# Presentation: 540321

## Basic Physics Concepts That Underlie Space Science

- Ruth Skoug
- LASSO Summer 2008 Workshop
- July 14-25, 2008

## Outline

-   Atoms and Atomic Structure
-   Forces
-   Electromagnetic Waves
-   Phases of Matter
-   Particle Distributions
-   Plasmas
-   Gravity/Orbits/Propulsion
-   Distance Scales
-   Time Scales

## Slide 3

**Notes:**
Listing that Scientists and teachers like of the elements.
Increasing numbers of protons/electrons - determines the commonly experienced properties - electrical forces dominent in chemistry.

Atomic mass, Atomic number, neutron number

Why are there more neutrons than protons?

## –

- 	element
- Central nucleus includes:
- 	positively charged particles – protons
- 	electrically neutral particles – neutrons
- Negatively charged particles (electrons      ) “orbit” the nucleus
- Number of protons identifies the element

**–**

**Notes:**
Matter, as we commonly experience it, is made up of protons, neutrons, electrons - though we generally don’t realize it.

Elements - broken down further into sub-atomic particles.

## –

- 	element
- Central nucleus includes:
- 	positively charged particles – protons
- 	electrically neutral particles – neutrons
- Negatively charged particles (electrons      ) “orbit” the nucleus
- Number of protons identifies the element

**–**

- H or 1H

- He or 4He

**Notes:**
Matter, as we commonly experience it, is made up of protons, neutrons, electrons - though we generally don’t realize it.

Elements - broken down further into sub-atomic particles.

## Protons

- 	positive electrical charge
- 	mass = 1.67x10-27 kg
- 	size: ~10-15 m     (0.000000000000001 m)
- Neutrons
- 	no electrical charge
- 	mass = 1.67x10-27 kg (approx. same as proton)
- 	size: ~10-15 m (approx. same as proton)
- Electrons
- 	negative electrical charge
- 	mass = 9.11x10-31 kg = proton mass / 1800
- 	size: ~10-18 m = proton size / 1000

**Notes:**
Which ones carry all the mass?
What does mass mean?  (Harder to push around)
Which ones carry all the charge?

## Ions: Charged atoms

- 	In space physics, “ion” is used only to refer to positively
- 	  charged atoms.

**Notes:**
If electrons are taken away, then we get ions.

## Ions: Charged atoms

- 	In space physics, “ion” is used only to refer to positively
- 	  charged atoms.

- H
- (or H0)

**Notes:**
If electrons are taken away, then we get ions.

## Ions: Charged atoms

- 	In space physics, “ion” is used only to refer to positively
- 	  charged atoms.

- H+

- H
- (or H0)

**Notes:**
If electrons are taken away, then we get ions.

## Ions: Charged atoms

- 	In space physics, “ion” is used only to refer to positively
- 	  charged atoms.

- H+

- He

- H
- (or H0)

**Notes:**
If electrons are taken away, then we get ions.

## Ions: Charged atoms

- 	In space physics, “ion” is used only to refer to positively
- 	  charged atoms.

- H+

- He

- He+

- H
- (or H0)

**Notes:**
If electrons are taken away, then we get ions.

## Ions: Charged atoms

- 	In space physics, “ion” is used only to refer to positively
- 	  charged atoms.

- H+

- He

- He++

- He+

- H
- (or H0)

**Notes:**
If electrons are taken away, then we get ions.

## Isotopes: Same element, different number of neutrons

## Isotopes: Same element, different number of neutrons

- 3H

- 2H

- 1H
- (or H)
- (or even 1H0)

## Isotopes: Same element, different number of neutrons

- 3H

- 2H

- 4He

- 3He

- 1H
- (or H)
- (or even 1H0)

## Ions: Charged atoms

## Ions: Charged atoms

- 16O+5

## “Plum pudding” model
negatively charged electron “raisins” floating in a positively charged “pudding”

- Evolution of the Atomic Model

- 1897 (Thomson)

- “Plum pudding” model
- negatively charged electron “raisins” floating in a positively charged “pudding”

**Notes:**
PROBLEM - recognized that light emissions had color lines.
JJ Thomson - electrons in a “positive fluid”.  Plum Pudding.
Can’t use only electric forces.  Can’t predict emission “lines”.
Rutherford - experiment - large angle scattering!

