Ganymede is the largest satellite of Jupiter and the largest moon in the solar system. It was discovered by Galileo Galilei in 1610 and was named by Simon Marius after one of the lovers of the Roman god Jupiter. Ganymede was the first moon, other than Earth’s moon, to be discovered.

Ganymede is 5,280 kilometers in diameter, which is larger than Mercury. It is revolving just over 1 million kilometers from Jupiter and is the seventh of the planet’s sixteen satellites. Ganymede is large enough to generate its own magnetic field – an unusual characteristic for a moon.

Ganymede always faces the Jupiter with the same side. Such relationships are quite common and are called synchronous. Another good example of a synchronous relationship between a planet and its satellite is Earth’s moon. Ganymede revolves in the same direction as Jupiter. Its orbit is almost circular, meaning that its eccentricity (the measure of how close to a circular orbit the satellite travels) is small. A circular orbit has an eccentricity of zero. Ganymede’s angle of inclination is less than a degree, meaning that this moon revolves almost exactly in the plane of Jupiter’s equator.

Although Ganymede is now locked into the same position facing Jupiter all the time, there are indications that this may not have always been the case. One clue is that the number of meteor craters should be greater on the leading side of Ganymede, as is the case with Callisto. However, this is not true for Ganymede. Another fact pointing to a change in the part of the ice shell facing Jupiter is the catenae that are found on the back side of Ganymede. Catenae are caused by a string of fragments from a comet that was broken up by the intense magnetic field of Jupiter but escaped capture to hit one of the satellites. They should occur only on the Jupiter-facing side of Ganymede.

Ganymede’s surface is covered by ice mixed with carbon-rich soil, which reflects large amounts of sunlight. When the ice underneath the surface is heated and melts, it erupts to the surface. The soil, which is denser than water, sinks below the water. The water then freezes, causing a bright spot on the surface. The water is heated either by radioactive decay or by tidal flexing. Not only does the gravity of Jupiter and Callisto pull on Ganymede; the moon also has Laplace resonance, which occurs because of the forces from the satellites Io and Europa. Every time Ganymede revolves around Jupiter once, Europa, the satellite just inside Ganymede, goes around Jupiter twice, and Io, the moon inside Europa, goes around four times. Thus, during every orbit the three satellites are aligned, magnifying the gravitational effect. This increased gravitational pull and then relaxation not only cause the orbits to become elliptical, but also causes stresses within the satellites themselves. This tidal flexing generates heat that melts ice and causes the surface of Ganymede to be smoother than what it would be on a dense planet/moon.

The percentage of ice on Ganymede has been measured at 45-55 percent. The bulk density of the moon is between that of ice and that of carbonaceous silicates, indicating a mixture of the two materials.

Ganymede has an intrinsic magnetic field that is opposite to the field of Jupiter. It also displays an induced magnetic field caused by the strong rotating, angled field of Jupiter. The induced field is an indication of a conducting ocean deep under the icy surface. If the ocean has enough minerals dissolved in it to make it strongly conducting, it could generate the intrinsic magnetic field. Jupiter’s strong magnetic field causes Ganymede to be bombarded by charged particles. This bombardment is thought to cause the molecular oxygen, O2, and ozone, O3, found in the surface of Ganymede.

Since Ganymede’s orbit is in the same plane as Jupiter, it is thought that they were formed by the same process. Jupiter was formed in the very hot, dense region. Ganymede was formed in a cooler region, where water did not boil away but instead froze to form part of the satellite.

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[Images: nasa.gov; solarsystem.nasa.gov/galileo/; Wikipedia]

“One small step for man, one giant leap for mankind” – this is what Neil Armstrong, an American and the first man to step on the surface of the Moon, had to say. Truly, the first steps of mankind on the Moon have been immortalized, literally and figuratively (since there’s no atmosphere on the Moon, the footprints of all the astronauts will be preserved for millions of years due to absence of erosion). This little satellite of our planet has been very popular among artists and scientists alike. Humans have found out a lot of information about the Moon and we are still sending unmanned voyages to the Moon to find out more – the latest being the ‘Chandrayaan’ by India.

Moon Formation

The Moon is estimated to have come into formation about 30-50 million years after the Solar System came into being. That makes the Moon more than 4.527 billion years old – the Earth itself is 4.54 billion years old. Some researchers and computer simulations suggest that in the initial days of the Earth, its spin was so great that the immense centrifugal force caused the fission of the Moon from the Earth’s crust. However, in order for the Earth to gravitionally capture the pre-formed Moon, its atmosphere would have had to be unfeasably extended from what exists today.

