from Kerbin, Laythe's apparent magnitude varies between +4.4 and +3.0. Its disk subtends an angle that ranges from 2.4" to 4.0". Laythe is in orbital resonance with Jool's second and third moons, Vall and Tylo. The three moons orbit in a 4:2:1 resonance. Because of its small apparent size and lack of large-scale surface variation, a telescopic view of Laythe probably appears rather featureless. However, this doesn't mean that astronomers are clueless about its surface composition. Different surfaces (soil, ice, water, vegetation, etc.) have different spectral reflectance signatures. Since Laythe is almost completely covered by a global ocean, it's possible that Kerbals would detect evidence of this liquid surface in the light that it reflects. It should also be noted that Laythe's albedo of 0.3 suggests that clouds contribute significantly. The presence of liquid on Laythe's surface requires a substantial atmosphere and sufficiently warm temperatures. Observations would confirm the existence of the atmosphere, but Laythe's effective temperature is only -160 oC. If the liquid on the surface is water, then Laythe requires a source of internal heat, possibly generated by tidal heating. Laythe is undoubtedly the subject of much discussion among Kerbal planetary scientists. Vall Mean distance from Jool: 43,000 km Orbital period: 4.91 days Diameter: 600 km Mass: 3.11×1021 kg Rotation period: 4.91 days Albedo: 0.5 Effective temperature: -169 oC Atmosphere: none Vall is Jool's second moon by distance and third by size. From Kerbin, Vall's apparent magnitude varies between +4.9 and +3.6. Its disk subtends an angle that ranges from 1.4" to 2.4". Through a telescope, Vall appears as a tiny featureless disk. Vall's high albedo and low density differentiates it from Laythe and Tylo, and suggests that Vall is likely composed of a substantial amount of ice. If Vall's interior is tidally heated, it may contain subsurface liquid water. Vall shows no evidence of an atmosphere. Calculations show, however, that Vall is capable of retaining at least some atmosphere. Its close proximity to massive Jool means that Vall is almost certainly tidally locked. Tylo Mean distance from Jool: 68,500 km Orbital period: 9.81 days Diameter: 1,200 km Mass: 4.23×1022 kg Rotation period: 9.81 days Albedo: 0.1 Effective temperature: -153 oC Atmosphere: none Tylo is the largest satellite in not only the Joolean system, but the entire the solar system. Its diameter is equal to Kerbin, but it has only 80% of Kerbin's mass. From Kerbin, Tylo varies in apparent brightness from magnitude +5.1 to +3.8. Its disk subtends an angle that ranges from 2.9" to 4.8". Close up views of Tylo shows what appears to be small variations in surface brightness between its eastern and western hemispheres. It might be possible for astronomers on Kerbin to detect these very slight changes as the planet rotates. If so, following these variations would confirm that Tylo's rotation is tidally locked to Jool. Tylo is certainly large enough to retain a substantial atmosphere, but none has been detected. It is the largest solar system body not to have an atmosphere. Bop Mean distance from Jool: 128,500 km Orbital period: 25.2 days Diameter: 120 km ? Mass: 3×1019 kg ? Rotation period: 25.2 days ? Albedo: 0.25 ? Effective temperature: -160 oC ? Atmosphere: none Bop, the fourth moon of Jool, is in a highly eccentric and inclined orbit. Its maximum brightness at opposition is magnitude +7.9. Bop is too small for telescopes to resolve its disk. It is assumed to be tidally locked to Jool. From its measured brightness and small gravitational effects, astronomers can make a best guess estimate of Bop's diameter and mass. Furthermore, stellar occultations by Bop can provide additional information about its size, though these events happen rarely. The characteristics listed above are a fictionalized example of the type of uncertain numbers that astronomers might derive. Pol Mean distance from Jool: 180,000 km Orbital period: 41.8 days Diameter: 100 km ? Mass: 2×1019 kg ? Rotation period: 41.8 days ? Albedo: 0.4 ? Effective temperature: -160 oC ? Atmosphere: none Pol, the farthest moon of Jool, is in a moderately eccentric and inclined orbit. Its maximum brightness at opposition is magnitude +7.8. Pol is too small for telescopes to resolve its disk. Like Bop, Pol's size and mass are estimated from its brightness and small gravitational effects. Pol is assumed to be tidally locked to Jool. It is believed that Pol and Bop may be captured asteroids. Eeloo Mean distance from Sun: 90,119,000 km Orbital period: 17.06 years Diameter: 400 km Mass: 1×1021 kg Rotation period: unknown Albedo: 0.5 Effective temperature: -182 oC Atmosphere: unlikely Satellites: none Among the orbits of the seven planets, Eeloo's is the largest, most eccentric, and second most inclined. The orbit is so eccentric that Eeloo's perihelion lies inside the orbit of Jool. As seen from Kerbin, Eeloo's apparent brightest varies from magnitude +7.5 at its farthest to +4.5 at its closest. Eeloo's disk subtends an angle of between 0.7" and 1.6". When near its closest distance to Kerbin, Eeloo can be spotted with the naked eye. Through a telescope it can be resolved into a tiny disk. Eeloo shows no features or brightness variations, thus its rotation period is unknown. Eeloo has no known satellites and its gravitational effects are very small, thus its mass is not well known. Eeloo's composition likely includes a substantial amount of ice. Eeloo's distance from the Sun and high albedo makes it the coldest place in the solar system. Its effective temperature varies between -167 oC at perihelion and -192 oC at aphelion. No atmosphere has been detected but, because of the very cold temperatures, the possibility of a thin atmosphere has not been completely ruled out. Imagery As a final exercise I Photoshopped some images that depict what I think the moons and planets would look like from Kerbin. I sized the images so that the image resolution matches normal naked eye or telescopic resolution. For example, naked eye resolution is typically 1 arcminute, so Mun, which has an angular size of 115 arcminutes, was sized to be 115 pixels across. For telescopic views the resolution is 1 arcsecond. Of course it is possible to increase magnification, but this won't increase resolution. You'll just get a bigger and fuzzier image, much as would happen if you simply zoomed in on the images below. Composite.png (750×1500) (braeunig.us)

