ENABLING & SUPPORT

Types of orbits
30/03/2020 894363 VIEWS 2429 LIKES

ESA / Enabling & Support /

Space Transportation

In brief
Our understanding of orbits, first established

by Johannes Kepler in the 17th century,

remains foundational even after 400 years.

Today, Europe continues this legacy with a

family of rockets launched from Europe’s

Spaceport into a wide range of orbits around

Earth, the Moon, the Sun and other planetary

bodies.

In-depth

What is
an orbit?
An orbit is the — Mass affects orbiting
curved path that an bodies

object in space (like

a star, planet, moon, asteroid or spacecraft)

follows around another object due to gravity.

Gravity pulls objects with mass towards each

other. If this attraction brings them together

with enough momentum, they can start to

orbit each other.

Objects of similar mass orbit each other with

neither object at the centre, while small

objects orbit around larger objects. In our

Solar System, the Moon orbits Earth and the

Earth-Moon system orbits the Sun, but that

does not mean the larger object remains

completely still. Because of gravity, Earth is

pulled slightly from its centre by the Moon

(which is why tides form in our oceans), and

our Sun is pulled slightly from its centre by

Earth and the other planets.

The early formation of our Solar System

about 4.6 billion years ago began with an

enormous swirling cloud of dust and gas, held

together by gravity. Its core became so dense

and massive that it collapsed under its own

gravity, igniting to become our Sun. Around

this early star, the remaining material from the

interstellar cloud continued to spin around the

Sun with great momentum, gradually

flattening due to centrifugal forces into a disc

called the solar nebula. The huge Sun at the

cloud’s core kept these bits of gas, dust and

ice in orbit around it, shaping it into a kind of

ring around the Sun.

Eventually, these particles started to settle

and clump together (or ‘coalesce’), growing

ever larger like rolling snowballs until they

formed what we now see as planets, comets

and asteroids, and moons – although not our

own. That the planets all formed together

from this cloud of rotating dust explains why

they all orbit the Sun in the same direction, in

roughly the same plane.

— Reaching orbit

When rockets launch our satellites, they put

them into orbit in space. There, gravity keeps

the satellite in its required orbit – in the same

way that gravity keeps the Moon in orbit

around Earth.

This happens in a way that is similar to

throwing a ball out of the window of a tall

tower – to get the ball going, you need to first

give it a ‘push’ by throwing it, making the ball

fall towards the ground on a curved path.

Whilst it is your throw that gives the ball its

initial speed, it is gravity alone that keeps the

ball moving towards the ground once you let

go.

Similarly, a satellite is put into orbit by being

placed a few hundred or even thousands of

kilometres above Earth’s surface, where it is

no longer in danger of being decelerated by

the denser parts of Earth’s atmosphere. From

here, as if at the top of a very tall tower, it is

then given a ‘push’ by the rocket’s engines to

make it begin its orbit.

As shown in the figure, throwing something off

the ‘tower’ will make it fall on a curved path

toward the ground – but a really powerful

throw will impart so much speed to the object

that the ground starts to curve away before

your object reaches it. Your object will fall

‘towards’ Earth indefinitely, causing it to circle

the planet repeatedly. Congratulations! You

have reached orbit.

In space, there is no air and therefore no air

friction, so gravity lets a satellite orbit around

Earth with almost no further assistance.

Putting satellites into orbit enables us to use

technologies for telecommunication,

navigation, weather forecasting, emergency

response and astronomy observations.

Launch to
orbit
The role of a
— Artist's view of
launcher is, Europe's launcher
therefore, first to family

pass through the

atmosphere with its cargo, and then to

provide the horizontal velocity push

necessary to get into orbit.

Europe’s rockets operate from Europe’s

Spaceport in French Guiana, and each

mission delivers one or more satellites –

payloads – into carefully designated orbits.

The choice of which launch vehicle to use

depends primarily on the mass of the

payload, but also on how far from Earth it

needs to go. A heavy payload or a high-

altitude or interplanetary orbit requires more

power to fight Earth’s gravity than a lighter

payload and a lower altitude.