Another hypothesis takes into consideration the fact that in the early days of the Solar System, giant impacts between planets and huge asteroids or meteorites were very common, and this theory suggests that a similar giant impact was responsible for the formation of the Moon. Possibilities exists that a Mars-sized planet collided with the newly formed Earth, thus blasting material from the Earth into the orbit resulting in the formation of the Earth-Moon system.

Moon Atmosphere and Temperatures

It would be incorrect to say that the Moon has no atmosphere at all, though it is so insignificant that it is easier to ignore its presence. Total weight of the Moon’s atmosphere is about 10 metric tons; compare that with the Earth’s atmpsphere which is about 5 quadrillion tons. This causes the temperature ranges to be at extremes on the Moon and the Sun is the only factor which influences it. The temperature on the side of the Moon facing the Sun can go upto a maximum of 123°C (253°F) while the side in darkness can record temperatures as low as -153°C (-243°F). The poles and the perpetually dark craters can sometimes experience temperatures close to the absolute zero.

Moon Size and Speed

The Earth’s Moon is the second largest moon of the Solar Syste, when the relative size is taken into consideration. In other words, the relative size of the Earth and the Moon is quite large when compared to the relative sizes of planets like Jupiter and Saturn and their moons. The diameter of the Moon is 2,000 miles ( 3,476 kilometres). To give an idea of the surface area of the Moon, it is comparable to the size of the continent of Africa. The speed at which the Moon orbits the Earth differs according to its distance from the Earth at different points in its orbit. On an average, the Moon revolves around the Earth at 2,288 miles per hour (3,683 kilometres per hour).

Moon Distance to Earth and Lunar Month

The center of the Moon is at a distance of 250,000 miles (384,400 kilometres) from the center of the Earth. A beam of light from the surface of the Earth to the surface of the Moon would take about 1.26 seconds to travel this distance. The Moon takes 27 days, 7 hours, 43 minutes, 11.6 seconds to complete one orbit around the Earth. It should be noted that the Lunar month (time between two new moons) is considered to be 29 days, 12 hours, 43 minutes, 11.6 seconds long. Though the two are basically based on the same phenomenon – i.e. the completion of one orbit around the Earth – the difference in time arises since the Earth is constantly moving around the Sun and thus the Moon has to cover more than 360 degrees to complete one revolution around the Earth.

The Moon has quite a few influences on the Earth and a lot of changes would affect the Earth if the Moon was to be taken out of the picture. For example, the gravitational pull of the Moon causes the water of the oceans to move away from the polar regions. Without the Moon, the water would redistribute itself near the polar regions and cause the sea levels to change a little. The spin of the Earth is slowed down by the Moon and without the Moon we would experience shorter days. Also, the axis of rotation of the Earth is said to be stabilized due to the Moon.

Though only two countries have so far reached the Moon, an international treaty has been signed which declares the Moon as a property of the whole of mankind. This restricts countries from using the Moon for anything other than peaceful purposes.

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[images: nasa.gov]

Scientists have found oxygen in the exosphere (the upper atmosphere) of Saturn’s moon Dione. The article about this discovery was published in the Geophysical Research Letters magazine. Dione is a relatively large satellite. Its diameter is 1,123.4 kilometers and it consists mainly of water ice and rock.

Researchers have used data collected by the Cassini probe in April 2010. During that period Cassini came close to the moon and was able to gather data about its atmosphere. The amount of oxygen in the atmosphere is extremely low - 0.01 to 0.09 oxygen ions per cubic centimetre of exosphere.

Scientists believe that oxygen ions are formed as a result of the bombing of Dione’s surface by charged particles trapped by Saturn’s magnetic field. Prior to Dione oxygen was found on Saturn’s moon Rhea, as well as Jupiter’s moons Ganymede, Europa and Callisto.

“Cassini” is a joint project of ESA, NASA and the Italian Space Agency. It was launched in 1997.  The probe’s mission will continue until 2017.

[image: nasa.org]

The first planet in the habitable zone around a Sun-like star was discovered by the Kepler telescope. The new object was named Kepler 22-b. It revolves around a star, which is located at a distance of 600 light years away from Earth. The star belongs to the G spectral class (which is the same spectral class as our Sun), but it is slightly smaller and cooler than the Sun – the brightness of the star is about ¾ of our Sun’s.

