GCSE Astronomy - Topic 2

State the main constituents of the Solar System.

* 8 planets,
* moons,
* dwarf planets,
* asteroids,
* comets,
* centaurs,
* Trans-Neptunian Objects.

Name three dwarf planets in the Solar System.

1. Ceres,
2. Pluto
3. Eris.

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==(Also: Haumea, Sedna, Makemake.)==

What is a comet?

Nuclei of ice, dust and rock that develop a gaseous coma and tails when close to the Sun.

What is a Trans-Neptunian object?

Objects orbiting the Sun beyond Neptune.

What is a centaur?

* Show **similarities to both comets and asteroids**
* generally ==orbit the Sun between the orbits of Jupiter and Neptune==
* e.g. Chiron, Hidalgo and Asbolus.

State the features that qualify an object as a ‘planet’.

* An object that is in orbit around the Sun
* large enough to be spherical
* **and** has ‘cleared its orbit’ of other objects.

What is an asteroid?

* **A minor planet**.
* Small soild bodies, with
* the majority orbiting in the Main or **Asteroid Belt** between the orbits of Mars and Jupiter,
* including Vesta (the brightest) and Pallas.

Name the planets in order of increasing distance from the Sun.

1. Mercury,
2. Venus,
3. Earth,
4. Mars,
5. Jupiter,
6. Saturn,
7. Uranus,
8. Neptune.

What is the ecliptic?

* **The projection of the Earth’s orbit onto the celestial sphere**;
* it may also be defined as the apparent yearly path of the Sun against the stars.
* It is inclined to the celestial equator by 23.5°

What is an astronomical unit?

1 AU is defined as **150 million km**

==(the mean distance between the Earth and the Sun).==

What is the Zodiac?

* A belt stretching right round the sky, **8° to either side of the ecliptic**
* in which the Sun, Moon, and planets (apart from Pluto) are always to be found.

Define the terms: perihelion, aphelion.

**Perihelion** = the position of a body in the Solar System when at its ==closest== to the Sun.

**Aphelion** = the position of a planet or other body when at its greatest distance from the Sun

Define the terms: greatest eastern elongation (GEE), greatest western elongation (GWE).

Mercury and Venus are best placed for observation when they are furthest from the Sun in the sky.

The positions in their orbits are known as GEE and GWE and ==**at these positions the angle between the lines planet-Earth and planet-Sun are 90**°==.

What is meant by the term retrograde motion?

**Orbital or rotational movement in the sense opposite to that of the Earth’s movement or rotation**.

A planet is said to move in an apparent retrograde direction when shifting from east to west on the celestial sphere.

Describe the orbit of the planets around the Sun.

* The planets orbit the Sun in **ellipses**.
* The plane of the Earth’s orbit is called the ecliptic and the planes of all the planets are inclined by only a few degrees to this.

Define the term: conjunction.

a) A planet is in conjunction with a star or another planet when it passes by it in the sky;

* A planet is in **superior conjunction** when it is on the ==far side of the Sun with respect to the Earth==,
* and in **inferior conjunction** when passing between the Sun and the Earth.

Define the term: transit.

a) The passage of a celestial object across the observer’s meridian

b) The projection of Mercury or Venus against the disc of the Sun.

Describe the physical characteristics of Mercury.

* Heavily cratered and contains highlands
* lava-filled basins
* many parts of its surface appear almost identical to the Moon.

What is the link between Venus and global warming?

Venus has a dense atmosphere containing CO2; this prevents infrared radiation leaving Venus, making the surface and lower atmosphere so hot – a ‘runaway greenhouse effect’.

Define the term: opposition.

The position of **a planet when exactly opposite to the Sun in the sky**.

Define the term: occultation.

**The covering-up of one celestial body by another**. Strictly speaking, a solar eclipse is an occultation of the Sun by the Moon.

Describe the physical properties of Mars.

* Mars has **iron-rich rocks**, **seasonal ice caps**
* **450 km-long canyon called Valles Marineris**
* the highest volcano in the Solar System – **Olympus Mons**
* violent **dust storms** rage across its surface.

Describe the physical properties of Venus.

* **Backwards-spinning,** **similar size to the Earth**.
* Clouds of sulfuric acid,
* s**urface pressure 90X** greater than the Earth,
* **surface temperature of 470 °C**.
* **Dense atmosphere containing carbon dioxide.**

Describe some of the physical characteristics of Jupiter.

* Jupiter **rotates on its axis in only 10 hours**, producing an equatorial bulge
* dynamic wind systems that split the atmosphere into a series of red-brown belts and yellow-white zones.
* The **Great Red Spot (GRS**) is an anticyclone weather system, larger than the planet Earth.

What are some of the physical characteristics of Saturn?

* Saturn is a similar gas giant to Jupiter but it reveals less structure in its atmosphere
* shows **no evidence of long-term features like Jupiter’s GRS**.
* Saturn’s most notable feature is its **majestic rings**.

What are the atmospheres of Uranus and Neptune made up of?