Ariane 6 is Europe’s most powerful rocket,

capable of lifting one, two, or multiple

satellites into their required orbits. Depending

on which orbit Ariane 6 is going to, it can

launch approximately 21.5 tonnes into space

– that is 21 500 kg, about the mass of a grey

whale.

Vega-C is smaller than Ariane 6, capable of

launching roughly 2.2 tonnes at a time,

making it an ideal launch vehicle for many

scientific and Earth observation missions.

Both Ariane 6 and Vega-C can deploy

multiple satellites at a time.

Ariane 6 and Vega-C are Europe’s new

generation of rockets, following on from

Ariane 5 and Vega. They are more flexible,

have greater payload capacity and will

expand Europe’s launch capacity, able also to

deliver payloads to several different orbits in a

single flight – like a bus making multiple

stops.

Types of orbit
Upon launch, a satellite or spacecraft is

usually placed in one of several specific orbits

around Earth, or it may be sent on an

interplanetary journey, meaning it no longer

orbits Earth, but instead orbits the Sun,

perhaps until it arrives at another final

destination, like Mars, Jupiter, or even a

comet.

Many factors influence the choice of an

optimal orbit for a space mission, all

depending on the mission’s objectives.

Geostationary orbit (GEO)

Low Earth orbit (LEO)

Polar orbit (PO)

Sun-synchronous orbit (SSO)

Medium Earth orbit (MEO)

Highly eccentric orbit (HEO)

Transfer orbits and geostationary

transfer orbit (GTO)

Lagrange points (L-points)

Heliocentric orbit

— Geostationary orbit

Geostationary orbit
(GEO)
Satellites in geostationary orbit (GEO) fly

above Earth’s equator, moving from west to

east, exactly matching Earth’s rotation: taking

23 hours 56 minutes and 4 seconds to

complete one full orbit, i.e. the duration of a

sidereal day. This makes GEO satellites

appear ‘stationary’ over one fixed spot. To

keep pace with Earth’s spin, they travel at

about 3 km per second at an altitude of 35

786 km, much farther than most satellites.

GEO is ideal for satellites that need to stay

fixed above a specific location, such as

telecommunication satellites, allowing

antennas on Earth to stay in a constant

position, always pointing at the satellite. It is

also valuable for weather satellites, enabling

continuous monitoring specific regions to

track evolving weather patterns over times

and see how weather trends emerge.

Satellites in GEO can cover a large portion of

Earth; just three evenly spaced satellites can

provide near-global coverage. Being far away

allows these satellites a view of larger areas,

similar to how you see more of a map

standing a metre away from it compared to

with your nose up close to it. To see all of

Earth at once from GEO, far fewer satellites

are needed than at lower altitudes.

ESA’s European Data Relay System (EDRS)

programme consists of a constellation of GEO

satellites that relay information and data

between non-GEO satellites, spacecraft and

ground stations that are otherwise unable to

permanently transmit or receive data. This

means Europe can always stay connected

and online, even with satellites in lower orbits

that have reduced visibility from the ground.

Low Earth
orbit
(LEO)
A low Earth orbit

(LEO) is, as the

name suggests, an — Low Earth orbit

orbit that is

relatively close to Earth’s surface. LEO is

considered to be under altitudes of 2000 km,

this upper limit a consequence of the Van

Allen belts above and the harsh environment

they create. The lower limit for how low a

satellite can fly is down to the impact of

Earth’s atmosphere. Generally, satellites do

not fly below 180 km for this reason – low

compared to most orbits, but still very far

above Earth’s surface.

By comparison, most commercial aeroplanes

do not fly higher than approximately 12 km,

so even the lowest LEO is more than ten

times higher.

Unlike satellites in GEO that orbit along

Earth’s equator, LEO satellites can have their

orbital planes tilted at various angles.

LEO’s close proximity to Earth is useful for

several reasons. It is ideal for satellite

imaging, as its closeness allows for higher

resolution images. The International Space

Station (ISS) also orbits here as the shorter

distance makes it easier for astronauts to

reach. Satellites in low-Earth orbit travel at

approximately 7.8 km per second with respect

to Earth, completing an orbit in about 90

minutes. This means the ISS circles Earth

roughly 16 times a day.

