The Solar System in Close-Up
Page 14
Mars also lacks a magnetosphere, which means its surface and atmosphere are exposed to attack by solar radiation. It is believed that fields detected on Mars are remnants of a magnetosphere that collapsed early in its history. The lack of a magnetosphere is thought to be one reason for Mars’s thin atmosphere. Solar wind induced ejection of atmospheric atoms has been detected by probes orbiting Mars.
Martian Moons
Mars has two small moons that were discovered in 1877 by American astronomer Asaph Hall. Hall named them Phobos (meaning fear) and Deimos (meaning panic).
Phobos is the larger and innermost of the two moons being 21 km in diameter. It takes 7 h 39 min to orbit Mars, at a orbital radius of 9380 km. Deimos is only 12 km in diameter, takes 30 h 18 min to orbit Mars, at an orbital radius of 23,460 km. Both moons have an irregular shape, low density and many craters. They are hard to see from Earth because they are so small and reflect little light. The densities of each moon are so low they cannot be pure rock. Both moons orbit in nearly circular orbits (see Fig. 7.11 and Table 7.3).
Fig. 7.11The two moons of Mars—Deimos and Phobos are both irregularly shaped and probably captured asteroids (Credit: NASA).
Table 7.3Details of the moons of Mars
Name
Distance from Mars (km)
Period (days)
Diameter (km)
Discovered
Phobos
9380
0.32
21
1877
Deimos
23,460
1.26
12
1877
Recent images from Mars Global Surveyor indicate Phobos is covered with a layer of fine dust about a metre thick. The Soviet spacecraft Phobos 2 detected a faint but steady out gassing from Phobos. Unfortunately, Phobos 2 failed before it could determine the nature of the material.
Phobos is slowly being pulled closer to Mars (1.8 m per century), and in about 50 million years it will either crash into the surface of Mars or break up into a ring. Deimos on the other hand appears to be getting further from Mars, slowing down as it does so.
Phobos has a potato shape and always has the same face turned towards Mars. If you were standing on Mar’s equator you would see Phobos rising in the west, move across the sky in only 5½ h and set in the east, usually twice a day. Deimos takes about 2½ days to cross the Martian sky. Both moons are heavily cratered but Phobos has one large crater called Stickney (named after Angelina Stickney, Hall’s wife) that is 10 km wide (this is visible on the right side of Phobos in Fig. 7.11). The grooves and streaks on the surface of Phobos were probably caused by the Stickney impact. Deimos is less cratered than Phobos. The largest crater on Deimos is only 2.3 km across.
It is thought that these two moons were asteroids captured by Mar’s gravitational field. Evidence to support the asteroid theory is that both moons reflect very little of the light that falls on them and are very light for their size. The cratering on each moon suggests their surfaces are equally old—about 3 billion years. They are similar to C-type asteroids, which belong to the outermost part of the asteroid belt.
Further Information
For information about Mars and the various space missions check out
http://mars.jpl.nasa.gov/
http://www.nasa.gov/maven
www.space.com/mars/
For fact sheets on any of the planets including Mars check out
http://nssdc.gsfc.nasa.gov/planetary/planetfact.html
© Springer International Publishing Switzerland 2016
John WilkinsonThe Solar System in Close-UpAstronomers' Universe10.1007/978-3-319-27629-8_8
8. The Asteroid Belt
John Wilkinson1
(1)Castlemaine, Victoria, Australia
Highlights
In February 2001, the NEAR probe landed on the asteroid Eros. A gamma-ray spectrometer on the probe was able to analyse material from the surface.
The Japanese Hayabusa probe landed on the surface of the asteroid Itokawa on 9 May 2003. It collected surface material and returned it to Earth.
The DAWN spacecraft has become the first spacecraft ever to orbit two worlds beyond Earth—Vesta in 2011/12 and Ceres in 2015.
In January 2014, ESA scientists detected plumes of water vapour on Ceres.
The largest asteroid, Ceres, was classified as a dwarf planet in 2006. A visit by the DAWN spacecraft in March 2015 captured some exciting images including unusual bright spots.
The asteroid belt is region between the orbits of Mars and Jupiter that contains about a million small rocky bodies, called asteroids. Most of these irregular shaped bodies are only a few kilometres in size, but they all orbit the Sun in much the same plane and direction. The belt contains about 200 asteroids larger than 100 km and about 750,000 up to 1 km in size. However, their combined mass is only about one-twentieth that of the Moon.
Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, and M-type. These were named after and are generally identified with carbon-rich, stony, and metallic compositions, respectively.
There are some asteroids that have very elliptical orbits and cross Earth’s orbit. These Earth-crossing asteroids have many craters on their surface because of impacts with other smaller bodies. Fragments of rock or iron ejected from the asteroids following impacts create bodies called meteoroids. A meteoroid entering Earth’s atmosphere often heats up and glows, giving off light and appears like a like a shooting star or meteor across our night sky. Most meteoroids are about as old as the solar system itself.
