The asteroid Vesta - summary of findings

The Dawn spacecraft was planned to orbit Vesta for a year until July 2012. In light of the spacecraft's success in fulfilling its mission, it was decided to extend its mission around Vesta for another month without budgetary allocation, and without harming the next target - Ceres

Among the asteroids, the second largest after Ceres is Vesta. Its diameter is 535 km. Since its discovery for over 100 years, observations of it have been made using telescopes from Earth. The first observations that enabled a global view, albeit with low resolution, were made in 1994 between December 1.12 and November 28.11 by the Hubble Space Telescope. A series of 24 photographs was taken during a complete rotation around itself, which enabled global mapping of it. Its day length is 5.34 hours. For the first time it was possible to recognize its surface, although it is a rough outline. A particularly striking item identified in these observations is a large crater with a diameter of 456 km and a depth of 12.8 km (1). In terms of size, it is in the same group as Enceladus and Mimes, the moons of Saturn and Miranda, the moon of Uranus.

Initial research using a spacecraft began with the launch of the Dawn spacecraft on August 27.8.2007, 2011. According to the spacecraft's work plan, it arrived at Vesta in July 2015, entered orbit around it to survey it for a year, and from there will be directed towards a meeting with the largest asteroid in the solar system - Ceres in XNUMX.

The first global mapping of an asteroid was done by the NEAR spacecraft which was put into orbit around the 33 km diameter asteroid Eros and at the end of its mission was placed on its surface even though it was not designed for this. Vesta is the second asteroid to be studied on a global scale. The Dawn flight is considered a great success. In light of this success, the question is asked about future plans in the study of the small bodies in the solar system, such as launching spacecraft that will enter orbit around Enceladus and Miranda, including landing on their surface.

From the photographs taken by the Hubble telescope it became clear that Vesta is not spherical but flattened. Due to its non-diagonal configuration, the force of gravity is also not arranged in a numerical form around it. The researchers knew that when the spacecraft was closer to the ground, the force of gravity would be stronger, while at a greater distance the force of gravity would be weaker. The design of the spacecraft's flight around it took into account the configuration of the asteroid (2).

flight objectives

The Dawn spacecraft was planned to orbit Vesta for a year until July 2012. In light of the spacecraft's success in fulfilling its mission, it was decided to extend its mission around Vesta for another month due to the fact that during its work there was winter in the northern hemisphere of the asteroid (not in the climatic sense known on Earth), they hoped Because with this extra time it will be possible to explore higher latitudes in the Northern Hemisphere. What made the extension of the mission possible were two reasons. The first reason was that this extra time would not affect the flight plan, to arrive as planned in Zeres. The second reason was that no budgetary allocation was required for this (3).

According to the accepted assumption, Vesta is essentially a protoplanet that survived the massive bombardment of bodies in the asteroid belt 4.5 billion years ago. Therefore, its investigation enables recognition of one of the oldest bodies in the solar system. The definition of Vesta as a protoplanet is also due to the fact that its density led to its internal differentiation into a crust, mantle and core and the fact that it orbits the Sun in the same pattern as the inner planets, but did not undergo full development in the direction of planets (4). From the data of the observations towards it we hope to learn about the early days of the solar system.

The missions of the spacecraft include photographing the surface up to a resolution of 10 meters per pixel (5). The flight path was planned so that it arrived at its destination when the South Pole was illuminated by the sun, which allowed a view towards the giant crater discovered by the Hubble telescope (4). The spacecraft's radio waves were also used to measure the spacecraft's motion around the asteroid. This way you can learn about its gravity, its mass and its internal structure, including its nucleus and potential mascons (6). Gamma radiation and neutron detectors allow the measurement of subatomic particles emitted from various elements in the ground up to a depth of one meter. Mineralogical mapping is done in visible and infrared light (7). Until the beginning of June 2012, the spacecraft transmitted over 25,000 photographs to Israel (8).