One thing that can get tricky when calculating ÃŽâ€v remaining is when you have some unusual stagging. If you have a SSTO, then that's probably not an issue. However I bring the point up in case it is ever an issue in the future. A perfect example of what I'm talking about is the Apollo missions to the moon. Let's say you are on your way to the moon and your vehicle includes the CSM and LM. You want to calculate the ÃŽâ€v remaining in your service module. Well, the CSM is 30 t with propellant and 12 t empty, the LM is 15 t, and the ISP is 314 s. So your calculation is easy, right? ÃŽâ€v = 314*9.81*LN(45/27) = 1573 m/s But hold on a minute. You perform orbit insertion with the LM docked to the CSM, but all your other manuevers are performed without the LM. Your dry mass is not constant, it changes after you ditch the LM. The above calculation is rather meaningless because it doesn't provide the data you really want to know. But the above it what KER will tell you. The first thing you have to do is calculate the amount of propellant burned during lunar orbit insertion. Let's say orbit insertion in 900 m/s. Therefore, 900 = 314*9.81*LN(45/(45-mp)) mp = 11.4 t Therefore the amount of ÃŽâ€v remaining in the CSM after the LM has been detached is ÃŽâ€v = 314*9.81*LN(18.6/12) = 1350 m/s So the total ÃŽâ€v that you're going to get out of the service propulsion system when you consider all the stagging is, 900+1350 = 2250 m/s. You have to be careful not to always take what KER tells you at face value. There are some calculations that the various tools in the game just can't give you because they can't anticipate what you plan to do. There are times when you're going to have to carefully think through each step of the mission and perform the calculations by hand.

That's hard to answer because Bop's orbit is so much different than Laythe's. Using Laythe to aerobrake will get you into a Jool orbit, but you'll then have to perform a series of burns to get the Bop. Bop's orbit is also 15 degrees out of plane with Laythe, so that doesn't help. By the time you do everything necessary to get a good intercept and orbit insertion at Bop, I'm not sure you've saved all that much by doing the aerocapture. I've only been to Bop a couple times. The method I used was chosen because I thought it was easiest, not necessarily the most efficient. I just used engines to brake into a circular orbit out near Bop's apoapsis, incorporating any necessary plane change. This would take a ÃŽâ€v of about 1500 m/s. From there I would drop my periapsis at the appropriate time to approximately match Bop's orbit. I would then just wait for an opportunity to create an intercept, much as I would if I were rendezvousing with another ship. All of that would take maybe another 300-400 m/s.

For typical entry into Jool's atmosphere, a spacecraft is probably traveling about 9800 m/s. For that same spacecraft swinging by Jool out at Laythe's orbit, it will be traveling only 5068 m/s. Laythe orbits Jool at a velocity of 3224 m/s. Therefore, if the spacecraft approaches Laythe from behind, the relative velocity is 5068 - 3224 = 1844 m/s. If it approaches Laythe head on, the relative velocity is 5068 + 3224 = 8292 m/s. We then have to account for the fact that Laythe's gravity will pull the spacecraft in and speed it up before it hits the atmosphere. In the minimum case (approach from behind), the spacecraft will enter the atmosphere at 3246 m/s. In the worst case (a head on approach), the spacecraft will enter the atmosphere at 8712 m/s. So even in the worse case scenario, the entry velocity is still nearly 1100 m/s slower than an entry into Jool's atmosphere. In the best case the velocity is about the same as a spacecraft returning to Kerbin from Mun.