The largest and first known asteroid, Ceres, is about 950 km in diameter. It contains about one-third the total mass of all the asteroids. In 2006, Ceres was classified as a dwarf planet because it orbits the Sun, has enough mass to form a spherical shape, has not cleared the area around its orbit and is not a satellite. The second largest asteroid is Vesta, with a diameter of around 550 km, closely followed by Pallas, an irregular shaped object about 540 km across. One of the smallest asteroids is 1991BA, discovered in 1991, and only 6 m across. The only asteroid that can be seen by the unaided eye is Vesta. It can be seen in dark skies when it is favorably positioned because it has a relatively high reflective surface.
Early Views About the Asteroids
The word ‘asteroid’ means ‘star-like’. This name probably arose because, viewed from a telescope from Earth, asteroids look like a star.
Ancient observers on Earth did not know the asteroids because most cannot be seen with the unaided eye. Johann Elert Bode first suggested the idea that a planet-like body might exist between the orbits of Mars and Jupiter in 1768. In January 1801, the Sicilian astronomer Giuseppi Piazzi discovered a body in a position similar to that predicted by Bode. This body was called Ceres in honour of the Roman goddess of plants and harvest, and was the first asteroid to be discovered. In March 1802 the German astronomer, Heinrich Olbers discovered another faint asteroid that he called Pallas (after the Greek goddess of wisdom). Two more asteroids were discovered in the early 1800s were called Juno and Vesta.
By 1850, ten asteroids were known to orbit at average distances from the Sun of between 2.2 and 3.2 AU. These early findings were made by astronomers who spent many hours at the telescope observing changes in positions of celestial objects against the background of stars. In 1891 the German astronomer Max Wolf made the first photographic discovery of an asteroid. The object was named Brucia (the 323rd asteroid to be found). By 1923, the list of asteroids had grown to over a thousand.
During the early years of discovery, mythological names were given to the asteroids. All the early names were female (this naming scheme was later abandoned). Some asteroids were named after countries, for example, 1125 China, while others are named after scientists, for example, 2001 Einstein. Permanent numbers are assigned to asteroids once their orbits have been calculated and confirmed.
Distance from Sun
413,700,000 km (2.76 AU)
Diameter
950 km in diameter
Mass
9.46 × 1020 kg
Density
2.07 g/cm3 or 2070 kg/m3
Orbital eccentricity
0.08
Period of revolution
1681 Earth days or 4.60 Earth years
Rotation period
9.07 h
Orbital velocity
17.9 km/s
Axial tilt
3°
Average temperature
−106 °C
Atmosphere
Tenuous, some water vapour
Strength of gravity
0.27 N/kg
Probing the Asteroids
In October 1991, the NASA space probe Galileo took the first detailed photograph of an asteroid while en route to Jupiter. The asteroid, named Gaspra, is an irregularly shaped object measuring about 19 by 12 km in size. The Galileo probe also passed by the asteroid Ida, in August 1993. Both Gaspra and Ida are classified as S-type asteroids since they are composed of metal-rich silicates. Both asteroids are probably fragments of larger parent bodies that were broken apart by catastrophic collisions. Ida’s surface is more heavily cratered than Gaspra’s, but Ida is much older. Also, Ida, has its own companion, Dactyl, which is 1.5 km is diameter and orbits Ida at a distance of 100 km (see Figs. 8.1 and 8.2).
Fig. 8.1The asteroid Gaspra as seen by the Galileo space probe (Credit: NASA).
Fig. 8.2The asteroid Ida and its companion Dactyl as seen by the Galileo probe (Credit: NASA).
In 1996, NASA launched the NEAR (Near Earth Asteroid Rendezvous) space probe. This probe flew within 1216 km of the asteroid Mathilde in 1997. This encounter gave scientists the first close-up look of a carbon-rich C-type asteroid. This visit was unique because NEAR was not designed for fly-by encounters. The next year, NEAR flew past the asteroid Eros at a distance of 3829 km, and it went into orbit around Eros in February 2000. In March 2000, the probe was renamed NEAR-Shoemaker in honor of American astronomer Eugene Shoemaker, who had died not long before. In February 2001, NEAR-Shoemaker became the first spacecraft to land on an asteroid when it landed on Eros.
In July 1999 NASA’s Deep Space 1 probe flew within 26 km of the asteroid Braille.
The Japan Aerospace Exploration Agency (JAXA) launched the Hayabusa probe on 9 May 2003. The probe’s mission was to land on the surface of the asteroid Itokawa, and to collect samples and return them to Earth. The first attempt at landing failed but the second, in September 2005, was successful. Problems with the probe’s engines delayed the return flight, however it eventually returned to Earth on 13 June 2010. The heat-shielded capsule made a parachute landing in the South Australian desert while the spacecraft broke up and incinerated in a large fireball. On 16 November 2010, JAXA confirmed that most of the particles found inside the Hayabusa sample container came from Itokawa. Japanese scientists found the samples were more similar to meteorites than known rocks from Earth, with concentrations of olivine and pyroxene. On 26 August 2011, scientists announced that the dust from Itokawa suggested it was probably part of a larger asteroid. The dust collected is believed to have been there for about 8 million years. Pictures from Hayabusa showed the surface of Itokowa is unlike any other solar system body yet photographed—a surface possibly devoid of craters.
