The Universe is a very big place, and we occupy a very small
corner of it. Known as the Solar System, our stomping grounds are not
only a tiny fraction of the Universe as we know it, but is also a very
small part of our galactic neighborhood (aka. the Milky Way Galaxy).
When it comes right down to it, our world is just a dot in an endless
cosmic sea.
The Solar System. Image Credit: NASA
Nevertheless, the Solar System is still a very big place, and one
which is filled with its fair share of mysteries. And in truth, it was
only within the relatively recent past that we began to understand its
true extent. And when it comes to exploring it, we’ve really only begun
to scratch the surface.
Discovery:
With very few exceptions, few people or civilizations before
the era of modern astronomy recognized the Solar System for what it was.
In fact, the vast majority of astronomical systems posited that the
Earth
was a stationary object and that all known celestial objects revolved
around it. In addition, they viewed it as being fundamentally different
from other stellar objects, which they held to be ethereal or divine in
nature.
Although there were some Greek, Arab and Asian astronomers during
Antiquity and the Medieval period who believed that the universe was
heliocentric in nature (i.e. that the Earth and other bodies revolved
around the Sun) it was not until
Nicolaus Copernicus developed his mathematically predictive model of a
heliocentric system in the 16th century that it began to become widespread.
Galileo
(1564 – 1642) would often show people how to use his telescope to view
the sky in Saint Mark’s square in Venice. Note the lack of adaptive
optics. Credit: Public Domain
During the 17th-century, scientists like
Galileo Galilei,
Johannes Kepler, and
Isaac Newton
developed an understanding of physics which led to the gradual
acceptance that the Earth revolves round the Sun. The development of
theories like gravity also led to the realization that the other planets
are governed by the same physical laws as Earth.
The widespread use of the telescope also led to a revolution in astronomy. After Galileo discovered the
moons of Jupiter in 1610, Christian Huygens would go on to discover that
Saturn also had moons in 1655. In time, new planets would also be discovered (such as
Uranus and
Neptune), as well as comets (such as
Halley’s Comet) and the
Asteroids Belt.
By the 19th century, three observations made by three separate
astronomers determined the true nature of the Solar System and its place
the universe. The first was made in 1839 by German astronomer Friedrich
Bessel, who successfully measured an apparent shift in the position of a
star created by the Earth’s motion around the Sun (aka.
stellar parallax). This not only confirmed the heliocentric model beyond a doubt, but revealed the vast distance between the Sun and the stars.
In 1859, Robert Bunsen and Gustav Kirchhoff (a German chemist and
physicist) used the newly invented spectroscope to examined the spectral
signature of the Sun. They discovered that it was composed of the same
elements as existed on Earth, thus proving that Earth and the heavens
were composed of the same elements.
With
parallax technique, astronomers observe object at opposite ends of
Earth’s orbit around the Sun to precisely measure its distance. Credit:
Alexandra Angelich, NRAO/AUI/NSF.
Then, Father Angelo Secchi – an Italian astronomer and director at the
Pontifical Gregorian University
– compared the spectral signature of the Sun with those of other stars,
and found them to be virtually identical. This demonstrated
conclusively that our Sun was composed of the same materials as every
other star in the universe.
Further apparent discrepancies in the orbits of the outer planets led
American astronomer Percival Lowell to conclude that yet another
planet, which he referred to as “
Planet X“,
must lie beyond Neptune. After his death, his Lowell Observatory
conducted a search that ultimately led to Clyde Tombaugh’s discovery of
Pluto in 1930.
Also in 1992, astronomers David C. Jewitt of the University of Hawaii and Jane Luu of the MIT discovered the
Trans-Neptunian Object (TNO) known as
(15760) 1992 QB1. This would prove to be the first of a new population, known as the
Kuiper Belt, which had already been predicted by astronomers to exist at the edge of the Solar System.
Further investigation of the Kuiper Belt by the turn of the century would lead to additional discoveries. The discovery of
Eris and other “plutoids” by Mike Brown, Chad Trujillo, David Rabinowitz and other astronomers would lead to the
Great Planet Debate – where IAU policy and the convention for designating planets would be contested.
Structure and Composition:
At the core of the Solar System lies the Sun (a
G2 main-sequence star) which is then surrounded by
four terrestrial planets (the Inner Planets), the main
Asteroid Belt, four gas giants (the
Outer Planets), a massive field of small bodies that extends from 30 AU to 50 AU from the Sun (the
Kuiper Belt) and a spherical cloud of icy
planetesimals that is believed to extend to a distance of 100,000 AU from the Sun (the
Oort Cloud).