Both are gas giants of similar size made up of **hydrogen**, **helium**, **methane** and **ammonia**.

State some differences in the physical characteristics of Uranus and Neptune.

* **Uranus spins almost on its side** and its ==surface is almost featureless==;
* **Neptune shows many surface markings** (including the Great Dark Spot) with dark banded features and cirrus-like clouds of frozen methane at high altitudes.

Name five general techniques by unmanned probes to investigate planets and other Solar System bodies.

1. Flyby
2. orbit
3. landing
4. impact collision
5. collecting and returning

State some of the problems associated with manned exploration of the Solar System.

* Space Adaptation Syndrome,
* problems associated with living in a zero-gravity environment (brittle bones, muscle fatigue, reduced red blood cell counts),
* communication delays,
* radiation risk,
* psychological problems.

What are the names of the two moons of Mars?

1. Phobos (Fear)
2. Deimos (Terror).

What is the believed origins of the moons of Mars?

Captured bodies from the Asteroid Belt.

What are the names of the 4 Galilean moons of Jupiter?

1. Io,
2. Europa
3. Ganymede,
4. Calliso

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Which is the only moon in the Solar System which has a dense atmosphere?

Saturn’s moon **Titan**, which has an atmosphere that extends 600 km from the moon’s surface, consisting of 95% nitrogen and methane and other hydrocarbons.

What is unusual about the orbit of Triton around Neptune?

* Its **orbit is highly-inclined** and revolves around Neptune
* and in the **opposite sense to that in which Neptune spins** (the only large moon in the Solar System to do so).

What is the proposed origin of Neptune’s moon Triton?

Probably a captured body, but because of its size and mass.

What is the proposed origin of Neptune’s moon Proteus?

As Proteus orbits in the plane of Neptune’s equator and in the same sense as the planet’s spin, it is suggested that Proteus formed at the same time as Neptune. (**Although some astronomers believe that Proteus and Saturn’s equally-dark moon Phoebe were formed in a different part of the Solar System and were subsequently captured by the two planets**.)

What is unusual about the orbit of Neptune’s moon Nereid?

It is the **most highly eccentric orbit of any moon or planet in the Solar System**; it takes 360 days to orbit Neptune. It is most likely a captured Kuiper Belt object.

What is the Kuiper belt?

Frozen objects (mainly methane, ammonia and water) that **lie mostly beyond Neptune**. ==Thought to be the source of comets.==

What is the Oort cloud?

The **outermost region of the Solar System** (at a distance of 1.5 light years from the Sun). Consists of billions of small lumps of rock and ice. The objects of the Oort cloud are too faint to be seen using visible light.

What is a ring system?

Billions of individual particles of ice, rock and dust that range from a few micrometres to several metres in size. The exact composition varies from planet to planet.

What are possible origins of ring systems?

* Debris left over after the formation of the planet;
* large impacts between moons;
* a moon that was torn apart through tidal gravitational forces;
* material ejected from the surfaces of moons by meteoric impacts.

What is a comet?

Balls of **rock and ice** (often called ‘dirty snowballs’ or ‘icy dirtballs’) that form tails in the course of their **highly-elliptical orbits around the Sun**.

What are the two classes of comet?

* **Short-period** comets (**periods of less than 200 years**)
* **long-period** comets (periods of **greater** **than 200 years**).

Where do the two classes of comet originate?

* Short-period comets originate from the **Kuiper belt** (**30-50 AU** from the Sun)
* long-period comets originate from the **Oort cloud** (about **50,000 AU** from the Sun).

Is the Halley Comet a short-period or a long-period comet? What is its period?

Short-period comet, with a period of 76 years.

What is the difference between the nucleus and the coma of a comet?

* **Nucleus** = the core of rock and ice.
* **Coma** = extremely rarefied sphere of gas and dust surrounding the nucleus (\~100,000 km across) formed as the temperature rises as a comet approaches the Sun.

Explain the two types of tail on a comet?

1. **Gas Tail**: Blue-coloured, straight ion tail consisting of atoms and molecules of gas (mostly carbon monoxide) that have been ionised by the solar wind.
2. **Dust Tail**: Lighter-coloured, shorter, broader and slightly curved dust tail produced by radiation pressure that pushes particles out of the nucleus; this tail shines by reflecting sunlight and its curvature is due to the individual dust particles following their own independent solar orbit.

Define the terms: meteoroid, meteor, meteorite.

* **Meteoroid** = small rocky (with perhaps some iron-nickel content) irregular lumps of debris in the Solar System.
* **Meteor** = when meteoroids enter the Earth’s atmosphere, friction causes the meteoroid and surrounding air to heat up producing a short streak of incandescent light.
* **Meteorite** = if part of a meteoroid survives its passage through the atmosphere, it lands on the Earth’s surface and is called a meteorite.

What is a fireball?

A meteor with a **magnitude of -3 or brighter**.

Name four annual meteor showers.

1. Perseids (August),
2. Quandrantids (January),
3. Leonids (November),
4. Geminids (December).

What is a radiant?

