#1

SATELLITE CHARACTERISTICS

Satellite Characteristics: Orbits and

Swaths
We learned in the previous section that remote sensing instruments can be placed on a variety of
platforms to view and image targets. Although ground-based and aircraft platforms may be used,
satellites provide a great deal of the remote sensing imagery commonly used today. Satellites
have several unique characteristics which make them particularly useful for remote sensing of
the Earth's surface.

The path followed by a satellite is referred to as its orbit. Satellite orbits are matched to the
capability and objective of the sensor(s) they carry. Orbit selection can vary in terms of altitude
(their height above the Earth's surface) and their orientation and rotation relative to the Earth.
Satellites at very high altitudes, which view the same portion of the Earth's surface at all times
have geostationary orbits. These geostationary satellites, at altitudes of approximately 36,000
kilometres, revolve at speeds which match the rotation of the Earth so they seem stationary,
relative to the Earth's surface. This allows the satellites to observe and collect information
continuously over specific areas. Weather and communications satellites commonly have these
types of orbits. Due to their high altitude, some geostationary weather satellites can monitor
weather and cloud patterns covering an entire hemisphere of the Earth.
Many remote sensing platforms are designed to follow an orbit (basically north-south) which, in
conjunction with the Earth's rotation (west-east), allows them to cover most of the Earth's surface
over a certain period of time. These are near-polar orbits, so named for the inclination of the
orbit relative to a line running between the North and South poles. Many of these satellite orbits
are also sun-synchronous such that they cover each area of the world at a constant local time of
day called local sun time. At any given latitude, the position of the sun in the sky as the satellite
passes overhead will be the same within the same season. This ensures consistent illumination
conditions when acquiring images in a specific season over successive years, or over a particular
area over a series of days. This is an important factor for monitoring changes between images or
for mosaicking adjacent images together, as they do not have to be corrected for different
illumination conditions.

Most of the remote sensing satellite platforms today are in near-polar orbits, which means that
the satellite travels northwards on one side of the Earth and then toward the southern pole on the
second half of its orbit. These are called ascending and descending passes, respectively. If the
orbit is also sun-synchronous, the ascending pass is most likely on the shadowed side of the
Earth while the descending pass is on the sunlit side. Sensors recording reflected solar energy
only image the surface on a descending pass, when solar illumination is available. Active sensors
which provide their own illumination or passive sensors that record emitted (e.g. thermal)
radiation can also image the surface on ascending passes.

As a satellite revolves around the Earth, the sensor "sees" a certain portion of the Earth's surface.
The area imaged on the surface, is referred to as the swath. Imaging swaths for spaceborne
sensors generally vary between tens and hundreds of kilometres wide. As the satellite orbits the
Earth from pole to pole, its east-west position wouldn't change if the Earth didn't rotate.
However, as seen from the Earth, it seems that the satellite is shifting westward because the
Earth is rotating (from west to east) beneath it. This apparent movement allows the satellite
swath to cover a new area with each consecutive pass. The satellite's orbit and the rotation of
the Earth work together to allow complete coverage of the Earth's surface, after it has completed
one complete cycle of orbits.

If we start with any randomly selected pass in a satellite's orbit, an orbit cycle will be completed
when the satellite retraces its path, passing over the same point on the Earth's surface directly
below the satellite (called the nadir point) for a second time. The exact length of time of the
orbital cycle will vary with each satellite. The interval of time required for the satellite to
complete its orbit cycle is not the same as the "revisit period". Using steerable sensors, an
satellite-borne instrument can view an area (off-nadir) before and after the orbit passes over a
target, thus making the 'revisit' time less than the orbit cycle time. The revisit period is an
important consideration for a number of monitoring applications, especially when frequent
imaging is required (for example, to monitor the spread of an oil spill, or the extent of flooding).
In near-polar orbits, areas at high latitudes will be imaged more frequently than the equatorial
zone due to the increasing overlap in adjacent swaths as the orbit paths come closer together
near the poles.