Bohr - particular orbits are non-emitting.  Transitions between orbits!
Schrodigner - Not really particles in orbits - wave nature - probability - allowable states for the waves.

## Rutherford Scattering

- • Expected scattering at small angles; found large-angle scattering
- • Implies a heavy nucleus
- • Disproves Thomson’s model

## Evolution of the Atomic Model

- 1909 (Rutherford)

- 1897 (Thomson)

- Scattering implies heavy nucleus
- Electrons orbit around nucleus

**Notes:**
PROBLEM - recognized that light emissions had color lines.
JJ Thomson - electrons in a “positive fluid”.  Plum Pudding.
Can’t use only electric forces.  Can’t predict emission “lines”.
Rutherford - experiment - large angle scattering!

Bohr - particular orbits are non-emitting.  Transitions between orbits!
Schrodigner - Not really particles in orbits - wave nature - probability - allowable states for the waves.

## Atomic/Molecular/Nuclear Transitions

- • Atoms/molecules/nucleii have a characteristic lowest energy state (ground state)
- • The addition of energy raises the energy level by a quantized amount
- • To get back to the ground state, this excess energy is released as an EM wave
- • The wavelength (color) of this light is a characteristic of the material

## Evolution of the Atomic Model

- 1909 (Rutherford)

- 1913 (Bohr)

- 1897 (Thomson)

- Characteristic colors for each atom imply electrons can only be in “special” orbits
- Electrons can jump from one orbit to another, but no partial jumps
- Energy can only change in small, discrete jumps -- quanta

**Notes:**
PROBLEM - recognized that light emissions had color lines.
JJ Thomson - electrons in a “positive fluid”.  Plum Pudding.
Can’t use only electric forces.  Can’t predict emission “lines”.
Rutherford - experiment - large angle scattering!

Bohr - particular orbits are non-emitting.  Transitions between orbits!
Schrodigner - Not really particles in orbits - wave nature - probability - allowable states for the waves.

## Two-Slit Experiment

- • Shine light through two slits -- see an interference pattern
- • The same pattern is seen when particles pass through two slits!
- • Implies wave-particle duality (particles can act like waves)

## Evolution of the Atomic Model

- 1909 (Rutherford)

- Present (Schrodinger)

- 1913 (Bohr)

- 1897 (Thomson)

- Wave/particle duality
- Only know probability of an electron being in a given location

**Notes:**
PROBLEM - recognized that light emissions had color lines.
JJ Thomson - electrons in a “positive fluid”.  Plum Pudding.
Can’t use only electric forces.  Can’t predict emission “lines”.
Rutherford - experiment - large angle scattering!

Bohr - particular orbits are non-emitting.  Transitions between orbits!
Schrodigner - Not really particles in orbits - wave nature - probability - allowable states for the waves.

## Slide 25

## Forces

- Forces:
- Gravity
- Electric force
- Magnetic force
- Strong and weak nuclear forces (not discussed here)
- Sources of magnetic fields (electricity & magnetism)
- Dipole magnetic fields
- Charged-particle motion in a dipole magnetic field

## Forces

- Force is a push or pull on an object
- Weight is a force: how hard the gravity of the Earth is pulling on something
- Force is a vector -- both magnitude and direction are important
- A force can change the speed of an object
- A force can change the direction of an object

## Particle motions are affected by forces

- _Gravity_: attraction between any two things which have mass
- 	• very weak force
- 	• F = Gm1m2/r2

- r = distance between objects
- M, m = mass of objects

## Particle motions are affected by forces

- _Electrical force_: acts between any two charged particles
- 	• opposite charges attract (positive & negative)
- 	• like charges repel (both + or both –)
- 	• neutral particles not affected
- 	• F = kq1q2/r2

