Should only spherical satellites be considered 'moons'?

Should only spherical satellites be considered 'moons'?

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Since Titan and Ganymede fall into the same category as Deimos and Phobos or the components of planetary rings, the category of moons, it's like if all asteroids were considered planets. Tiny irregular satellites are called 'moonlets' but still fall into the category of moons.

Is there any distinction between the natural satellites and should there be one? From my point of view, only equilibrium-shaped spherical moons should be considered moons. While natural satellites are all satellites, the spherical ones should be moons. Otherwise it would be extremely weird that Deimos and Phobos fall into the same category as our Moon or the Galilean Moons. For example, Pluto should be considered having five natural satellites, and one of them (Charon) is a moon.

Apparently not! From's Here's Why Saturn's Inner Moons Are Shaped Like Ravioli and Potatoes

The odd shapes of Saturn's inner moons, which resemble objects ranging from ravioli to potatoes, may be due to mergers of tiny moonlets, a new study finds.

The new finding may help to explain how moons in general may form, researchers said in a new paper describing the work.

The paper in 21-May-2018 Nature Astronomy is The peculiar shapes of Saturn's small inner moons as evidence of mergers of similar-sized moonlets (arXiv, slideshare) so these are definitely considered to be non-spherical moons made from mergers of "moonlets".

More (cropped) images of Pan from the NASA News item Cassini Reveals Strange Shape of Saturn's Moon Pan

The need to distinguish between these potato-shaped and spherical moons hasn't arisen. As such, there aren't two different words to designate these two types of moons. And as long as the community of astronomers who study natural satellites doesn't feel the need for there to be an extra category, there likely won't be one created preemptively.

Perhaps, one day, we will discover that potato-shape and spherical satellites form in very different ways, of have very different properties (not just esthetic). Then the case might be made to sort them into two different categories.

On a side-note, defining arbitrary categories is never trivial. How spherical is spherical? The boundary has to be set somewhere, but there always will be edge cases where an object could reasonably fall in either category.

Should only spherical satellites be considered 'moons'? - Astronomy

A moon is an object that orbits a planet or something else that is not a star

Earth Science, Astronomy, Geology, Physics

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A moon is an object that orbits a planet or something else that is not a star. Besides planets, moons can circle dwarf planets, large asteroids, and other bodies. Objects that orbit other objects are also called satellites, so moons are sometimes called natural satellites. People have launched many artificial satellites into orbit around Earth, but these are not considered moons.

The planet or body that a moon orbits is called its primary. Just as gravity holds the planets in our solar system in orbit around the sun, gravity also keeps moons in orbit around their primaries.

Many moons formed at the same time as their primary. Gravity pulled bits of dust and gas together into larger and larger clumps of material. Eventually, the smaller clump of material (moon) began orbiting the larger clump (primary).

Some moons formed in other ways. Earth's moon may have formed when an object the size of Mars crashed into the planet. The collision sprayed a huge amount of material into orbit around Earth. This material slowly accumulated into one large body, our moon. Other moons in our solar system were once asteroids, chunks of rock that are too small to be planets. These asteroids came too close to their primary and were pulled into orbit by the force of gravity.

Most moons are made of rock, but many also contain a large amount of ice, gas, and other chemicals. Europa, a large moon orbiting Jupiter, has an icy surface that may cover a liquid oceanof water.

Some moons have volcanic or geologic activity. For example, scientists have observed volcanic plumes rising 300 kilometers (190 miles) from the surface of Io, another one of Jupiters moons. Other moons, including Earths moon, show little or no signs of geologic activity, though they may have been more active in the past.

As of 2010, astronomers had discovered 166 moons circling planets in our solar system. Ninety-nine of these have been discovered since 2000. Jupiter has the most known moons, with 63. Saturn has 60 named moons, Uranus has 27, and Neptune has 13. Mars has just 2, and Earth has only 1. Venus and Mercury have no moons.