SPACE CHARACTERISTICS
Space is not a friendly place. Mostly, it is nothing: vacuum, and apparent lack of gravity.
What little there is in space is mostly hazardous, even the things that are assets.
One asset abundant in space, at least out to Mars orbit, is power, in the form of energy
from the Sun that can be converted into electricity. The supply of power from the Sun is
continuous and uninterruptible, and it is 6 to 15 times as intense in Earth orbit as on the
Earth's surface (solar energy intensity is absorbed by Earth's atmosphere). Solar energy
does, however, present hazards, in that it is harmful to humans who are not shielded
from it. The sun is also the source of flares producing intense radiation deadly to
unprotected humans and equipment.
The Sun is not the only source of hazardous radiation in space: cosmic rays, gamma
rays, and X-rays are also part of the ambient space environment, at sufficient intensities
to be damaging to humans. Earth's magnetic field prevents most of this radiation from
reaching low earth orbits and the planet's surface; it also, however, traps dangerous
levels of protons and electrons in the Van Allen radiation belts, which begin at 250 to
750 miles altitude, and extend to between 37,000 and 52,000 miles altitude.
The primary law of space is Orbital Mechanics, which determines where things can stay
and where they can go. Everything in space is moving, attracted by gravitational fields
(usually interacting) of every major object in the vicinity. Which orbit an object is in
depends on its position and velocity at any particular time. Orbits are changed with
acceleration due to thrust, usually from rockets.
Satellites are constrained to be far enough above most of any atmosphere that an orbit
can be sustained for a few days before decaying through friction with the atmosphere
(about 100 miles above Earth's surface; a 150-mile Earth orbit can be maintained for
about five years, a 250-mile orbit for about ten, with variations depending on solar
activity and other factors that can cause the atmosphere to expand or contract).
Objects with bigger cross-sections experience faster orbital decay. Higher velocities put
objects in higher orbits; an object achieving ``escape velocity'' (25,000 mph for Earth)
will leave the influence of its ``host'', and go into orbit around something else. Orbits are
also not perfectly stable, requiring that satellites use small rockets for ``station-keeping''
to stay where desired. Some quirks of orbital mechanics cause gravitational forces due
to two large bodies (e.g., moon and Earth, or Earth and Sun) to balance in some
locations, creating ``Libration Points'' that are either unusually stable (L4 and L5) or are
unstable but provide opportunities for maintaining spacecraft in otherwise impossible
locations (L1, L2, and L3). All five libration points can host multiple small objects in local
orbits: around stable L4 and L5, these orbits are large and apparently elliptical; around
unstable L1, L2, and L3, orbits are convoluted but viable with station-keeping
propulsion.
Solid objects in space are also both hazards and assets. Debris is a nuisance: at orbital
velocities, even paint flecks can do damage, and small bolts or rocks can puncture
spacecraft cabins. Most human-caused debris is in Low Earth Orbit (LEO),
Geosynchronous Earth Orbit (GEO), and Molniya orbits used for Russian
communications satellites; all objects larger than three inches in diameter are tracked
by the United States Air Force, and warnings are provided when collisions are
predicted.
Natural debris is primarily dust, which can degrade materials that stay in orbit for long
periods of time. The Moon and near-Earth asteroids, however, represent vast reserves
of relatively easily accessible resources; getting to them requires much less energy than
launching materials from the Earth. The lunar surface is 99% oxides, of silicon, iron,
aluminum, calcium, magnesium, and titanium. Asteroids are of several types, and can
contain nickel, iron, water, carbon, and carbon compounds.
The space environment near Mars is even less hospitable than near Earth. With no
magnetic field, Mars lacks the equivalent of Van Allen radiation belts that provide some
protection for human operations in GEO. Solar radiation in Mars orbit is only 36% of the
intensity near Earth, requiring larger solar panels to produce the same amount of
electricity. Moons Phobos and Deimos are close to the planet; at 5825 miles altitude,
Phobos is below Mars synchronous orbit. Neither of the two tiny moons has enough
gravity to produce a Martian equivalent of libration points.
#2
Our solar neighborhood is an exciting place. The Solar System is
full of planets, moons, asteroids, comets, minor planets, and
many other exciting objects. Learn about Io, the explosive moon
that orbits the planet Jupiter, or explore the gigantic canyons and
deserts on Mars.

