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Interdisciplinary "Space"
Studying the Universe
for IJSO training course
1
Content
1.
Solar system – an overview
2.
Order of the planets
3.
Key features of each planet
4.
Asteroids, comets and meteoroids
5.
Stars and their colors
6.
Constellations
7.
Galaxy
8.
Space exploration
9.
Scale model of planets
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1. Solar system
• Sun: its mass is about 300,000 times more massive
than the Earth. Its radius is 700,000 km, about 110
times that of the Earth.
• Energy source: thermonuclear reactions (熱核反應)at
the core.
• Its atmosphere is divided into 3 layers:
• Photosphere (~ 500 km thick) (光球層)
• Chromosphere (色球層)
• Corona (日冕)
3
1. Core
2. Radiative zone
3. Convective zone
4. Photosphere
5. Chromosphere
6. Corona
7. Sunspot
8. Granules
9. Prominence
(Wikimedia Commons)
4
• Sunspots (太陽黑子): cool, dark areas of the solar
surface, each consists of a darker, cooler (~ 4,000 K)
region called umbra (本影), surrounded by a less
cool region called penumbra (半影).
A large group of sunspots in
year 2004. The grey area
around the spots can be
seen very clearly, as well as
the granulation of the sun's
surface. (Wikimedia
Commons)
5
• Planets (行星): 8 planets and their
satellites
• lie close to a common plane.
• Planets move in nearly circular orbits around
the sun in counter-clockwise sense as seen
from “above”.
• The average distance between the sun and the
earth is about 1.5  1011 m, which is also called
1 AU (Astronomical unit).
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• Self-rotation is also in the counter-clockwise
sense as seen from “above”, except for Venus
and Uranus.
• Orbits of planets are not evenly spaced distances between successive planets increase
with their distances from the sun.
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(Wikimedia Commons)
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Scale model of planets
• Earth: a grain of table salt (0.3 mm in diameter)
• Moon: A speck of pepper 1 cm away
• Sun: A plum 4 m away
• Mercury, Venus, Mars: grains of salt
• Jupiter: Apple seed 20 m from the sun
• Saturn: Smaller apple seed 36 m from the sun
• Uranus: lighter than salt grain
• Neptune: lighter than salt grain, 115 m from the sun
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• Dwarf planets (矮行星): “Minor” planets.
The first three members are
• Ceres (穀神星) --- in the Asteroid Belt
• Pluto (冥王星)
• Eris [formerly known as 2003 UB313 or Xena
(齊娜)]
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The orbit of Eris (blue) compared to those of Saturn, Uranus, Neptune, and
Pluto (white/grey). The arcs below the ecliptic are plotted in darker colours,
and the red dot is the Sun. The diagram on the left is a polar view while the
diagrams on the right are different views from the ecliptic.
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• Small solar-system bodies: include
• Asteroids (小行星): Most can be found in the
Asteroid belt (小行星帶) that lies between
the orbits of Mars and Jupiter.
• Comets (彗星): “Dirty snow balls” moving in
highly elliptical orbits around the sun
In the solar system, there are the sun, the planets, the
dwarf planets and small solar-system bodies.
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Universal gravitation (萬有引力)
• In hammer throw (投鏈球),
the tension in the chain
keeps the ball moving
around a centre. Without
the tension, the ball will fly
straight away.
(Wikimedia Commons)
13
• The gravitational force from the sun is just like the
tension in an invisible chain that keeps a planet in
its orbit around the sun.
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• Inverse square law: The attractive force (F)
between any two bodies is directly proportional to
the product of their masses (M1 and M2) and is
inversely proportional to the square of their
separation (r).
GM1M 2
F 
2
r
G = 6.674  10-11 Nm2kg-2
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Circular motion (圓周運動)
• An object is moving in a
circle of radius r. Its
speed is v.
• What is the
acceleration of the
object?
r
v
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Time for the particle to travel
from A to B is given by
 1 r
t  2r 
 
2 v
v
To find the acceleration, we
have to know the change in
velocity.