Kepler 22-b revolves around its star in 290 days. The radius of the planet is about 2.4 times larger than the Earth. Scientists do not know the mass of the planet, so it is not possible to estimate its density and hence the approximate composition. It is quite possible that the planet may consist almost entirely of gas.

The planet is also known as Christmas Planet. It got it’s nickname because it took three snapshots for the Kepler telescope to determine the planet was really there, and the snapshots had to be taken 290 days apart (the length of planet’s year). The last of those three encounters happened during the 2010 holiday season.

According to the scientists, to date, Kepler telescope has found 54 candidates for a potentially habitable planet.  Habitable zone around a star is a region where the planet’s surface (if it exists) can potentially have liquid water.

Space telescope Kepler was launched into space in March 2009. It continuously scans the area in the sky, containing about 4.5 million stars, which is located between Cygnus and Lyra constellations. Scientists find extrasolar planets by monitoring changes in star brightness caused by the passage of a body across the disk of a star (so-called transit method).

[images - nasa.gov]

Jupiter’s moon Europa is one of the most interesting bodies of the Solar System. It was discovered by Galileo Galilei and Simon Marius (Simon Marius did it a bit later, and it was him who has offered the names to moons discovered by Galilei) in 1610. According to Greek mythology, Europa was a Phoenician princess stolen by Zeus who had transformed her into a white bull.

Tidal forces inside Europa are much smaller compared to another Jupiter’s moon – Io. First readings from spacecrafts orbiting Jupiter did not show any signs of eruptions on Io. But further examinations found a sultan over the moon’s limb, which included water, ammonia and other products. Nevertheless, gas eruptions that are so typical for Io seem to be very rare on Europa.

Photos of Europa show that the entire surface of the moon is covered by ice. It looks very unusual. A thrilling idea of artificial origin of channels on Mars was proposed around 100 years ago. However, these lines turned out to be an optical illusion caused by hardly distinguishable details on the distant planet. But here on surface of orange-brown Europa scientists found a really dense network of crossed lines. Pictures of Europa’s surface look very similar to the pictures of the Arctic Ocean made from the orbit. In the beginning scientists were very careful about arising analogy. But spectral measurements did not leave a place for doubts — Europa’s surface is covered by ice.

The size and average density of Europa allowed scientists to calculate the amount of ice in the total mass of the moon. With a diameter of 3138 km and average density of 3,04 g/sm3, Europa should have much more water than Io or the Moon. Because of this, first calculations predicted that Europa’s ice cover should be about 100 km thick. Further research, however, has led to smaller figures.

According to the latest research Europa’s ocean could be tens of kilometers deep, and its ice cover is probably just a few kilometers thick. This cover is fragile and sometimes breaks under tidal pressure. These breaks give liquid water access to moon’s surface, which has no atmosphere.

It is likely that a global network of lines visible on Europa’s surface is cracks in the thick ice cover, caused by tectonic processes. These breaks are not accompanied by any movements of the ice cover, and cracks are quickly filled with orange liquid. Breaks can be tens of kilometers to 100s of kilometers wide and more than 3000 km long. Water instantly begins to boil and simultaneously freezes. Evaporated water returns to the moon’s surface in the form of snow and frost. The boiling carries away a lot of heat, and it only takes few minutes for water to form a half-meter layer of ice.

Europa is a very flat moon. Its highest ‘mountains’ are less than 50 meters high.  All of this can be explained in two ways: either Europa is a very young moon or there is a ‘mechanism’, which smoothens its surface. Few facts speak in favor of the second option – relatively high temperature (ocean of liquid water) and ability of ice to move.

Interest to the ocean lying under the ice of Europa had been stimulated by the assumption that life could exist in it. Even if this life is in elementary form. If Europa’s ocean is 50—60 km deep, its volume should be close to the terrestrial oceans. Free fall acceleration on the surface of the moon is 1.32 m/s2. This means that the pressure at the bottom of Europa’s ocean is the same, as it is at the 4-kilometer depth on Earth. It is well known that life on Earth has started in oceans. But there is one fundamental difference – Europa’s ocean does not have a constant energy source. Sunlight is such source on Earth. Life and photosynthesis are inseparable. However, there is one exception – sulfur compounds formed at rather high temperatures of underwater volcano eruptions are used by some microorganisms in chemosynthesis (chemical synthesis under the influence of heat). There are other, equally speculative ideas, such as light absorption by microorganisms while new cracks are still free of ice.

Existence of an ice cover on Europa had been proven and there are no doubts about it.  As for the ocean and the assumptions associated with it – very little is known about what is hidden under the ice cover and most hypotheses are only speculative at this stage.