What Is The Solar System?

The Solar System is made up of all the planets that orbit our Sun. In
addition to planets, the Solar System also consists of moons, comets,
asteroids, minor planets, and dust and gas.

Everything in the Solar System orbits or revolves around the Sun. The Sun
contains around 98% of all the material in the Solar System. The larger an
object is, the more gravity it has. Because the Sun is so large, its powerful
gravity attracts all the other objects in the Solar System towards it. At the
same time, these objects, which are moving very rapidly, try to fly away
from the Sun, outward into the emptiness of outer space. The result of the
planets trying to fly away, at the same time that the Sun is trying to pull
them inward is that they become trapped half-way in between. Balanced
between flying towards the Sun, and escaping into space, they spend
eternity orbiting around their parent star.

What is Mercury like?

Mercury is the smallest planet in our solar system. It’s just
a little bigger than Earth’s moon. It is the closest planet to
the sun, but it’s actually not the hottest. Venus is hotter.
Along with Venus, Earth, and Mars, Mercury is one of the
rocky planets. It has a solid surface that is covered with
craters. It has a thin atmosphere, and it doesn’t have any
moons. Mercury likes to keep things simple.
This small planet spins around slowly compared to Earth,
so one day lasts a long time. Mercury takes 59 Earth days
to make one full rotation. A year on Mercury goes by fast.
Because it’s the closest planet to the sun, it doesn’t take
very long to go all the way around. It completes one
revolution around the sun in just 88 Earth days. If you lived
on Mercury, you’d have a birthday every three months!
A day on Mercury is not like a day here on Earth. For us,
the sun rises and sets each and every day. Because
Mercury has a slow spin and short year, it takes a long
time for the sun to rise and set there. Mercury only has
one sunrise every 180 Earth days! Isn't that weird?
What is Venus like?
Even though Venus isn't the closest planet to the sun, it is
still the hottest. It has a thick atmosphere full of the
greenhouse gas carbon dioxide and clouds made of
sulfuric acid. The gas traps heat and keeps Venus toasty
warm. In fact, it's so hot on Venus, metals like lead would
be puddles of melted liquid.
Venus looks like a very active planet. It has mountains and
volcanoes. Venus is similar in size to Earth. Earth is just a
little bit bigger.
Venus is unusual because it spins the opposite direction of
Earth and most other planets. And its rotation is very slow.
It takes about 243 Earth days to spin around just once.
Because it's so close to the sun, a year goes by fast. It
takes 225 Earth days for Venus to go all the way around
the sun. That means that a day on Venus is a little longer
than a year on Venus.
Since the day and year lengths are similar, one day on
Venus is not like a day on Earth. Here, the sun rises and
sets once each day. But on Venus, the sun rises every
117 Earth days. That means the sun rises two times
during each year on Venus, even though it is still the same
day on Venus! And because Venus rotates backwards, the
sun rises in the west and sets in the east.
Just like Mercury, Venus doesn’t have any moons.
What is Earth like?
Our home planet Earth is a rocky, terrestrial planet. It has
a solid and active surface with mountains, valleys,
canyons, plains and so much more. Earth is special
because it is an ocean planet. Water covers 70% of
Earth's surface.
Our atmosphere is made mostly of nitrogen and has plenty
of oxygen for us to breathe. The atmosphere also protects
us from incoming meteoroids, most of which break up in
our atmosphere before they can strike the surface as
meteorites.
Since we live here, you might think we know all there is to
know about Earth. Not at all, actually! We have a lot we
can learn about our home planet. Right now, there are
many satellites orbiting Earth taking pictures and
measurements. This is how we can learn more about
weather, oceans, soil, climate change, and many other
important topics.
What is Mars like?
Mars is a cold desert world. It is half the size of Earth.
Mars is sometimes called the Red Planet. It's red because
of rusty iron in the ground.
Like Earth, Mars has seasons, polar ice caps, volcanoes,
canyons, and weather. It has a very thin atmosphere
made of carbon dioxide, nitrogen, and argon.
There are signs of ancient floods on Mars, but now water
mostly exists in icy dirt and thin clouds. On some Martian
hillsides, there is evidence of liquid salty water in the
ground.
Scientists want to know if Mars may have had living things
in the past. They also want to know if Mars could support
life now or in the future.
What is Jupiter like?
Jupiter is the biggest planet in our solar system. It's similar
to a star, but it never got big enough to start burning. It is
covered in swirling cloud stripes. It has big storms like the
Great Red Spot, which has been going for hundreds of
years. Jupiter is a gas giant and doesn't have a solid
surface, but it may have a solid inner core about the size
of Earth. Jupiter also has rings, but they're too faint to see
very well.
What is Saturn like?
Saturn isn’t the only planet to have rings, but it definitely
has the most beautiful ones. The rings we see are made
of groups of tiny ringlets that surround Saturn. They’re
made of chunks of ice and rock. Like Jupiter, Saturn is
mostly a ball of hydrogen and helium.
When Galileo Galilei saw Saturn through a telescope in
the 1600s, he wasn't sure what he was seeing. At first he
thought he was looking at three planets, or a planet with
handles. Now we know those "handles" turned out to be
the rings of Saturn.
What is Neptune like?
Neptune is dark, cold, and very windy. It's the last of the
planets in our solar system. It's more than 30 times as far
from the sun as Earth is. Neptune is very similar to
Uranus. It's made of a thick soup of water, ammonia, and
methane over an Earth-sized solid center. Its atmosphere
is made of hydrogen, helium, and methane. The methane
gives Neptune the same blue color as Uranus. Neptune
has six rings, but they're very hard to see.
What is Uranus like?
Uranus is made of water, methane, and ammonia fluids
above a small rocky center. Its atmosphere is made of
hydrogen and helium like Jupiter and Saturn, but it also
has methane. The methane makes Uranus blue.
Uranus also has faint rings. The inner rings are narrow
and dark. The outer rings are brightly colored and easier
to see. Like Venus, Uranus rotates in the opposite
direction as most other planets. And unlike any other
planet, Uranus rotates on its side.
#3
Sun's Effect on Earth
Energy from the Sun is very important to the Earth. The Sun warms our planet, heating the surface, the
oceansand the atmosphere. This energy to the atmosphere is one of the primary drivers our weather.
Our climate is also strongly affected by the amount of solar radiation received at Earth. That amount changes
based on the Earth’s albedo, that is how much radiation is reflected back from the Earth’s surface and clouds.

