Greek and Trojan Asteroids

Jupiter has a two groups of asteroids lagging behind it in its orbit and leading it as it goes around the Sun. These asteroids are called Trojan asteroids and the two groups are broken down into the two camps during the Trojan War; the Greek camp which is the leading group and the Trojan camp which are the ones lagging behind.

Both of these groups are situated at Lagrangian points which are gravitational balanced points in a three-body system. The Greek asteroids are located at L4 in the Sun-Jupiter system and the Trojan asteroids are at L5.



From our post about Lagrangian points, we know that the Greeks are at 60° ahead of Jupiter in its orbit and the Trojans are 60° behind. Where did these asteroids come from?

 

The leading theory is that these asteroids are remnants of the formation of the Solar System and in as they moved in space, the asteroids got caught in the L4 or L5 Lagrangian points, which we learned is stable. Once they got to those points, they were there permanently.

 

One last thing about Trojan and Greek asteroids is that all the ones that have been discovered are named after Greek or Trojan heroes from the Trojan war.

Lagrangian Points

In a three-body system in orbital mechanics, there are five points where the combined gravity of the two larger objects can affect the orbit of the smallest body in the system. The combined gravity will provide the right centripetal force to allow the small body to orbit with the other two objects. These points are called Lagrangian Points, after Joseph-Louis Lagrange who published an essay on three-body physics.

Three of these points are collinear, i.e. lie along the line connecting the centers of the two larger masses. However, these points are unstable. A slight deviation in the location or the speed will cause the smallest mass to move towards either other mass, depending on which direction it moves. The other two points are coplanar, i.e. lying in the same plane as the other two masses. These two points create an equilateral triangle with the two larger masses.

Note: there is a point between the yellow mass and the blue mass where the gravity from each mass is completely cancelled out. L1 is not at this point.

L1 is the first Lagrangian Point and is located between the two masses. It is closer to the smaller mass since an object would need to be closer to the smaller mass to have a larger influence from the smaller mass. For the Sun-Earth system, L1 is 1.5 million km from Earth, outside the radius of the Moon's orbit.

L2, the second Lagrangian point, is located opposite L1 around the smaller mass. Its distance is approximately the same as L1.

L3 is opposite the smaller mass on that mass's orbit. It may seem counterintuitive that L3 balances the forces from the two masses, but the small mass does have a gravitational influence on the larger mass, L3 has the same orbital period as the small mass. L3 is very unstable and it would be practically impossible to keep a satellite there for any extended period of time without making adjustments with small thrusts of rockets.

L4 and L5 lie approximately on the orbit of the small mass, either 60° ahead or 60° behind. These two points are the most stable because the distance from either mass is the same. The gravitational force from each object is at the same ratio as the two masses, causing the total net gravitational force to act through the common center of mass.



It should be noted, that L3, L4, and L5 do not lie exactly on the smaller mass's orbit, but a little outside.

This post is not intended to make you an expert on Lagrangian Points, but to give you an idea of what they are. The Greek asteroids and Trojan asteroids near Jupiter lie near the L4 and the L5 points of the Sun-Jupiter system and will be the next topic.

If you have any questions, please feel free to ask me. Post a comment here, write on my Google+ wall, or tweet me (patronaut0709). I'll try to answer your question as well as I can.

Ida and Dactyl

Galileo Probe image from NASA

Typically, asteroids are too small to have their own satellites. The gravitational force exerted by an asteroid is too minor to hold on to any object larger than a boulder if that object passes close by. The object are moving faster than the escape velocity of the asteroid. However, there are exceptions. The prime example is the dual system of Ida and Dactyl.

243 Ida was the 243rd asteroid discovered in the Asteroid Belt. It was originally discovered by Austrian astronomer Johann Palisa in 1884. Based on the spectroscopy, Ida is an S-type asteroid with an albedo of 0.2383. It has a semi-major axis of 2.862 AU, taking 4.84 Earth years to orbit the Sun. It has an average diameter 31.4 km across which is kinda weird to use since it is longer than it is wide.

In 1993, the space probe Galileo visited Ida on its way to explore Jupiter. It was in this visit where Dactyl was discovered. Dactyl is only 1/20th the size of Ida, only about 1.4 km in diameter. It is difficult to determine Dactyl's orbital characteristics without much more observation, but because it is so small in relation to Ida, to determine how it orbits Ida, Dactyl and Ida will have to be visited. Constraints to its orbit did allow a density to be roughly detemined and Dactyl is lacking metallic minerals. Ida and Dactyl share similar characteristics, so it is possible that they formed at the same time.