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[images: nasa.org]

The equator of Vesta asteroid was photographed by the Dawn probe. High definition photos and descriptions are available on the project’s site.

Dark hill on the surface of the asteroid is of great interest to scientists. It looks dark on different photos, regardless of the shooting angle. It is likely that this is its natural color, and not a result of light and shadow effects. Scientists are discussing possible mechanisms of formation of the hill.

The photograph was taken on September 20, 2011, but only published now. When taking the image Dawn was at an altitude of 673 kilometers above the surface of the asteroid. The resolution of the photograph is about 66 meters per pixel.

Scientists working with the Dawn probe gave a report on the planetary science conference EPSC-DPS in French city of Nantes in early October. They reported that they have found the second largest mountain in the solar system on Vesta. The exact height of the peak, which is located on asteroid’s south pole, is not yet determined, but scientists believe it is “almost as high as the Olympus volcano on Mars” (the height of the volcano is 21 km).

Dawn probe was launched into space on September 27, 2007. It reached the orbit of Vesta – the second largest asteroid in the Solar System on July 16, 2011. Its mission will last approximately until 2015. During this time, the device should examine the relief of Vesta, its composition and determine the history of the asteroid.

Jupiter has a number of moons. Four of these were known since Galileo days – Io, Europa, Ganymede and Callisto. They stand out form the rest of the moons due to their size (varies from the size of the Moon to the size of Mercury) and their proximity to the planet. There are few other satellites that rotate even closer to Jupiter: three very small ones and Amalthea, which has an irregular shape (its size is approximately 130 x 80 km). These small satellites together with the Galilean satellites form a so-called accurate system, which has a distinctive coplanarity (location of satellite orbits in the equatorial plane of the planet) and almost circular shape of the orbits.

If you compare them to the position of Earth’s Moon, Io is 10% further, and Callisto – 4.9 times further from the Jupiter than Moon is from Earth. But because of Jupiter’s huge mass it only takes them 1.8 and 16.7 days to orbit the planet.

Since their discovery, those four moons remain one of the most popular objects of astronomical observations. But if only astronomers knew then what wonders are hidden in these celestial bodies!

Io

Murphy’s Law

Relatively short history of space exploration is full of accidents and unexpected discoveries. Gradually experts have developed a specific slang, which is only known to people related to space exploration. No wonder space experts came up with their own version of Murphy’s law: “Everything that can go wrong – will go wrong. Anything that can not go wrong – will go wrong too.” But, fortunately, this law can also work the opposite way. The discovery outlined below is a prime example of such amazing luck.

In order to accurately determine spacecraft’s position on the orbit of a distant planet, specialists do not rely on navigation information only. They also use pictures of visible satellites, which are transmitted by the spacecraft. Relative positions of celestial bodies are then entered into a computer, which determines the coordinates of the spacecraft. One legend says that when Voyager 1 was approaching Jupiter, a computer reported an error in the image of Io. The reason for the error was unclear, but in the end scientists found that Io’s image transmitted by the spacecraft did not match the expected image of a round celestial body. Something was protruding on the side of Io. That “something” turned out to be a huge gas cloud, which rose about 250-300 km above the active volcano.

An emission of oxygen, sodium and sulfur was discovered along the orbit of Io several years ago. The question is – How does such cloud remain in space? At first, scientists thought that images of Io explained this phenomenon: 7-8 active volcanoes on its surface constantly emit gaseous fountains that rise hundreds of kilometers above its surface. If we assume that only part of the eruption products is dissipated into space, the origin of the gas cloud along Io’s orbit can be explained.

But the fact is that Io is rather massive celestial body – its mass is 20% greater than the Moon. The average density of Io is 3.53 g/cm3. Its diameter is 3620 kilometers (Moon is 3476 kilometers). Calculations show that the free fall acceleration at its surface is also large enough – 1.81 m/s2. Heavy sulfur dioxide and sulfur vapor ejected from the volcanic caldera quickly condense in low temperatures, and fall on Ios` surface in the form of frost and snow. This process is faster than the destruction of gas molecules by ultraviolet radiation from the Sun. At the same time, the free-fall acceleration is not enough to retain an atmosphere, as it is on Mars, although Io has some traces of the atmosphere. In order for gas clouds to be several hundred kilometers high, it requires gas outflow of around about 1 km / sec from the volcano. Io’s atmosphere has very small density – 10 to 100 million times smaller than Earth. This is another factor contributing to the high gas outflow.