The amount of radiation given off by the Sun changes with solar activity like solar flares or sunspots. Solar
activity is known to vary in cycles, like the 11-yr sunspot cycle (and longer cycles). Some scientists have
wondered if changes in our weather and climate might be linked with short or long term solar cycles. Weather is
the current atmospheric conditions, including temperature, rainfall, wind, and humidity for a given area, while
climate is the general weather conditions over a longer amount of time. This has been an active area of
research for decades. It is an example of the scientific process.

Some scientists tried to find a link between changes in Earth’s weather and solar variability. Although some
scientists reported such correlations, later studies have not been able to find the same result, casting in doubt
or disproving the original studies. Examples include studies of the relationship between the number of sunspots
and changes in wind patterns, or between cosmic rays and clouds.

More researchers have looked at the influence of solar variation on Earth’s climate, again with mixed success.
Changes in sunspot cycles do change the amount of solar radiation given off by the Sun, but only by a little bit.
These changes aren't enough to account for the majority of the warming observed in the atmosphere over the
last half of the 20th century. The only way that climate modelscan match the observed warming of the
atmosphere is with the addition of greenhouse gases. If you would like to learn more about the relationship
between solar variation and climate, visit the Intergovernmental Panel on Climate Change’s Frequently Asked
Questions section of their recent report.

