Exploration and Colonization of the Solar System

With the Apollo program we made our first visit to another object in our solar system: the Moon. As we discussed in the last class, the next logical/likely step is a visit to the planet Mars. NASA currently has a multi-mission, unmanned program to investigate Mars, and determine if a manned mission is necessary, and determine if the prospects for colonizing Mars anytime in the distant future are feasible.

Sometime in the future, humans will visit Mars, and other bodies in the solar system. During this class I want to discuss the nature of these bodies, the logistics of visiting them, and issues involved with their eventual colonization by humans.

Let's first talk about the solar system. At the center of the solar system there is a normal star, the sun. The sun supplies the heat and light essential for the majority of life to exist on Earth. As discussed in the last class, the sun is a huge fusion reactor where hydrogen nuclei (protons) are smashed together ("fused") to form helium.

The radius of the sun is 7 X 10⁵ km (4.2 X 10⁵ miles), about 110 times the radius of the Earth. The sun has a mass of 2 X 10³⁰ kg, thus the average density of the sun is 1.41 gm/cm³, 50% greater than that of water! But at its center, the density is much higher: 160 gm/cm³. The surface temperature of the sun is about 6000^oF, but at its center the temperature is 12 million degrees!

At the current rate of burning, and in its current configuration, the sun will live for five billion more years. Compare this with human civilization: humans have been civilized (for example reading and writing) for about 6,000 years. The Earth will be a stable platform for life for a time span that is 1 million times longer than this! (If we don't kill ourselves off, we have plenty of time to branch out into the solar system.)

The distances in astronomy are very large, and even in the solar system become unwieldy. To remedy this, astronomers use two convenient definitions: the light year, and the astronomical unit. A light year ("ly") is simply the distance a photon of light can travel in one year. The speed of light is 3 X 10⁵ km/s, a year is 3.15 X 10⁷ seconds, so a light year is 9.5 X 10¹² km (5.7 trillion miles). An astronomical unit ("AU") is the average Earth-sun distance: 1.5 X 10⁸ km (93 million miles).

There are nine "major" planets that orbit the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto (ordered by distance). Data for these bodies can be found in the table, below.

The planets are classified into two distinct categories, terrestrial (mostly rock), or Jovian (mostly gaseous). Mercury, Venus, Earth and Mars are terrestrial planets. Jupiter, Saturn, Uranus, and Neptune are Jovian planets. This division is apparent when comparing their average densities (Pluto does not fit into either category, it is believed to have a large ice component).

Terrestrial planets:

Mercury is an inhospitable place-on the day side it is hot enough to melt lead, on the night side it is extremely cold. This is due to the fact that Mercury, like our moon, has no atmosphere to moderate its climate. Mercury rotates very slowly, a single "day' requires 57 Earth days. Its "year" is only 88 days. The surface of Mercury is heavily catered, more so than any other planet. Without a protective atmosphere, the surface of Mercury is bombarded by meteors and solar radiation. Mercury has no natural satellites.

Venus is probably even more inhospitable than Mercury. Due to a dense atmosphere and runaway greenhouse effect, the surface of Venus is just as hot as that of Mercury. The atmospheric pressure at its surface is 90 times that of the Earth, equivalent to an ocean depth of 1 km! Venus probably started out much like Earth, but the slightly closer distance to the sun allowed any existing water to escape into space, making Venus a dry, hot world, and inhospitable to life.

The surface of Venus is quite young, with the oldest formations dating back to about 800 million years. Apparently, Venus had a long history of volcanic activity, though very little is present today. The dense atmosphere means that few meteors have impacted its surface. The dense clouds hide the surface from view.

A "year" on Venus is 225 days, while a "day" is 243 Earth days. The atmosphere is toxic, including sulfuric acid droplets. The chemical composition of Venus is quite similar to the Earth, as are most other gross properties. Venus has no natural satellites.

Mars is a small, cold, apparently dry world. It is possible that Mars has a considerable quantity of water locked into the surface soil/crust. The atmosphere of Mars is very thin (1% or Earth's), and mostly carbon dioxide. The rotation period of Mars is 24 hours and 37 minutes (similar to Earth). The Martian "year" is 1.88 Earth years. Mars has two very small satellites, Phobos and Diemos (12, and 7 miles in diameter, respectively). In its distant past, Mars could have been wetter and warmer, possibly supplying the conditions necessary for life to evolve and exist there.