Below is a re-post from another thread in which this was discussed.

The length of Kerbin's sidereal year is calculated using orbital mechanics. It is based on the gravitational parameter of the sun (constant of gravitation * mass) and Kerbin's semimajor axis (mean distance from sun). It uses the following equation, P2 = 4*pi2*a3/GM where P is the period, a is the semimajor axis, and GM is the gravitational parameter. For Kerbin this number computes to 9,203,545 seconds, or 426.09 6-hour days. Of course this only takes into account the mass of the Sun. The more correct method also considers the mass of the planet. Since the mass of the planet is so small in comparison to the sun, the difference is small but not insignificant. The corrected formula is, P2 = 4*pi2*a3/(G(M+m)) where m is the mass of the planet. Using this method, the period computes to 9,203,531 seconds. I'm not 100% certain which method is used in KSP, though it seems it is likely the first method. At least this is what the writers of the Wiki articles believe because all the periods of revolution seem to be based on the first formula. It makes sense that the KSP team would want to simplify the computation by ignoring the mass of the secondary body when its effect on the result is so small. Interestingly, my method of computing the sidereal year based on observations of the sun and stars works out to 9,203,554 seconds. This is closer to the first method, though either of the above solutions are within my margin of error. I'm just delighted that I got so close. If I were to extend my observations out to 100 years, rather than 10 years, I could gain another decimal point of accuracy, however that's more work than I'm willing to do.

I'm just weird. Seriously though, I was just thinking about how the Kerbin days are all messed up being based on the sidereal day rather than the solar day. I started checking to see how much later the sun rose each day and I started to notice some descrepencies. One thing led to another and before you know it I discovered the 42-second year-to-year difference in the rising time of the sun. I knew something was out of whack so I checked the sidereal time using a reference star. I then worked out the math and the rest is history. As far as playing the game goes, nothing. It's just a curious side note. I find this kind of stuff interesting because it just makes the game feel more real to me. The fact that we can make observations and perform experiments to learn things about these fictional worlds as if there were real planets I think is pretty cool.

That is what they should have done, but it looks like they either didn't do it or they failed to get it right. They should slow Kerbin's sidereal rotation period down to 21549.41452 seconds. This would make the solar day exactly 6 hours long and make the calander year 426 solar days. The sidereal year would still be a little longer than the calendar year, but not enough the worry about. It they wanted to make the sidereal year eactly 426 days, they would just have to decrease Kerbin's semimajor axis by 1916 km. - - - Updated - - - That change occured before my time. I think I started playing soon after the switch to 6-hour Kerbin days, because I recall reading something about that in the version update notes. I briefly played the demo and I vaguely remember that it used 24-hour days, but I have no idea what the orbital parameters where at that time. EDIT: Clarification ... In the demo version Kerbin had a 6-hour long day, but I believe the game used a 24-hour clock and measured everything in Earth days.

The length of a Kerbin calendar day should really be based on the the solar day rather than the sidereal day (like here on Earth). This would synchronize the motion of the sun with the length of the day so that the sun rises and sets at the same time each day. As it is now, the sun rises about 50.7 seconds later each day. If the sun rises at 0:00 on the first day of the year, by mid-year it is rising at 3:00. That's screwed up and would really mess with schedules, work hours, etc. The length of Kerbin calendar day should be 21650.7245 seconds, which would make the year 425 days long rather than 426. This would make the calendar year 1987 seconds shorter than the actually sidereal year. Reconciling this would require the addition of 10 leap days every 109 years (there would still have to be some periodic minor adjustments to keep perfectly in sync). Of course a 21650.7245-second day dosen't work very well with our earthly units of measure. To keep the day 6 hours long, the simple solution is to make a Kerbin second equal to 1.002348356 Earth seconds. Correct. Kerbin rotates 426.091778 times per one complete revolution.

My finding is just a minor footnote that really doesn't make a bit of difference to anybody around here. Nonetheless, I found it interesting. It made me feel like I was doing real science. I find it strange that Squad would intentionally use such an oddball value for Kerbin's sidereal rotation period. I wonder if what I observed is an inaccuracy in the game's internal computations rather than something intentional?

I hadn't originally observed the sunsets, but I just checked it out. The sunsets show the same 42-second year-to-year difference as observed with the sunrises. Because of the internal consistency of the numbers, I feel pretty confident that my observations and computations are correct. For instance, the calculation of the length of the sidereal year from the observations is extremely sensitive to error. You can see that just a ±1" error in the measurements over a ten year period results in a ±85" error in the sidereal year. That fact that my sidereal year calculation is just 9 seconds off from the accepted value I believe is very strong corroboration. I don't think there is any way I could get that close by chance.