The **point in the sky from which meteors of any particular shower appear to radiate**.

What is a Near Earth Object (NEO)?

Any asteroid or comet with an orbit that brings them close to the Earth (**closer than 0.3 AU**)

What is a Potentially Hazardous Object (PHO)?

An NEO whose orbit brings it within **0.05 AU** of Earth

What evidence is there for collisions between astronomical bodies?

* Craters on the Moon,
* asteroids and other planets;
* the unusual rotations of Venus and Uranus;
* evidence for the Giant Impact Hypothesis;
* direct observation of the collision between Jupiter and Shoemaker-Levy in 1994;
* evidence of PHO impact craters on Earth (Barringer, Chicxulub, Tunguska).

What scale is used to categorise impact hazards and potential risks?

The Torino scale.

What are the names of the two models of the Solar System?

Geocentric and heliocentric.

What was Copernicus’ contribution to the geocentric vs. heliocentric argument?

* Proposed a heliocentric model to explain the occasional retrograde motion of Mars, Jupiter and Saturn.

What was Tycho’s contribution to the geocentric vs. heliocentric argument?

Undertook meticulous observations of the skies with meticulous precision.

What is Kepler’s first law of planetary motion?

**Planets move in elliptical orbit around the Sun**, with the Sun at one focus of each ellipse.

What is Kepler’s second law of planetary motion?

An **imaginary line from a planet to the Sun sweeps out equal areas in equal intervals of time**.

Mathematically, what is Kepler’s third law of planetary motion?

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**T^2 = r^3**

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where:

**T** is the orbital period of a planet in years

**r** is its mean distance from the Sun in AU.

What were Galileo’s three major astronomical discoveries relating to the Solar System?

1. Phases and apparent size of Venus;
2. relief features of the Moon;
3. principal satellites of Jupiter (Ganymede, Io, Callisto, Europa).

Who discovered Uranus and how?

**William** **Herschel** in 1781. He first mistook it for a comet, but subsequent observations allowed him to determine an orbit from which he deduced the object to be a planet.

Who discovered Ceres and how?

**Giuseppe Piazzi** in 1801. Several astronomers had predicted that an undiscovered planet lay in the obvious gap between the orbits of Mars and Jupiter. He noted a faint ‘star’ had moved in position on several successive nights.

Who discovered Neptune and how?

**Johann Galle** in 1846. Discovered first night of telescopic observation based on the predictions of the existence of Neptune to explain observed irregularities in the motion of Uranus.

Who discovered Pluto and how?

Clyde Tombaugh in 1930. Located by photographic techniques.

What is gravitation?

The force of attraction that exists between all particles of matter in the Universe.

Mathematically, what is Newton’s law of gravitation?

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What is meant by an inverse square law?

As the distance between two objects doubles, the force between them is one quarter of its previous value.

What are the three main techniques for detecting or inferring the presence of exoplanets?

1. Astrometry,
2. transit method,
3. radial-velocity method (using Doppler shifts).

What are the difficulties with detecting individual planets?

Current methods do not allow the discovery of small, rocky exoplanets similar to Earth because atmospheric turbulence and the fact that **planets with a relatively small mass have less significant effect on their host star**.

What do scientists think are the two essential chemical ingredients for life?

1. Carbon
2. liquid water

What are the two principal theories for the origin of water on Earth?

1. Outgassing of hydrogen and oxygen from volcanoes that combined to produce steam that condensed into water;
2. deposited by comets (containing ice) striking Earth

What methods have astronomers used to determine the origin of water on Earth?

* space probe placed a lander on comet
* scientific instruments then analysed the comet’s water content and examine whether it has the same relative abundances of isotopes as water on Earth.

What factors are included in the Drake equation for assessing the likelihood of life existing elsewhere in our galaxy?

* Number of stars in our galaxy;
* fraction of stars with planetary systems;
* number of planets capable of sustaining life;
* fraction of life-forms that are intelligent;
* fraction of these that can and wish to communicate;
* fraction of a planet’s lifetime for which such civilisations can live

What is meant by the term “habitable zone”?

**A narrow range of distances from the star in which the temperature allows liquid water to exist** (sometimes called the Goldilocks Zone because it is “neither too hot nor too cold”)

Name two other bodies in our Solar System that are candidates for micro-organisms to exist.

1. Mars,
2. Jupiter’s moon Europa.

State some methods astronomers use to search for signs of life on other planets?

* Space probes,
* spectral analysis of planetary atmospheres above rocky exoplanets to **search for gases such as oxygen and methane that are produced by living organisms**;
* analysis of **radio waves** from space to try to **detect signals that have originated from ET intelligent forms** of life in our galaxy

What are the possible disadvantages of discovering ET life?

* Would it be wise to transfer organisms from one environment into a totally different one in which they might become extinct or adapt too well and flourish?
* We know nothing of the intent or capabilities of possible alien life-forms – could continued life on Earth be threatened?
* Do we really want to discover that we are not alone in the Universe?