Another six moons in our solar system circle dwarf planets. Dwarf planets are planetlike objects that do not fit the full definition of a planet. Pluto is the most famous dwarf planet. Pluto has three moons. Many other moons in our solar system orbit smaller bodies. Because moons are relatively small, none have yet been discovered outside the solar system, but there are likely trillions of moons throughout the universe.

Photograph by Bonnie Kelley

Man in the Moon
The surface of Earth's moon is pockmarked with millions of craters left when asteroids and other space rocks crashed into its surface over millions of years. Sometimes, from Earth, the pattern of craters looks like a face peering down.

The largest moon in the solar system is Ganymede, which orbits Jupiter. Its diameter, or maximum distance across, is 5,262 kilometers (3,270 miles), larger than the planet Mercury. In 1610, Italian astronomer Galileo Galilei discovered Ganymede and three other planet-size moons circling Jupiter. They were the first moons discovered orbiting a planet other than Earth.

Difference between Planets and Moons

A planet is a large object that orbits around a star or a stellar remnant. This is mainly due to its own gravity and gravity of the star that allows the planet to have an orbit around the star. The orbit is usually elliptical in shape, mainly depending on the gravitational force of the planet and the star.

The gravitational force of the planet is strong enough that it leads the planet to be rounded, i.e. compound its matter in a spherical shape. A planet has also cleared its neighboring region of any other debris. The planetesimals, i.e. other debris, should either get absorbed into the planet, or if it big enough to have a gravitational force of its own, it might become a satellite of the planet, i.e. moon, or will just float away into space.

As per the International Astronomical Union (IAU), “A "planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.”

Hence, as per this definition, there are currently eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, in order of distance from the Sun.

Moons are the natural satellites of the planets. These natural satellites orbit around a primary body, mainly the planets or large enough asteroids. Natural satellites were rocks that had been left over after the solar system and the planets in it were formed. These rocks then either fell into an orbit around their nearest largest asteroid or planet or floated away into space.

These natural satellites can range in sizes, some are bigger than planets. For example, Ganymede, a moon of Jupiter and Titan, a moon of Saturn are bigger in size than Mercury, which is a planet. Furthermore, it is not necessary that all planets have moons or have one moon: Mercury and Venus do not have moons at all Earth has a single natural satellite, which we call, the Moon Jupiter and Saturn have more than 50 to 60 moons Neptune and Uranus have more than 20 moons even Pluto, Haumea, and Eris, which are considered as minor planets or dwarf planets have various moons, Pluto has five known moons. Furthermore, there are more natural satellites being constantly discovered.

As of January 2012, there are 176 known moons orbiting six of the eight planets, eight orbiting three of the five dwarf planets, and there are 76 asteroids located in the asteroid belt, between Mars and Venus, that have satellites orbiting them. Many of these satellites are believed to be capable of sustaining life. Some have proof of ice and water, while Titan even has an atmosphere.

Still, the main difference between planets and moons is the fact that planets orbit around the Sun, while moons orbit around their planets or dwarf planets or asteroids or primary objects. Furthermore, the natural satellites also orbit around the Sun, in collaboration with their primary objects, For example, the Moon orbits around the Earth, but as the Earth orbits the Sun, the Moon also orbits the Sun, while following the Earth.

Physical properties

Table 1 presents a log of all the resolved measurements of Pluto’s small moons. Some examples of the resolved images are shown in Fig. 1 (see also fig. S1 and figs. S7 to S14). Systematic measurements of the brightness variations of Pluto’s moons between May and early July 2015 were used together with the resolved measurements to constrain the sizes, shapes, rotation periods, and rotation poles of all four moons (12) (Table 2). Figure 2 shows the observed brightness variations after phasing by the best-fit rotational periods (see also figs. S2 and S3). Unlike the case for Pluto and Charon—each of which synchronously rotates with a period of 6.3872 days, equal to their mutual orbital period around their common barycenter—Pluto’s small moons rotate surprisingly rapidly (Hydra has the fastest rotational period,