The Sun contains 99.86% of the system’s known mass, and it’s gravity
dominates the entire system. Most large objects in orbit around the Sun
lie near the plane of Earth’s orbit (
the ecliptic)
and most planets and bodies rotate around it in the same direction
(counter-clockwise when viewed from above Earth’s north pole). The
planets are very close to the ecliptic, whereas comets and
Kuiper belt objects are frequently at greater angles to it.
It’s four largest orbiting bodies (the gas giants) account for 99% of the remaining mass, with
Jupiter and
Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the
dwarf planets, moons,
asteroids, and comets) together comprise less than 0.002% of the Solar System’s total mass.
The Sun and planets to scale. Credit: Illustration by Judy Schmidt, texture maps by Björn Jónsson
Astronomers sometimes informally divide this structure into separate
regions. First, there is the Inner Solar System, which includes the four
terrestrial planets and the Asteroid Belt. Beyond this, there’s the
outer Solar System that includes the four gas giant planets. Meanwhile,
there’s the outermost parts of the Solar System are considered a
distinct region consisting of the objects beyond Neptune (i.e.
Trans-Neptunian Objects).
Most of the planets in the Solar System possess secondary systems of
their own, being orbited by planetary objects called natural satellites
(or moons). In the case of the four giant planets, there are also
planetary rings – thin bands of tiny particles that orbit them in
unison. Most of the largest natural satellites are in synchronous
rotation, with one face permanently turned toward their parent.
The Sun, which comprises nearly all the matter in the Solar System,
is composed of roughly 98% hydrogen and helium. The terrestrial planets
of the Inner Solar System are composed primarily of silicate rock, iron
and nickel. Beyond the Asteroid Belt, planets are composed mainly of
gases (such as hydrogen, helium) and ices – like water, methane,
ammonia, hydrogen sulfide and carbon dioxide.
Objects farther from the Sun are composed largely of materials with
lower melting points. Icy substances comprise the majority of the
satellites of the giant planets, as well as most of
Uranus and
Neptune (hence why they are sometimes referred to as “ice giants”) and the numerous small objects that lie beyond Neptune’s orbit.
Together, gases and ices are referred to as
volatiles.
The boundary in the Solar System beyond which those volatile substances
could condense is known as the frost line, which lies roughly 5 AU from
the Sun. Within the Kuiper Belt and the Oort Cloud, objects and
planetesimals are composed mainly of these materials and rock.
Formation and Evolution:
The Solar System formed 4.568 billion years ago from the
gravitational collapse of a region within a large molecular cloud
composed of hydrogen, helium, and small amounts of heavier elements
fused by previous generations of stars. As the region that would become
the Solar System (known as the pre-solar nebula) collapsed, conservation
of angular momentum caused it to rotate faster.
The center, where most of the mass collected, became increasingly
hotter than the surrounding disc. As the contracting nebula rotated
faster, it began to flatten into a
protoplanetary disc with a hot, dense
protostar
at the center. The planets formed by accretion from this disc, in which
dust and gas gravitated together and coalesced to form ever larger
bodies.
Due to their higher boiling points, only metals and silicates could
exist in solid form closer to the Sun, and these would eventually form
the terrestrial planets of
Mercury,
Venus, Earth, and
Mars.
Because metallic elements only comprised a very small fraction of the
solar nebula, the terrestrial planets could not grow very large.
In contrast, the giant planets (Jupiter, Saturn, Uranus, and Neptune)
formed beyond the point between the orbits of Mars and Jupiter where
material is cool enough for volatile icy compounds to remain solid (i.e.
the frost line).
The ices that formed these planets were more plentiful than the
metals and silicates that formed the terrestrial inner planets, allowing
them to grow massive enough to capture large atmospheres of hydrogen
and helium. Leftover debris that never became planets congregated in
regions such as the asteroid belt, Kuiper belt, and Oort cloud.
Within 50 million years, the pressure and density of hydrogen in the
center of the protostar became great enough for it to begin
thermonuclear fusion. The temperature, reaction rate, pressure, and
density increased until
hydrostatic equilibrium was achieved.
At this point, the Sun became a main-sequence star. Solar wind from the Sun created the
heliosphere
and swept away the remaining gas and dust from the protoplanetary disc
into interstellar space, ending the planetary formation process.
