Astronomy

The Solar SystemThe Solar System consists of the Sun orbited by eight planets and their moons, some dwarf planets along with many asteroids and comets.

PlanetsA planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has cleared its neighbourhood of smaller objects around its orbit.

Based on this, International Astronomical Union’s definition of 2006, there are only eight planets in orbit around the Sun.

In order of distance from the Sun:

MercuryVenus

EarthMars

Jupiter

Saturn

Neptune

Uranus

Dwarf PlanetsA dwarf planet is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity. but has not cleared its neighbouring region of other similar bodies.As of 2011 there are five dwarf planets in the Solar System.Between Mars and Jupiter:

CeresBeyond Neptune:

Pluto, Haumea, Makemake

and Eris (the largest)

AsteroidsAn asteroid is a celestial body orbiting the Sun that is not massive enough to be spherical as a result of its own gravity.

Most asteroids are found between the orbits of Mars and Jupiter – a region called ‘The Asteroid Belt’.

There are about 750 000 asteroids larger than 1km across.

A few, called ‘Near Earth Asteroids’ can pass very close to the Earth.

Asteroid Vesta – image taken on July 17th 2011 by

the Dawn spacecraft

MoonsA moon orbits a planet.

Planet Moons (2011)

Mercury 0

Venus 0

Earth 1

Mars 2

Jupiter 64

Saturn 62

Uranus 27

Neptune 13The Earth’s only natural satellite

Note: A number of dwarf planets and asteroids also have moons, for example Pluto has three moons.

This is the time taken for a planet to complete one orbit around the Sun.

It increases with a planets distance from the Sun.

Mercury 88 days

Venus 225 days

Earth 1 year

Mars 2 years

Jupiter 12 years

Saturn 29 years

Neptune 165 years

Uranus 84 years

Time period (T )

Gravitational attractionThe force of gravity is responsible for the orbits of planets, moons, asteroids and comets.

In 1687 Sir Isaac Newton stated that this gravitational force:

- is always attractive- would double if either the mass of Sun or

the planet was doubled- decreases by a factor of 4 as the distance

between the Sun and a planet doubles.

Gravitational field strength (g)This is a way of measuring the strength of gravity.

The gravitational field strength is equal to the gravitational force exerted per kilogram.

Near the Earth’s surface, g = 10 N/kg

In most cases gravitational field strength in N/kg is numerically equal to the acceleration due to gravity in m/s2, hence they both use the same symbol ‘g’.

Gravitational field strength (g) varies from planet to planet.

It is greatest near the most massive objects.

Planetary orbits

The orbits of the planets are slightly squashed circles (ellipses) with the Sun quite close to the centre.

The Sun lies at a ‘focus’ of the ellipse

Planets move more quickly when they are closer to the Sun.

faster slower

The above diagram is exaggerated!

What would happen to an orbit without gravity

As the red planet moves it is continually pulled by gravity towards the Sun.

Gravity therefore causes the planet to move along a circular path – an orbit.

If this gravity is removed the planet will continue to move along a straight line at a tangent to its original orbit.

CometsA comet is a body made of dust and ice that occupies a highly elongated orbit.

When the comet passes close to the Sun some of the comet’s frozen gases evaporate. These form a long tail that shines in the sunlight.

Comets are most visible and travel quickest when close to the Sun.

Comets are approximately 1-30km in diameter.

Halley’s CometThis is perhaps the most famous comet.

It returns to the inner Solar System every 75 to 76 years. It last appeared in 1986 and is due to return in 2061.

It has been observed since at least 240BC. In 1705 Edmund Halley correctly predicted its reappearance in 1758.

Orbital speed (v)

orbital speed = (2π x orbital radius) / time period

v = (2π x r ) / T

orbital speed in metres per second (m/s)

orbital radius in metres (m)

time period in seconds (s)

Communication satellitesThese are usually placed in geostationary orbits so that they always stay above the same place on the Earth’s surface.

VIEW FROM ABOVE THE NORTH POLE

Geostationary satellites must have orbits that:- take 24 hours to complete- circle in the same direction as the Earth’s spin- are above the equator- orbit at a height of about 36 000 km

Uses of communication satellites include satellite TV and some weather satellites.

The Milky Way

The Milky Way is the name of our galaxy.

From Earth we can see our galaxy edge-on. In a very dark sky it appears like a ‘cloud’ across the sky resembling a strip of spilt milk.

A very dark sky is required to see the Milky Way this clearly

GalaxiesGalaxies consist of billions of stars bound together by the force of gravity.

There are thought to be at least 200 billion galaxies in our Universe each containing on average 2 billion stars.

Types of galaxy-spiral,elliptical.irregular

The Andromeda Galaxy

Question 1

Calculate the orbital speed of the Earth around the Sun. (Earth orbital radius = 150 million km)v = (2π x r ) / T

= (2π x [150 000 000 km] ) / [1 year]but 1 year = (365 x 24 x 60 x 60) seconds= 31 536 000 sand 150 000 000 km = 150 000 000 000 metres

v = (2π x [150 000 000 000] ) / [31 536 000]orbital speed = 29 900 m/s

Question 2

Calculate the orbital speed of the Moon around the Earth. (Moon orbital radius = 380 000 km; orbit time = 27.3 days)v = (2π x r ) / T

= (2π x [380 000 km] ) / [27.3 days]but 27.3 days = (27.3 x 24 x 60 x 60) seconds= 2 359 000 sand 380 000 km = 380 000 000 metres

v = (2π x [380 000 000] ) / [2 359 000]orbital speed = 1 012 m/s

Question 3Calculate the orbital speed of the ISS (International Space Station) around the Earth. (ISS orbital height = 355 km; orbit time = 91 minutes; Earth radius = 6 378 km)

The orbit radius of the ISS = (355 + 6 378) km = 6 733 kmv = (2π x r ) / T

= (2π x [6 733 km] ) / [91 minutes]but 91 minutes = (91 x 60) seconds= 5 460 sand 6 733 km = 6 733 000 metres

v = (2π x [6 733 000] ) / [5 460]orbital speed = 7 748 m/s

Question 4Calculate the orbital time of a satellite that has a speed of 3 075 m/s and height above the earth of 35 906 km. (Earth radius = 6 378 km)

The orbit radius of the satellite = (35 576 + 6 378) km = 42 284 kmv = (2π x r ) / Tbecomes: T = (2π x r ) / v= (2π x [42 284 km] ) / [3 075 m/s]

but 42 284 km = 42 284 000 metresT = (2π x [41 954 000 ] ) / [3 075 ]orbital time = 86 400 seconds= 1440 minutes= 24 hours