10 hours) and all are far from synchronous. The rotational poles (Table 2 and table S4) are clustered nearly orthogonal to the direction of the common rotational poles of Pluto and Charon: The inclination angles relative to the Pluto-Charon pole direction are 91°, 123°, 96°, and 110° for SNKH, respectively. Nominally, all the small moons have retrograde rotation, but Nix is the only one significantly so (i.e., retrograde with greater than 1σ confidence). This collection of inclinations is inconsistent with an isotropic distribution even with only four points, a Kolmogorov-Smirnov test shows a less than 1% probability that this is a uniform distribution in inclination (i.e., the pole inclinations are nonuniform to a 2σ to 3σ confidence level). These results on the rotational properties have not been seen in other regular satellite systems in the solar system. Rapid rotations and large obliquities imply that tidal despinning has not played a major role in the moons’ rotational histories. The moons have probably never reached the state of near synchronicity where chaotic perturbations by Charon have been predicted to dominate (5) determining whether chaos plays a role in the moons’ current rotational dynamics is deferred to a future study.

All observations of Pluto’s small satellites with a resolution better than 15 km per pixel and downlinked to Earth before 15 December 2015 are listed. The dates are the mid-observation times at the New Horizons spacecraft. Resolution refers to the projected distance at the object subtended by a single instrument pixel. The phase angle is the Sun–object–New Horizons angle. All observations were taken with the LORRI panchromatic camera (10), except “N_COLOR_2” and “N_MPAN_CA,” which were taken with the MVIC color camera (11).

Celestial north is up east is to the left. The Styx image is a deconvolved (12) composite of six images from U_TBD_1_02 (Table 1) that has been resampled with pixels one-eighth of the native pixel scale for cosmetic purposes. The Nix image is a deconvolved single image from N_LEISA_LORRI_BEST and is displayed with the native pixels. The Kerberos image is a deconvolved composite of four images from U_TBD_2 and has been resampled with pixels one-eighth of the native pixel scale for cosmetic purposes (12) (fig. S1). The Hydra image is a deconvolved composite of two images from H_LORRI_BEST with pixels one-half of the native scale. Some surface features on Nix and Hydra appear to be impact craters (12).

The sizes (diameters) are three-dimensional ellipsoidal best fits to the resolved and unresolved (light curve) measurements (12). Uncertainties are ±3 km (±1σ) for Styx, Kerberos, and Nix and ±10 km (±1σ) for Hydra. Kerberos has a dual-lobed shape that is not fit well by a single ellipsoid. The orbital periods are from (5). The rotation rates are determined from analyses of light-curve data taken over several months (12). Rotational pole directions are determined from a model that attempts to match both the light-curve measurements and the resolved measurements (12). The pole positions listed below are accurate to ±10° (±1σ, see also fig. S4) the rotational poles of Pluto and Charon both point at [RA, DEC] = [132.993°, −6.163°]. The geometric albedos listed here may not fully account for any potential rapid increase in brightness near 0° phase angle (see text for further details). On the basis of a recent (November 2015) analysis of stellar calibration data, we have reduced LORRI’s sensitivity by 20% relative to the preflight value, which raises the derived geometric albedo values (tabulated below) by 20% relative to the values based on the original calibration. LORRI’s sensitivity has been stable at the

1% level since launch, and a more definitive absolute calibration is expected from stellar observations planned in July 2016.

Systematic measurements of the brightnesses of Pluto’s small satellites were obtained by LORRI during the approach to Pluto from May through early July 2015. Hv refers to the total (i.e., integrated over the entire target) visible magnitude (V band) referenced to a heliocentric distance of 1 AU, a spacecraft-to-target distance of 1 AU, and a solar phase angle of 0° (using a phase law of 0.04 mag/deg). Different colors are used for the seven different observing epochs (12) (table S4) ±1σ error bars are shown for each measurement (some error bars are smaller than the symbols). Three different algorithms were used to search for periodic variations in the data (12). The rotational periods derived from that analysis (Table 2) were then used to phase the brightness data, producing the light curves displayed above. These double-peaked light curves presumably result from the rotation of elongated bodies, with the light-curve amplitude determined by the variation in the cross-sectional area presented to the observer, which depends on the body’s shape and the angle between the rotational pole and the line of sight to the body. The rotational phases for all the resolved observations of the small satellites (Table 1) are indicated by the vertical red lines, although the angle between the observer and the rotational pole may be different for these observations relative to the earlier ones. The dashed curves are sinusoids with the best-matched periods. The amplitudes for Styx, Nix, Kerberos, and Hydra, respectively, are 0.30, 0.20, 0.37, and 0.07 mag. The dashed horizontal lines are the mean Hv values, which are 11.75, 8.28, 11.15, and 7.77 mag for Styx, Nix, Kerberos, and Hydra, respectively.