The
terrestrial planets of our Solar System at approximately relative
sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and
Planetary Institute
The Solar System will remain roughly as we know it today until the
hydrogen in the core of the Sun has been entirely converted to helium.
This will occur roughly 5 billion years from now and mark the end of the
Sun’s main-sequence life. At this time, the core of the Sun will
collapse, and the energy output will be much greater than at present.
The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a
red giant. The expanding Sun is expected to vaporize Mercury and Venus and render Earth uninhabitable as the
habitable zone
moves out to the orbit of Mars. Eventually, the core will be hot enough
for helium fusion and the Sun will burn helium for a time, after which
nuclear reactions in the core will start to dwindle.
At this point, the Sun’s outer layers will move away into space, leaving a
white dwarf
– an extraordinarily dense object that will have half the original mass
of the Sun, but will be the size of Earth. The ejected outer layers
will form what is known as a
planetary nebula, returning some of the material that formed the Sun to the interstellar medium.
Inner Solar System:
In the inner Solar System, we find the “Inner Planets” –
Mercury, Venus, Earth, and Mars – which are so named because they orbit
closest to the Sun. In addition to their proximity, these planets have a
number of key differences that set them apart from planets elsewhere in
the Solar System.
For starters, the inner planets are rocky and terrestrial, composed
mostly of silicates and metals, whereas the outer planets are gas
giants. The inner planets are also much more closely spaced than their
outer Solar System counterparts. In fact, the radius of the entire
region is less than the distance between the orbits of Jupiter and
Saturn.
Generally, inner planets are smaller and denser than their
counterparts, and have few to no moons or rings circling them. The outer
planets, meanwhile, often have dozens of satellites and rings composed
of particles of ice and rock.
The terrestrial inner planets are composed largely of refractory
minerals such as the silicates, which form their crusts and mantles, and
metals such as iron and nickel which form their cores. Three of the
four inner planets (Venus, Earth and Mars) have atmospheres substantial
enough to generate weather. All of them have impact craters and tectonic
surface features as well, such as rift valleys and volcanoes.
Of the inner planets, Mercury is the closest to our Sun and the
smallest of the terrestrial planets. Its magnetic field is only about 1%
that of Earth’s, and it’s very thin atmosphere means that it is hot
during the day (up to 430°C) and freezing at night (as low as -187 °C)
because the atmosphere can neither keep heat in or out. It has no moons
of its own and is comprised mostly of iron and nickel. Mercury is one of
the densest planets in the Solar System.
Venus, which is about the same size as Earth, has a thick toxic
atmosphere that traps heat, making it the hottest planet in the Solar
System. This atmosphere is composed of 96% carbon dioxide, along with
nitrogen and a few other gases. Dense clouds within Venus’ atmosphere
are composed of sulphuric acid and other corrosive compounds, with very
little water. Much of Venus’ surface is marked with volcanoes and deep
canyons – the biggest of which is over 6400 km (4,000 mi) long.
Earth is the third inner planet and the one we know best. Of the four
terrestrial planets, Earth is the largest, and the only one that
currently has liquid water, which is necessary for life as we know it.
Earth’s atmosphere protects the planet from dangerous radiation and
helps keep valuable sunlight and warmth in, which is also essential for
life to survive.
Like the other terrestrial planets, Earth has a rocky surface with
mountains and canyons, and a heavy metal core. Earth’s atmosphere
contains water vapor, which helps to moderate daily temperatures. Like
Mercury, the Earth has an internal magnetic field. And our Moon, the
only one we have, is comprised of a mixture of various rocks and
minerals.
Mars, as it appears today, Credit: NASA
Mars is the fourth and final inner planet, and is also known as the
“Red Planet” due to the oxidization of iron-rich materials that form the
planet’s surface. Mars also has some of the most interesting terrain
features of any of the terrestrial planets. These include the largest
mountain in the Solar System (Olympus Mons) which rises some 21,229 m
(69,649 ft) above the surface, and a giant canyon called Valles
Marineris – which is 4000 km (2500 mi) long and reaches depths of up to 7
km (4 mi).
Much of Mars’ surface is very old and filled with craters, but there
are geologically newer areas of the planet as well. At the Martian poles
are polar ice caps that shrink in size during the Martian spring and
summer. Mars is less dense than Earth and has a smaller magnetic field,
which is indicative of a solid core, rather than a liquid one.