 

The coffee routes around Vesta

3 types of orbits were planned for the spacecraft from the moment it begins to orbit Vesta until the end of its mission. Upon reaching the asteroid, it entered a survey orbit. After that, it moved to HAMO (Orbit Mapping High Altitude) and then to LAMO (Low (Orbit Mapping Altitude Low). With the completion of the work program, the spacecraft climbs to the high flight path from which it will begin its journey towards the asteroid Ceres ( 9).

The spacecraft began circling the asteroid on July 16.7.2011, 16,000 when it was 2700 km from the ground. From here it slowly moved to the review route which is 680 km from the ground. At this stage, the spacecraft photographed the asteroid and measured it in wavelengths reflected from the surface of the ground and tested its thermal characteristics. From this route she moved to the high mapping route which is 200 km from the ground. In this route, the entire surface that is illuminated by the sun is mapped. Taking stereo photographs is done to examine the topography. The high resolution photographs are intended for mapping rock types and learning about the thermal properties. From here the spacecraft descended to the low mapping orbit which is 10 km from the ground. The main goal in this orbit is to discover by-products of cosmic radiation hitting the surface and to identify the types of elements that make up the ground and to examine the internal structure (XNUMX).

The transition from orbit to orbit was extremely slow because the spacecraft's engines are ionic. These engines cannot develop the same power as the chemical engines. The purpose of using the ion drive is to save fuel. Therefore the duration of the flight towards the asteroid was several years and the transition from orbit to orbit is very slow. In transitions from track to track, the spiral movement track technique is used.

The duration of the activity in the review track was 20 days. The whole coffee lasted 3 days. 7 laps in this orbit were planned for the spacecraft. The transition to the high mapping route lasted a month. The entire coffee lasted 12.3 hours. 60 laps were made on this route (11,12). In the low mapping orbit the duration of the entire lap is 4 hours, a little less than the time the asteroid revolves around itself 5.34 hours (9).

With the completion of its planned mission and after the additional working time given to the spacecraft, it was in the high flight path. The asteroid's departure is scheduled for August 26.8.2012, 15.6. She stayed in this route from 5. In order to free herself from the force of gravity, she needs 26.8 weeks. Since the trajectory is spiral, each frigate is higher than the previous one. On August 13, the flight path will be far enough to allow the spacecraft to fly towards the next destination (XNUMX).

The totality of the 25,000 photographs taken until June of this year in the various trajectories made it possible to construct a map of the asteroid, its inclination and mapping from the South Pole to latitude N ° 50. All the formations and the surface were given names.

 

The configuration of the asteroid

As mentioned, it turned out that Vesta does not have a spherical configuration but an elliptical one, it is very flattened and in its other characteristics it is more reminiscent of the Earth's moon (14). Because of its configuration, its gravity is strong in part of the equatorial region and in the section of the North Pole (15). Its inclination angle is 27°, 4° more than they thought. For this reason it has seasons. Each pole is completely dark during its winter period or completely illuminated during its summer period (5,16).

 

topography

The impression obtained from all the photographs is that Vesta is a fossil from the early days of the solar system and that its surface is more diverse than thought. For this reason it is more similar to the Earth's moon than to the other asteroids. Its topography is very diverse (17). The topographic height differences are on the order of 20 km, which can indicate a powerful process of formation of the asteroid as evidenced by the variation of the surface and the large number of craters, valleys and canyons and different brightness differences of the materials that make up the surface (18) . A big surprise was the presence of a mountain rising to a height of 21 km at the South Pole. It is believed to have been created by the impact of another body on the asteroid. Much material piled up around it to form this high mountain. The surface around the mountain is smooth, which can teach about the history of craters around the mountain.

On Vesta there is a hill 42.5 km long and 28 km wide. It was probably formed by impact craters. The dark color of the hill raised the hypothesis that it was a volcano. The absence of volcanic features raises the possibility that this is a dike or trending intrusion that has undergone weathering and was exposed by impact craters (20).