1 Ceres

NASA's Solar System Explorations Page



Ceres was the first asteroid discovered in the Asteroid Belt on 1 January 1801 by Giuseppe Piazzi. When he first discovered it, he thought it was a planet, as it was a common belief at the time that there might be a planet between Mars and Jupiter because of the large gap between the two planets.
Because there was not enough information to determine the orbit of Ceres, it wasn't confirmed to exist until December of 1801. After close observation of Ceres, it was determined that it was not a planet, but something new. It was then that William Herschel coined the term "asteroid" as the object had a star-like appearance so was difficult to distinguish from the background stars. However, after observing Ceres and other asteroids over the course of days and weeks, it was apparent that asteroids move faster in the sky than the stars since they are much closer to Earth.

Ceres is small compared to the planets and many satellites, but it is the largest object in the Asteroid Belt by a wide margin. 

• It has a diameter of about 950 km or the size of Texas
• It's mass is 9.47x10²° kg, or about 0.00016% of the Earth. Despite its mass, Ceres contains 25% of all the mass in the Asteroid Belt
• It has a semi-major axis (orbital radius) of 2.76 AU

As shown in the opening picture, Ceres is relative spherical. Much like the planets, is an oblate spheroid with a wider diameter at the equator than the poles. This shows that it rotates on an axis uniformly. Other asteroids with irregular shapes kinda tumble and wobble in space as they rotate.

We've learned more about Ceres as telescopic observations became more sophisticated and advanced. Ceres does not fit into the three main types of asteroids mentioned in the previous post. In fact, like planets, it is differentiated (layered) with a rocky core, an icy mantel, and an outer crust.

NASA's Solar System Explorations Page

For the next two centuries, Ceres was still considered an asteroid as there was no other category of object it fit in. It's official designation was 1 Ceres as it was the first asteroid discovered and Ceres is its proper name. However, in 2006 when Pluto was demoted from planet status to minor planet, it was concluded that Ceres should be upgraded. Ceres is now officially considered a dwarf planet. It is too small to be a planet (and where the lower limit to be considered a planet is still murky) but too large and too regularly shaped to be an asteroid.

One final weird thing discovered on Ceres is that it is slightly active. An object as small as Ceres should not have any sort of eruptions going on, but in January of 2014, the Herschel Space Telescope discovered water vapor plumes emitted from the surface. We will learn more about Ceres in the coming years when the space probe Dawn arrives at Ceres and begins its exploration. Ceres is also the third most likely spot for crewed missions to visit after the Moon and Mars.

Asteroids

Asteroids are believed to be remnants from the formation of the Solar System. The majority of these objects are found in the space between Mars and Jupiter called the asteroid belt.



As seen in the above image, there are also a few asteroids that are 60° ahead of Jupiter called "Greek" asteroids and some that are 60° behind Jupiter called "Trojan" asteroids. Not seen, but will be discussed are the three-A asteroid groups: Amor, Apollo, and Aten. These are near-Earth asteroids that are a concern for scientists and are searched for and catalogued by astronomers.

In our previous post about Phobos and Deimos, we learned a little about C-type and D-type asteroids. There are two other main types (and no, they are not A-type and B-type) called S-type and M-type. As mentioned before, C-type asteroids (and to an extent, D-type) have strong carbon lines in their spectra. S-type asteroid are stony asteroids, made of silicates, with densities similar to Earth's density, and make up about 17% of all asteroids. M-type asteroids are metallic, with the most common metal found in them iron and some having nickel. This is one reason why mining asteroids might be a lucrative business in the future, when travelling easily from Earth to the outer reaches of the Solar System will be achieved.

253 Mathilde - C-type Asteroid

NASA

15 Eunomia - S-type Asteroid

Astronomical Institute of the Charles University

16 Psyche - M-type Asteroid

Astronomical Institute of the Charles University

 



 Asteroids, generally, come in many sizes, with the largest being 1 Ceres at 950 km in diameter. However, most asteroids are only a few miles in size and are irregularly shaped. It is typically believed that asteroids are planetessimals (baby planets) that were not able to accrete into a planet because of the presence of Jupiter. Measurements of the mass of the asteroid belt show that there is only enough mass there to create a planet with the quarter of the size of a planet.

Asteroids have two parts to their names, a number designation and a proper name. The first few asteroids were just given names, but as more and more were discovered, astronomers started placing a number in front to give an indication of the sequence of discovery. So 1 Ceres was the first asteroid discoved and 253 Mathilde was the 253rd. As of 2013, there are several hundred thousand asteroids named with more than a million probably out there. Many of those asteroids are in the asteroid belt, but as mentioned above, there are thousands leading and trailing Jupiter in its orbit and thousands that are near Earth's orbit. We will learn more about Amor, Aten, and Apollo asteroids in a future post.

Most asteroids orbit independently around the Sun, but there are some that mutually orbit each other.  The most well known pair is Ida and Dactyl, where Dactyl was the first satellite discovered around an asteroid.  The Solar System is a strange place.

Martian Canals?

Hubble Image of Mars (Mercator Projection)

 

By the mid 19th century, telescopes had improved enough that surface features on Mars could be easily observed and examined. In 1877, Italian astronomer Giovanni Schiaparelli noted what he thought were long straight lines on the surface which he named after rivers on Earth and identified them on maps he drew of Mars as "canali" which is Italian for channels or grooves. English speakers misinterpreted canali as canals and thought that the channels on Mars were artificial, created by Martians (which is the correct term for someone from Mars - if there were such a creature).

Schiaparelli Map of Martian "canals"

Percival Lowell Map of Martian "canals"

The Martian canals led rise to a plethora of science fiction which believed that Mars was inhabited by intelligent creatures including "War of the Worlds" by H.G. Wells and "The Martian Chronicles" by Edgar Rice Burroughs (who also created Tarzan the Apeman). 

As telescopic resolution improved with new technology, it became clear that the channels on Mars were not artificial, but were created naturally in the ancient past by running water. In a previous post, we learned that water once flowed freely on the surface, but as the atmosphere was stripped away, allowing the planet to cool and the air pressure to drop, surface water no longer exists on Mars.

What's in the Sky this Weekend?

If you get up early enough, in the pre-dawn eastern sky, you will be able to see Venus, Jupiter, and the crescent moon relatively close together in the sky.

Both Venus and Jupiter will be to the left of the Moon, with Venus being the brighter of the two objects.

Areavoices Via Astrobob

Also, there is Comet Jacques, a magnitude 5.66 comet in the constellation Cassiopeia, a W shaped (or M shaped) constellation in the northern sky.  It should be pretty easy to see with a good pair of binoculars.  Just look for the fuzzy object with a bluish-green hue.

Astronomy Picture of the Day

The Satellites of Mars

Mars has two satellites, Phobos and Deimos. They are named after creatures summoned by Ares (the Greek equivalent of the Roman war god Mars) in the Iliad and their names mean Fear (Phobos - where the word Phobia comes from) and Fright (Deimos). Unlike the Moon, these satellites did not form in the same location in the solar nebula as Mars but rather formed elsewhere (likely, the asteroid belt between Mars and Jupiter) and wandered too close to Mars and were captured by its gravity.

AreaVoices.com

 

Phobos is an 11-km diameter, irregularly shaped object that only orbits 6000 km from the surface of Mars. Compare that to the Moon, which orbits 384,400 km from the surface of the Earth. If the Moon orbited only 6000 km from the Earth, not only would our tides be much higher (see post on tides) but the Moon would appear 64 times bigger in the sky making it about 32° across. At that apparent diameter, it would fill up a sixth of the sky!  Because Phobos is so close to Mars, it only takes about seven and a half hours to orbit Mars, which means that it crosses the Martian sky twice in one Martian day, taking only about four and a quarter hours to cross the sky.  It also orbits retrograde around Mars, meaning that it rises in the west and sets in the east.

 

Deimos is smaller than Phobos, being only 6.2 km in diameter, but orbits much farther away, at 23,500 km from the Martian surface.  At that distance, Deimos takes about 30.3 hours to complete one orbit around Mars, or about a Martian day and a quarter.

 

Both Phobos and Deimos were discovered in August of 1877 by Asaph Hall at the United States Naval Observatory in Washington, D.C. (Quick fact - the USNO is home to the official Master Clock for the US and is also the official residence of the Vice President.)  Despite being smaller, Deimos was actually discovered first on August 12th and Phobos was discovered on August 18th.  The names were suggested by Henry Madan from the Iliad.

 

The reason why Phobos and Deimos are believed to be captured asteroids is because they are similar in composition, albedo, densities of C- or D-type asteroids.

• C-type asteroids (carbonaceous asteroids)
• The most common type of asteroids (make up about 75% of all asteroids
• They have a low albedo which means they do not reflect a lot of light, almost appearing black
• Their compositions are similar to the early solar nebula except for the lack of volatile elements (gases, water, etc) but do contain hydrated minerals (water-containing minerals)
• D-type asteroids
• They have a lower albedo than C-type asteroids
• Their spectra are the strongest toward the red end of the electromagnetic spectrum
• They contain organic, carbon, and anhydrous (lacking-water) silicates
• However, they may have water ice cores

 

Aurorae

NASA APOD

 

The aurorae are the bright, dancing light display seen in the northern and southern latitude on Earth.  The aurorae are caused by ionized particles from the solar wind being captured by magnetic field lines from the Earth's magnetosphere  The particles spiral around the field lines and slam into the atmosphere, creating plasma, which we see as the aurorae.  Depending on the atoms involved, different colors can be seen.

 

 

If there were not a magnetosphere around the Earth, the Earth would be a barren planet.  The magnetosphere not only creates the beautiful aurorae, but also prevents the solar wind from stripping away the outer layers of the atmosphere as it has done to Mars and completely strips away the atmosphere or Mercury.

 

Why do we have a magnetosphere and Mars does not?  The simple answer is the Earth's core.  The Earth's core is a molten ball of iron and nickel, two ferromagnetic metals.  The molten core then rotates, which creates a magnetic field around the core, which in turn encompasses the entire planet.  Mars, however, has a core that solidified over 4 billion years ago and over time, the solar wind was able to strip the outer layers of Mars' atmosphere away, leaving behind the planet we see today.  Our core stays molten from intense heat from the outer layers of the Earth pushing on the core as well as radioactivity in the core.

 

Other planets also demonstrate aurorae, especially the Jovian planets which all have molten iron/nickel cores.

NASA



Earth has two types of aurorae, the Aurora Borealis and the Aurora Australis.  The names only refer to the location of the aurora.  The Aurora Borealis are seen in the northern latitudes in the northern hemisphere and the Aurora Australis are seen at the southern latitudes in the southern hemisphere.

Here is a link to a page that shows the forecast for the Aurora Borealis.  Maybe you may be able to see aurora in your neighborhood!

The Atmosphere of Mars

Mars has a very tenuous atmosphere.  As mentioned before, at the surface, the pressure is only 0.636 kPa, 200 times less than that of Earth.  Because of the low pressure, any liquid water on the surface of water would be immediately vaporized.  Mars may at one time had a more substantial atmosphere, but over time, the gas was stripped by the solar wind because of the lack of a magnetosphere.  Other gases escaped because their average speed is faster than the escape velocity required to leave Mars.  We do know that Mars had a thicker atmosphere in the distant past because of evidence of flowing water (see previous post).

The atmosphere of Mars is mostly carbon dioxide, much like Venus.  However, that is where the similarities end. Venus' atmospheric pressure is 90 times that of Earth.  Because Mars is so much thinner, it did not undergo a runaway greenhouse effect.  Besides carbon dioxide, Mars' atmosphere also contains argon, nitrogren, oxygen, carbon monoxide, and trace water vapor.

Mars would have difficult time maintaining a thick atmosphere because of the lack of a magnetosphere. 

The magnetosphere prevents solar wind from reaching the outer layers of the atmosphere and stripping the gas in Earth's atmosphere.  The magnetosphere is created by the rotation of the molten core in Earth's interior.  Our molten core is made of iron and nickel which are ferromagnetic materials, i.e. they can become magnetic.  As the core rotates, it turns the Earth into a giant magnet which pushes the solar wind away from Earth.  Mars' core does not rotate, so it does not have a magnetosphere.  Not only does the magnetosphere keep Earth safe, but it also creates the beautiful aurora we see near the poles of Earth.

If we wanted to terraform Mars, not only would we have to find a way to thicker the atmosphere, we would also have to find someway to get the core to rotate again or some other way of creating a giant magentosphere around Mars to save the atmosphere from the solar wind.

The Geography of Mars

Technically, the correct term for the title of this post is Areology, from Ares (the Greek god of war) and -logy (the study of something).
Mars has mountains much like Earth.  However, unlike many of the mountain ranges on Earth, Martian mountains were not created by plate techtonics.  Olympus Mons, Ascraeus Mons, Arsia Mons, and Elysium Mons are all taller than Mount Everest. Pavonis Mons is slightly shorter, but has a much wider base.

All five of those mountains are shield volcanoes, which erupt but have low viscosity lava. The lava flows down the sides of the volcano, which lead to wide bases and low profiles.  For the Martian volcanoes, lack of plate tectonics lead to all the shield volcanoes on Mars to get really wide and really tall.

Olympus Mons is the largest of these volcanoes.  It is 21.4 km tall (from the peak to average surface elevation around its locality)and as shown in the picture below, the base has an area equivalent to the size of Arizona.  This means that the slope of Olympus Mons is only 5 degrees from base to peak.  Olympus Mons is not the tallest mountain in the solar system. That honor belongs to Rheasilvia on the asteroid Vesta. It has a height from base to peak of 22 km, though only 12 km are above the average surface elevation of Vesta.  Mauna Kea and Mauna Loa are two shield volcanoes in the Hawaiian island chain. They have peaks only 4.2 km above sea level, but from base to peak they are 10.2 km.


Olympus Mons

Tharsis Montes

Elysium Mons

Mars is also home to one of the most extensive canyon systems in the solar system.  Valles Mariners is a huge scar running across the landscape in the southern hemisphere.  If we were able to place it on Earth, it would run from New York to Los Angeles.  It is approximately 4000 km long send has a maximum depth of 7 km.  Compare this to the Grand Canyon in Arizona, which is "only" 446 km long and 1.8 km deep.  The Grand Canyon was shaped by the Colorado River, but it is unknown what formed the Valleys Marineris.  It is believed that liquid water or volcanism formed it, but it could be a combination of both.

Valles Mariners



Lastly, Mars may have at one time liquid oceans. It does not anymore because of the atmospheric pressure (future post) and the low temperature. Images from Martian probes show evidence of ancient shorelines.

Astronomy Picture of the Day 

Mars Global Surveyor

 



Mars

Space.com



 

Mars is the fourth planet from the Sun, the third largest terrestrial planet, and the seventh largest planet in the Solar System (only Mercury is smaller). 

• Distance from the Sun: 1.5 AU
• Solar Day equivalent to 24.5 Earth hours
• Tropical Year (how long it takes to complete one orbit around the Sun: 687 Earth days 
• Inclination of 5.65° from the Solar equator and 1.85° from the ecliptic
• Martian density is 3.9 g/cm³ or about 3.9 times that of water
• Mass is 10.7% of Earth and Radius is 53.2% of Earth
• Gravity on Mars is 0.376 times that of Earth (a 100-lb person would weigh 37.6 lbs on Mars)

Mars has a geography that you would find on Earth, but to a larger scale.  Mars boasts the largest mountain in the solar system, the longest and deepest canyon, and strange polar caps.  Mars is also well known for its reddish color, which you can see above, but is also apparent when seeing it in the sky.  Another strange feature is that Mars is criss-crossed by a multitude of channels, has high spots and low spots, and has features that on Earth, were created by flowing water.

 

Mars has two satellites that did not form in the same location as Mars.  Phobos and Deimos were probably captured by Mars and will be discussed in a later post.

 

Mars has an atmosphere, which can be seen as the bluish ring in the above photo of Mars. The major component of its atmosphere is carbon dioxide, like Venus, but that is where the similarities end.  It has an atmospheric pressure at the surface of 0.636 kPa, compared to Earth's surface pressure of 101.3 kPa and Venus' surface pressure of 9.2 MPa (9,200 kPa).  Despite the low pressure, the surface of Mars can reach 35°C (95°F) in direct sunlight, but drops to as low as -143­­­°C (-225.4°F) at night.  Unlike Earth and Venus, it does not retain heat as well because of its thin atmosphere.  The average surface temperature of Mars is -63°C (-81.4°F) so liquid water does not really exist on the surface.

Tides

The tides on Earth are driven by gravity, but not Earth's gravity.  The main reason we have tides is from the Moon.  The Sun affects the tides to some extent, but the Moon is the chief driver of the tides.  The gravitational pull from the Moon on the Earth attracts the oceans in a way to create tides.

High tides occur when the Moon is directly overhead or overhead on the opposite side of the Earth. Low tides are when the Moon is on the horizon.  When the Moon is directly overhead, the Moon is pulling water towards it, creating a bulge.  This is called a sublunar tide.  When the Moon is at the other side, it is pulling the Earth away from the water.  We call this an antipodal tide.  At the horizon, it is basically pulling the water along the surface of the Earth, and we have low tide.

In reality, since the Earth is rotating, the tides actually follow a couple hours after the location of the Moon.  But for our purposes, we can safely assume the Moon is directly overhead or at the horizon.

During the lunar cycle, there are times when the Sun, Earth, and Moon line up in a condition called syzygy.  These are when the Moon is full or at new phase.  Tides are higher than normal because of the combined gravitational attraction of the Sun and the Moon.  During the Full Moon and New Moon, the high tides are called spring tides. During the first quarter or third quarter, the Moon and the Sun are 90° apart in the sky and high tides are at the lowest heights.  We refer to these high tides as neap tides.

If there hadn't been a Moon, we would still have tides, but they would only be affected by the Sun.  High tides and low tides would be much different than we have today.  In fact, if there hadn't been a Moon, ground-based life might not exist.  Biologists believe that life began in the oceans and as tides rose and fell, some of that life might have been left behind on the shores, especially during spring tides.  This would force that life to adapt to life on land and evolve into air-breathing creatures.  If there hadn't been a Moon, intelligent life might still have evolved, but would have developed in cetaceans, rather than primates.

The Moon also affects atmospheric tides, but since air is less dense than water, the tides are not as pronounced.  Atmospheric tides do add to weather and climate on Earth.

Lastly, as mentioned in a previous post (The Origin of the Moon), the Moon is slowly receding from the Earth.  As it gets farther and farther away, the size of the tides will decrease.  Since the recession is only 2 cm/century, it will take millenia for the size of the tides to be noticable.

Perseid Meteor Shower

If you go outside at night this week, you might be able to see an annual event called the Perseid meteor shower.  It occurs yearly when the Earth passes through the comet Swift-Tuttle's debris trail.  This year, the shower will peak on the night of August 12-13, with the maximum being before dawn on August 13th.

The Perseids are called that because if you were to trace the origin of the meteors, they would appear to originate in the constellation Perseus.   This origination point is called the radiant, since the meteors seem to radiate out from this point.

When we experience meteor showers, the Earth passes through a debris trail left by comets or asteroids that have orbits that cross near our orbit around the Sun.  The debris is caught by the Earth and falls through the sky and what we see is the meteor.

There are a couple of definitions you should learn.

• Meteoroid - the actual object falling through the sky.  This can be as tiny as a dust particle or as big as an asteroid (which we really don't want)
• Meteor - this is actually what we see.  As the meteoroid falls through the atmosphere, friction causes the gas to heat up around the meteoroid and glows.
• Meteorite - if the meteoroid is large enough, it will not completely burn up as it falls and what is left when it hits the ground is a meteorite.

For your viewing pleasure, this week, Perseus is a little north of the Moon.

Supermoon

I probably should have posted this a couple of days ago, but oh well.

The Supermoon of 2014 just occured this past weekend (August 10). 

The Supermoon is not the newest superhero of DC or Marvel.

The Supermoon is the full moon happened to occur during perigee.  The Moon appeared the largest because it happened to be at its closest in its orbit to Earth.  Tides (which will be discussed further in a future post) happened to be a tad higher, but nothing else really is affected by a Supermoon.  It's just cool to see the Moon so large.

Return to the Moon

We have not been to the Moon since Apollo 17 in 1972.  The reason the United States went to the Moon, originally, was not for science or exploration, but rather for political reasons.  The US wanted to beat the Soviet Union to the Moon.  On July 20, 1969, when Neil Armstrong made one small step for man and a giant leap for all mankind, a human being stood on another celestial body other than Earth for the first time in history.

Why haven't we been back?  One, it is extremely expensive to travel to the Moon.  Not only do we have to have enough fuel to get there, but we need enough to get back.  We'd also have to worry about keeping the astronauts safe while on the Moon.  Two, there really is no economic or political gain from going to the Moon.  At the moment, the only gains we would receive would be purely scientific.  There is no profit to travelling to the Moon, though sometime in the future, it may be profitable to mine the Moon.  Politically, it wouldn't make one country better than any other.  The only advantage would be if there was a multi-nation coalition to go the Moon and make it worthwhile for all humanity.  Lastly, we don't have the technology to go back to the Moon.  A whole class of new spacecraft would have to be designed, tested, and constructed for man to go to the Moon once again.

Why is this important?  There has been talk of a crewed mission to Mars, which is all well and good.  But to skip going back to the Moon first would be a huge mistake.  The Moon is much easier to get to from the Earth than Mars; it would only take a few days travel to get to the Moon, with a round trip only taking about a week.  To get to Mars, it would require at least 6 months of travel from Earth to Mars and almost two years for a round trip.  If humanity built a lunar base, it would be easier to use as a launching point for exploration of the rest of the Solar System.  It would require less energy to launch a ship from the Moon than the Earth because the Moon is much smaller and its gravity would not work as hard against launching a rocket or spacecraft.  Once we set up a permanent presence on the lunar surface, exploration of the Solar System should follow, with Mars being the most logical first step.

Another nice thing about using the Moon as a launching pad is that the materials needed to build rockets and habitats and create fuel for spacecraft are already on the Moon.  The challenge would be to harvest the material and convert it into useful products. That is obviously many years in the future, but we still need to return to the Moon before thinking about going to Mars.

The Origin of the Moon

When we first explored the Moon, we weren't sure what we would find.  We expected to find similar material that we find on Earth.  What we found is even more unusual.

First, why did we expect similar material on the Moon?  Based on the location of the Earth in the Solar System (and by default, the location of the Moon), we expected to find refractory elements because they have a higher boiling point than volatile elements, i.e. they vaporize at high temperatures which in the early Solar System included the location of the Earth.  All planets close to the Sun are generally made up of this type of material because they are close to the Sun.  This is why we believed that terrestrial planets are close to their central star and Jovian planets are far from their star.  Because of this, we expected to find the Moon was made up of refractory elements.

However, when we actually went to the Moon, we discovered something really strange; the composition of the Moon is nearly identical to that of the interior of the Earth.  Not that we found the same material, but that the concentrations were the same.  So what does this tell us?

This tells us that when the Earth was very young, something collided with the infant Earth to create the Moon.  A Mars-sized body collided with the Earth to create the Moon and to leave some material on the Earth.  This helps explain three things:

1. The Moon's composition and why it is nearly identical to the Earth's composition
2. The Moon is receding from Earth. Recall that the Moon is moving about 2 cm away from the Earth every century (See blog post on the eclipses)
3. The 1:1 Moon-Earth resonance since the Moon formed from the Earth

Daily Galaxy

Synodic Period vs. Sidereal Period

As mentioned before, the sidereal period of an object depends on that object's alignment with the body it orbits and with a distant, background star.  When we talk about the Moon, we need to clarify the difference between the sidereal month and the synodic month.

Last time, we talked about the lunar resonance, explaining why the same face of the Moon is always pointed towards Earth.  When we measure the time from Full Moon to Full Moon (or New Moon to New Moon), it takes about 29.5 days to complete one cycle of phases.  We call this the synodic month and it is also the reason why months are approximately 30 days.  But the sidereal month is only 27.3 days.  The reason why it is shorter than the synodic month is because as the Moon orbits the Earth, the Earth is also orbiting the Sun.

Lunar Resonance

Despite what my wife thinks (and the Pink Floyd albums says), there is no such thing as a "Dark Side of the Moon". Granted, the face of the Moon facing away from the Sun is dark, but that face changes as the Moon orbits the Earth.  A better description of the faces of the Moon would be to call them the Near Side of the Moon and the Far Side of the Moon.  The near side is the face that always pointed to the Earth, and the far side is always pointed away.  Why is this?

The reason why we should use near side and far side is because the Moon is in a 1:1 resonance with the Earth.  Remember that the Sun-Mercury system has a 3:2 resonance, so for every two orbits around the Sun, Mercury rotates three times on its axis.  For the Moon-Earth system, this means that for every one complete orbit around the Earth, the Moon rotates on its axis just one time.  Because of this, the same side of the Moon is always facing Earth.  It wasn't until 1959, when the Soviet Union's Luna 3 space probe photographed the far side.  In 1968, it wasn't observed by human eyes for the first time during the Apollo 8 mission.