- _Electric field E_: Electric force per unit charge   _**F**_ = q _**E**_

- r = distance between particles
- q = charge of particle

## Magnetism

- People have known for thousands of years that certain stones attract iron
- Magnets have north and south poles (dipoles)
- 	Like poles repel
- 	Opposite poles attract
- Isolated magnetic poles are NOT observed

**Notes:**
Physician to Queen Eliz I.
Realized he could make a little Earth - with a magnet - spurred great interest for navigation.

## Compass needles (which are themselves magnets) are attracted to magnets

- North pole of compass attracted to south pole of magnet

## In 1600, William Gilbert determined that compass needles point in specific directions because the Earth is a permanent magnet

## The Earth’s magnetic axis is currently ~11 ̊ away from the geographic spin axis

- The Earths’ geographic North pole is a magnetic South pole

## Magnetic field line patterns of a bar magnet

## Near the Earth, the Earth’s field lines look like those of a bar magnet

- Particles can be trapped on these field lines (Van Allen belts)
- First discovery of the space age, 1958

## Magnetism

- There is a connection between electricity and magnetism
- 	1821: Oersted finds that an electric current causes a compass needle to move
- Upon learning this, Ampere finds:
- 	electric currents attract iron
- 	parallel electric currents attract each other
- 	proposes that electric currents are the source of all magnetism
- Magnetism is due to electric currents, that is to moving 		electric charges

## Magnetism

- Magnetic force on charged particles given by:
- 	_**F**_ = q (_**v**_ x _**B**_)
- 	• q = charge, _**v**_ = velocity, _**B**_ = magnetic field
- 	• force is proportional to speed v
- 	• force is always perpendicular to the direction of motion
- 	• force is perpendicular to the magnetic field B

**B**

- So, how do particles move in a magnetic field?

## Magnetism

- _Magnetism_
- Magnetic force on charged particles given by:
- 	_**F**_ = q (_**v**_ x _**B**_)
- 	• force is proportional to speed v
- 	• force is always perpendicular to the direction of motion
- 	• force is perpendicular to the magnetic field B

-  In circles around the field line
-  In a straight line along the field direction
-  Putting those together gives spirals

## Slide 39

## Magnetism

- So, back to magnetism in matter...
- Atoms contain electrons which
- orbit the nucleus
- spin on their own axis
- These moving charges create magnetic fields
- There are 3 types of magnetism:
- Paramagnetism: permanent magnetic dipoles are randomly oriented, dipoles can be aligned by an external magnetic field
- Ferromagnetism: permanent magnetic dipoles are aligned within domains, domains can be aligned by an external magnetic field, and will stay aligned after the field is removed
- Diamagnetism: no permanent magnetic dipoles, but dipoles can be induced by an applied magnetic field

## Magnetic Domains

- Unmagnetized material

- Magnetized material

## Electromagnetic waves:

-    • time-varying combination of electric and magnetic fields
-    • propagate in vacuum at the speed of light:
- 		3 x 108 m/s 	(300,000,000 m/s)
-    • carry energy
-    • have no mass

## Wavelength is the distance between wave crests or troughs

- Period is how long (time) it takes to cover one wavelength.
- Frequency = 1 / period
- Speed = Distance / Time = Wavelength / Period
- 			    = Wavelength * Frequency
- For electromagnetic waves (in vacuum), speed is a constant: 	the speed of light (c = 3x108 m/s)

- Electromagnetic Waves

## Slide 44

## Electromagnetic Spectrum

## Electromagnetic Spectrum

## Electromagnetic Spectrum

## Radiation

- Energy traveling through space
- Waves (gamma rays, light, UV, microwaves, radio waves, ...)
- Particles (alphas (He++), betas (electrons), neutrons)
- Some atoms and nuclei are unstable and give off radiation when they decay (break apart)
- Energy can also be released when atoms come together (fusion)
- Radioactive substances are those that decay naturally