Surface

Vesta's surface appears rougher than those of the other asteroids. A preliminary analysis of the age of the craters shows that various regions in the Southern Hemisphere are 1-2 billion years younger than those found in the Northern Hemisphere (21). A mosaic image of the asteroid constructed from the set of photographs shows that Vesta is one of the brightest bodies in the Solar System and that the brightness of several places on the surface is greater than its average brightness. These spots are scattered over the surface of the asteroid. It is estimated that these regions have undergone the least changes in mineralogy throughout the geological history of the asteroid and therefore represent the material from which it was formed (22).

 

The size of those bright spots ranges from hundreds of meters to 16 km. The dark materials are in a variety of colors such as dark gray, brown and red. There are places where they are small and deposited around impact craters. There are also larger ones that surround impact craters and received the name "snowman". They appear to be related to surface trauma and the consequences of these traumas. According to the researchers' assessment, meteorites rich in carbon hit Vesta at low speeds and created the small deposits without throwing material to the sides. Larger bodies that hit the asteroid at high speeds melted the basaltic crust and darkened the original surface. Collections of molten materials are found on the sides and floor of the impact craters, on hills, on ridges and under bright areas. It is estimated that the dark materials in the asteroid come from the asteroid belt and beyond and originate in the early days of the solar system (23).

In the photographs, cracks were identified that were probably created during the impact of various bodies on the ground and they are probably made of minerals rich in iron and magnesium that are found in volcanic rocks on Earth. Pond-like deposits were also found. probably formed when the fine dust grains formed during the impact, sank in low areas. In the South Pole, anomalous areas were observed, the density of which is probably greater. This raises the possibility that dense material originating from the asteroid's interior was exposed by the impact that created the Rheasilvia basin. Lighter and younger materials covering different parts of the surface were blown out of the basin (24).

Makhteshim

Following the mapping of the asteroid and giving names to all the craters observed on its surface, a Semitic list of the 50 largest craters was prepared (25). According to this list, 46 craters are less than 100 km in diameter, the number of craters with a diameter between 101-200 km is 0, the number of craters with a diameter between 300-201 is 1, the number of craters with a diameter between 301-400 km is 1 and the number of craters with a diameter of Between 401-500 km is 2. You can see that the total number of craters with a diameter greater than 100 km is very small. Due to the dimensions of these craters, some of them are treated as basins, such as the Rheasilvia basin in the South Pole, which is 500 km in diameter and has a 15 km high relief from its center (26). Another Veneneia basin with a diameter of 400 km is also found in the South Pole. These basins created shock waves that penetrated to the core of the asteroid. Evidence of this is a number of depressions found along the equator. A clear geological connection can be made to the centers of the basins (14). It is estimated that the Veneneia basin formed 2 billion years ago and the Rheasilvia basin formed 1 billion years ago (17).

As for the rest of the craters in terms of their geographical distribution on the surface of Vesta, the Northern Hemisphere has more craters than the Southern Hemisphere, at least in those parts imaged by the Dawn spacecraft (21). As for the craters with a diameter smaller than 1 km, a linear chain of craters of this size was observed. And in some of them they go into one of the craters and come out of it from the other side (27). It is likely that there are craters on Vesta that are tens of meters in diameter or less. For the sake of illustration, several craters will be mentioned here.

1. A crater whose sides show clear signs of rockslides and in terms of the shape of the sides they resemble the Stickney crater found on Phobos, a moon of Mars. The diameter of the crater is about 10 km (28).
2. Crater of Numisia. It is 30 km in diameter and has characteristics similar to the crater in example no. 1 . Its location is near the equator (29).
3. A crater that has material flowing into it on one of its sides. This material is darker than the wall on which it is found (30).
4. The Vilidia crater is 10 km in diameter and around it is a dark material thrown from it in the form of rays and it is bright. Some of the bright matter is also inside it (31).
5. Canuleia Crater. Its diameter is 10 km. Around it is a bright substance that is thrown out of it (32).