The ESA’s Rosetta probe passed within 3162 km of the asteroid 21 Lutetia in July 2010.
Lutetia has an irregular shape and is heavily cratered, with the largest impact crater reaching 45 km in diameter. The surface was found to be geologically heterogeneous and intersected by a system of grooves and scarps, which are thought to be fractures. It has a high average density, suggesting it is made of metal-rich rock (see Fig. 8.3).
Fig. 8.3The ESA’s Rosetta space probe took this image of the asteroid 21 Lutetia at closest approach in July 2010. The asteroid is 100 km in diameter and made of metal rich rock (Credit: ESA).
In September 2007, NASA launched the DAWN spacecraft on a mission to the asteroid belt. Seeking clues about the birth of the solar system, the craft orbited Vesta between July 2011 and September 2012, before moving on to encounter Ceres in 2015. Both these asteroids are believed to have evolved more than 4.5 billion years ago, about the same time Mercury, Venus, Earth and Mars formed. Images taken by the Hubble Space telescope show these two asteroids are geologically diverse, but mysteries abound. The DAWN probe was the first spacecraft to go into orbit around a main belt asteroid, enabling a detailed and intensive study of the object (Table 8.2).Table 8.2Significant space probes to asteroids
Probe
Country of origin
Launched
Visited
Galileo
USA
1989
Gaspra 1991, Ida 1993
NEAR-Shoemaker
USA
1996
Mathilde 1997, Eros 2001
Deep space 1
USA
1998
Braille 1999
Hayabusa
Japan
2003
Itokawa 2011
Rosetta
ESA
2004
Lutetia 2010
DAWN
USA
2007
Vesta 2011, Ceres 2015
On 13 December 2012, China’s lunar orbiter Change 2 flew within 3.2 km of the asteroid 4179 Toutatis on an extended mission. Around 2015 Japan Aerospace Exploration Agency plans to launch the improved Hayabusa 2 space probe and to return asteroid samples by 2020. Current target for the mission is the C-type asteroid 1999JU3. On 19 June 2014, NASA reported that asteroid 2011 MD was a prime candidate for captures by a robotic mission, perhaps in the early 2020s.
Position and Orbit
The asteroids orbit the Sun in a region between Mars and Jupiter between 2.0 and 3.5 AU from the Sun. The largest asteroid, Ceres, orbits at an average distance from the Sun of 413,700,000 km (2.76 AU). Ceres is spherical but is not to be considered a planet because it has not cleared its neighbourhood of other objects; instead it is classed as a dwarf planet. Its diameter is only one quarter the diameter of Earth’s moon. Pallas orbits the Sun in a slightly elliptical orbit; it is dimmer than Ceres and has a diameter of only 540 km. Vesta is the brightest asteroid, orbiting at an average distance from the Sun of 2.36 AU.
Asteroids, which remain within the main asteroid belt, are referred to as main belt asteroids. In 1857 the American astronomer Daniel Kirkwood suggested that there would be gaps in the asteroid belt, created by gravitational perturbations of Jupiter. The existence of what became known as Kirkwood Gaps was confirmed in 1866. Such gaps were made through repeated alignments of asteroids with Jupiter. For example, an asteroid with an orbital period of exactly half that of Jupiter would, on every second orbit, be aligned with Jupiter.
The Trojan asteroids are two groups of asteroids, which travel around the Sun in the same orbit as Jupiter. These two groups are held in orbit by the combined gravitational forces of Jupiter and the Sun (see Fig. 8.4).
Fig. 8.4The main belt of asteroids (white) is located between the orbits of the planets Mars and Jupiter. There are three other groups (Trojans, Hildas, Greeks) that share the orbit of Jupiter (Credit: NASA).
Some larger asteroids have been found to have smaller asteroid fragments orbiting them. The asteroid Toutatis consists of two similarly sized bodies orbi
ting each other. The asteroid Ida has a companion called Dactyl (see Fig. 8.2).
During 2007, astronomers announced that they have found strong evidence that sunlight can cause asteroids to spin more quickly by ‘pushing’ on the irregular surface features, and this accelerates or decelerates the rotation rate. The theory is that the Sun’s heat serves as a propulsion engine on the irregular features of an asteroid’s surface (Fig. 8.5).
Fig. 8.5The asteroid Eros is irregular in shape because of a series of collisions. It is 31 km long and its surface is covered by dust and rock fragments. This image was taken by the NEAR spacecraft in February 2000 from a distance of 289 km (Credit: NASA/NEAR).
Asteroid Collisions with Earth
Although there may be a million asteroids in the asteroid belt, their average separation is actually 10 million km, so collisions are not as common as one might expect.
On June 14, 1968, the asteroid Icarus passed within 6 million km of Earth. On 23 March 1989, the asteroid 1989 FC passed within 800,000 km of earth, and on 9 December 1994, asteroid 1994 XM1 passed within 105,000 km. This latter asteroid is only about 10 m in diameter but it could have done a lot of damage had it hit Earth.