## Phases of Matter

-   Temperature
-   Solids
-   Liquids
-   Gases
-   Plasmas

## Energy

- Work is the transfer of energy
- Work is moving an object by means of a force
- Something moving has kinetic energy
- E = 1/2 * m * v2
- m = mass, v = speed
- Stored energy is called potential energy
- Lift an object in a gravitational field
- Compress a spring
- Units of energy
- Electron volt (eV)
- Air molecules: ~1/40 eV
- Atom at Sun’s surface: ~0.5 eV
- Auroral particle: ~1000–10,000 eV (1–10 keV)
- Ring current particle: ~50,000 eV (50 keV)
- Radiation belt particles: ~10,000,000 eV (10 MeV)

## Heat and Temperature

- _Heat and Temperature_
- Heat can be thought of as the internal energy of the atoms and molecules that make up a substance.
- Temperature is a relative term, and refers to the average kinetic energy (KE = 1/2 * m * v2) of the atoms and molecules in a substance.
- Three temperature scales: Farenheit, Celsius, Kelvin

-  ̊F = (9/5) x  ̊C + 32
-  ̊C = (5/9) x ( ̊F – 32)
- K =  ̊C + 273

|  |  ̊C | K |  ̊F |
| --- | --- | --- | --- |
| Water boils | 100 | 373 | 212 |
| Water freezes | 0 | 273 | 32 |
| Absolute zero | -273 | 0 | – 459 |

## Cold substances are solid

## When they warm up, they become liquid

## When they warm up further, they become a gas

## Further warming can ionize the particles, forming a plasma

## Slide 56

## Phases of Matter

## Plasma

- Ionized gas, consisting of positively and negatively charged particles (ions and electrons)
- “Fourth state of matter”
- 99% of the known material in the Universe consists of plasma

## Plasma

-  Ionized gas, consisting of positively and negatively charged particles (ions and electrons)
-  “Fourth state of matter”
-  99% of the known material in the Universe consists of plasma
- Examples:
-  Sun, solar wind, magnetosphere, ionosphere, lightning discharge
-  Fluorescent light bulb, neon sign
-  Formed in reactors for fusion research

## Slide 60

## Particle Distributions

-   Velocity distributions
-   Moments
-   density
-   velocity
-   temperature

## Describing a Gas

- Each particle is characterized by its speed and its direction of motion, i.e., its vector velocity, **v**
- Full description:  Number of particles N within a given volume of space **x** that are moving with velocities in a range **v**  about **v**:

**v**

## Distribution Function

## Particle Distributions – the bell-shaped curve 

- Particle Distributions – the bell-shaped curve

## Shorthand Description: Average Properties

-   Density = Average number of particles (all energies) within a given volume of space
-   Flow Velocity = Average vector velocity of all the particles in a given volume of space
-   Temperature = Average kinetic energy of the random motion relative to the average flow

## Average Properties (“Moments”)

- f(

**x,v**

- )

- v

- x

- 0

- Flow

- Speed

- Density

- Temperature

- Density: area under the curve
- Flow speed: location of peak
- Temperature: width of peak

## Particle Distributions: Spectrogram Display

- 104

- 103

- 102

- 101

- 100

- Energy (eV)

- Time ->

## Plasmas

-   Examples
-   Collective behavior
-   Frozen-in magnetic field
-   Reconnection
-   Shock waves

## Plasma Distributions

- fe(x,v)
- fi1(x,v)
- fi2(x,v)

## Plasma Distributions: Multiple Species

## Plasma Behavior

- Charged particles respond to electric and magnetic fields:
**F** = q**E**
**F = **q**v**x**B**
- Charged particles create electric and magnetic fields:
- E ~ q/r2
- B ~ qv/r

- Therefore, each particle feels the combined effects of all the others:
- Collective motion!

- The fields and particles evolve self-consistently.

