Chapter 7 Minor Bodies Of The Solar System

Question 1

Keplers first law

Answer 1

Planets move in elliptical orbits, with the sun at one focus.

Question 2

What are the semi-major and semi-minor axis of an ellipse?

Answer 2

The Major axis is the diameter of the longest part of an ellipse. The semi-major axis is half the major axis ( radius of the widest part ).

The minor axis is the diameter of the narrowest part of an ellipse. The semi-minor axis is half the minor axis ( radius of the narrowest part ).

Question 3

What are the focus points of an ellipse?

Answer 3

An ellipse has two foci that lie along the semi-major axis. The position of the foci depend on now elongated the ellipse is.

Question 4

What is eccentricity?

Answer 4

This is a dimensionless number whose value lies between 0 and 1. For a circle e=0 where both foci are at the centre. For a very flattened ellipse e approaches 1.

Question 5

Calculating the focus point from the centre of an ellipse

Answer 5

To find the distance of the focus point from the centre multiply the semi-major axis (a) by the eccentricity (e).

ae

Question 6

What is the perihelion?

Answer 6

If the sun is at one of the focus points and a planet is orbiting around the edge of the ellipse, then the point where the planet is closest to the sun is known as perihelion.

Question 7

What is the aphelion?

Answer 7

If the sun is at one of the focus points and a planet is orbiting around the edge of the ellipse, then the point where the planet is farthest from the sun is known as aphelion.

Question 8

What is the perihelion distance (q)? (Equation)

Answer 8

This is the distance from the sun to the perihelion. It is related to the semi-major axis (a) and eccentricity (e) by the expression:

q = a(1 - e)

Question 9

What is the aphelion distance (Q)?

Answer 9

This is the distance from the sun to the aphelion. It is related to the semi-major axis (a) and eccentricity (e) by the expression:

Q = a(1 + e)

Question 10

Keplers second law

Answer 10

A line connecting the sun to a planet would sweep out equal areas of space in equal times.

Question 11

Keplers third law (equation)

Answer 11

The square of a planets orbital period (P) is proportional to the cube of its semi-major axis (a).

P^2 = ka^3

Where k is the constant of proportionality and who’s value depends on the mass of the body that is being orbited. If the units of P is years an a is Au then the value of K is 1.

Question 12

What is the ecliptic plane?

Answer 12

The ecliptic plane is defined by the plane of the earths orbit around the sun.

Question 13

What is orbital inclination (i)?

Answer 13

This is the angle between the plane of an orbit and the ecliptic plane. The inclination angle is between 0° and 180°.

Question 14

What does prograde mean?

Answer 14

This is when planets orbit the seen in an anticlockwise direction when observed from above. They will have an orbital inclination (i) of between 0° and 90°.

Question 15

What does retrograde mean?

Answer 15

This is when a planet orbits around the sun clockwise when viewed from above. it will have an orbital inclination (i) of between 90° and 180°.

Question 16

What is synchronous rotation?

Answer 16

This is when the orbital period of a body is equal to the rotational period, so the same face is always in the direction of the larger body.

Question 17

What is libration?

Answer 17

This is the small variations of rotation back and forth of a synchronously rotating object causing the tidal bulge to he pulled back and forth inducing Frictional heating (tidal heating).

Bodies can only undergo libration if their eccentricities are sufficiently far from 0.

Question 18

What is orbital evolution?

Answer 18

This is a change in a body’s orbital parameters over time.

Question 19

Can asteroids around the earth be replenished?

Answer 19

Yes, asteroids from the asteroid belt can be influenced by the gravity of Jupiter and mars and pulled into the inner solar system as a result.

Question 20

Classes of asteroids

Answer 20

• C-type (carbonaceous): Dark, non reflective, carbon rich and rocky. Primitive
• s-type (stony or stony-metallic mix): more reflective and more red.
• e-type ( mainly mineral enstatite MgSiO3): often highly reflective.
• d-type (dark types): extremely dark and red. Primitive
• m-type (metallic): mostly made of iron and nickel.
• p-type: also thought to be mostly metallic.

Question 21

What is orbital resonances?

Answer 21

This is when the orbits of 2 bodies have gravitational interactions causing a stretching of the orbit.