Pluto’s small moons have highly elongated shapes with maximum to minimum axial ratios of

1.5 to 2 (Table 2). Highly asymmetrical shapes are typical of many other small bodies in the solar system and presumably reflect a growth process by agglomeration of small objects into loosely bound, macroporous bodies whose gravity was insufficient to pull them into more spherical shapes. Kerberos, in particular, has a double-lobed shape, suggesting the merger of two smaller bodies. Hydra also has a highly asymmetrical shape that may also indicate the merger of smaller bodies, but the divots in Hydra’s surface may plausibly have been produced by impacts from the local Kuiper Belt population. The nonspherical shapes of Pluto’s small satellites are consistent with their formation in the remnant disk produced by the collision of two large Kuiper Belt objects (KBOs) that formed the Pluto-Charon binary (1315).

Large uncertainties in the masses of the small moons (up to

100%), as well as large uncertainties in their volumes, preclude determining accurate values for their densities at this time (densities of 0 are within the current error estimates). However, the New Horizons results on Kerberos (see below) clearly demonstrate that the current dynamical estimate for its mass (4) is an overestimate, possibly by a factor of

Double trouble

That formula yields one peculiar consequence. Margot defines a pair of orbiting objects that are both above the critical mass as a double planet. So, New Scientist asked him at the meeting, what about Earth and the moon? With a quick glance at a graph, Margot confirmed that the moon is above the critical mass. So by his proposed definition, it’s a planet too.

“But we should be careful here,” he adds. “The IAU has not defined the term ‘satellite’. When they do, that will affect what they might decide about double planets versus satellites.” The next opportunity for the IAU to reopen the case would be their general assembly in Vienna in 2018.

As for the rest of the solar system, Margot’s criterion leaves a gulf between planets and dwarf planets. Pluto would retain its dwarf status, because it still has so much company in the Kuiper belt. The least planet-like planet, Mars, has more than 50 times the orbit-clearing mass whereas the most dominant dwarf, Ceres, has only a few per cent of the mass required. It also means that all known exoplanets are indeed planets, except in the few cases where measurements aren’t yet good enough to tell. And conveniently, the proposal makes the iffy matter of “roundness” redundant – anything above orbit-clearing mass is so big that its gravity must pull it into a round shape.

“Of course it’s just a proposal,” says Margot. “I don’t know whether it will stick, whether people will love it, hate it or be indifferent.”

His suggestion certainly won’t satisfy those who think the IAU was wrong to require orbit clearance in the first place. “I’m sympathetic with what he’s trying to do,” says Richard Binzel of the Massachusetts Institute of Technology. “But to me, it’s about the body itself, not its location.” Binzel prefers an earlier suggestion that roundness be the main criterion – which would planetise many more objects.

Spherical Astronomy

the branch of astrometry that deals with mathematical methods for solving problems connected with the study of the apparent positions and motions, on the celestial sphere, of celestial bodies, such as stars, the sun, the moon, planets, and artificial celestial bodies. Spherical astronomy is made use of in various areas of astronomy. It arose in antiquity and constituted the first step in the study of astronomical phenomena.