MOON’S EFFECT ON EARTH

The earth has an interesting effect on the moon called as tidal locking.

Ever wondered why this is the only side of the moon we’re able to see?

If the moon was not rotating at all or if it was not tidally attached to the earth, we would be
able to see the other side of the moon, commonly called the ‘dark side’.

Tidal locking results in the Moon rotating about its axis in about the same time it takes to
orbit Earth. Observe the below image. The circle is Earth and shaded object revolving it is
moon. Observe how the ‘left moon’ is tidally locked and the ‘right moon’ is not.
Types of Eclipses
From Earth, we can see 2 types of eclipses – eclipses of the Sun (solar eclipses), and eclipses of
the Moon (lunar eclipses). These occur when the Sun, Earth, and the Moon align in a straight or
almost straight configuration. Astronomers call this a syzygy, from the ancient Greek word syzygia,
meaning to be yoked together or conjoined.
The term eclipse also finds its roots in ancient Greek – it comes from the word ékleipsis, meaning to
fail or to abandon.
Eclipses, solar and lunar, have fascinated scientists and lay people for centuries. In ancient times,
eclipses were seen as phenomena to be feared – many cultures came up with stories and myths to
explain the temporary darkening of the Sun or the Moon. In recent centuries, eclipses have been
sought after by scientists and astronomers who use the events to study and examine our natural
world.
Eclipses and Transits 1900-2199
Solar Eclipses

Infographic: Types of solar eclipses. Click image for full version.

Solar eclipses can only occur during a New Moon when the Moon moves between Earth and the
Sun and the 3 celestial bodies form a straight line: Earth–Moon–Sun.
There are between 2 and 5 solar eclipses every year.
There are 3 kinds of solar eclipses: total, partial, and annular. There is also a rare hybrid that is a
combination of an annular and a total eclipse.

Total Solar Eclipses

A total solar eclipse occurs when the Moon completely covers the Sun, as seen from Earth. Totality
during such an eclipse can only be seen from a limited area, shaped like a narrow belt, usually about
160 km (100 mi) wide and 16,000 km (10,000 mi) long. Areas outside this track may be able to see a
partial eclipse of the Sun.
Looking at a solar eclipse without any protective eyewear can severely harm your eyes. The only
way to safely watch a solar eclipse is to wear protective eclipse glasses or to project an image of the
eclipsed Sun using a DIY Pinhole Projector.
Total Solar Eclipses 1900-2199

Partial Solar Eclipses

A partial solar eclipse happens when the Moon only partially covers the disk of the Sun.
All Partial Solar Eclipses 1900-2199

Annular Solar Eclipses

An annular solar eclipse occurs when the Moon appears smaller than the Sun as it passes centrally
across the solar disk and a bright ring, or annulus, of sunlight remains visible during the eclipse.
All Annular Solar Eclipses 1900-2199

Hybrid Solar Eclipses

A hybrid solar eclipse is a rare form of solar eclipse, which changes from an annular to a total solar
eclipse, and vice versa, along its path.

Lunar Eclipses

Partial lunar eclipse in 2008 seen in Germany.©iStockphoto.com/cinoby

The Moon does not have its own light. It shines because its surface reflects the Sun's rays. A lunar
eclipse occurs when Earth comes between the Sun and the Moon and blocks the Sun's rays from
directly reaching the Moon. Lunar eclipses only happen at Full Moon.
All Lunar Eclipses 1900-2199
There are 3 kinds of lunar eclipses: total, partial, and penumbral.

Total Lunar Eclipses

A total lunar eclipse occurs when Earth's umbra – the central, dark part of its shadow – obscures all
of the Moon's surface.
All Total Lunar Eclipses 1900-2199

Partial Lunar Eclipses

A partial lunar eclipse can be observed when only part of the Moon's surface is obscured by Earth’s
umbra.
All Partial Lunar Eclipses 1900-2199
Penumbral Lunar Eclipses
A penumbral lunar eclipse happens when the Moon travels through the faint penumbral portion of
Earth’s shadow.
All Penumbral Lunar Eclipses 1900-2199

A transit of Mercury. (Illustration not to scale.)