As such, communications satellites in LEO

often work as part of a constellation, a

networking of several of the same or similar

satellites working together to provide

continuous coverage in a ‘net’ around Earth.

The same can apply to observation or

navigation constellations.

Ariane 5 carried its heaviest 20-tonne

payload, the Automated Transfer Vehicle

(ATV), five times to the ISS. In the future,

Ariane 6 may take over this task and launch

supplies to space stations in Earth or lunar

orbit.

Polar orbit
(PO)
Polar orbits are a

type of low Earth

orbit, typically

between 200 to — Polar and Sun-

synchronous orbit
1000 km in altitude.

Satellites in polar orbits usually travel around

Earth from, roughly, one pole to the other,

rather than from west to east. They do not

need to pass exactly over the North and

South Poles; a deviation of 10 degrees is still

classed as a polar orbit.

Polar orbits are particularly useful for global

Earth coverage, as satellites orbiting ‘up’ and

‘down’ Earth’s surface can see every inch of

the planet over time as it also rotates below.

Sun synchronous orbit

(SSO)
Sun-synchronous orbit (SSO) is a particular

kind of polar orbit in which satellites are in

sync with the Sun. Matching Earth’s rotation

around the Sun, they always appear in the

same position relative to our star. This means

they pass over the same spot on Earth at the

same local time every day – for example

passing over the city of Paris daily at noon.

This consistency allows for the accurate

monitoring of changes over days, weeks,

months or even years, as images are more

comparable in terms of light and shadows.

Scientists use SSO and this ability to

compare like-for-like images over time to

investigate how weather patterns emerge,

help predict extreme weather events, monitor

emergencies like forest fires or flooding and

to accumulate data on long-term problems

like deforestation or rising sea levels.

Often, satellites in SSO are synchronised so

that they are in constant dawn or dusk,

constantly riding a sunset or sunrise and

ensuring as consistent as possible

illumination/shadows. A satellite in a Sun-

synchronous orbit usually flies at an altitude

of between 600 to 800 km. At 800 km, it will

travel at a speed of approximately 7.5 km per

second.

Medium Earth orbit

(MEO)
Medium Earth orbit (MEO) covers a wide

range of altitudes anywhere between LEO –

usually over the top of the Van Allen belts –

and GEO. Like LEO, satellites in MEO do not

need to follow specific paths around Earth

and the orbit is used by a variety of satellites

for many different purposes.

MEO is very commonly used by navigation

satellites, like the European Galileo system

(pictured). Galileo provides navigation

services across the world, helping with

everything from tracking large jumbo jets to

giving directions on your smartphone. Galileo

consists of a constellation of satellites that

provide simultaneous coverage over large

areas of the world.

Transfer
orbits and

— Once the satellite

reaches the furthest
point from Earth in
Geostationary
transfer orbit (green),
it fires its engines in
such a way that it
enters into a circular
GEO orbit (blue)

Geostationary Transfer
Orbit (GTO)
Transfer orbits are a special kind of orbit used

to get from one orbit to another. When

satellites are launched from Earth by rockets

like Ariane 6, they are not always placed

directly into their final orbit. Instead, they are

often put into an initial transfer orbit where,

using energy from onboard motors, the

satellite or spacecraft can move from one

orbit to another.

For example, to reach a high-altitude orbit like

GEO, a satellite doesn’t need the launch

vehicle to take it all the way. Instead, the

satellite is placed in a special type of transfer

orbit called a geostationary transfer orbit

(GTO), which serves as a shortcut.

Orbits have different shapes or

‘eccentricities’ – a measure of how circular

(round) or elliptical (squashed) they are. In a

perfectly round orbit, a satellite is always the

same distance from Earth’s surface, but in a

highly eccentric orbit, the satellite moves

closer and farther from Earth as it travels.

In transfer orbits, satellites or spacecraft use

their engines to go from an orbit of one

eccentricity to another, which puts it on track

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