Tag Archives: NASA

Tesla Orbiting Earth

Credit: SpaceX.

Explanation: Last week, a Tesla orbited the Earth. The car, created by humans and robots on the Earth, was launched by the SpaceX Company to demonstrate the ability of its Falcon Heavy Rocket to place spacecraft out in the Solar System. Purposely fashioned to be whimsical, the iconic car was thought a better demonstration object than concrete blocks. A mannequin clad in a spacesuit — dubbed the Starman — sits in the driver’s seat. The featured image is a frame from a video taken by one of three cameras mounted on the car. These cameras, connected to the car’s battery, are now out of power. The car, attached to a second stage booster, soon left Earth orbit and will orbit the Sun between Earth and the asteroid belt indefinitely — perhaps until billions of years from now when our Sun expands into a Red Giant. If ever recovered, what’s left of the car may become a unique window into technologies developed on Earth in the 20th and early 21st centuries.

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A Brief History of the Universe

The universe is the biggest and oldest thing we know. It contains all existing matter and space. And its origin marks the beginning of time as far as we understand it. We don’t know what made the formation of the universe possible, nor why it occurred. The visible universe is currently about 93 billion light years wide.

A light-year is a distance that light travels in a year, which makes the universe about 880 trillion trillion metres wide. The visible universe is, however, still expanding, and we can measure that rate of expansion. Then, working backwards, we can figure out when the universe would have begun. To the best of our knowledge, the universe formed about 13.8 billion years ago in what is commonly referred to as the Big Bang.

This image shows the universe about 370000 years after the Big Bang, which is the oldest light that we’ve been able to record with the greatest precision. The image records ancient light or cosmic microwave background. The colours show tiny temperature fluctuations from an average temperature. These indicate areas of different densities, which became the stars and galaxies of today. Red spots are a bit hotter and blue spots a bit cooler. The image was recorded between 2009 and 2013, during the Planck mission, when the space observatory was operated by the European Space Agency, in conjunction with NASA, the National Aeronautics and Space Administration. Today, the universe is very cold. On average, it is 2.7Kelvin. Kelvin is a measure of temperature with the same magnitude as degrees Celsius. But 0 Kelvin equals minus 273.15 degrees Celsius.

In the universe, the hot parts, such as stars, make up only a tiny fraction. If we wind the clock backwards, the universe gets smaller. And this means the universe was hotter in the past. When matter gets hot, solids melt and liquids boil. The hot matter glows – red at first, but it becomes bluer as the temperature goes up. Eventually, all matter is gas. So we have a bright, glowing blob of gas. Going further back in time, as the gas gets hotter, the electrons are separated from the nuclei and a plasma is made. The temperature at this point is about 3000 to 6000 Kelvin and the glowing blob is white hot. As we go back further in time, the universe gets even smaller and hotter.

The nuclei themselves, containing protons and neutrons, are broken up. The reason for the breakup of nuclei is that the individual particles and the energy of the radiation are so great that the collisions of all this hot stuff are incredibly violent. The light is no longer in the visible spectrum. It is energetic enough to be x-rays and even gamma rays. Between just 10 seconds and 1000 seconds after the Big Bang, subatomic particles, including neutrons and protons, were formed. Neutrons live for just 9 minutes when they are free. Hence only those that stuck to protons during this period survived. All of the ordinary matter present today formed in this short window of time.

At about 1 microsecond after the Big Bang, the universe was very hot, at 10 to the 10 Kelvin, and quarks formed stable particles called hadrons. Before 1 picosecond, or 10 to the minus 12 seconds, the universe was an exotic place. The gas was hotter still and the laws of physics appeared different to how we see them today. The distinction between matter and radiation, such as light, cannot be detected. The forces of electromagnetism and the weak nuclear force also become indistinguishable. At the very earliest times, the universe was so hot and dense that we cannot yet describe them accurately.

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The Structure of the Earth

Physical Properties of the Earth

The Earth is an oblate spheroid, being slightly flattened at the

Equatorial radius = 6378 km Polar radius = 6357 km

These measurements are calculated on the assumption that the Earth’s surface is smooth, but this is only an approximation since it disregards mountains and ocean depths. However, the difference between the height of Mount Everest and the depth of the Marianas trench is only about 20 km. Most land is concentrated in seven continents each fringed by shallow seas (flooded continent). Separating these are a number of major oceans including the Pacific, Atlantic and the Indian oceans.