Mars’ thin atmosphere has led some astronomers to believe that the
surface water that once existed there might have actually taken liquid
form, but has since evaporated into space. The planet has two small
moons called Phobos and Deimos.
Outer Solar System:
The outer planets (sometimes called Jovian planets or gas
giants) are huge planets swaddled in gas that have rings and plenty of
moons. Despite their size, only two of them are visible without
telescopes: Jupiter and Saturn. Uranus and Neptune were the first
planets discovered since antiquity, and showed astronomers that the
solar system was bigger than previously thought.
The
outer planets of our Solar System at approximately relative sizes. From
left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary
Institute
Jupiter is the largest planet in our Solar System and spins very
rapidly (10 Earth hours) relative to its orbit of the sun (12 Earth
years). Its thick atmosphere is mostly made up of hydrogen and helium,
perhaps surrounding a terrestrial core that is about Earth’s size. The
planet has
dozens of moons, some
faint rings and a
Great Red Spot – a raging storm that has happening for the past 400 years at least.
Saturn is best known for its prominent
ring system
– seven known rings with well-defined divisions and gaps between them.
How the rings got there is one subject under investigation. It also has
dozens of moons.
Its atmosphere is mostly hydrogen and helium, and it also rotates
quickly (10.7 Earth hours) relative to its time to circle the Sun (29
Earth years).
Uranus was first discovered by William Herschel in 1781. The planet’s
day takes about 17 Earth hours and one orbit around the Sun takes 84
Earth years. Its mass contains water, methane, ammonia, hydrogen and
helium surrounding a rocky core. It has
dozens of moons and a faint ring system. The only spacecraft to visit this planet was the
Voyager 2 spacecraft in 1986.
Neptune is a distant planet that contains water, ammmonia, methane,
hydrogen and helium and a possible Earth-sized core. It has more than
a dozen moons and six rings. NASA’s
Voyager 2 spacecraft also visited this planet and its system by 1989 during its transit of the outer Solar System.
How many moons are there in the Solar System? Image credit: NASA
Trans-Neptunian Region:
There have been more than a thousand objects discovered in the
Kuiper Belt, and it’s theorized that there are as many as 100,000
objects larger than 100 km in diameter. Given to their small size and
extreme distance from Earth, the chemical makeup of KBOs is very
difficult to determine.
However, spectrographic studies conducted of the region since its
discovery have generally indicated that its members are primarily
composed of ices: a mixture of light hydrocarbons (such as methane),
ammonia, and water ice – a composition they share with comets. Initial
studies also confirmed a broad range of colors among KBOs, ranging from
neutral grey to deep red.
This suggests that their surfaces are composed of a wide range of
compounds, from dirty ices to hydrocarbons. In 1996, Robert H. Brown et
al. obtained
spectroscopic data on the KBO 1993 SC,
revealing its surface composition to be markedly similar to that of
Pluto (as well as Neptune’s moon Triton) in that it possessed large
amounts of methane ice.
Water ice has been detected in several KBOs, including
1996 TO66, 38628 Huya and 20000 Varuna. In 2004,
Mike Brown et al. determined the existence of crystalline water ice and ammonia hydrate on one of the largest known KBOs,
50000 Quaoar.
Both of these substances would have been destroyed over the age of the
Solar System, suggesting that Quaoar had been recently resurfaced,
either by internal tectonic activity or by meteorite impacts.
Keeping Pluto company out in the Kuiper belt are many other objects worthy of mention. Quaoar,
Makemake,
Haumea,
Orcus and
Eris
are all large icy bodies in the Belt and several of them even have
moons of their own. These are all tremendously far away, and yet, very
much within reach.
Oort Cloud and Farthest Regions:
The Oort Cloud is thought to extend from between 2,000 and
5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the
Sun, though some estimates place the outer edge as far as 100,000 and
200,000 AU (1.58 and 3.16 ly). The Cloud is thought to be comprised of
two regions – a spherical outer Oort Cloud of 20,000 – 50,000 AU (0.32 –
0.79 ly), and disc-shaped inner Oort (or Hills) Cloud of 2,000 – 20,000
AU (0.03 – 0.32 ly).
The outer Oort cloud may have trillions of objects larger than 1 km
(0.62 mi), and billions that measure 20 kilometers (12 mi) in diameter.
Its total mass is not known, but – assuming that Halley’s Comet is a
typical representation of outer Oort Cloud objects – it has the combined
mass of roughly 3×10
25 kilograms (6.6×10
25 pounds), or five Earths.