Planet Transits
When a planet comes between Earth and the Sun, it is called a transit. The only 2 planets that can
be seen transiting the Sun from Earth are Venus and Mercurybecause they are the only planets
which orbit inside Earth's orbit.
From 2000–2199, there will be 14 transits of Mercury. However, Venus transits are even rarer with
only 2 this century, in 2004 and 2012.

Why does the Moon seem to change shape?

The half of the Moon that points toward the Sun looks bright because it is lit by sunlight.
The Moon appears to change shape because we see different amounts of the lit part as the
Moon orbits Earth. When the Moon is between Earth and the Sun, the lit side is hidden from
us. As it moves around Earth, more and more of the lit side comes into view. Then it begins
to disappear again.
#4 Exploration of Mars
From Wikipedia, the free encyclopedia
Not to be confused with Colonization of Mars.
The exploration of Mars is the study of Mars by spacecraft. Probes sent from Earth, beginning in
the late 20th century, have yielded a dramatic increase in knowledge about the Martian system,
focused primarily on understanding its geology and habitability potential.[1]

Current status[edit]
Engineering interplanetary journeys is complicated and the exploration of Mars has experienced a
high failure rate, especially the early attempts. Roughly two-thirds of all spacecraft destined for Mars
failed before completing their missions and some failed before their observations could begin. Some
missions have met with unexpected success, such as the twin Mars Exploration Rovers, which
operated for years beyond their specification.[2] On 24 October 2016, two scientific rovers were on
the surface of Mars beaming signals back to Earth (Opportunity of the Mars Exploration
Rover mission and Curiosity of the Mars Science Laboratory mission), with six orbiters surveying the
planet: Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, Mars Orbiter Mission, MAVEN,
and the Trace Gas Orbiter, which have contributed massive amounts of information about Mars.
No sample return missions have been attempted for Mars and an attempted return mission for
Mars' moon Phobos (Fobos-Grunt) failed.[3]
On 24 January 2014, NASA reported that current studies on the planet Mars by
the Curiosity and Opportunity rovers will search for evidence of ancient life, including
a biospherebased on autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as
well as ancient water, including fluvio-lacustrine environments (plains related to
ancient riversor lakes) that may have been habitable.[1][4][5][6] The search for evidence
of habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a
primary NASA objective.[1]

PAST AND CURRENT MISSIONS

Starting in 1960, the Soviets launched a series of probes to Mars including the first intended flybys
and hard (impact) landing (Mars 1962B).[10] The first successful fly-by of Mars was on 14–15 July
1965, by NASA's Mariner 4.[11] On November 14, 1971 Mariner 9 became the first space probe to
orbit another planet when it entered into orbit around Mars.[12] The amount of data returned by probes
increased dramatically as technology improved.[10]
The first to contact the surface were two Soviet probes: Mars 2 lander on November 27 and Mars
3 lander on December 2, 1971—Mars 2 failed during descent and Mars 3 about twenty seconds
after the first Martian soft landing.[13] Mars 6 failed during descent but did return some corrupted
atmospheric data in 1974. [14] The 1975 NASA launches of the Viking program consisted of two
orbiters, each with a lander that successfully soft landed in 1976. Viking 1 remained operational for
six years, Viking 2 for three. The Viking landers relayed the first color panoramas of Mars.[15]
The Soviet probes Phobos 1 and 2 were sent to Mars in 1988 to study Mars and its two moons, with
a focus on Phobos. Phobos 1 lost contact on the way to Mars. Phobos 2, while successfully
photographing Mars and Phobos, failed before it was set to release two landers to the surface of
Phobos.[16]
Roughly two-thirds of all spacecraft destined for Mars have failed without completing their missions,
and it has a reputation as a difficult space exploration target.[17]
Missions that ended prematurely after Phobos 1 & 2 (1988) include (see Probing difficulties section
for more details):

 Mars Observer (launched in 1992)