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  
vc  2v sin 

 2 
Magnitude of acceleration a is then
vc v 2 2
  
a
 
sin 

t r   2 
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To find the instantaneous acceleration at A, we
let  tend to 0. We also note that sin(x) = x
when x is very small, hence
v 2
   v 2  v
a 
sin 


 
r   2  r  2
r
2
Acceleration is
2
2
v2
a
r
Direction: Perpendicular to the velocity and
towards the centre, hence centripetal.
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Example 1
• It is given that the gravitational acceleration on
the Earth is 9.81 ms-2 and the radius of the
Earth is 6373 km.
• Find the mass of the Earth.
Solution:
Let m be an object on the Earth, M the mass of
the Earth, R the radius of the Earth, the weight
of the object is
GmM
W
R2
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Hence
W GM
g
 2
m
R
gR 2
M
G
9.81 (6373000) 2

kg
11
6.674  10
 5.97  10 24 kg
From table: 5.97  1024 kg
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Example 2
• It is given that the mean radius R of the Earth’s
orbit is 149600000 km. Mass of the sun M is
1.98911030 kg. Find the period of revolution of
the Earth around the sun.
Solution:
The centripetal acceleration of the Earth is caused
by the gravitational attraction from the sun.
v 2 F GM
  2
R m
R
v: speed of the Earth
m: mass of the Earth
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GM
v
R
The period T is therefore
2R
R
T
 2
v
GM
3
(149600000000) 3
 2
s
11
30
6.674 10 1.989110
 3.16 107 s
 365.26 days
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2. Order of the planets
Radius
Mass/Earth
Rotation
Period
Orbital Radius
Revolution
Period
No. of
Satellites
Mercury
2439 km
0.055
59 days
57.9  106 km
88 days
0
Venus
6052 km
0.815
244 days
108.2  106km
224.7 days
0
Earth
6378 km
1
1 day
149.6  106km
365.2 days
1
Mars
3397 km
0.107
24.7 hours
227.9  106km
1.88 yrs
2
Jupiter
71492 km
317.8
9.9 hours
778.6  106km
11.8 yrs
63
Saturn
60268 km
95.2
10.7 hours
1433.5  106km
29.4 yrs
56
Uranus
25559 km
14.5
17.2 hours
2872.5  106km
83.8 yrs
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Neptune
24764 km
17.2
16.1 hours
4495.1  106km
164 yrs
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Two kinds of planets: Terrestrial (類地行星)
• Mercury, Venus, Earth, Mars, all lie in the inner solar
system
• Relatively dense (~3-5 g cm-3), with cores of iron and
nickel surrounded by a mantle of dense rocks.
• Small in size and mass
 weak gravity
 have a few satellites (e.g., one for Earth, two for
Mars) and thin atmospheres, no ring systems
• Their surfaces are scarred with craters.
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Two kinds of planets: Jovian (類木行星)
 Jupiter, Saturn, Uranus, Neptune, all lie in the
outer solar system
 Gaseous-like, mainly made up of hydrogen and
helium, low-density (1 g cm-3)
 They do not have solid surfaces, but have thick
liquid layers inside, possibly with small rocky core
of Earth’s size.
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 Large in size and mass
 strong gravity
 all have ring systems (光環系統), many
satellites and thick atmospheres of hydrogen,
high atmospheric pressure and a lot of
weather activities.
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3. Key features of each planet
A. Mercury (水星)
• Too hot and gravity too weak to
hold a thick atmosphere.
• Results:
– retains a lot of craters
– No thick atmosphere to retain
heat, large temperature
difference between day and
night (-173oC – 430oC)
(NASA)
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• Mercury in fact has a thin
layer of atmosphere, which
is mainly made up of
sodium and a little helium.
The atmospheric pressure
is almost zero. The
presence of gaseous
sodium means the
temperature is high enough
to allow sodium in rock be
released. This is expected
as Mercury is so near the
Sun.
(NASA)
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B. Venus (金星 )
(NASA)
Rotation of Venus
• Firstly, its self-rotation is in the clockwise sense.