The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA
Based on the analyses of past comets, the vast majority of Oort Cloud
objects are composed of icy volatiles – such as water, methane, ethane,
carbon monoxide, hydrogen cyanide, and ammonia. The appearance of
asteroids thought to be originating from the Oort Cloud has also
prompted theoretical research that suggests that the population consists
of 1-2% asteroids.
Earlier estimates placed its mass up to 380 Earth masses, but
improved knowledge of the size distribution of long-period comets has
led to lower estimates. The mass of the inner Oort Cloud, meanwhile, has
yet to be characterized. The contents of both Kuiper Belt and the Oort
Cloud are known as Trans-Neptunian Objects (TNOs), because the objects
of both regions have orbits that that are further from the Sun than
Neptune’s orbit.
Exploration:
Our knowledge of the Solar System also benefited immensely from the
advent of robotic spacecraft, satellites, and robotic landers. Beginning
in the mid-20th century, in what was known as “
The Space Age“, manned and robotic spacecraft began exploring planets, asteroids and comets in the Inner and Outer Solar System.
All planets in the Solar System have now been visited to varying
degrees by spacecraft launched from Earth. Through these unmanned
missions, humans have been able to get close-up photographs of all the
planets. In the case of landers and rovers, tests have been performed on
the soils and atmospheres of some.
Photograph
of a Russian technician putting the finishing touches on Sputnik 1,
humanity’s first artificial satellite. Credit: NASA/Asif A. Siddiqi
The first artificial object sent into space was the Soviet satellite
Sputnik 1,
which was launched in space in 1957, successfully orbited the Earth for
months, and collected information on the density of the upper
atmosphere and the ionosphere. The American probe
Explorer 6, launched in 1959, was the first satellite to capture images of the Earth from space.
Robotic spacecraft conducting flybys also revealed considerable
information about the planet’s atmospheres, geological and surface
features. The first successful probe to fly by another planet was the
Soviet
Luna 1
probe, which sped past the Moon in 1959. The Mariner program resulted
in multiple successful planetary flybys, consisting of the
Mariner 2 mission past Venus in 1962, the
Mariner 4 mission past Mars in 1965, and the
Mariner 10 mission past Mercury in 1974.
By the 1970’s, probes were being dispatched to the outer planets as well, beginning with the
Pioneer 10 mission which flew past Jupiter in 1973 and the
Pioneer 11 visit to Saturn in 1979.
The
Voyager
probes performed a grand tour of the outer planets following their
launch in 1977, with both probes passing Jupiter in 1979 and Saturn in
1980-1981.
Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989.
Launched on January 19th, 2006, the
New Horizons
probe is the first man-made spacecraft to explore the Kuiper Belt. This
unmanned mission flew by Pluto in July 2015. Should it prove feasible,
the mission will also be extended to observe a number of other Kuiper
Belt Objects (KBOs) in the coming years.
Orbiters, rovers, and landers began being deployed to other planets in the Solar System by the 1960’s. The first was the Soviet
Luna 10 satellite, which was sent into lunar orbit in 1966. This was followed in 1971 with the deployment of the
Mariner 9 space probe, which orbited Mars, and the Soviet
Venera 9 which orbited Venus in 1975.
The
Galileo probe became the first artificial satellite to orbit an outer planet when it reached Jupiter in 1995, followed by the
Cassini–Huygens probe orbiting Saturn in 2004. Mercury and
Vesta were explored by 2011 by the
MESSENGER and
Dawn probes, respectively, with
Dawn establishing orbit around the asteroid/dwarf planet
Ceres in 2015.
The first probe to land on another Solar System body was the Soviet
Luna 2 probe, which impacted the Moon in 1959. Since then, probes have landed on or impacted on the surfaces of Venus in 1966 (
Venera 3), Mars in 1971 (
Mars 3 and
Viking 1 in 1976), the asteroid 433 Eros in 2001 (
NEAR Shoemaker), and Saturn’s moon
Titan (
Huygens) and the comet Tempel 1 (
Deep Impact) in 2005.
Curiosity
Rover self portrait mosaic, taken with the MAHLI camera while sitting
on flat sedimentary rocks at the “John Klein” outcrop in Feb. 2013.