 Mars 96 (1996)
 Mars Climate Orbiter (1999)
 Mars Polar Lander with Deep Space 2 (1999)
 Nozomi (2003)
 Beagle 2 (2003)
 Fobos-Grunt with Yinghuo-1 (2011)
 Schiaparelli lander (2016)
Following the 1993 failure of the Mars Observer orbiter, the NASA Mars Global Surveyor achieved
Mars orbit in 1997. This mission was a complete success, having finished its primary mapping
mission in early 2001. Contact was lost with the probe in November 2006 during its third extended
program, spending exactly 10 operational years in space. The NASA Mars Pathfinder, carrying a
robotic exploration vehicle Sojourner, landed in the Ares Vallis on Mars in the summer of 1997,
returning many images.[18]
Phoenix landed on the north polar region of Mars on May 25, 2008.[19] Its robotic arm dug into the
Martian soil and the presence of water ice was confirmed on June 20, 2008.[20][21] The mission
concluded on November 10, 2008 after contact was lost.[22] In 2008, the price of transporting material
from the surface of Earthto the surface of Mars was approximately US$309,000 per kilogram.[23]
Rosetta came within 250 km of Mars during its 2007 flyby. [24] Dawn flew by Mars in February 2009
for a gravity assist on its way to investigate Vesta and Ceres. [25]

International Space Station

From Wikipedia, the free encyclopedia
The International Space Station (ISS) is a space station, or a habitable artificial satellite, in low
Earth orbit. Its first component launched into orbit in 1998, and the ISS is now the largest human-
made body in low Earth orbit and can often be seen with the naked eye from Earth.[8][9] The ISS
consists of pressurised modules, external trusses, solar arrays, and other components. ISS
components have been launched by Russian Proton and Soyuz rockets, and American Space
Shuttles.[10]
The ISS serves as a microgravity and space environment research laboratory in which crew
members conduct experiments in biology, human biology, physics, astronomy, meteorology,
and other fields.[11][12][13] The station is suited for the testing of spacecraft systems and equipment
required for missions to the Moon and Mars.[14] The ISS maintains an orbit with an altitude of
between 330 and 435 km (205 and 270 mi) by means of reboost manoeuvres using the engines of
the Zvezda module or visiting spacecraft. It completes 15.54 orbits per day.[15]
The ISS is the ninth space station to be inhabited by crews, following the Soviet and later
Russian Salyut, Almaz, and Mirstations as well as Skylab from the US. The station has been
continuously occupied for 16 years and 304 days since the arrival of Expedition 1 on 2 November
2000. This is the longest continuous human presence in low Earth orbit, having surpassed the
previous record of 9 years and 357 days held by Mir. The station is serviced by a variety of visiting
spacecraft: the Russian Soyuz and Progress, the American Dragon and Cygnus, the Japanese H-II
Transfer Vehicle,[16] and formerly the Space Shuttleand the European Automated Transfer Vehicle. It
has been visited by astronauts, cosmonauts and space tourists from 17 different nations.[17]
After the US Space Shuttle programme ended in 2011, Soyuz rockets became the only provider of
transport for astronauts at the International Space Station, and Dragon became the only provider of
bulk cargo return to Earth (called downmass). Soyuz has very limited downmass capability.
The ISS programme is a joint project among five participating space
agencies: NASA, Roscosmos, JAXA, ESA, and CSA.[16][18]The ownership and use of the space
station is established by intergovernmental treaties and agreements.[19] The station is divided into two
sections, the Russian Orbital Segment (ROS) and the United States Orbital Segment (USOS), which
is shared by many nations. As of January 2014, the American portion of ISS is being funded until
2024.[20][21][22] Roscosmos has endorsed the continued operation of ISS through 2024[23] but has
proposed using elements of the Russian Orbital Segment to construct a new Russian space station
called OPSEK.[24]
On 28 March 2015, Russian sources announced that Roscosmos and NASA had agreed to
collaborate on the development of a replacement for the current ISS.[25][26] NASA later issued a
guarded statement expressing thanks for Russia's interest in future co-operation in space
exploration but fell short of confirming the Russian announcement.[27][28]