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• Secondly, the axis of
rotation is almost
perpendicular to the
orbital plane. (For the
Earth, the rotational
axis tilts 23.5o.) As a
result, there is no
seasonal change on
Venus.
(NASA)
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The atmosphere of Venus
• Venus has a thick atmosphere. The pressure
is 90 times that of the Earth. The atmosphere
consists of 90% of CO2 , 3% of N2 , and
some SO2 . The whole planet is completely
covered by clouds made up of sulphuric acid
(H2SO4). As a result, the rain on Venus is
acidic.
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• Much carbon dioxide
 Greenhouse effect (溫室效應) : CO2
traps the heat of solar radiation
 very hot surface (470C); the atmosphere
is full of vapour of chemical compounds.
A schematic representation
of the exchanges of energy
between outer space, the
Earth's atmosphere, and the
Earth surface. The ability of
the atmosphere to capture
and recycle energy emitted
by the Earth surface is the
defining characteristic of the
greenhouse effect.
(Wikimedia Commons)
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Volcano Eista
(NASA)
Crator Cunitz:
diameter ~ 48.5 km
(NASA)
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C. Mars (火星)
• Like Earth, the axis of
rotation tilts 24o.
Hence, there are
seasonal changes on
Mars.
• Mars looks red
because its soil
contains minerals of
iron (like rust).
(NASA)
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The atmosphere of Mars
• Mars has a mass less than 11% of Earth’s, its
gravity is weak
• the atmosphere was much denser billions of years ago, but
volatile gases escaped, leaving a thin atmosphere (1% of
Earth’s). The chemical composition is mainly carbon
dioxide (95%) and nitrogen (3%).
• Long ago water was dissociated by the solar
radiation (unlike the earth, Mars has no ozone layer
to shield the solar ultraviolet radiation)
• no liquid water on surface, a little water combined with
minerals in soil; polar caps (極冠) contain layers of frozen
CO2 (dry ice) with frozen water beneath.
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(NASA)
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• Although the atmosphere consists mainly of
carbon dioxide, it is too thin to trap heat. So,
the surface temperature varies enormously,
from -100oC to -10oC. Moreover, owing to the
long distance from the Sun, the temperature is
quite low on average.
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Features on the surface
• Mars is a cratered world having gigantic volcanoes
(e.g. Olympus Mons 奧林匹斯山), deep canyons (e.g.
Valles Marineris 水手谷), dry channels, and vast dust
storm.
25 km above the surface and is
600 km in diameter (NASA)
5000 km long, 200 km wide and
7 km deep (NASA)
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Large bodies of liquid water may
have existed on Mars
(NASA)
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 Evidence of old channels and signs of erosion,
seemingly carved by running liquid
 billions of years ago Mars was much warmer
(with a thicker atmosphere)
 Large bodies of liquid water may have existed
(NASA)
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Its two satellites
Deimos
Phobos
(NASA)
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D. Jupiter (木星)
• The largest and most
massive planet in our solar
system. The mass of
Jupiter is about 300 times
that of the Earth, however
its density is low. In fact,
these are general features
of Jovian planets.
• Almost completely made
up of gases.
(NASA)
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• The rotational period of Jupiter is about 10 hours,
and such a high velocity flattens Jupiter at the two
poles.
• Mainly made up of hydrogen, helium, and a small
amount of methane and ammonia.
• The atmospheric pressure is extremely high, over
1000 times than that of the Earth. Because of the
great pressure, the core of Jupiter is made up of
metallic hydrogen. The rapid rotation of such
metallic core explains the strong magnetic field of
Jupiter.
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Feature I: Dark belts and light zones
(NASA)
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Feature II: Great Red Spot
(NASA)
• A great cyclone lasting for at least 300 years.
• 3 times the size of the Earth.
• Red: presence of sulphuric compounds.
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(NASA)
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Feature III: Ring system
(NASA)
• The dark and thin ring of Jupiter. It is composed of
small particles.
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Satellites
• Jupiter has 63
satellites, the four
largest ones were
discovered by
Galileo.