Credit: NASA/JPL-Caltech/MSSS/Marco Di Lorenzo/KenKremer
To date, only two worlds in the Solar System, the Moon and Mars, have
been visited by mobile rovers. The first robotic rover to land on
another planet was the Soviet
Lunokhod 1, which landed on the Moon in 1970. The first to visit another planet was
Sojourner, which traveled 500 meters across the surface of Mars in 1997, followed by
Spirit (2004),
Opportunity (2004), and
Curiosity (2012).
Manned missions into space began in earnest in the 1950’s, and was a
major focal point for both the United States and Soviet Union during the
“S
pace Race“. For the Soviets, this took the form of the
Vostok program, which involved sending manned space capsules into orbit.
The first mission –
Vostok 1 – took place on April 12th, 1961, and was piloted by Soviet cosmonaut
Yuri Gagarin (the first human being to go into space). On June 6th, 1963, the Soviets also sent the first woman –
Valentina Tereshvoka – into space as part of the
Vostok 6 mission.
In the US,
Project Mercury was initiated with the same goal of placing a crewed capsule into orbit. On May 5th, 1961, astronaut
Alan Shepard went into space aboard the
Freedom 7 mission and became the first American (and second human) to go into space.
After the Vostok and Mercury programs were completed, the focus of
both nations and space programs shifted towards the development of two
and three-person spacecraft, as well as the development of long-duration
spaceflights and extra-vehicular activity (EVA).
Bootprint in the moon dust from Apollo 11. Credit: NASA
This took the form of the
Voshkod and
Gemini
programs in the Soviet Union and US, respectively. For the Soviets,
this involved developing a two to three-person capsule, whereas the
Gemini program focused on developing the support and expertise needed
for an eventual manned mission to the Moon.
These latter efforts culminated on July 21st, 1969 with the
Apollo 11 mission, when astronauts Neil Armstrong and Buzz Aldrin became the first men to walk on the Moon. As part of the
Apollo program,
five more Moon landings would take place through 1972, and the program
itself resulted in many scientific packages being deployed on the Lunar
surface, and samples of moon rocks being returned to Earth.
After the Moon Landing took place, the focus of the US and Soviet
space programs then began to shift to the development of space stations
and reusable spacecraft. For the Soviets, this resulted in the first
crewed orbital space stations dedicated to scientific research and
military reconnaissance – known as the
Salyut and
Almaz space stations.
The first orbital space station to host more than one crew was NASA’s
Skylab, which successfully held three crews from 1973 to 1974. The first true human settlement in space was the Soviet space station
Mir,
which was continuously occupied for close to ten years, from 1989 to
1999. It was decommissioned in 2001, and its successor, the
International Space Station, has maintained a continuous human presence in space since then.
Space Shuttle Columbia launching on its maiden voyage on April 12th, 1981. Credit: NASA
The United States’
Space Shuttle,
which debuted in 1981, became the only reusable spacecraft to
successfully make multiple orbital flights. The five shuttles that were
built (
Atlantis, Endeavour, Discovery, Challenger, Columbia and
Enterprise) flew a total of 121 missions before being decommissioned in 2011.
During their history of service, two of the craft were destroyed in accidents. These included the
Space Shuttle Challenger – which exploded upon take-off on Jan. 28th, 1986 – and the
Space Shuttle Columbia which disintegrated during re-entry on Feb. 1st, 2003.
In 2004, then-U.S. President George W. Bush announced the
Vision for Space Exploration, which called for a replacement for the aging Shuttle, a return to the Moon and, ultimately, a
manned mission to Mars. These goals have since been maintained by the Obama administration, and now include plans for an
Asteroid Redirect mission, where a robotic craft will tow an asteroid closer to Earth so a manned mission can be mounted to it.
All the information gained from manned and robotic missions about the
geological phenomena of other planets – such as mountains and craters –
as well as their seasonal, meteorological phenomena (i.e. clouds, dust
storms and ice caps) have led to the realization that other planets
experience much the same phenomena as Earth. In addition, it has also
helped scientists to learn much about the history of the Solar System
and its formation.
As our exploration of the Inner and Outer Solar System has improved
and expanded, our conventions for categorizing planets has also changed.
Our current model of the Solar System includes eight planets (four
terrestrial, four gas giants), four dwarf planets, and a growing number
of Trans-Neptunian Objects that have yet to be designated. It also
contains and is surrounded by countless asteroids and planetesimals.
Given its sheer size, composition and complexity, researching our Solar System in full detail would take an entire lifetime.
Source:
Universe Today, written by Matt Williams
Traces of
salty water have been detected in in gullies and craters on Mars, such
as this impact crater 