It was Cavendish in 1798 who first calculated the mass of the Earth as 5.977 x 1024kg, and since its volume is known (from 4/3 ∏ r^3 where r is the radius of the Earth), then it can be calculated that the average density is 5.516 g/cm3. However, most rocks exposed at the surface have densities of less than 3g/cc, for example:

sandstone: 1.9 - 2.4 g/cm3
limestone: 1.9 - 2.7 g/cm3
granite: 2.6 - 2.7 g/cm3
basalt: 2.8 - 3.0 g/cm3 

Therefore, a material of greater density must exist at deeper levels within the Earth. The Earth has a series of layers or “shells”, but only the outer few km of the Earth can be directly observed; the upper crust, and the deepest boreholes which reach to only about 12.5 kms. Earthquakes provide the key to the structure at depth.


Stresses which develop in the Earth may become great enough to break the rocks, and cause slip along the resulting in fractures (faults). Although the slip distance in a given earthquake may be small (cm to metres), the rock masses involved are large and so the energy released is great. The resulting shock waves, or earthquakes, may cause great damage; greatest near the centre or focus, and less further away. The epicentre is the point on the surface of the Earth vertically above the focus.

Detection of seismic waves.
Earthquake energy is transmitted by several types of waves. Two types will be described:

P waves (primary or compressional) are transmitted by vibrations oscillating in the direction of propagation (push/pull).

S waves (secondary or shear), which vibrate at right angles to the direction of propagation. S waves cannot be transmitted through liquids because liquids have no elastic strength.

Recording Earthquakes

The arrival of earthquake waves is recorded by a seismograph. A mass is loosely coupled to the Earth by a spring. A chart is firmly coupled to the Earth. A pen linking them traces the difference in motion between the mass and the Earth’s surface. The arrival of waves from a distant earthquake is recorded as a seismogram on the rotating drum.

Consider what happens to P and S waves as they travel through the Earth.

The most important property of seismic waves is their speed of propagation. The velocity is governed by the physical properties (density, compressibility, rigidity) of the medium through which the wave is travelling.

Earlier in this lecture, it was deduced that the density of the Earth increases with depth. The wave propagation velocity must, therefore, change with depth, and this causes the wave to refract.


If a wave travelling through a medium with a fixed density encounters a new medium with a different density, the wave will change its direction. This “bending” of the wave is called refraction.

Data from seismometers located around the world can record waves from any given earthquake. The differences between recordings at different seismometers reveal properties of the sub-surface and hence the internal structure of the Earth.

For example, it has been discovered that the mantle is solid rock, but the outer core is a liquid. This was discovered, because for any given earthquake:-

  1. Both P and S waves are recorded by seismometers at distances of up to 103o from the epicentre.
  2. At distances greater than 103o, no S waves are recorded. This means that S waves that would have reappeared at > 103o have not propagated. The material at depths travelled by such waves must be liquid and be unable to transmit S waves.

Also, it has been discovered that the outer core must have a lower P wave velocity than the mantle. This is because at distances of 103o to 142o, no strong P waves are recorded. The liquid outer core has a lower P wave velocity, causing the P waves to be refracted to a steeper angle, so they cannot re-emerge between 103o to 142o. They actually re-emerge at angles > 186o. There is one small caveat to this observation. The inner core appears to be solid because some weak P wave arrivals occur between 103o to 142o. This is thought to be due to a slight increase in P wave velocity as waves enter the inner core, causing them to be refracted to a shallower angle, to re-emerge between 103o to 142o. If the inner core is solid, S waves could propa- gate there. The graph shows some calculations of what expected S wave velocities would be, but the inner core structure is still a source of controversy.


In the early 20th century a Yugoslavian seismologist by the name of Mohorovicic was studying seismograms from shallow focus earthquakes (< 40 km) that were nearby <800km. He noticed that there were 2 distinct sets of P waves and S waves involved. He interpreted these waves as a direct set and a refracted set. In the refracted set, waves travel down and are refracted at a boundary by a medium of higher velocity.

This boundary separates the crust with VP of 6-7km/sec from the upper mantle where VP starts at 8km/sec. It is called the Mohorovicic discontinuity but is commonly known as the MOHO.

Today, seismologists use artificial explosions to determine the structure beneath the surface and it is from these data that the depth of the MOHO can be calculated and thus the thickness of the crust. The MOHO is at 5-15 km under ocean crust and 35 km beneath normal thickness continental crust. The MOHO can be as much as 70 km deep beneath mountain belts where converging plates have caused an orogeny or mountain building event.