## Plasmas Behave Like a Fluid

- Magnetic field line:

- Magnetic flux tube:

- Magnetohydrodynamics: Flux tube maintains its integrity as the plasma flows, and two parcels of plasma linked by a flux tube remain linked.

- Implications:
-   The magnetic field is “frozen-in” to the flow.
-   The flow “carries” the imbedded magnetic field along.
-   Plasmas have a hard time mixing across magnetic field lines.

## Magnetic Reconnection:Local Violation of Frozen-In Flow

**Plays a crucial role in** **many plasma systems!**

## Waves in Plasmas

- Waves are the means for transmitting information from one point to another.

- Example: Sound waves launched from the nose of an airplane propagate into the incoming air flow and tell it to deflect around the airplane.

- In neutral gases, there is only one wave mode: sound waves.
- In plasmas, there are several wave modes, with different propagation speeds.

## Shock Waves

- What if the oncoming flow speed is greater than the speed of the sound waves?  How can the flow know that it needs to deflect around the airplane?

## Shock Waves

- What if the oncoming flow speed is greater than the speed of the sound waves?  How can the flow know that it needs to deflect around the airplane?

**Shock Wave**

- Sonic Boom!

## Shock Waves in Plasmas

- Same principle, different physics!

- Examples of Shock Waves in Plasmas:
-   Earth’s bow shock
-   Interplanetary shocks
-   Supernova shocks
-   Shocks in the solar atmosphere
-   High-altitude nuclear explosions

- Shock waves in plasmas can accelerate some particles to extremely high energies (e.g., cosmic rays)!

## Gravity/Orbits/Propulsion

- Acceleration of a body experiencing a force:
- F = m a   (or  a = F/m)
- Gravitational force exerted by body of mass M on another body of mass m:
- F = GMm/r2
- Resulting acceleration:
- A  = GM/r2
- Orbital radius of a body in a circular orbit:
- R = GM/v2  (or  v2 = GM/r)
- Orbital period:
- T = (42/GM)1/2 r3/2

## Gravity/Orbits/Propulsion

- Escape velocity:
- vesc = (2GM/R)1/2
- Amount of energy needed to move a body from R to infinity:
- E = GMm/R
- (from the Earth, E/m~6 x 107 joules/kg)

## Distance Scales

-   Radius of the hydrogen atom: 5 x 10-11 m
-   Distance between ions in the sun’s visible surface: 2 x 10-8 m
-   Distance between air molecules at Earth’s surface: 7.4 x 10-6 m
-   Distance between solar wind ions: 5 x 10-3 m (= 0.5 cm)
-   Distance between students in the classroom: 1 m
-   Height of Mt. Everest: 8.8 x 103 m
-   Space shuttle altitude: 3 x 105 m (= 300 km)

## Distance Scales

-   Radius of Earth: 6370 km (RE)
-   Radius of Sun: ~100 RE  (also, ~ 1 million miles)
-   Earth - Moon distance:
-   Earth - Sun distance:

## Distance Scales

-   Radius of Earth: 6370 km (RE)
-   Radius of Sun: ~100 RE  (also, ~ 1 million miles)
-   Earth - Moon distance: ~60 RE
-   Earth - Sun distance: 1.5 x 108 km = 1 AU
- (also, ~100 million miles)
-   Size of Magnetosphere: ~10 RE on front & sides
-   Mars - Sun distance:
-   Jupiter - Sun distance:
-  Saturn - Sun distance:
-   Pluto - Sun distance:

## Distance Scales

-   Radius of Earth: 6370 km (RE)
-   Radius of Sun: ~100 RE  (also, ~ 1 million miles)
-   Earth - Moon distance: ~60 RE
-   Earth - Sun distance: 1.5 x 108 km = 1 AU
- (also, ~100 million miles)
-   Size of Magnetosphere: ~10 RE on front & sides
-   Mars - Sun distance: ~1.5 AU
-   Jupiter - Sun distance: ~5 AU
-   Saturn - Sun distance: ~10 AU
-   Pluto - Sun distance: ~39 AU
-   Light year: distance light travels in 1 year:
-   Nearest star:

## Distance Scales

-   Radius of Earth: 6370 km (RE)
-   Radius of Sun: ~100 RE  (also, ~ 1 million miles)
-   Earth - Moon distance: ~60 RE
-   Earth - Sun distance: 1.5 x 108 km = 1 AU
- (also, ~100 million miles)
-   Size of Magnetosphere: ~10 RE on front & sides
-   Mars - Sun distance: ~1.5 AU
-   Jupiter - Sun distance: ~5 AU
-   Saturn - Sun distance: ~10 AU
-   Pluto - Sun distance: ~39 AU
-   Light year: distance light travels in 1 year: ~63000 AU
-   Nearest star: 4.3 light years

## Time Scales

- Distance = (speed) x (time)
- Light from Sun to Earth:
- Solar wind particles (400 km/s) from Sun to Earth:
-   Radio signals (light):
- Earth to moon:
- Mars (1.5 AU) to Earth (1 AU):
- Saturn (10 AU) to Earth (1 AU):
- Pluto (39 AU) to Earth (1 AU):
-   Travel time for automobile (100 km/hr):
- Earth to the moon:
- Earth to the Sun:
- Earth to Saturn:
-   Travel time for Cassini spacecraft to Saturn:

## Time Scales

- Distance = (speed) x (time)
- Light from Sun to Earth:  8.3 minutes
- Solar wind particles (400 km/s) from Sun to Earth:
-   Radio signals (light):
- Earth to moon:
- Mars (1.5 AU) to Earth (1 AU):
- Saturn (10 AU) to Earth (1 AU):
- Pluto (39 AU) to Earth (1 AU):
-   Travel time for automobile (100 km/hr):
- Earth to the moon:
- Earth to the Sun:
- Earth to Saturn:
-   Travel time for Cassini spacecraft to Saturn:

## Time Scales

- Distance = (speed) x (time)
- Light from Sun to Earth:  8.3 minutes
- Solar wind particles (400 km/s) from Sun to Earth:  4.3 days
-   Radio signals (light):
- Earth to moon:
- Mars (1.5 AU) to Earth (1 AU):
- Saturn (10 AU) to Earth (1 AU):
- Pluto (39 AU) to Earth (1 AU):
-   Travel time for automobile (100 km/hr):
- Earth to the moon:
- Earth to the Sun:
- Earth to Saturn:
-   Travel time for Cassini spacecraft to Saturn:

## Time Scales

- Distance = (speed) x (time)
- Light from Sun to Earth:  8.3 minutes
- Solar wind particles (400 km/s) from Sun to Earth:  4.3 days
-   Radio signals (light):
- Earth to moon:  1.3 seconds
- Mars (1.5 AU) to Earth (1 AU):
- Saturn (10 AU) to Earth (1 AU):
- Pluto (39 AU) to Earth (1 AU):
-   Travel time for automobile (100 km/hr):
- Earth to the moon:
- Earth to the Sun:
- Earth to Saturn:
-   Travel time for Cassini spacecraft to Saturn:

## Time Scales

- Distance = (speed) x (time)
- Light from Sun to Earth:  8.3 minutes
- Solar wind particles (400 km/s) from Sun to Earth:  4.3 days
-   Radio signals (light):
- Earth to moon:  1.3 seconds
- Mars (1.5 AU) to Earth (1 AU):	4 - 21 minutes
- Saturn (10 AU) to Earth (1 AU):
- Pluto (39 AU) to Earth (1 AU):
-   Travel time for automobile (100 km/hr):
- Earth to the moon:
- Earth to the Sun:
- Earth to Saturn:
-   Travel time for Cassini spacecraft to Saturn:

## Time Scales

- Distance = (speed) x (time)
- Light from Sun to Earth:  8.3 minutes
- Solar wind particles (400 km/s) from Sun to Earth:  4.3 days
-   Radio signals (light):
- Earth to moon:  1.3 seconds
- Mars (1.5 AU) to Earth (1 AU):	4 - 21 minutes
- Saturn (10 AU) to Earth (1 AU):  1.25 hours
- Pluto (39 AU) to Earth (1 AU):
-   Travel time for automobile (100 km/hr):
- Earth to the moon:
- Earth to the Sun:
- Earth to Saturn:
-   Travel time for Cassini spacecraft to Saturn:

## Time Scales

- Distance = (speed) x (time)
- Light from Sun to Earth:  8.3 minutes
- Solar wind particles (400 km/s) from Sun to Earth:  4.3 days
-   Radio signals (light):
- Earth to moon:  1.3 seconds
- Mars (1.5 AU) to Earth (1 AU):	4 - 21 minutes
- Saturn (10 AU) to Earth (1 AU):  1.25 hours
- Pluto (39 AU) to Earth (1 AU): 	5.3 hours
-   Travel time for automobile (100 km/hr):
- Earth to the moon:
- Earth to the Sun:
- Earth to Saturn:
-   Travel time for Cassini spacecraft to Saturn:

## Time Scales

- Distance = (speed) x (time)
- Light from Sun to Earth:  8.3 minutes
- Solar wind particles (400 km/s) from Sun to Earth:  4.3 days
-   Radio signals (light):
- Earth to moon:  1.3 seconds
- Mars (1.5 AU) to Earth (1 AU):	4 - 21 minutes
- Saturn (10 AU) to Earth (1 AU):  1.25 hours
- Pluto (39 AU) to Earth (1 AU): 	5.3 hours
-   Travel time for automobile (100 km/hr):
- Earth to the moon:  159 days
- Earth to the Sun:
- Earth to Saturn:
-   Travel time for Cassini spacecraft to Saturn:

## Time Scales

- Distance = (speed) x (time)
- Light from Sun to Earth:  8.3 minutes
- Solar wind particles (400 km/s) from Sun to Earth:  4.3 days
-   Radio signals (light):
- Earth to moon:  1.3 seconds
- Mars (1.5 AU) to Earth (1 AU):	4 - 21 minutes
- Saturn (10 AU) to Earth (1 AU):  1.25 hours
- Pluto (39 AU) to Earth (1 AU): 	5.3 hours
-   Travel time for automobile (100 km/hr):
- Earth to the moon:  159 days
- Earth to the Sun: 154 years
- Earth to Saturn:
-   Travel time for Cassini spacecraft to Saturn:

## Time Scales

- Distance = (speed) x (time)
- Light from Sun to Earth:  8.3 minutes
- Solar wind particles (400 km/s) from Sun to Earth:  4.3 days
-   Radio signals (light):
- Earth to moon:  1.3 seconds
- Mars (1.5 AU) to Earth (1 AU):	4 - 21 minutes
- Saturn (10 AU) to Earth (1 AU):  1.25 hours
- Pluto (39 AU) to Earth (1 AU): 	5.3 hours
-   Travel time for automobile (100 km/hr):
- Earth to the moon:  159 days
- Earth to the Sun: 154 years
- Earth to Saturn:  1541 years
-   Travel time for Cassini spacecraft to Saturn:

## Time Scales

- Distance = (speed) x (time)
- Light from Sun to Earth:  8.3 minutes
- Solar wind particles (400 km/s) from Sun to Earth:  4.3 days
-   Radio signals (light):
- Earth to moon:  1.3 seconds
- Mars (1.5 AU) to Earth (1 AU):	4 - 21 minutes
- Saturn (10 AU) to Earth (1 AU):  1.25 hours
- Pluto (39 AU) to Earth (1 AU): 	5.3 hours
-   Travel time for automobile (100 km/hr):
- Earth to the moon:  159 days
- Earth to the Sun: 154 years
- Earth to Saturn:  1541 years
-   Travel time for Cassini spacecraft to Saturn: 6.7 years