But let’s leave a question of how do sulfur and sodium get into space for now, and look at Io’s magnificent volcanos.

Energy for Io’s volcanoes

Io is not large enough for the radioactive decay of elements in its core to cause strong heating of the crust as it does on Earth. Energy for heating is drawn from an entirely different source: from tidal effects of the second Galilean satellite – Europe, Jupiter itself, and, to a lesser extent, the third satellite – Ganymede.

These tidal effects force Io’s lithosphere to bend, and it heats up – just like a wire does when you bend it. Enormous energy is released because of the tidal effects – 60-80 million MW. This energy is most likely distributed unevenly – the majority of it is probably released in the surface layers of the moon.

Volcanoes and hot spots on Io

Around 2% of Io’s surface is covered by active hot spots. There are more than 10 of these known to scientists. Temperatures in these spots are believed to be between 310 and 600 K, with spot sizes varying from 75 to 250 km. “Voyager 1″ found 8 active giant eruptions. When 4 months later “Voyager-2” reached Io, it found that 7 of them still continued to erupt. The only one that stopped erupting was one of the largest volcanoes called Pele (in honor of the god of volcanoes).

It is interesting to note that the center of the eruption is for some reason dark, while products of eruptions are orange. Scientists believe that these products accumulate in a molten state in deep reservoirs under Io’s surface.

Io’s volcanoes can be divided into two main categories. Volcanoes of the first group have temperatures of around 350 – 400 K and release gaseous products at a rate of about 500 m/sec. Their gas clouds are usually around 100 km high, and precipitations are white in colour. The majority of Io’s volcanos are of the first type. Volcanos in the second group have very high caldera temperatures; have emission rate of about 1 km/s and heights of their gas clouds can reach around 300 km. Abovementioned Pele, and later found Surt and Aten are examples of volcanos in the second group.

One of the volcanic objects, known as Patera Ra, has a very unusual form and deserves special mentioning. The serpentine lava flows that start at Patera Ra extend over distances of up to 200 km. Their colours vary from shades of brown to light orange and snow-white tones. The nature of these volcanic flows remains unclear, as is nature of even more mysterious objects – lava lakes.

The strongest signal recorded by Voyager’s thermal radiometers, came from a strange object, which was later called Loki. On high-resolution images it appears as a slightly truncated dark circular formation. This formation is around 250 km in diameter and has a bright yellow object of an angular shape in its center. It is believed that the yellow object is a 100-kilometer “iceberg” of solid sulfur, which floats in the center of a lake of molten sulfur! More of these small floating fragments can be seen around the main one.

Topography of Io

Few words about Io’s relief. The moon is mostly flat. There are several large mountainy areas and the mountains in the center of the Pele area. There are also high mountains at the South Pole, covering an area of ​​about 150×80 km. An interesting object was detected on one of the photos – 2.5 km high conical shape mountain with a base diameter of about 85 km.

Interaction with Jupiter 

Io’s orbit is located in in the center of Jupiter’s radiation belt – a part of magnetosphere, where the streams of charged particles are especially dense.

Electric and magnetic phenomena are very intense in Jupiter’s magnetosphere. Io is a part of one of the nature’s wonders – a powerful natural electrical generator. Perhaps “powerful” is not the right word though. A current of around 5 million Amperes flies between Jupiter and Io. This is more than 20 times more than the total energy produced by all Earth power plants. The mechanism by which this fantastic power is produced, is probably associated with a very peculiar structure of the so-called current shells in the Jupiter’s plasmasphere. Some scientists argue that strong electrical currents on Io’s surface can focus on small areas. And potentially volcanic eruptions could be related to these electric activities. So, to summarize, Io “works” as a part of a giant natural particle accelerator.
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[Images: www.nasa.org]

There are eight planets in the solar system. They rotate around the Sun in the following order, starting from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. In addition to the full size planets there are 5 dwarf planets that also make their way around the Sun: Ceres, Pluto, Haumea, Makemake, Eris.

All of the major planets orbit the Sun in one direction. The orbits of all major planets are ellipses that are very close to the circles, and the planes of their orbits are inclined to the ecliptic plane at small angles. The masses of all the planets put together, make up only 0.0013 of the Sun. In addition to these planets, a large number of smaller bodies, called minor planets or asteroids, is moving mostly between Mars and Jupiter. The number of known asteroids is constantly growing as a result of new discoveries, and currently there are more than 1,600 different bodies known to the scientists.