The Structure of the Earth

Recent advances in seismology now allow tomographic images of the interior of the Earth to be produced from P and S wave velocity data. Just as tomographic images of the interior of human bodies are produced by density contrasts in human tissue and bone subject to wave propagation, density contrasts in the Earth can be mapped by combining wave velocity data from large numbers of earthquakes.

The basic idea is that where the solid mantle is relatively hot, the P and S wave velocities should be anomalously low because the heat will result in a density decrease. One should be able to image hot, ascending plumes of mantle asthenosphere by looking for areas of anomalously low seismic velocity. Conversely, where the solid mantle is relatively cool, the P and S wave velocities should be anomalously fast because the lack of heat will result in a relatively high density.

One should be able to image cool, descending slabs of mantle lithosphere by looking for areas of anomalously high seismic velocity. Such images allow us to study subduction zones and constrain how deep the slabs penetrate. It appears that some slabs do not penetrate beneath 670 km whereas others continue down to the core-mantle boundary. This is an area of controversy in geology.

Orionid Meteors – Astronomy Photo of the Day

See Explanation.  Clicking on the picture will download  the highest resolution version available.

Orionid Meteors Over Turkey 
Credit & Copyright: Tunc TezelExplanation: Meteors have been flowing out from the constellation Orion. This was expected, as mid-October is the time of year for the Orionids Meteor Shower. Pictured above, over a dozen meteors were caught in successively added exposures over three hours taken this past weekend from a town near BursaTurkey. The above image shows brilliant multiple meteor streaks that can all be connected to a single point in the sky just above the belt of Orion, called the radiant. The Orionids meteors started as sand sized bits expelled from Comet Halley during one of its trips to the inner Solar System. Comet Halley is actually responsible for two known meteor showers, the other known as the Eta Aquarids and visible every May. Next month, the Leonids Meteor Shower from Comet Tempel-Tuttle might show an even more impressive shower from some locations.


Source:  https://apod.nasa.gov/apod/ap061023.html

Astronomy Picture of the Day – Dark Molecular Cloud Barnard 68

See Explanation.  Clicking on the picture will download
 the highest resolution version available.Dark Molecular Cloud Barnard 68 
Image Credit: FORS Team8.2-meter VLT AntuESOExplanation: Where did all the stars go? What used to be considered a hole in the sky is now known to astronomers as a dark molecular cloud. Here, a high concentration of dust and molecular gas absorb practically all the visible light emitted from background stars. The eerily dark surroundings help make the interiors of molecular clouds some of the coldest and most isolated places in the universe. One of the most notable of these dark absorption nebulae is a cloud toward the constellation Ophiuchus known as Barnard 68pictured here. That no stars are visible in the center indicates that Barnard 68 is relatively nearby, with measurements placing it about 500 light-years away and half a light-year across. It is not known exactly how molecular clouds like Barnard 68 form, but it is known that these clouds are themselves likely places for new stars to form. In fact, Barnard 68 itself has been found likely to collapse and form a new star system. It is possible to look right through the cloud in infrared light.


From: https://apod.nasa.gov/apod/ap171008.html

Astronomy Picture of the Day – Eclipsosaurus Rex

See Explanation.  Clicking on the picture will download
 the highest resolution version available.

Eclipsosaurus Rex 
Image Credit & CopyrightFred Espenak (MrEclipse.com)Explanation: We live in an era where total solar eclipses are possible because at times the apparent size of the Moon can just cover the disk of the Sun. But the Moon is slowly moving away from planet Earth. Its distance is measured to increase about 1.5 inches (3.8 centimeters) per year due to tidal friction. So there will come a time, about 600 million years from now, when the Moon is far enough away that the lunar disk will be too small to ever completely cover the Sun. Then, at best only annular eclipses, a ring of fire surrounding the silhouetted disk of the too small Moon, will be seen from the surface of our fair planet. Of course the Moon was slightly closer and loomed a little larger 100 million years ago. So during the age of the dinosaurs there were more frequent total eclipses of the Sun. In front of the Tate Geological Museum at Casper College in Wyoming, this dinosaur statue posed with a modern total eclipse, though. An automated camera was placed under him to shoot his portrait during the Great American Eclipse of August 21.


From: https://apod.nasa.gov/apod/ap171007.html