They are represented by ratios i.e. 2:1 which would mean the planet closer to the sun orbits two times for every one orbit of the planet further than the sun.

(The closer body always goes on the left and will always be larger)

Question 22

What are centaurs?

Answer 22

These are objects who’s orbits are in the same region as the giant planets.

These objects are thought to originate in the kuiper belt and migrate inwards as their orbits evolve.

Question 23

What is a comet and what are its constituents?

Answer 23

A comet is a large body that consists of ice and dust.

When seen from earth the bright head is knows as the coma, which leads onto a wispy tail.

The actual body of the comet is known as the cometary nucleus.

Question 24

Why does a comet have a tail?

Answer 24

As the comet comes near the sun, the ices are heated and turn directly into gas (i.e. The ices sublime) and escape from the surface into space, carrying dust with them.

Question 25

What is the surface of a comet like?

Answer 25

As ices at the surface sublime some dust is left behind on the surface. This layer of dust protects the ice underneath from the sun so the cometary nucleus cannot release gas from its entire surface. **active spots** are areas of uncovered ice where approximately 90% of degassing occurs.

Question 26

Where died the comets form?

Answer 26

As comets are predominantly icy we can assume that they formed in the outer solar system similar to the giant planets. Many would have accreted into planetary embryos, while other may have been thrown into the inner solar system where they bombard the terrestrial planets, or thrown out to form the** Oort Cloud** by the gravity of the giant planets.

Question 27

What are **kuiper belt objects (KBO)**?

Answer 27

These are large bodies of between 100 - 1000 km sized bodies found in the kuiper bet belt, such as Pluto.

Question 28

Why are the colours of kuiper belt objects so diverse?

Answer 28

As many KBO's have a comet like composition degassing may occur from the energy released from impacts. This degassing could form a thin transient atmosphere which could recondense on the surface, changing the appearance/ colour of the object.

Question 29

What types of orbit can a comet have?

Answer 29

**long period comets** typically come from the Oort cloud, this means their orbital inclinations can be any value as the Oort Cloud is a sphere. They can have semi-major axis of hundreds or thousands of Au, thus have very long orbital periods. **short- period comets** generally have inclinations of low angles as they originate from the kuiper belt. They have mostly prograde orbits with high eccentricity with relatively short orbital periods (few hundred years).

Question 30

Why might a short period comet have a retrograde orbit?

Answer 30

Comets from the Oort Cloud can have retrograde orbits. They can he affected by the gravity of Jupiter and have their orbits altered to form a short-period comet.

Question 31

Why are comets orbital periods hard to predict?

Answer 31

As degassing occurs, the resulting jets can act as small rockets / boosters applying non - gravitational forces on the comet affecting its orbital period by days or weeks.

Question 32

What is interplanetary dust and where does it come from?

Answer 32

This is simply the dust found between planets (< 1mm). This can be created by comets passing through the solar system or expected by the impact of bodies (impacts on planets or in the asteroid belt).

Question 33

What is a **meteoroid stream**?

Answer 33

Dust particles ejected from comet take up keplerian orbite very similar to the parent comet. This means that there is a multitude of dust particles who's orbits form a tube of sorts around the comets orbit.

Question 34

What is a **meteor**?

Answer 34

When a small meteoroid (usually dust) enters the atmosphere, the air infront of it is compressed and heated, this heating raises the energy level of an electron leading to it emitting light. The streak of light is known as a meteor (shooting star).

Question 35

What is a **meteor shower**?

Answer 35

This is when many meteors are observed associated with the same meteor stream.

Question 36

What is a **sporadic meteorite**?

Answer 36

These are random background dust particles that enter earths atmosphere.

Question 37

What is **radiation pressure**?

Answer 37

When photons of light hit an object they can transfer momentum to that object. This is most effective for extremely small object. Radiation pressure has the greatest effect when the cross sectional area to mass ratio is maximised. Effective for particles sized 0.1μm to 1μm, blowing them away from the sun.

Question 38

What is the **poynting - Robertson effect**?

Answer 38

For dust particles larger than 1 μm that are traveling perpendicular to photons emitted from the seen, the radiation pressure can have a braking effect. This causes eye semimajor axis to decrease over time and the dust particles slowly spiral into the inner solar system from outer areas, such as the asteroid belt.