Planets in the solar system can be easily divided into two groups based on their size. The first group consists of relatively small planets that are closest to the Sun: Mercury, Venus, Earth and Mars. This group is commonly known as the Earth group. The second group consists of the largest planets in the Solar System: Jupiter, Saturn, Uranus and Neptune. This group is also known as the Jupiter group. The asteroid belt separates these groups. Pluto stands alone in this grouping, as it is insufficiently explored.

Looking at the physical characteristics of the major planets, we can easily divide them into two groups. For example, the average density of planets in the first group is 4.5 g/cm3 and an average density of planets in the Jupiter group is 1.21 g/cm3. Judging by the density, we can say that the planets in the Earth group are solid. The densities of the planets in the Jupiter group, however, are similar to the density of the Sun and cannot be considered solid.

There is quite a simple explanation for the strong differences between the four giant planets, and four terrestrial planets.  Pluto is an exception to this scheme; however it looks more like one of the moons of distant planets. Hydrogen-rich planets consist of relatively little altered original substance from which the Solar System was formed. Solid planets of the Earth group on the other hand have lost much of the light gases that form the basis of this substance. Instead of primary atmosphere captured from the gas-dust cloud, from which the Solar System was formed, they have secondary atmospheres that emerged after the planets were formed.

Comparing the rotation periods, we, once again, can notice a big difference between the two groups. Terrestrial planets take more than a day to revolve around their axis – they rotate slowly. Planets of the Jupiter group revolve around their axis much faster – on average the rotation period is less than half a day. Huge Jupiter revolves in only 9h 50m – it is clear that its linear velocity at the equator will be much larger than the corresponding rate of rotation of the Earth (28 times larger). Because of this, planets in the Jupiter group have a higher rate of compression, reaching 0.1 on Saturn, and the planets in the Earth group are almost perfect spheres.

The number of satellites of the planets of both groups is also quite different. The entire first group has only three satellites; the second group has more than 160 moons, and this number is constantly increased by new discoveries.

Thermonuclear reactions started on the Sun around 4.6 billion years ago. Because of this, temperatures of the Solar System planets, especially the interior ones noticeably increased. There were two crucial factors that determined the future look of the planets – their size and distance from the Sun. Small size planets were unable to keep the lightest gases – hydrogen and helium. Planet’s ability to retain hydrogen and helium was the decisive factor during the formation of the Solar System. Another determining factor was the distance from the Sun. The fact is that the closer the planet is to the Sun, the more it is heated, and the more difficult it is to retain light gases on its surface. The combined effect of these factors had a significant influence on the formation of planets in the Solar System and divided them into the two main groups.

[images: www.nasa.org]

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In 2007 NASA launched its Dawn spacecraft on a mission to the solar system’s second biggest asteroid – Vesta. Vesta is located in the main Asteroid Belt and is approximately 117 million miles (188 million kilometres) from Earth. It has a circumference of 329 miles.

Dawn reached its destination last Saturday July 16. It transmitted information to confirm it entered Vesta’s orbit, but the exact time this milestone took place is not known. The time of Dawn’s capture depended on Vesta’s mass and gravity, which could only be estimated until this historic visit. The asteroid’s mass determines the strength of its gravitational pull. If Vesta is bigger than expected, its gravity is stronger, and consequentially it pulled the spacecraft into orbit quicker. If the asteroid is smaller than expected, its gravity is weaker and it would have taken Dawn longer to arrive at the orbit. With the spacecraft now in orbit, the researchers will be able to take more accurate readings of Vesta’s gravity and gather more accurate timeline information.

Dawn navigated toward the asteroid belt, a space rock-rich zone between the orbits of Mars and Jupiter, using gravitational energy from Mars and by firing its ion-powered thrusters.

Down spacecraft is the first human object to visit the asteroid. It is also the first spacecraft to orbit two solar system destinations beyond Earth. Dawn will hover about 9,900 miles above Vesta’s surface for a year and use two different cameras, a gamma-ray detector and a neutron detector to study the object. Next July, Dawn’s ion thrusters will fire up and slowly propel the spacecraft toward the dwarf planet Ceres – the largest object in the Asteroid Belt.

Dawn’s observations of Vesta will help scientists understand the earliest chapter of our solar system’s history. The findings will also be used for planning future manned missions to the Asteroid Belt.

Planetary scientists know very little about either Vesta or Ceres, but they suspect each has planet-like qualities. Vesta may have once had a molten core that oozes lava before going cold after a few million years. Ceres, meanwhile, may have an icy mantle and active mud volcanoes.

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[Images - NASA]