Triton (Neptune)

Significant geological activity, possibly induced by tidal interactions.Varied geology over the surface.

Of particular note, Triton orbits Neptune backwards (i.e. in the opposite sense of pretty much everything in the Solar System). This strongly suggests that it was a body like Pluto that was captured in Neptune’s gravity.

What is one possible reason why moons of Jovian planets are more geologically active than terrestrial planets?

A. They are smaller.B. They are icier.C. They are in orbital resonancesD. They are younger.E. They are darker.

What is one possible reason why moons of Jovian planets are more geologically active than terrestrial planets?

A. They are smaller.B. They are icier.C. They are in orbital resonancesD. They are younger.E. They are darker.

Ice Geology

•Note that many of the moons (including those we haven’t discussed in class) are geologically active.

•These are smaller than the terrestrial planets, so they should cool faster. So, what’s up?

•Tidal forces provide a source of heating that doesn’t exist in the inner solar system.

•The compositions contain much more water which melts/boils at lower temperatures than rock.

RINGS

The rings of Saturn. These show significant structure established by orbits and gravity.

The rings are only ~100 m thick and are comprised of many small rocks/snowballs all on similar orbits.Collisions are frequent!

Gaps in the rings are usually cleared by small moons.

Some gaps are cleared by orbital resonances moons outside their orbits.

All the Jovian planets have rings. 1) Where did they come from?

2) Why “rings” and not donuts or something else?

Rings form out of tidal destruction of small moons.

Gravity pulls more on one side of the planet than on the other.

This differential force pulls the moons apart into chunks of rock.

Collisions pulverize rocks into a finer pieces forming rings (1 collision every 5 hours on average).

Gravity keeps the disk quite thin since there’s nothing to puff it up and collisions stop vertical motions.

Rings

•Collisions constantly make smaller and smaller particles, eventually making fine dust.

•Drag on this dust causes the ring particles to fall into the planet.

•Rings will disappear over the course of time!

Asteroids -- Leftover planetesimals (or pieces) from the formation of the inner solar system.

They are found within the frost line of the inner solar system, so they are similar to terrestrial planets in composition.

Interactive Figure 12.1

Asteroids were first noticed by their motions with respect to the background stars. Since they were in the ecliptic, it was suspected they were Solar System objects.

Which asteroid has the largest mass?

A. B.

C. D.

Asteroid Census

•There are lots of known asteroids: >105

•Most asteroids are found in a belt between Mars and Jupiter.

•There isn’t a lot of mass in the asteroids, not even enough to copy the Moon.

•Small asteroids (10 km) are irregular, medium (100 km) are oblong and the largest, Ceres is spherical due to self-gravity.

Greeks(L4)

Trojans(L5)

Meteor(ites)

•Meteroites are small pieces of asteroid that have fallen through the Earth’s atmosphere and hit the planet.

•Meteors are the actual falling bright lights that we see (shooting stars).

Comets, like asteroids, are planetesimals. They formed beyond the frost line and we expect their dominant chemical component to be:

A. Hydrogen gasB. RockC. MetalD. IcesE. Helium

Comets, like asteroids, are planetesimals. They formed beyond the frost line and we expect their dominant chemical component to be:

A. Hydrogen gasB. RockC. MetalD. IcesE. Helium

CometsLike asteroids, comets are the remnants from the formation of the Solar System.

Unlike asteroids, they formed beyond the frostline and have a high concentration of hydrogen compounds in the form of ices.

While we usually associate comets with long tails (even the name), most of the time comets are dark, dirty little snowballs.

Comets•Historically comets were thought to be near Earth

phenomena, usually thought to be the harbingers of doom.

•With the theory of gravitation, Newton predicted the orbit of a comet, assuming it orbited the Sun.

The most famous comet is probably Halley’s comet, which he predicted to return every 76 years.

Comet orbits are usually highly eccentric (squished)

ellipses or hyperbolas (open orbits).

Because of Kepler’s Laws, they spend most of their

time in the outer reaches of the solar system.

Halley’s comet, for example, has a period of 76

years, but only spends about 1 year inside Mars’

orbit

Many comet orbits are one-time trips into the inner Solar System before being ejected on a hyperbolic orbits (i.e., they don’t return on known orbits).

Other orbits plunge right into

the Sun!

Some comets get ejected from the solar system. Other orbits plunge right into the Sun!

Comet dust tails are left in the inner solar system. When a tail is left in the plane of the ecliptic, planets pass through the dust tail.

The small dust particles burn up in the atmosphere, becoming visible as a meteor shower.

Meteor showers are named based on the constellation they seem to appear out of (e.g. Leonids, Nov. 17-18, Geminids Dec. 12-14)

Pluto -- The first Kuiper Belt Object which was discovered in 1930. Observers were searching for a large body which was pulling on Neptune’s orbit.

They found Pluto which is different from the Jovian planets in many ways:1) Highly inclined orbit2) Comet-like composition3) Large distance from Sun4) Low mass

For a long time, Pluto was considered the 9th planet.

Pluto’s highly inclined orbit actually intersects Neptune’s orbit. However, the two objects won’t collide because of orbital resonances.

Pluto is part of a complex orbital system. It has one “moon” Charon which is almost 1/8th its mass.

It also has a couple smaller moons that orbit farther out. These were discovered recently.

Artist’s conception of Pluto/Charon viewed from Nix

The planet compositions are likely dominated by hydrogen compound ices with some rock.

Not much soot like comets have.