(NASA)
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Io (木衛一)
(NASA)
• Famous for its active volcanic activity that emits sulphuric
compounds, and has a geologically young surface.
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(NASA)
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Erupting
volcano
(NASA)
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A volcano
spewing out gas.
(NASA)
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Europa (木衛二)
(NASA)
• A rocky world with an icy crust.
• There may be a lake under the icy surface.
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Ganymede (木衛三)
• The largest
satellite in the
solar system, its
surface is old and
is heavily cratered,
crossed with
grooved (有溝槽)
terrain.
(NASA)
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Callisto (木衛四)
(NASA)
• A heavily cratered, dark surface.
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E. Saturn (土星)
• The second largest planet. It has 47 satellites.
• Atmospheric condition is similar to Jupiter, but the
belts and zones seem less distinct.
• Average density is lower than water (0.7 g cm-3).
(NASA)
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Ring system
• Three concentric
rings (A, B and C) can
be easily observed on
Earth.
• Thickness ~ I km
• Made of dust and ice.
• The most obvious gap
is Cassini’s division.
(Wikimedia Commons)
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Shepherd
satellites
for inner F
ring
Pandora (pan-DOR-uh)
Prometheus (praMEE-thee-us)
(NASA)
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Mimas
Rhea
Tethys
(NASA)
Dione
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Titan (土衛六)
• The most famous
satellite.
• It is cold enough to
hold an atmosphere
of nitrogen (氮) and
methane (甲烷).
(NASA)
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F. Uranus (天王星)
 Discovered in 1781 by William Herschel.
 Uranus appears blue because of the methane in its
atmosphere. It has much less distinct atmospheric
circulation than Jupiter.
(NASA)
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(NASA)
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Shepherd
satellites
(NASA)
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G. Neptune (海王星)
 Astronomers in 19th century found that Uranus’
orbit deviated from a perfect ellipse, it was under
the gravitational pull of an unknown outer planet.
 Newtonian mechanics predicted the mass and
orbit of this planet.
 discovery of Neptune in 1846.
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(NASA)
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 It is similar to Uranus in size, mass, and
atmospheric condition.
 Cyclone patterns have been discovered (e.g.
Great Dark Spot 大黑斑).
(NASA)
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(NASA)
68
Changing
(NASA)
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(NASA)
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4. Asteroids, comets and meteoroids
Asteroids (小 行 星)
 Small rocky debris that revolve around the sun.
 Most orbits lie in the asteroid belt ( 小 行 星 帶 )
between those of Mars and Jupiter.
 Only two dozens or so are larger than 200 km, most
as small as 0.1 km, irregular in shape.
 Asteroids are either fragments of a planet broken
up long ago, or primal rocks never managed to
accumulate into a planet. Researchers favour the
latter view.
71
Gaspra：
19  12  11 km
Spins once
every 7 hours
(NASA)
72
Ida and its satellite
Dactyl
(NASA)
73
Comets (彗星)
Comet Hale-Bopp
(Wikimedia Commons)
Comet Hyakutake
(Wikimedia Commons)
74
• They are dirty “snow balls”.
• Nucleus (彗核): is very small (a few km), it is the
main solid body of a comet. Only this frozen part
exists when a comet is far from the sun.
• Coma (彗髮): Dust and evaporated gas
surrounding the nucleus. Its maximum size could
be as large as Jupiter.
• Tail (彗尾): Vapourized materials directed away
from the sun by solar wind (particles from the sun)
and pressure of the sunlight.
• Coma and tail are most pronounced when the
comet is closest to the sun.
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Comets have highly elliptical orbits. Note the two distinct
tails:Cyan for gas tail (controlled by the solar magnetic field),
grey for dust tail (bends due to the comet’s motion).
(Wikimedia Commons)
76
Meteoroids (隕星)
• Meteoroids are interplanetary debris hitting
Earth, heated up by friction in Earth’s
atmosphere.
• appear as bright streaks of “shooting stars”
called meteors (流星).
• Most meteoroids are destroyed in the
atmosphere; any parts that reach the ground
are called meteorites (隕石).
