Introduction to the Solar System

Pluto is the only planet in the solar system discovered in this century
Recall that Neptune was found by calculating from the irregularities of Uranus' orbit, and by luck
But it turned out that Neptune's gravitational interaction with Uranus did not explain all it's irregularity
And Neptune itself had some unexplained motion
These two things suggested that there was some mass out there, beyond Neptune's orbit
Percival Lowell, in 1905 began a program of searching for the 9th planet
Percival Lowell had founded an observatory in Flagstaff, AZ, partly based on a program to look closely for the 'canals' on Mars
Lowell created a lot of controversy, but in the end was an enthusiastic supporter and popularizer of astronomy
In 1930, Clyde Tombaugh discovered Pluto with the Lowell Observatory telescope, by comparing photos
To search for asteroids or for things moving in the solar system against the background of stars (like Pluto) astronomers used
He looked at thousands of photographic plates, 2 million stars, using a blink microscope
These devices are still used -- you 'blink' between 2 photographic images taken a few days apart. Anything that moves against the background stars pops out at you
Pluto is named after a god of the underworld
Pluto was given the symbol 'PL', which also happen to be Percival Lowell's initials
Pluto was found to be quite small -- smaller than our moon, not a gaseous giant
It is the sixteenth largest body in the solar system -- many moons are bigger
We can calculate its mass from observations of its orbit and Newton's Laws
We can figure out how big it is by its brightness and distance

Just a dot -- like the 1930 discovery photo
Pluto's orbit is inclined to the ecliptic, and it is quite eccentric
In fact, Pluto's orbit crosses Neptune's, and for several decades every 248 years (its year) Neptune becomes the farthest out planet
Pluto's orbit takes it as far out as 50 AU, and as close in as 30 AU
Recall from the discussion of impacts that whenever one body crosses the orbit of another, there is a possibility of impact
Why doesn't this happen with Pluto and Neptune?
Pluto goes around the Sun exactly twice in the time it takes Neptune to go around 3 times
This is called a resonance, and it always keeps Pluto and Neptune apart when the orbits are crossing
In 1978, people were photographing Pluto at the US Naval Observatory to get a better 'fix' on it and calculate a more accurate orbit
They saw a slight elongation on one side -- Pluto's moon, Charon, was discovered
But Charon is very big -- half the size of Pluto
Unusual in the solar system -- only Earth has a moon of comparable size ratio
SL: With Hubble, we can see the Pluto/Charon system quite clearly
Charon was the boatman in Greek mythology who operated the ferry across the River Styx in the underworld
Charon orbits Pluto every 6.4 days
Pluto rotates every 6.4 days (a plutonian day)
They are both tidally locked to each other -- they both present only one side to each other
BOARD: Charon orbits perpendicularly to the plane of the solar system
It appears that Pluto also spins like this
Length of Pluto's day found by looking at its light curve
How the brightness of a planet changes over time tells us the length of the day

Using infrared spectroscopy, we look for characteristic lines in the sunlight reflected off the surface
Being the furthest out planet, it may also be the coldest (Triton may be colder)
It's probably about 46 K, and the surface is covered mostly with N2 frost, with methane and CO frost mixed in
In the early 1980's, it became obvious that the plane of Charon's orbit was going to shift to be in a direct line with Earth
That meant that Charon would go directly in front of Pluto, and then directly behind -- mutual occultations
Astronomers were excited, because this meant that by looking at the light curves, and knowing how one disk was going to cover another, you could deduce where patches of light and dark were on the surface
And, with one covered up (Charon behind Pluto, for instance) you could look at the spectrum of just one, to see if there was a difference between what the two surfaces were made of
You would expect that its moon, presumably formed in the same region, would have a similar surface
The amazing thing was that this only happens once every 124 years, and we discovered it just a couple of years before the next one was to happen
But spectroscopy reveals that Charon has a darker surface that is mostly water ice, with some dirt mixed in
It's possible that because Charon is so light, all its methane has escaped and been recaptured onto Pluto's surface as a frost
Charon's size relative to Pluto, strange orbit, and different composition hint at an unusual origin
It's possible that Charon was formed during a giant impact between Pluto and another body, the way the Earth's Moon was formed
This kind of thing is an example of a shifting paradigm in planetary astronomy
It looks like giant, random impacts were a major stochastic force shaping the early solar system
By analyzing the Pluto-Charon light curves with a computer, we could calculate rough maps of the surface of Pluto and Charon
Once again, then came Hubble
We could actually get a direct look at the surface, although it takes considerable computer enhancement (and attendant errors) to get the rough maps
But they compare well with the maps derived from the mutual occultations