 

Deep Grooves

Surprisingly, grooves were also found on this asteroid, some of which are deep (grooves). For example, several grooves were found running parallel to the equator. They are hundreds of kilometers long. In several sections they are inclined to each other and it looks as if they were dug by a plow (14). In the northern hemisphere there are many grooves in different directions and their width is less than 1 km. It is estimated that the grooves were formed as a result of the deposition of regolith on the surface (33). Another type of grooves are those adjacent to chains of craters. All the grooves are on the surface of a smooth layer of material thrown from a large crater located nearby (34). Another group of small grooves near the Rheasilvia basin cross each other and form a complicated network of grooves and are less than 500 meters wide (35).

Volcanic activity

Although volcanic structures such as volcanoes were expected to be present, these were not found on Vesta's surface at least in the parts of the asteroid that were imaged. It is possible that a thick layer of regolith formed following many impacts covered ancient traces of volcanic activity (14). On the other hand, observed minerals exposed by cuts in the surface of the ground created by the bodies that hit the asteroid, can support the hypothesis that Vesta was once an ocean of magma below the ground (17).

 

Mineralogy and geological history

According to the accepted estimate, Vesta was larger in the past than it is today. Large parts were blown off it as a result of collisions with various bodies. Vesta was large enough to allow internal separation (differentiation) into a mantle nucleus and a membrane. This assumption was confirmed by mineralogical analysis of the surface. This analysis showed that some of the minerals found in it are identical to the minerals found in rare meteorites that hit the Earth (14). One of the minerals is pyroxene, a mineral rich in iron and magnesium (17).

 

Temperatures

The temperature range is greater than -23°C in the sunlight to -100°C in the shadowed areas. It turned out that the soil reacts quickly to sunlight because there is no mediating factor of the atmosphere (24).

 

Internal structure

Vesta's internal structure is a core, mantle and crust that makes it more like a planet than an asteroid. It also has enough radioactive material which, upon cooling, releases heat, melting rocks and allowing light material to rise to the surface (4). The diameter of the nucleus is 110 km (17) and it is made of high density iron (2).

Sources

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14. "Vesta - a planet like asteroid" 11.5.2012
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15. PIA15602: Vesta shape and gravity
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16. "Vesta likely cold and dark enough for ice" 26.1.2012
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18. "Vesta asteroid full of surprises" 19.9.2011
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19. "Space mountain produces terrestrial meteorites" 3.1. 2012
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20. PIA15148: Dark hill on asteroid movie
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21. "Dawn at Vesta massive mountains rough surface and old-young dichotomy in hemispheres" 6.10.2010
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22. PIA15233: Map of bright areas on Vesta
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23. "Dawn sees new surface feathers on giant asteroid" 23.3.2012
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25. Nomenclature search results target Vesta
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26. PIA14711: A false color topography of Vesta south pole
http://photojournal.jpl.nasa.gov/Catalog/ PIA14711

27. PIA15296 : Unusual crater chains and grids
http://photojournal.jpl.nasa.gov/Catalog/ PIA15296

28. PIA14960 : Unusual craters on Vesta ll
http://photojournal.jpl.nasa.gov/Catalog/ PIA14960

29. PIA15234 : Bright material at Numisia crater
http://photojournal.jpl.nasa.gov/Catalog/ PIA15234

30. "Dawn spirals down to lowest orbit above Vesta" 13.12.2011
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31. PIA15455 : Vibidia crater
http://photojournal.jpl.nasa.gov/Catalog/ PIA15455

32. PIA15235 : Bright rays from Canubia crater
http://photojournal.jpl.nasa.gov/Catalog/ PIA15235

33. PIA15773 : Grooved surface
http://photojournal.jpl.nasa.gov/Catalog/ PIA15773

34. PIA15046 : Grooves and craters on Vesta
http://photojournal.jpl.nasa.gov/Catalog/ PIA15046

35. PIA15324 : Complex surface texture in Vesta's southern hemisphere
http://photojournal.jpl.nasa.gov/Catalog/ PIA15324