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 Some are fragments dislodged from
comets, spreading along the comets’
orbits.
Marília Meteorite, a chondrite H4,
which fell in Marília, São Paulo state,
Brazil, on October 5, 1971, at
5:00p.m. (Wikimedia Commons)
78
5. Stars and their colors
• When we heat something up, it will radiate
electromagnetic waves. When the object is not very
hot, it will be red. If it is hotter, it will be yellow, then
white and finally blue. The color of a star depends
only on its surface temperature, and nothing else.
79
6. Constellations (星座)
• Visual groupings of stars.
• There are totally 88 constellations today,
some added in modern days (e.g.,
Telescopium 望 遠 鏡 座).
• Usually no real correlation among the stars in
the same constellations; they could be very
far away from one another.
80
7. Galaxy (星系)
• Almost all the stars visible by naked eyes are
in our galaxy, the Milky Way Galaxy.
• A galaxy is a collection of hundred billions of
stars.
• Galaxies are categorized into three basic
classes according to their shapes: Elliptical
galaxies, spiral galaxies and irregular
galaxies.
81
Milky Way Galaxy
• About 200 billion stars whirling in a great wheel-like
system; the sun is 8.5 kpc (1 pc  3.3 light years) from
the galactic centre.
Artist's conception of the
spiral structure of the Milky
Way with two major, stellar
arms and a bar. (Wikimedia
Commons)
82
• Disk component
contains almost all of the
gas and dust in the
galactic plane.
– Spiral arms (旋臂): Long
spiral patterns of bright
stars, star clusters, gas
and dust.
Observed and extrapolated
structure of the spiral arms
(Wikimedia Commons)
83
• Spherical component
– Halo (銀暈): Thin scattering of
old, lower mass stars, globular
star clusters; almost no gas and
dust.
– Nuclear bulge (核心): The most
crowded part of spherical
component around the galactic
core; about 20000 light years in
diameter; the center is obscured
at visual wavelengths and
requires radio or infrared
observations.
84
• The universe contains 100 billion galaxies.
• Along the plane of Milky Way, dust clouds block
our view of distant galaxies.
85
Elliptical galaxies (橢圓星系)
• Spherical or elliptical in
shape, lacking in gas and
dust, they contain
relatively old, low-mass
stars.
– Disk component is not
obvious or missing
The giant elliptical galaxy ESO
325-G004. (Wikimedia Commons)
86
Spiral galaxies (旋渦星系)
• contain gas, dust, and hot
bright stars outlining spiral
arms, having a mixture of
star types.
• Obvious disk component.
• They are very luminous
and therefore easy to find.
An example of a spiral galaxy, the
Pinwheel Galaxy (also known as
Messier 101 or NGC 5457)
(Wikimedia Commons)
– 2/3 of all known galaxies
are spiral, but they may
make up only a small
fraction of all galaxies
87
Irregular galaxies (不規則星系)
• Irregular in shape, clouds of gas and dust mixed
with both young and old stars.
– e.g., the Large Magellanic Cloud and Small Magellanic
Cloud are neighbors of the Milky Way Galaxy.
NGC 1427A, an example of an
irregular galaxy about 52 Mly
distant. (NASA)
88
Hubble classification of galaxies
Types of galaxies according to the Hubble classification scheme.
An E indicates a type of elliptical galaxy; an S is a spiral; and SB is
a barred-spiral galaxy. (Wikimedia Commons)
89
8. Space exploration
1957
Sputnik
First Earth orbiter
1969
Apollo 11
First Manned Lunar Landing (Total six
manned lunar landings, Apollo 17 shown
below)
1972
Pioneer 10
First Jupiter Flyby
1977
Voyager 1 and 2
Multiple Planet Flybys (still active)
1989
Galileo
First Asteroid Flyby (on trip to Jupiter)
1990
Hubble Space
Telescope
1996
Mars Pathfinder
First Mars Rover
1997
Cassini-Huygens
First Saturn Orbiter
1998
International
Space Station
(date of first section)
2003
Shenzhou 5
First Chinese Manned Earth orbiter
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