Although we didn't know the real mass of Pluto (Pluto-Charon system) until Charon was discovered in 1978, we suspected that it's mass wasn't sufficient to explain the orbital irregularities of Neptune
So there was a decades long search for a 10th planet, Planet X
As we looked into the outer solar system, we found objects that challenged the neat division of solar system bodies into planets, moons, asteroids and comets
BOARD:In 1977, astronomers found an object 1/10 the size of Pluto orbiting in an eccentric orbit between Uranus and Saturn
It appears to be a comet, but it is huge!
Objects that formed in the outer solar system are some rock, mostly ice and frozen gases
All these things are comets of a sort, no matter how big they are
What matters is that comets are objects that have not been subjected to the heating that comes with formation near a giant planet
So they contain primordial matter -- stuff directly condensed and frozen from the original solar nebula
BOARD: When Chiron got closer to the Sun, by 1987 we observed a sublimed atmosphere around it. Frozen gases had started to sublimate, and created a giant 'active comet'
Other smaller objects have been found between Saturn and Neptune, in orbits which appear to be unstable.
They can only have been there for a few million years, and their orbits will only be stable for a few million more
BOARD: In 1993, astronomers found another large body on a very eccentric orbit between Saturn and Neptune
It is called Pholus, and it has the deep red color associated with what we think the primordial organic matter in the outer solar system should look like
The carbonaceous material has been exposed to space, UV, cosmic rays, and dust particles which probably turns it into complex organics, which have that kind of redness
These giant comets in transition between the Kuiper belt and the outer solar system are called the Centaurs
But in 1992, CCDs became so sensitive that they could see small (100's of km) objects out beyond Pluto

BOARD: The first Kuiper belt object -- 200 km in diameter, orbiting just outside Pluto's orbit, but crossing it like Pluto crosses Neptune's orbit
Since then, 32 Kuiper Belt objects have been found
About half are at the same distance from the Sun as Pluto, (40 AU)
Half are farther out -- 43 AU
All between 100 and 400 km
Most of the dull red color associated with old, unaltered cometary material (carbonaceous)
The rapid discovery after the introduction of CCDs with a known sensitivity allows us to estimate, probabilistically, how many objects there may be in the Kuiper belt
It looks like there are about 35,000 giant comets over 100 km in diameter
And many, many more that go all the way down to snowballs
The means that the Kuiper itself would have a mass 100 x as much as the main asteroid belt between Mars and Jupiter

It was known that comets coming into the solar system could not be permanent objects, because they would sublime away after too many passes by the Sun
Halley's comet, for instance would only last for 10,000 orbits, or 1 million years
Oort in 1950 had proposed that 10 Billion comets existed way outside the solar system (50,000 to 100,000 AU)
The existence of these neatly explained the number and orbits of long-period comets
But the prevailing view was that short period comets just came indirectly from the Oort cloud, perturbed by the giant outer planets
Up until 1993, the Kuiper belt was a theoretical idea put forth by Gerard Kuiper in 1951
Kuiper thought that the Kuiper belt could better explain the origin of the short period comets
He envisioned a belt of comets beyond Neptune, out where there was still plenty of cold gas and dust in the solar nebula
But area wasn't dense enough for planets to form -- they stopped at planetesimals
In the 70's and 80's, computers became sophisticated enough to simulate the effects of the giant planets' gravity perturbing Oort cloud comets
They found that it wasn't really possible for the gravity of the outer planets to turn long period comets into short period comets
With the discovery of the Kuiper belt in the 90's we have another example in astronomy of visionary theoretical work being confirmed by observation

The Kuiper belt has been pummeling the inner solar system with small to medium size comets at the rate of about 1 per two years
Because it is unstable, some people have suggested (notably Alan Stern at SWRI in Boulder, who is teaching a graduate class in this next fall) that the Kuiper belt once had many more objects
In particular, he has suggested that there were many more very massive objects, and that Pluto, Charon and Triton are the last survivors of this population of gargantuan comets.
Recall that Triton, Neptune's giant moon, orbits around Neptune the wrong way
It is probably a captured massive object from the Kuiper Belt
Likewise, because Pluto, Charon and half the other Kuiper belt objects are in a 3:2 resonance with Neptune, they are also there because of Neptune
And because of Pluto and Charon's relative size, strange orbit and differing compositions, it is possible that the pair was formed by a collision of two giant Kuiper Belt objects