Jovian Planets:

Jupiter is the largest planet in the solar system, nearly 11 times the diameter of the Earth. If it had been about 100 times more massive, it would have undergone fusion in its core and become a star. Jupiter is one large ball of hydrogen (90%) and helium (10%). There may be a small rocky crust at its center, but most of it is gaseous-though near the center the pressure is so high this gas behaves as a metal! Jupiter probably has no true solid surface, thus there is no place to "land" a spacecraft. It is unlikely that life could ever evolve on such a planet.

The disk of Jupiter is crossed by bright and dark bands. These are due to zonal wind patterns, and cloud chemistry. Near the edges of these bands, great rotating storms form. These create enormous vortices that are like hurricanes, but larger and more violent.

Jupiter is very cold. It also has a strong magnetic field that creates zones of intense radiation (the Jovian equivalent to the van Allen belts), that are too hostile even for unmanned space probes. Jupiter rotates very quickly, once every 9 hours and 55 minutes. It takes about 12 years to make a complete trip around the sun.

Jupiter has at least 16 known satellites, and a very tenuous ring system. Most of its moons are tiny, a few tens of miles in diameter, similar to those of Mars. Four of them, however, are much larger. These are the Galilean satellites (named after their discoverer, Galileo). Three of these have diameters that are larger than our moon.

The largest of these is Ganymede, about the same size as Mercury (though of much lower mass due to its lower density). The closest to Jupiter of these four is Io. Due to the gravitational tug of war between Jupiter and the other big moons, Io is repeatedly stretched. This causes its core to melt, and creates the most volcanically active object in the solar system.

Europa is the smallest of the Galilean satellites, but perhaps the most interesting. The surface of Europa resembles the pack ice of the Arctic ocean: numerous cracks and fissures. It is likely that below this layer of ice there is a deep ocean of liquid water. If hot water volcanoes exist at the "sea" floor, it is possible that life could have evolved and presently exist in the oceans of Europa. Missions to send a probe to Europa to investigate the ocean hypothesis are being contemplated.

Saturn is known by its beautiful ring system. Saturn is about 20% smaller in diameter than Jupiter. Its average density is the lowest of all of the planets, less than that of water! It has a similar composition and structure to Jupiter, and probably has no true solid surface. At its great distance from the sun, the cloud tops are very cold (about -300F), but in its interior it is warmer due to gravitational contraction (as seen in Jupiter). Saturn's disk shows weak banding structure, and small storms. Both are much less prominent than those seen on Jupiter.

Saturn also rotates rapidly, its "day" is only 10.5 hrs long, though its "year" is 29.5 Earth years!

The rings of Saturn are small particles of ice and rock that are the remnants of a satellite that either never formed, or was destroyed by the gravitational pull of Saturn. The rings are 250,000 km in diameter, but only about 2 km thick! The amount of material in the rings is actually quite small, enough to make up a spherical satellite 100 km in diameter.

Saturn has at least 18 satellites (more seem to be discovered every year), half of them are less than 100 km in radius. The largest Saturnian satellite is Titan, similar in size to the Galilean satellites of Jupiter (its radius is 2575 km), and larger than Mercury (though less massive).

Titan is unique among planetary satellites in that it has a dense atmosphere. While several planetary satellites have tenuous atmospheres, that of Titan is dense, the surface pressure is 50% higher than that at the surface of the Earth!

We know little about what is going on at Titan's surface due to the opaque clouds. There are hydrocarbons (methane, ethane) in the atmosphere, and these could feasible precipitate out forming a hydrocarbon rain, and seas and oceans on its surface. The Huygens probe aboard the Cassini spacecraft will enter that atmosphere (in November of 2004) and help us better understand this unique place.

Uranus is not quite a smaller version of Jupiter as one might suppose. It is mostly rock and ice, with only 15% of the planet being composed of hydrogen. The atmosphere, however, is mostly hydrogen and helium (+2% methane). In a telescope, Uranus appears a light blue color due to the absorption of red light by methane in its atmosphere. Unlike Jupiter, the atmosphere has very little cloud structure visible. This maybe due to a high-altitude haze that obscures these features, but it may also be due to climatic effects.

Uranus has a peculiar orientation: its rotation axis lies very close to the orbital plane. So Uranus appears to roll around its 84 yr orbit, with each polar axis having long periods of constant daylight (40+ yrs!), followed by equal periods of darkness. In 2007 the sun will be directly over the equator, and may give rise to larger storms. It rotates rapidly, a "day" takes 10.7 hours to complete.

Uranus also has a small ring system, consisting of dirty ice and rock. Uranus has at least 15 natural satellites, only five of them are more than 100 miles in diameter, the largest (Titania) is about 1000 miles in diameter.