That's because I edited the Wiki yesterday.

It has been generally accepted that Kerbin's sidereal rotation period is exactly 6 hours (21,600 seconds), however I'm here to dispel that myth. I started playing around by observing the time of sunrise each morning. This lead to the discovery that if you observe the sunrise on the same day one year apart, the sun rises 42 seconds earlier on the second year than it did on the first. I then decided to observe the rising times of identifiable stars. In this case I observed that a reference star rises about 37 seconds earlier than it did one year earlier. To refine these numbers I warped ahead 10 years and found that sunrise differed by 421 seconds, or 42.1 s/yr, and the stars differed by 374 seconds, or 37.4 s/yr. I estimate my accuracy on the accumulative totals to be about ±1 s on both measurements. From this we can compute the actual length of Kerbin's sidereal rotation period and its solar day: Sidereal day = (426 * 21600 - 37.4) / 426 = 21599.9122 ±0.00024 seconds Solar day = (426 * 21600 - 42.1) / 425 = 21650.7245 ±0.00024 seconds Using these numbers we can compute the length of Kerbin's sidereal year: Sidereal year = (-21599.9122 * 21650.7245) / (21599.9122 - 21650.7245) = 9,203,554 ±85 seconds The generally accepted duration of Kerbin's sidereal year is 9,203,545 seconds, which is very close to my computation and clearly within the margin of error. The math matches the observations, so I feel quite confident in my results. Of course Kerbin's calendar day remains 21,600 seconds regardless.

Unless you've already been captured into a closed elliptical orbit, the velocity at atmospheric entry can never be less than escape velocity, which at the top of Jool's atmosphere is 9547 m/s. That's an improvment but I seriously doubt it will improve the rapid overheating problem in any significant way. I agree with those that say using Laythe to aerobrake would be a more survivable option. If the timing is such that the spacecraft can approach Lathye from behind (i.e. both moving the same direction) than I think the entry velocity could be as little as 3000-4000 m/s. Even a head on encounter with Laythe will likely produce an entry velocity less than an encounter with Jool.

I did some experiments to see if it were possible to do a Jool aerocapture under v1.04. I found that it is possible under just the right conditions, but barely. The entry corridor is so narrow that if you miss the correct periapsis altitude by just a couple hundred meters you will either blow up or fail to capture. The design of the spacecraft is also very important. The spacecraft requires a large ablator and a low ballistic coefficient, i.e. low mass per cross-sectional area. A low ballistic coefficient means that the spacecraft will decelerate more quickly for every unit of dynamic pressure produced. This allows aerocapture to occur higher in the atmosphere and with less aerodynamic heating. The vehicle I used in my tests had a mass at atmospheric entry of only 2915 kg and was equipped with a 2.5 m heat shield. This means its mass was less than 600 kg per square meter of ablator surface. I don't think that anything much higher than this would have a very good chance of survival. My velocity at atmospheric entry (200 km altitude) was 9783 m/s, which is about typical for a Kerbin-to-Jool transfer. For this vehicle and conditions, I found that the ideal periapsis was about 196,200 m ±200 m. On one attempt I came in with a periapsus a little below 196,000 m and just barely made it, having burned off all but 3 kg of my ablator (a few meters lower and I probably would have been destroyed). On another attempt I came in with a periapsis just a little below 196,500 m and failed to aerocapture (I actually got my eccentricity below 1 but was still on an escape trajcetory due to the patched conics). Although, in this example, a periapsis altitude >196.5 km would not result in an aerocapture, the vehicle would be significantly slowed down to the point that just a small burn would be necessary to finish off the capture. This might be a good strategy to use - a combination of aerobraking and propulsive braking. Obviously the higher the periapsis altitude, the less the heating and the greater your chance of survival. I've proven that small probes can survive a Jool aerocapture with proper design and targeting. Unfortunately, I think that large massive vehicles are a greater challenge. The need for a low ballistic coefficient means that a large craft would have to be equipped with an unwieldy number of heat shields. For Jool aerocapture you want to use a "pancake" design.

I'm not sure about the heat shields and such. I'd have to go check. The large reaction wheel is an open faced part, and probably has a drag cube manually assigned to it. It might be out of balance. Open faced parts need to be handled manually by inserting drag cubes into their part.cfg files. So you can imagine how much of a pain this is any why some of them might have gotten missed in the updating process (or updated with incorrect values). The above issue appears not to have been fixed in 1.03/1.04. Do you know if Squad is planning to fix these problems, and when?