The very last solar system object Voyager passed by before it said goodbye to the solar system was Triton
The path of the spacecraft had to be altered to fly out of the plane of the solar system, over the pole of Neptune
It was able to get within 25,000 miles and take close-up pictures of the surface
We knew Triton had a very thin atmosphere, from spectroscopic studies
Voyager found one -- a thin nitrogen atmosphere, with a little bit of methane
Similar to Titan's but much thinner
The surprising thing was that when Voyager got close, we saw an object way out in deep, frozen space like this, that apparently was geologically active
There were very few craters seen on the surface, indicating vigorous volcanic activity going on right up to the present
The surface is made up of dirty frozen ices of N2 and methane that move around with the season
SL: Neptune's year is 165 years long, and polar caps of N2 and methane grow and subside over the seasons
The surface temperature was the coldest yet measured 38 K or -400 F
The really amazing surprise was finding active volcanoes on this world
Dark, smoke-like material seems to be erupting vertically into the thin atmosphere, and then getting caught in the prevailing wind
The geysers or plumes seem to reach 26,000 ft into the atmosphere
Sooty material falls downwind, as the methane reacts with the weak solar UV to form dark organics
Frost can be seen condensing around the base of the geysers
A close inspection of the surface has a hilly terrain, caused by ice-tectonics (ice crust crumpling), and frozen ice lava lakes
But what could cause all this geological activity so far out in the solar system?
If Triton was captured, it was probably captured in an elliptical orbit
Most captured orbits are highly elongated, because objects pass each other going fairly fast
But Triton's orbit is circular
It has been suggested that as tidal forces, the friction of planetary motion, circularized the orbit, they caused Triton to heat up internally
Perhaps the N2 and methane volcanism we see there is a result of this left-over heat

So we have this trio of giant objects, the great icy bodies of the Kuiper Belt
And we see a plethora of smaller objects, in a deep freeze, tempting us with what the solar system may have been like when it formed
We have studied biggest one, Triton, and have found that it has an evolution more like a moon
That leaves Pluto, the king on the edge of the Kuiper belt, cold and enigmatic, and totally unexplored
Our best maps are fuzzy, speculative patchworks
But we have a chance to go to the Kuiper belt, and see its objects directly
We have shown that we can travel to the outer reaches of the solar system, the Voyagers are way past Pluto
Pluto Express -- two tiny, low cost spacecraft, consistent with the cost pressures felt by NASA
They could launch in a few years and get there by 2010 (12 years)
It would be good to get there by then because Pluto will still be as close to the Sun as it could get (1989 perihelion)
It acts like a giant comet, with a coma from the warming of the distant Sun
If you consider Pluto a planet, then it is called an atmosphere
But as Pluto recedes from the Sun, the atmosphere will freeze out as frost
Some argue that we should get there before that happens, but that's just good advertising
The mission is not funded, but Dan Goldin, the maniacal manager of NASA, seems to be behind the idea
Would use the most lightweight high-tech hardware that it could
Science goals are to:
Characterize the global geology and geomorphology of Pluto and Charon, imaging both sides of each
Map the surface composition (spectrometers)
Characterize Pluto's neutral atmosphere, including composition, thermal structure, and aerosol particles.

In the past 35 years, we have conducted a reconnaissance of our solar system
All the planets but Pluto have been explored, and we have sent orbiters to Mars, Venus, Jupiter, with an orbiter destined for an asteroid in 2.5 years (the NEAR spacecraft)
But the Space Age was born in the 1950's at the depth of the Cold War, and during a nearly unanimous belief in the 'endless frontier' of science
Because of societies' consensus that there was so much good in developing technology to stay ahead of the Soviets, our powers to explore space grew in leaps and bounds
But today, science is under intense scrutiny by society. The limitations and destructiveness of technology in our lives is constantly being examined.
It is no longer necessary for science to save us from the Soviet nuclear threat, and it is no longer clear that technology is capable of providing us with a utopia
What we are beginning to see is that our consumptive lifestyles and exponentially growing population are beginning to change our planet on a global scale
Because we have the power to into space and explore and monitor other planets, it now looks like we have another way of achieving a perspective we didn't have before
We can turn our space technology around and look at our home as a planet
We can monitor the weather, the health of crops, and look for global changes
This is along the lines of the exploration of the other planets, but we can do it in fantastic detail
We can look at the rate of rainforest clearing precisely
We can measure how much CO2 we are putting into the global atmosphere through cars and slash-and-burn agriculture
So although space technology has developed along with B1 Bombers and VCRs, products of a technologically and resource-decadent society, it can actually help to illuminate the consequences of our choices
The latest large scale mission at NASA is called the Mission to Planet Earth
It is an ambitious one to monitor every aspect of the health of the planet from space
It is also by far the program that is most under fire at NASA.
Conservative politicians, worried about their industrial constituencies, fight to have the threat of increased global environmental data available
Studying the Earth and becoming sensitive to the man made changes will bring up new questions as to the long term effects of the way we use resources such as air and water
It will necessitate investment in cleaner, more conservation-minded ways of doing things, from the latest and greatest green technologies to the realization that we have to lead simpler lives
The last video I'm going to show in this class is about this new phase in space exploration, where we endeavor to find explore our own spaceship through the cosmos, Earth

VIDEO: MISSION TO PLANET EARTH, SPACE AGE SERIES (First half)

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