Hence, Pluto is surprisingly reflective for an object inits class.

1) Nitrogen atmosphere which freezes out seasonally.2) Ices3) Mix of silicate rocks and ices.

Structure of Pluto

All was well and good until...

More Kuiper Belt Objects were discovered!

2005 1930

2004 2002

In particular, in 2005, Eris was discovered and it was calculated to be larger than Pluto. This meant there are probably more objects out there of Pluto’s size.

So, astronomers had to formalize their

definition of planet.

A Planet

•...is massive enough to be spherical because of self-gravitation.

•... orbits the Sun.

•... has cleared its orbit around the Sun of other objects (i.e. not Pluto or Ceres).

Pluto, Eris and all the rest are called dwarf planets because they fulfill the first two criteria.

Do you buy it? Pluto is:

A. A Planet!B. Not a Planet!C. Can’t really care right now.

Space Travel and Colonization

Exploring the Solar System

•Modern rocketry has a practical limit on the velocity that spacecraft can obtain of ~100 km/s.

•This means months or years in transit to Solar System objects.

•Solar flares sweep the inner Solar System showering craft with a storm of charged particles.

Harsh Reality Time

What makes big rocket go zoom?

A. Newton’s Third LawB. Conservation of momentumC. Conservation of Angular MomentumD. Kepler’s First LawE. Tomato Juice

Rocket Science

Rockets work based on Newton’s Third Law (every action has an equal reaction).

By throwing material out the back end quickly (by burning the fuel), the rocket moves faster.

ActionReaction

Saturn V Rocket

95% of mass doesn’t reach orbit.

Of what does reach orbit, >80% is fuel for the return!

The “payload” is <1% of the original mass.

Breaking out of Earth’s gravity well is the hard part.

Space Elevators -- A clever alternative.

Geosynchronous Orbit

Need materials with incredibly high tensile strength to build a space elevator cable.

One excellent possibility is carbon nanotubes. Current technology can make nanotube material with strength 4% of what’s needed for a space elevator. Not too bad!

Getting There

•The first step is getting out of Earth’s gravity well.

•Starting higher is better.

This is why you want to build a space elevator!

This allows you to leave Earth with 10% the required energy as from the surface or less (if you’re clever!).

Moon distance

Geosynch.

Tricks out of the gravity well...•Orbital elevators (already discussed).

•Ramjets -- a novel type of engine that has no moving parts, but controls thrust and ignition by varying engine shape and openings.

A ramjet cross-section

Ramjets can theoretically reach orbital velocity in the atmosphere!

Hohmann Transfer OrbitsThe lowest energy way to get anywhere in the Solar System. Simply change from one accepted orbit to another with “burns” of fuels.

Example Earth-Mars transfer orbit. One idea is to leave a “cycler” craft in the Transfer Orbit to provide links between the two ends.

Earth

Mars

PropulsionChemical Rockets and their analogues are the most likely form of technology. Requires trucking reaction mass with you.

Solar Sails are another possibility!

Light carries a significant amount of momentum. By absorbing or reflecting that momentum, the sail will accelerate.

Sunlight

Aerobraking

•A good way to shed velocity when you want to enter into orbit around a planet.

•Skim the upper part of the atmosphere and use friction to slow down the space craft.

•The Mars Reconnaissance Orbiter successfully used aerobraking to enter and stability orbit around Mars.

Difficulties en route•Time: Typical transfer orbit times on the Earth-

Mars orbit are 250 days or more.

•Generally, not a lot of space to move around in. Space = more mass = more energy required.

•Psychological issues.

•Significant time lags to link with Earth (4-12 minutes one way depending on alignment).

•Long term physiological changes like muscle and bone deterioration.

Coronal Mass Ejections from the Sun

Large ejections of charged particles at high energies.

Particle radiation can kill humans quickly. Requires shielding -- a big lead-lined vault.

Meteorites -- small rocks moving at high relative speeds in the solar system.

A piece of gravel moving at 2 km/s packs the same energy as 100 kg moving at 100 km/h

These can punch through spaceship walls.

Requires shielding or active defense.

Supplies! What humans need...

•Air, or specifically, free Oxygen (O2). A human body requires 1 kg of O2 per day.

•Heat and Pressure -- You need to live at a pressure of 100 kPa and a temperature of 290 K. Space is ~0 kPa and T=10 K to 10,000 K depending on where you are.

•Water -- You need about 2-4 kg of water per day.•Food -- 1-2 kg of food per day•1.5 metric tons per person for a one-way trip to

Mars. Less if there’s active recycling, e.g. by plants, but this costs mass as well!

Once you’re there...

•An outpost needs to continue to supply the basic needs humans.

•At least you get back gravity.

•Hostile environments are discouraged: i.e. not Venus or Mercury.

•Try to use some ambient materials: metals for building, minerals or ices for volatile compounds like water.

What about power?

•There won’t be fossil fuels wherever you go.

•You need power from nuclear or solar (or, in certain situations, wind or thermoelectric).

•Bringing solar panels is a lot of mass and manufacturing them there is hard.

Terrestrial Planet Colonization

Mars is probably the best planet for long term isolated colonization owing to its terrestrial like seasons and days, presence of water ices, and thin atmosphere.

The moon is closer and far easier to support (2-3 day travel time).

No atmosphere or volatile elements. Good for telescopes!

Should we, as a civilization, invest a major effort in colonizing another planet?A. YesyesyesyesyesB. YesC. <shrug>D. NoE. Oh Heck No!