Neptune is the near twin of Uranus. Its diameter is just a few percent smaller, while its density is a bit higher. Thus, it is slightly more massive than Uranus. The structure appears to be almost identical, an icy/rocky core with a dense atmosphere of hydrogen and helium. The storms on Neptune, however, seem more violent, and appear to change rapidly. The cause of these changes remains unknown. Neptune rotates rapidly (16 hours), but takes 165 years for one trip around its orbit.

Neptune has a tenuous ring system, and at least 8 moons. Seven of these are small (< 250 miles in diameter), but one is large: Triton (1700 miles in diameter). Triton is the only large planetary satellite that orbits its parent body in a retrograde fashion (counterclockwise as seen from the north pole). This suggests it was captured early in the history of the solar system as it passed too close to Neptune.

Triton is believed to have a large fraction (25%) of its mass in the form of water ice. Its surface has few craters, suggesting it is young, and is being shaped by recent volcanic activity. At the surface temperature of Triton (-391F), methane, nitrogen and carbon dioxide freeze! It is possible that the volcanoes that have been observed to erupt on its surface spew liquid nitrogen!

Pluto is the oddball of the planets. In many ways it is more like Triton than any other major body in the solar system, and is actually smaller than the seven largest planetary satellites! Pluto's density suggests it is about 70% rock, and 30% water ice. Pluto is covered by bright and dark regions, the origin of which remains unknown. Pluto was discovered in 1930 by Clyde Tombaugh, a member of the NMSU Astronomy department until his death in 1995.

Pluto has an unusual orbit which carries it inside that of Neptune. It is likely that some sort of interaction with Neptune in the distant past led to it being "captured" into its current orbit. It has a small moon named Charon. Pluto is the only planet that has not been explored by unmanned missions, and thus remains a great unknown. Its enormous distance means that even with the Hubble, it is a tiny target. Pluto takes 248 years to complete one circuit around the sun.

The Scale of the Solar System

We noted the distances of the planets from the sun in the preceding, but due to their size, it is hard to get a feel for these numbers. Let's use some familiar means to get a handle on the enormous distances we are talking about. A normal passenger jet flies at a speed of just under 1,000 km/hr. The circumference of the Earth is 40,000 km. Thus, it takes about 40 hours to fly around the Earth at this speed.

The moon is 380,000 km from Earth, if we could fly our jet to the moon, it would take 380 hours, or about 16 days, to get there. To reach the sun, it would take 17 years at this speed, and to get to Pluto, 646 years!

Of course, a velocity of 1,000 km/hr is much to slow to escape the Earth's gravity. As we have discussed in previous classes, the escape velocity of the Earth is 11.2 km/s = 40,000 km/hr. This is the speed required to visit the moon, and was achieved (or exceeded) by the Apollo spacecraft, and all interplanetary space probes.

If we send a rocket to Mars at this velocity, how long would it take? At closest approach, Mars gets to within about 60 million kilometers of Earth. Thus, if we could shoot straight to Mars, it would take our spacecraft 62 days to fly that distance. But this is, of course, impossible. Both Earth and Mars are orbiting the sun, and thus are constantly moving. You have to shoot to a position well ahead of Mars' position at launch to actually hit it! In fact, planned manned missions to Mars take 180 days or longer (for example, see the trajectory that the Mars Global Surveyor used).

For example, the Earth travels 2(pi)R in one year, with R= 1AU (1.5X10⁸, or 9.42 X 10⁸ km per year. This works out to an orbital speed of 29.9 km/s (about 65,000 mph). Mars is a tad slower, 24 km/s. In the 180 day transit period to Mars, the planet will have moved about 94^o in its orbit. So you aim ahead to that point with your spacecraft.

Even radio communication to the various corners of the solar system is difficult due to the great distances. Radio waves are a low energy form of electromagnetic radiation, and hence they travel at the speed of light. The speed of light is 186,000 miles per second = 3.0 X 10⁵ km/s. Even though the moon seems relatively close, a message to the Apollo astronauts and their reply would take several seconds. The round-trip time for a signal to the moon is 2.5 seconds. For Mars, the round trip time is 400 seconds (6.7 min, when it is nearby! More when further away.).

A trip to Pluto (at 11.2 km/s) would take nearly a decade! The round trip communication time for a spacecraft visiting Pluto would be 11 hours! [Look here for the plans and trajectory of a possible Pluto probe.]

Any type of normal communication can not be made when humans begin to leave the near-Earth environment.

Can humans visit worlds such as Mars? During any trip you have to be concerned with: