Guide to Space Archives

March 19, 2010January 15, 2016 by John Carl Villanueva

What Is A Moon?

Before the invention of the telescope in the early 1600’s, man just knew of the Moon — a round, mysterious astronomical object that people would gaze up to in the night sky. As time progressed however, astronomers discovered that the moon isn’t exactly unique to earthlings, and other planets had their own moons. So exactly what is a moon?

A moon is defined to be a celestial body that makes an orbit around a planet, including the eight major planets, dwarf planets, and minor planets. A moon may also be referred to as a natural satellite, although to differentiate it from other astronomical bodies orbiting another body, e.g. a planet orbiting a star, the term moon is used exclusively to make a reference to a planet’s natural satellite.

The first moons to be discovered outside of the Earth’s moon were the Galilean moons of Jupiter, named after astronomer and discoverer Galileo Galilei. The moons Io, Europa, Ganymede, and Callisto are Jupiter’s largest and only the first four to be revealed, as to date, the planet has 63 moons.

Other than the four Galilean moons, Saturn’s Titan and Neptune’s Triton are two other moons which are comparable in size to the Earth’s Moon. In fact, these seven moons are the largest natural satellites in the solar system, measuring more than 3,000 kilometers in diameter. Only the inner planets Mercury and Venus have no moons.

An interesting fact about some of the solar system’s largest moons that most people may not be aware of is that a few of them are geologically active. While we may not see the Moon spewing lava or displaying any evidence of tectonic activity, Jupiter’s Io and Europa, Saturn’s Titan and Enceladus, and Neptune’s Triton have been found to be volcanically active bodies.

If the moon count had a grand total of just one in the olden times, that number has ballooned to 336 as of July 2009, with 168 moons orbiting the six planets, while the rest are moons of dwarf planets, asteroids moons, and natural satellites of Trans-Neptunian objects.

As more and more discoveries are made however, astronomers may find it more difficult to put a really defining line on what can or what can’t be classified as a moon. For instance, can you consider a 10-inch rock that’s orbiting Jupiter a moon? If yes, then there could be thousands or even millions of moons out there. If not, then where do you draw the line? Obviously, even the size of an “official” moon is still up for debate, so other than the simple definition of it being a natural satellite of a planet, there really is no clear cut answer to the question, “What is a moon?”.

Here in Universe Today, we have a nice collection of articles that explain why the Moon landings could not have been faked. Here are some of them:

Moon Rocks – Discusses how the Moon rocks are one of the most tangible objects that prove the landings took place.

Moon Landing Hoax – An explanation that counters some of the points raised by skeptics

Apollo 11 Hoax – another point for point discussion by Jerry Coffey

TV – Alert: Mythbusters and the Moon Hoax Myth – a teaser for the Mythbusters episode featuring the so-called hoax. You’ll find the comments below that article equally interesting, by the way.

Here’s an article from NASA that debunks the hoax theory using the Moon rock arguments. Another article about Moon rocks from the same site.

Episodes about the moon from Astronomy Cast. Lend us your ears!

Shooting Lasers at the Moon and Losing Contact with Rovers
The Moon Part I

References:
NASA Solar System Exploration: Moons of Jupiter
NASA Solar System Exploration: Moons

March 19, 2010December 24, 2015 by John Carl Villanueva

Radiation from the Sun

[/caption]Radiation from the Sun, which is more popularly known as sunlight, is a mixture of electromagnetic waves ranging from infrared (IR) to ultraviolet rays (UV). It of course includes visible light, which is in between IR and UV in the electromagnetic spectrum.

All electromagnetic waves (EM) travel at a speed of approximately 3.0 x 10 ⁸ m/s in vacuum. Although space is not a perfect vacuum, as it is really composed of low-density particles, EM waves, neutrinos, and magnetic fields, it can certainly be approximated as such.

Now, since the average distance between the Earth and the Sun over one Earth orbit is one AU (about 150,000,000,000 m), then it will take about 8 minutes for radiation from the Sun to get to Earth.

Actually, the Sun does not only produce IR, visible light, and UV. Fusion in the core actually gives off high energy gamma rays. However, as the gamma ray photons make their arduous journey to the surface of the Sun, they are continuously absorbed by the solar plasma and re-emitted to lower frequencies. By the time they get to the surface, their frequencies are mostly only within the IR/visible light/UV spectrum.

During solar flares, the Sun also emits X-rays. X-ray radiation from the Sun was first observed by T. Burnight during a V-2 rocket flight. This was later confirmed by Japan’s Yohkoh, a satellite launched in 1991.

When electromagnetic radiation from the Sun strikes the Earth’s atmosphere, some of it is absorbed while the rest proceed to the Earth’s surface. In particular, UV is absorbed by the ozone layer and re-emitted as heat, eventually heating up the stratosphere. Some of this heat is re-radiated to outer space while some is sent to the Earth’s surface.

In the meantime, the electromagnetic radiation that wasn’t absorbed by the atmosphere proceeds to the Earth’s surface and heats it up. Some of this heat stays there while the rest is re-emitted. Upon reaching the atmosphere, part of it gets absorbed and part of it passes through. Naturally, the ones that get absorbed add to the heat already there.

The presence of greenhouse gases make the atmosphere absorb more heat, reducing the fraction of outbound EM waves that pass through. Known as the greenhouse effect, this is the reason why heat can build up some more.

The Earth is not the only planet that experiences the greenhouse effect. Read about the greenhouse effect taking place in Venus here in Universe Today. We’ve also got an interesting article that talks about a real greenhouse on the Moon by 2014.

Here’s a simplified explanation of the greenhouse effect on the EPA’s website. There’s also NASA’s Climate Change page.

Relax and listen to some interesting episodes at Astronomy Cast. Want to know more aboutUltraviolet Astronomy? How different is it from Optical Astronomy?

References:
NASA Science: The Electromagnetic Spectrum
NASA Earth Observatory

March 19, 2010December 24, 2015 by John Carl Villanueva

Proxima Centauri

[/caption]As the nearest star from our Solar System, Proxima Centauri is a prime candidate for future interstellar travel and space colonization missions.

In the meantime, scientists are trying to determine whether this star has super Earths orbiting within its habitable zone. Habitable zones are regions around a star where planets are believed to receive just the right amount of heat. For instance, Earth is within the Sun’s habitable zone.

If we were slightly nearer, say on Venus’ orbit, the heat would have evaporated all our oceans. On the other hand, if we were slightly farther, the temperature would have been too cold to support life.

So far, searches in the neighborhood of Proxima Centauri have revealed nothing. Even companion stars or supermassive planets that may be accompanying the star have not yet been discovered (if they are ever there at all). Although the search continues, some scientists believe Proxima Centauri’s flares can be a big obstacle for life even inside the star’s habitable zone.

Proxima Centauri’s flares are believed to be caused by magnetic activity. When a flare occurs, the brightness of all electromagnetic waves emitted by the star increases. This includes radio waves as well as harmful X-rays. The most common flare stars are red dwarfs, just like Proxima Centauri.

Now, even if Proxima Centauri is the nearest star, it is still 4.2 light years away. That’s about 4 x 10 ¹³ km. The spacecraft that would take the first explorers to that system would have to rely on a virtually unlimited supply of energy. Furthermore, sufficient shielding against cosmic radiation should be in place.

Proxima Centauri is smaller than our Sun with a mass of approximately 0.123 solar masses and a radius of only about 0.145 solar radii. Its interior is believed to be totally dependent on convection when it comes to transferring heat from the core to the exterior.

Discovered in 1915 by Robert Innes, the Director of the Union Observatory in Johannesburg, South Africa, the star was observed to have the same proper motion as Alpha Centauri. Further studies confirmed that it was in fact very close to Alpha Centauri. The current distance between the two is estimated to be about only 0.21 light years.

Here are some articles in Universe Today that talk about Proxima Centauri:

What is the nearest star to the Sun?

How far is the nearest star?

Can’t get enough of stars? Here’s Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage..

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Source: Wikipedia

March 1, 2010December 24, 2015 by Tega Jessa

Earth Formation

One of the oldest questions for mankind is how the Earth was formed. However, no one has an exact answer. First by the best estimates it occurred over 4 billion years ago before any life appeared. So there are no eyewitness accounts and other pieces of evidence. The best we can do is look at the geologic record and the stars to get our answers. While we may not have the entire picture we have a good idea and it all starts with how stars are born.

Just like the formation of the Earth and other planets stars take a long time to be be born. Stars are essentially formed from clouds of gas in space. We know these as nebulas. You can basically consider them to be star forges. Over time gravity causes the atoms of gases and space dust to start coming together and gathering. Over time this gather of gases gains more mass and with it stronger gravity. This is a process that can take millions of years. In time the gravity causes the gases, mainly hydrogen to fuse in a nuclear reaction and a star is formed.

The formation of the Earth occurred after this intial phase happened for our Sun. After the Sun was formed we know from observations and other indirect evidence that there were left over gases and heavier elements. The gravity of the Sun helped to flatten these left overs into a disk and start to fuse them together. This created the planetesimals and planetoids which would later make up the planets. Over time these planetesimals would collide creating even bigger masses. It was in this method that the Earth was eventually formed.

Now we need to know that fusion eventually creates heavier elements such as carbon and iron. These elements were to compose a significant part of young Earth. The pressure and heat from radioactive decay of elements and the aftershocks of massive collisions caused the Earth to be molten. Over time the surface of the Earth cooled and became the Crust. However the molten layers that remained became our mantle and the core. The currents of this massive underground ocean of magma cause volcanic activity that released gases. These would lead to the creation of the atmosphere and the oceans starting the water cycle.

The formation of the Earth was only the beginning and we still see the Earth changing year by years through erosion and plate tectonics. However in learning more about the formation of the Earth we are able to better understand what makes life possible on our planet.

If you enjoyed this article there are several others on Universe Today that you will enjoy. There is a great article on plate boundaries and an interesting piece on early Earth.

You can also find some great resources online. There is a great web page on the University of Oregon web site that goes into detail about the formation of the Earth. You can also look at the Hadean page on the Smithsonian website. It talks about the Hadean period the period of geologic time when the Earth was formed.

You can also listen to Astronomy Cast. Episode 108 is about the life of the Sun.

Reference:
NASA

February 22, 2010December 24, 2015 by Jerry Coffey

Atom Structure

[/caption]We know that atoms are parts of an element that can not naturally be broken down any further. What is the atom structure, though? The concept that atoms existed was first written about in ancient India in the 6th century B.C. The theory stayed just that, a theory, until the late 19th century. As microscopes and spectrometers developed, scientists were better able to develop their theories and finally observe the small scale structure of elements.

Atoms are made up of three particles: protons, electrons, and neutrons. Electrons are the smallest and lightest of the the three particles and they have a negative charge. The protons are much heavier and larger than electrons. Protons have a positive electrical charge. Neutrons are as large and massive as protons, but do not have an electrical charge at all. Every atom contains these particles in varying numbers. To understand exactly how small an atom is, you have to know that a single hydrogen atom is 5 x 10^-8mm in diameter. It would take at least 60 million hydrogen atoms to fill the space of any one of the letters on this page.

The simplest atom is that of hydrogen: 1 electron and one proton. In every stable, neutrally charged atom there is the exact same number of protons as electrons. These particles work together like two magnets with the opposite electrical charges attracting each other. The reason that they do not crash together is that the electron is constantly revolving around the nucleus(usually a proton/neutron combination, but hydrogen, uniquely, does not contain any neutrons). The centrifugal force of the electron keeps it in place at a constant distance from the nucleus. Actually, representing the electron as spinning around the nucleus is somewhat misleading. Electrons act like waves. That is how they are seen on a spectrometer. It is just easier to think of them as spinning around.

Atoms can have an electrical charge, positive or negative. This happens when an atom gains or loses electrons. The number of protons never changes in an atom. More electrons means a negative charge and fewer means a positive charge. Once an atom has an electrical charge it is called an ion. In an ion the atomic number and atomic mass do not change from the original. If an atom were to gain or lose neutrons it becomes an isotope. Remember the hydrogen atom I mentioned earlier. It did not have a neutron attached to its proton. If it gains a neutron it become an isotope called deuterium. Since the atomic mass is the total of the number of protons and neutrons, an isotope would have a different atomic mass, but the same atomic number as the original atom.

Alright, that is a very basic rendition of atom structure. The University of Colorado has an interesting website to help you understand more complex versions of atoms. Here on Universe Today we have a great article about the many theorized models of the atom. We discussed ions. Astronomy Cast offers a good episode about interstellar travel using ion propulsion.

Sources:
Wikipedia
GSU Hyperphysics

February 22, 2010December 24, 2015 by Abby Cessna

Naiad

[/caption]

Naiad is one of the 13 moons of Neptune. Neptune was not discovered until 1989 through studying photos taken by the Voyager 2 probe. Thus, the Voyager Science Team is credited with its discovery. It was the last moon discovered by the probe, which helped scientists find five moons altogether. The last five new moons were discovered in the first decade of the 21st century.

The satellite was given its official name on September 16, 1991. At first the satellite was designated S/1989 N 6. Neptune’s moons are named after figures from mythology that have to do with the Roman god Neptune – or its Greek equivalent Poseidon – or the oceans. The irregular satellites of Neptune are named after the Nereids, which are the daughters of Nereus and Doris, who are Neptune’s attendants in Roman mythology. Naiad was named after a type of nymph in Greek mythology that presided over brooks, streams, wells, springs – all fresh water things.

Naiad is the closest satellite to the planet Neptune. It orbits about 48,230kilometers from the top of the planet’s atmosphere. Naiad is a very small satellite with a diameter of only approximately 58 kilometers. That is about one-sixtieth the size of the Earth’s Moon. Naiad’s mass is so small that it is only 0.00001% of the Moon’s mass. It takes Naiad less than one day – seven hours and six minutes to be precise – to orbit Neptune because of its proximity to its planet. With a decaying orbit, the satellite may crash into Neptune or be ripped apart and become part of one of its planetary rings. This may happen soon.

Naiad is an irregularly shaped satellite, which some have compared to a potato. In one of the pictures Voyager 2 took of it, the moon appears to be elongated because of smearing in the picture. Astronomers believe that the moon is made up of fragments from Neptune’s original satellites, some of which were destroyed when Neptune’s gravity captured Triton as a satellite. They do not think the moon has changed at all geologically since it was formed.

After the Voyager 2 probe passed by Neptune, the planet and its satellites have been studied by many observatories as well as the Hubble Space Telescope and the Keck telescope. Although scientists have been trying to observe Naiad and some of the other smaller irregular moons, scientists still do not know very much about the satellite. This is especially true because Naiad and similar satellites are so small.

Universe Today has articles on Neptune’s moons and moons of Neptune.

You should also check out Neptune’s moon Naiad and Naiad.

Astronomy Cast has an episode on Neptune you will want to see.

Source: NASA

February 19, 2010December 24, 2015 by Jerry Coffey

Atom Model

[/caption]The most widely accepted atom model is that of Niels Bohr. Bohr’s model was first introduced in 1913. This model of the atom depicts a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus much like the planets travel around the Sun, but the electrostatic forces product attraction instead of gravity. The model’s key success was in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen. It is, basically, a modification of the Rutherford model used for quantum physics purposes.

The Bohr model was an improvement on older atomic models, but it too has been rendered obsolete by ongoing scientific research. Although considered to be obsolete, it is still taught as an introduction to quantum mechanics and in early secondary school science classes. Once students are advanced enough in their comprehension, they are introduced to the more accurate valence shell atom. At some time in the future this model of the atom may be proven to be too rigid in its scope.

The Bohr model built on the Rutherford theory. Rutherford proposed that electrons orbited the nucleus much like a planet around the Sun. The drawback to the theory was that based on his theory, electrons would be emitting(losing) their charge and spiral into the nucleus, making all atoms unstable. Bohr proposed several changes to that model: electrons can only travel in special orbits at a certain set of distances from the nucleus with specific energies, electrons do not continuously lose energy as they travel. They can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation with a frequency determined by the energy difference of the levels according to the Planck relation, and that the frequency of the radiation emitted at an orbit is the reciprocal of the classical orbit period. This model is restricted in a few ways, but does allow for classical mechanics to explain many things while having an allowance for quantum rules.

The Bohr model begins to run into problems with heavier atoms. Other shortcomings of the model are:gives an incorrect value for the ground state orbital angular momentum, fails to explain much of the spectra of larger atoms, and the model also violates the uncertainty principle because it considers electrons to have known orbits and definite a radius. These two things can not be directly known at the same time.

Here is a good ink about the Bohr atom model. Here on Universe Today we have a couple of great articles on the topic: one is about the Bohr model and the other is about Dr. Bohr himself. Astronomy Cast offers a good episode about how molecules behave in space.

February 17, 2010December 24, 2015 by Jerry Coffey

Atom Diagram

[/caption]The image on the left is a basic atom diagram. This one shows the protons, neutrons, and electrons of a carbon atom. Each is in a group of six. That makes the atom very stable. There have been many atomic models over the years, but this type of model is now widely considered a sound basic version. Atomic diagrams were developed to explain the interaction of the elements of the Earth and space long before atoms could be observed. Nowadays, scientists can see particles that are smaller than an atom. These sub-atomic particles are the basis of particle physics.

Scientists have used atomic diagrams to explain the workings of the world for centuries. The ancient Greeks and, before them, the Chinese and Babylonians believed that there were forces that could not be seen that allowed certain metals to be combined and worked to man’s advantage. They did not know it, but that was simply heated metals exchanging subatomic particles to become a new metal.

Basic chemistry explains the atom best. It states that the fundamental building block of matter is the atom. An atom consists of three main parts: protons, neutrons, and electrons. Protons have a positive electrical charge. Neutrons have no electrical charge. Electrons have a negative electrical charge. Protons and neutrons are found together in what is called the nucleus of the atom. Electrons circle around nucleus. Chemical reactions involve interactions between the electrons of one atom and the electrons of another atom. Atoms which have different amounts of electrons and protons have a positive or negative electrical charge and are called ions. When atoms bond together, they can make larger building blocks of matter called molecules. If science did not have the atom modeled out, it would never have understood this exchange of electrons and we could still be stuck in the Dark Ages.

Earlier, I mentioned that there had been many atom models developed. Some of them are the Bohr model, the cubic model, the plum pudding model, the Saturnian model, and the Rutherford model.
Each of these models improved on the other and propelled science closer to a perfect atomic model. The Bohr and Rutherford models were developed for quantum mechanics and used for astronomical applications. As a matter of fact, an improvement on the Bohr model, called the Bohr-Summerfield model, is responsible for some of the many things we now know about quantum mechanics.

The atom diagram is under constant revision as science uncovers more information about sub-atomic particles. Follow this link to get information about the Bohr model and its enhancements. Here on Universe Today we have two great articles: one about the proton and the other about electrons. Astronomy Cast offers a good episode about matter from stars.

Sources:
Wikipedia
Chemistry Help

February 17, 2010December 24, 2015 by Jerry Coffey

Atom Definition

[/caption]The atom definition is: A unit of matter, the smallest unit of an element, having all the characteristics of that element and consisting of a dense, central, positively charged nucleus surrounded by a system of electrons. The entire structure has an approximate diameter of 10^-8 centimeters and characteristically remains undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons. Essentially, it is the smallest possible part of an element that still remains the element.

Under normal circumstances an atom can be broken down into any smaller particles, but we humans, have devised ways to break the atom apart. That is the entire basis of the atom bomb, particle colliders, and quarks. It takes high speed, high energy smashing of particles to break an atom. A particle collider does that and helps us understand some of the theories in particle physics. The results of an atom bomb are widely known. Quarks are extremely small particles that do not exist alone. They must group to form the protons, electron, and neutrons normally found in a single atom of an element. They have only been found as a result of a particle collider and in theory.

An atom itself is made up of three tiny kinds of particles called subatomic particles: protons, neutrons, and electrons. The protons and the neutrons make up the center of the atom called the nucleus and the electrons fly around above the nucleus in a small cloud. The electrons carry a negative charge and the protons carry a positive charge. In a normal (neutral) atom the number of protons and the number of electrons are equal. Often, but not always, the number of neutrons is the same, too. The opposing forces(negative and positive) attract and repel each other in the right way so as to keep the atom together. The universe could be looked at as one large atom. Everything in space attracts and repels just right so as to keep the whole together.

One type of theoretical ion propulsion spacecraft would have to take advantage of this atomic attraction and repulsion to operate. It takes advantage of magnetism and electricity to push a ship through space. Electricity, generated by the ship’s solar panels, gives a positive electrical charge to atoms inside the chamber. They are pulled by magnetism towards the back of the ship and then pushed by magnetic repulsion out of the ship. This steady stream of atoms going out of the spacecraft gives it the thrust it needs to go forward through space. NASA has tested other types of ion propulsion and found them lacking.

Here is another atom definition. Here on Universe Today we have a great article about atoms. Astronomy Cast has a good question and answer episode about interstellar travel, including a NASA link about ion propulsion.

Source:
Wikipedia

February 15, 2010May 4, 2017 by Jean Tate

Gravity Equation

There is not one, not two, not even three gravity equations, but many!

The one most people know describes Newton’s universal law of gravitation:

F = Gm₁m₂/r²,
where F is the force due to gravity, between two masses (m₁ and m₂), which are a distance r apart; G is the gravitational constant.

From this is it straightforward to derive another, common, gravity equation, that which gives the acceleration due to gravity, g, here on the surface of the Earth:

g = GM/r²,
Where M is the mass of the Earth, r the radius of the Earth (or distance between the center of the Earth and you, standing on its surface), and G is the gravitational constant.

With its publication in the early years of the last century, Einstein’s theory of general relativity (GR) became a much more accurate theory of gravity (the theory has been tested extensively, and has passed all tests, with flying colors, to date). In GR, the gravity equation usually refers to Einstein’s field equations (EFE), which are not at all straight-forward to write, let alone explain (so I’m going to write them … but not explain them!):

G_?? = 8?G/c⁴ T_??

G (without the subscripts) is the gravitational constant, and c is the speed of light.

Finally, here’s a acceleration of gravity equation you’ve probably never heard of before:

a = ?(GMa₀/r),

where a is the acceleration a star feels, due to gravity under MOND (MOdified Newtonian Dynamics), an alternative theory of gravity, M is the mass of a galaxy, r the distance between the star in the outskirts of that galaxy and its center, G the gravitational constant, and a₀ a new constant.

Some websites which contain more on gravity equations, for your interest and enjoyment: Newton’s Theory of “Universal Gravitation” (NASA), Einstein’s equation of gravity (University of Wisconsin Madison – heavy), and Gravity Formula (University of Nebraska-Lincoln).

Universe Today, as you would expect, has several stories relevant to gravity equations; here are a few: See the Universe with Gravity Eyes, A Case of MOND Over Dark Matter, and Flyby Anomalies Explained?. Here’s an article about 0 gravity.

Gravity, an Astronomy Cast episode, has more on gravity equations, as do several Astronomy Cast Question Shows, such as September 26th, 2008, and March 31st, 2009.

Sources:
University of Nebraska-Lincoln
NASA
UT-Knoxville