The basic concept of spherical astronomy is the celestial sphere. Each direction to a heavenly body in space is represented on the sphere by a point, and planes are represented by great circles. The use of the celestial sphere permits a considerable simplification of the mathematical relations between directions to celestial bodies, since complex spatial representations are reduced to simpler figures on the surface of the sphere hence the term &ldquospherical astronomy.&rdquo

In order to study the relative positions and motions of points on the celestial sphere, coordinate systems are established on it. Spherical astronomy makes use of the horizon coordinate system, two equatorial systems, and the ecliptic coordinate system (seeCELESTIAL COORDINATES). The relationships between the different coordinate systems are determined by means of the formulas of spherical trigonometry. Since spherical astronomy studies phenomena associated with the apparent diurnal rotation of the celestial dome, that is, the apparent motions of bodies due to the rotation of the earth, the celestial sphere is regarded as rotating from east to west about the extended axis of the earth at an angular speed equal to that at which the earth rotates. This kinematic model almost exactly reproduces the appearance of the sky to an observer on the rotating earth. The general relations between the horizon and equatorial coordinate systems make it possible to determine, for example, the times and azimuths at which a celestial body rises and sets, the time of transit of a celestial body, the elongation of a celestial body, and the position of a celestial body at a given time. One of the tasks of spherical astronomy is the determination of the conditions under which two suitably chosen stars are at the same altitude. Knowledge of these conditions is important in determining from astronomical observations the geographic coordinates of points on the earth&rsquos surface.

Measurement of time. An important problem in spherical astronomy is the establishment of the theoretical foundations of the astronomical system of reckoning time. Spherical astronomy thus studies units of time and the relationships between them. The measurement of time is based on the natural periodic phenomena of the rotation of the earth about its axis and the revolution of the earth about the sun.

The duration of one rotation of the earth is, depending on whether the vernal equinox or the sun is used as the reference point on the celestial sphere, one sidereal or solar day. In reckoning sidereal days, it is taken into account that the vernal equinox, owing to precession and nutation, does not remain at the same position on the celestial sphere but moves translationally and, at the same time, executes oscillations about its mean position. For the reckoning of solar days, the concept of the mean sun is introduced. The mean sun is a fictitious point that moves uniformly along the equator in coordination with the complicated apparent motion of the true sun along the ecliptic.

The period of one revolution of the earth about the sun is one tropical year. The calendar is based on the tropical year, which corresponds to the time required for one cycle of the four seasons to be completed. Since a tropical year does not contain an integral number of mean days, the duration of a calendar year is set at 365 days in some years and 366 days in other years in order that the average duration of the calendar year over a long interval of time be equal to the length of one tropical year. In astronomy, time is reckoned in tropical years, in calendar years with a mean duration of 365.25 days, or in Julian days.

Observed positions of celestial bodies. The coordinates of celestial bodies obtained directly from observation are distorted by a number of factors. First of all, the coordinate axes associated with the earth&rsquos axis of rotation and with the vernal equinox do not maintain a constant direction but move as a result of precession and nutation. Because of aberration, the apparent positions of celestial bodies on the celestial sphere are somewhat displaced from the positions the bodies would have if the earth were stationary. Observations are also distorted by the refraction of light in the earth&rsquos atmosphere. In addition, parallax effects must be taken into account in processing observational data.

Corrections must be applied to the coordinates of celestial bodies in order to eliminate the enumerated distortions from the observed positions of the celestial bodies and in order to determine positions in the same coordinate system for all observations. The coordinate system used is associated with the position of the earth&rsquos axis of rotation and the vernal equinox at some epoch, such as 1900.0 or 1950.0 (seeMEAN POSITION). The corrections applied take into account the effects of precession, nutation, aberration, parallax, and refraction. Astronomical yearbooks give the values of special reduction quantities that are used in allowing for the effects of precession, nutation, and aberration. The yearbooks also give the values of other quantities necessary for processing astronomical observations.

PRECESSION AND NUTATION. As a result of precession, the earth&rsquos axis slowly changes its direction, with a period of about 26,000 years, so as to describe a conical surface. Nutational oscillations are superimposed on this motion of the earth&rsquos axis (seeNUTATION). The position in space of the plane of the ecliptic also changes very slowly associated with this change is a motion of the vernal equinox. Since the vernal equinox is used as a reference point in the equatorial and ecliptic coordinate systems, the coordinates of celestial bodies in these systems change.

ABERRATION. Aberration is the apparent displacement of the position of a celestial body on the celestial sphere from the true position as a result of the observer and the celestial body being in motion relative to each other. In observations of stars, annual and diurnal aberration are taken into account. The former is the aberration due to the motion of the earth about the sun the latter is the aberration produced by the earth&rsquos rotation about its axis. In observations of artificial earth satellites, the aberration due to the motion of the satellite about the earth is also calculated.

PARALLAX. Because the observer moves in space as a result of the earth&rsquos rotation and the revolution of the earth about the sun, the directions to celestial bodies also change. To obtain comparable quantities, observation results are referred in the first case (when bodies in the solar system are observed) to the center of the earth and in the second case (when stars are observed) to the center of the solar system&mdashthe sun. The magnitude of the parallactic displacement depends on the distance to the celestial body.

REFRACTION. Because the light from celestial bodies is refracted in the earth&rsquos atmosphere, the celestial bodies appear displaced in the direction of the zenith. The magnitude of the displacement depends on the refractive index of the air&mdashthat is, on such factors as temperature and pressure&mdashand on the zenith distance of the celestial body. In the observation of celestial bodies near the earth, particularly artificial earth satellites, displacements due to refraction parallax are also taken into account. These displacements result from the different effects of refraction on celestial bodies that are located in the same direction from the terrestrial observer but at different distances from him.

Other concerns of spherical astronomy. The effects of the distorting factors listed above must be eliminated before data from the observation of celestial bodies can be used for theoretical studies or for such practical purposes as the determination of geographic coordinates, of azimuths, or of time. To calculate the appropriate corrections, the astronomical constants are used these constants are numerical quantities characterizing the described phenomena. The determination of the astronomical constants from the data of astronomical observations is a problem that links spherical astronomy with such fields as fundamental astrometry, celestial mechanics, and the study of the structure of the earth.

Practical astronomy makes extensive use of spherical astronomy. Among the matters dealt with in spherical astronomy are problems associated with the determination of coordinates on the surfaces of bodies in the solar system, especially on the surface of the moon, where librations must be taken into account. With the beginning of the age of interplanetary flight and the landing of astronauts on the moon, the determination of coordinates on the moon has taken on particular importance. Spherical astronomy also studies methods of calculating solar and lunar eclipses and similar phenomena, such as occultations of stars by the moon and transits of planets across the solar disk.

Moons around other worlds

Most planets in our solar system have natural satellites, which we also call moons. For the inner planets: Mercury and Venus each have no moons. Earth has one relatively large moon, while Mars has two asteroid-sized small moons called Phobos and Deimos. (Phobos is slowly spiralling into Mars and will likely break apart or fall into the surface in a few thousand years.)

Beyond the asteroid belt, are four gas giant planets that each have a pantheon of moons. As of late 2017, Jupiter has 69 known moons, Saturn has 53, Uranus has 27 and Neptune has 13 or 14. New moons are occasionally discovered &ndash mainly by missions (either past or present, as we can analyze old pictures) or by performing fresh observations by telescope.

Saturn is a special example because it is surrounded by thousands of small objects that form a ring visible even in small telescopes from Earth. Scientists watching the rings close-up over 13 years, during the Cassini mission, saw conditions in which new moons might be born. Scientists were particularly interested in propellers, which are wakes in the rings created by fragments in the rings. Just after Cassini's mission ended in 2017, NASA said it's possible the propellers share elements of planet formation that takes place around young stars' gassy discs.

Even smaller objects have moons, however. Pluto is technically a dwarf planet. However, the people behind the New Horizons mission, which flew by Pluto in 2015, argue its diverse geography makes it more planet-like. One thing that isn't argued, however, is the number of moons around Pluto. Pluto has five known moons, most of which were discovered when New Horizons was in development or en route to the dwarf planet.

A lot of asteroids have moons, too. These small worlds sometimes fly close to the Earth, and the moons pop out in observations with radar. A few famous examples of asteroids with moons include 4 Vesta (which was visited by NASA's Dawn mission), 243 Ida, 433 Eros, and 951 Gaspra. There are also examples of asteroids with rings, such as 10199 Chariklo and 2060 Chiron.

Many planets and worlds in our solar system have human-made "moons" as well, particularly around Mars &mdash where several probes orbit the planet doing observations of its surface and environment. The planets Mercury, Venus, Mars, Jupiter and Saturn all had artificial satellites observing them at some point in history. Other objects had artificial satellites as well, such as Comet 67P/Churyumov&ndashGerasimenko (visited by the European Space Agency's Rosetta mission) or Vesta and Ceres (both visited by NASA's Dawn mission.) Technically speaking, during the Apollo missions, humans flew in artificial "moons" (spacecraft) around our own moon between 1968 and 1972. NASA may even build a "Deep Space Gateway" space station near the moon in the coming decades, as a launching point for human Mars missions.

Fans of the movie "Avatar" (2009) will remember that the humans visited Pandora, the habitable moon of a gas giant named Polyphemus. We don't know yet if there are moons for exoplanets, but we suspect &mdash given that the solar system planets have so many moons &mdash that exoplanets have moons as well. In 2014, scientists made an observation of an object that could be interpreted as an exomoon circling an exoplanet, but the observation can't be repeated as it took place as the object moved in front of a star.


To date, there have been over a 100 missions to the moon since 1958 and 21 missions proposed or under development for the next four years. Some of the most notable are the Luna 2 which was the first spacecraft to reach the surface of the moon in 1959, Luna 3 was the first to see and send back images of the far side of the moon in 1959. Apollo 11 which not only successfully landed on the moon but was a manned spacecraft in which Neil Armstrong became the first human to ever set foot on another world in 1969.

Luna 16 was the first successful spacecraft to collect samples of rocks and return them to Earth in 1970, and Chandrayaan-1 discovered water ice on the moon in 2008. The 21 missions that have been proposed or are under development are being run by various agencies in hopes of gaining enough knowledge to implement a plan to erect a base on the moon in the near future.

Note: this section is only available in SkySafari Plus and Pro.

Show Planet Orbits: Shows orbital paths of the major planets around the Sun. Since the planets orbit in the nearly the same plane as the Earth (the Ecliptic plane), their orbits appear near the Ecliptic line - the Earth's orbit as seen from the Earth - in the sky. Note: this option is only available in SkySafari Plus and Pro.

Show Moon Orbits: Shows orbital paths of the moons around their primary (parent) planet. You may need to zoom in on a planet to see its moon orbits Mercury and Venus have no moons!

Selected Object Orbit: Shows the orbit of the selected planet, moon, asteroid, comet, or satellite. You need to select such an object and turn on this option to show its orbit.

Selected Object Path: Shows the apparent path of a solar system object across the sky, with its position at specific dates labelled. The solar system object must be selected, and you must be viewing it from the Earth's surface, in order to see the path.

Selected Object Path: Shows the apparent path of a solar system object across the sky, with its position at specific dates labelled. The solar system object must be selected, and you must be viewing it from the Earth's surface, in order to see the path.

Earth & Moon Shadow Circles: Shows the Earth's shadow (when viewing from Earth) or the Moon's shadow (when viewing from the Moon). When this option is turned on, the Earth's (or Moon's) umbral and penumbral shadows are shows as concentric circles. Inside the smaller umbral shadow, the Sun is totally hidden inside the larger penumbral shadow, the Sun is only partially blocked. This can be helpful for simulating lunar and solar eclipses, and illustrating the difference between total and partial eclipses.

Phobos: the first step to Mars?

The road to the Red Planet may yet see humans set foot on the closest of Mars’s tiny moons, something we have yet to do even with a lander.

Phobos has been mooted as a potential pit stop as part of a phased approach to a manned Mars mission, as it would allow NASA to use technology already in development, including the Orion capsule and the Space Launch System.

Not only would this mean astronauts could reach the vicinity of Mars quicker, it would give the space agency a forward operating base from which it could control rovers on the Red Planet until we are ready to send humans in their stead.

Kev Lochun is a science journalist and production editor on History Revealed magazine.