Posted on by Jerry Coffey

What Causes Earthquakes?

False-color composite image of the Port-au-Prince, Haiti region, taken Jan. 27, 2010 by NASA’s UAVSAR airborne radar. The city is denoted by the yellow arrow; the black arrow points to the fault responsible for the Jan. 12 earthquake. Image credit: NASA
False-color composite image of the Port-au-Prince, Haiti region, taken Jan. 27, 2010 by NASA’s UAVSAR airborne radar. The city is denoted by the yellow arrow; the black arrow points to the fault responsible for the Jan. 12 earthquake. Image credit: NASA

The two main answers to ‘how earthquakes happen’ is: as a result of tectonic plates colliding and volcanic eruption. The shock waves associated with nuclear weapons testing and other man-made explosions. To be considered an earthquake a shock wave has to be of natural origin.

Earthquakes Caused By Tectonic Plates:
The theory of plate tectonics explains how the crust of the Earth is made of several plates, large areas of crust which float on the Mantle. Since these plates are free to slowly move, they can either drift towards each other, away from each other or slide past each other. Many earthquakes happen in areas where plates collide or slide past each other. The Elastic Rebound Theory applies to these quakes.

Major earthquakes are sometimes preceded by a period of changed activity. This might take the form of more frequent minor shocks as the rocks begin to move,called foreshocks, or a period of less frequent shocks as the two rock masses temporarily ‘stick’ and become locked together. Following the main shock, there may be further movements, called aftershocks, which occur as the rock masses settle into their new positions. Aftershocks cause problems for rescue services because they can bring down buildings that were weakened by the main quake.

Earthquakes Caused By Volcanoes:
Volcanic earthquakes are far less common than tectonic plate related ones. They are triggered by the explosive eruption of a volcano. When a volcano explodes the associated earthquake effects are usually confined to an area 16 to 32 km around its base.

The volcanoes which are most likely to explode violently are those which produce acidic lava. Acidic lava cools and sets very quickly when it contacts air. This chokes the volcano’s vent and blocks the escape of pressure. The only way a blockage can be removed is by the pressure building up until it literally explodes the blockage outward.

The volcano will explode in the direction of its weakest point, so it is not always upward. Extraordinary levels of pressure can produce an earthquake of considerable magnitude. The shock waves have been known to produce a series of tsunami in some instances.

There you have the answer to ‘how earthquakes happen’. Keep in mind that there have been man-made shock waves following large explosions, but they are not considered earthquakes because of their artificial origin.

We have written many articles about earthquakes for Universe Today. Here’s an article about the biggest earthquake, and here are some pictures of earthquakes.

If you’d like more info on earthquakes, check out the U.S. Geological Survey Website. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded related episodes of Astronomy Cast about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.

Sources:
http://earthquake.usgs.gov/learn/topics/plate_tectonics/rift_man.php
http://www.geo.mtu.edu/UPSeis/where.html
http://www.geo.mtu.edu/volcanoes/hazards/primer/eq.html
http://news.discovery.com/earth/are-volcanoes-and-earthquakes-related.html

Posted on by Matt Williams

Chromatic Aberration

Chromatic Aberration
Chromatic Aberration. Source: Wikipedia

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Some colours just can’t keep up with the others! Well, that’s probably the simplest way to put it. But when scientists talk about the characteristics of light, it would be more accurate to say that different colours of light propagate at different speeds, orhave different wavelengths, and therefore refract differently. A well-known example of this is the prism effect, where a beam of white light is broken into a rainbow of colours. The result of this is that when objects are viewed through a simple lens, light will refract (bends) at different angles, meaning that it will not image all in the same place. A distortion results in which “fringes” of color appear along the boundaries that separate dark and bright parts of the image. This effect, known as Chromatic Aberration, can be a real pain for astronomers, surveyors, photographers, or just about anyone who wants to view an object (or objects) through a lens and needs to do so clearly!

Sir Isaac Newton was the first to demonstrate this effect some two-hundred years ago when he discovered that light was composed of multiple wavelengths. These colours refract unevenly, with blue-appearing light refracting at shorter wavelengths and red-appearing light refracting at longer, with green refracting in the middle. Since that time, scientists, astronomers and opticians have come to identify two basic kinds of aberration. The first is axial (or longitudinal) where different wavelengths are focused at a different distance because the lens in unable to focus different colours in the same focal plane. The second is transverse (or lateral) aberration, where different wavelengths are focused at different positions in the focal plane and the effect is a sideward displacement of the image. In the former case, distortion occurs throughout the image whereas in the latter, distortion is absent from the centre but increases towards the edge.

There are many ways to remedy Chromatic Aberration. During the 17th century, telescopes had to be very long in order to correct for colour distortions. Sir Isaac Newton remedied this problem by creating the comparably compact, reflecting telescope in 1668 that used curved mirrors to get around this problem. The achromatic lens (or achromatic doublet) is another; a double lens that uses two kinds of glass that focuses all white light coming in at the same point on the other side of the lens. Many types of glass, known as low dispersion glasses, have been developed to reduce chromatic aberration, the most notable being glasses that contain fluorite.

The discovery of Chromatic Aberration and the development of corrective lenses were major steps in the development of the optical microscope, the telescope; which in turn was a boon for astronomers and biologist who were able to gain a greater understanding of the universe and the natural world as a result.

We have written many articles about chromatic aberration for Universe Today. Here’s an article about optical aberration, and here’s an article about achromatic lens.

If you’d like more info on Chromatic Aberration, check out Hyperphysics for a great article on chromatic aberration, and here’s a link to Wise Geek’s discussion about chromatic aberration.

We’ve also recorded an entire episode of Astronomy Cast all about Choosing and Using a Telescope. Listen here, Episode 33: Choosing and Using a Telescope.

Sources:
http://en.wikipedia.org/wiki/Chromatic_aberration
http://toothwalker.org/optics/chromatic.html
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/aber2.html
http://www.yorku.ca/eye/chroaber.htm
http://www.yorku.ca/eye/achromat.htm

Posted on by Jerry Coffey

What are the Different Types Of Earthquakes?

Earthquakes

There are two main types of earthquakes: natural and man-made. Naturally occurring(tectonic) earthquakes occur along tectonic plate lines(fault lines) while man-made earthquakes are always related to explosions detonated by man.

Tectonic earthquakes will occur anywhere there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. Plate boundaries move past each other smoothly and aseismically if there are no irregularities or asperities along the boundary that increase the frictional resistance; however, most boundaries do have such asperities that lead to stick-slip behavior. Once the boundary has locked, continued relative motion between the plates leads to increasing stress and stored strain energy around the fault surface. The energy increases until the stress breaks through the asperity, suddenly allowing sliding over the plate and releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating, and cracking of the rock, which all adds up to an earthquake. This process is called the elastic rebound theory. It is estimated that only 10 percent or less of an earthquake’s total energy is radiated as seismic energy. Most of the earthquake’s energy is used to power the fracture growth or is converted into heat generated by friction.

Occasionally, naturally occurring earthquakes happen away from fault lines. When plate boundaries occur in continental lithosphere, deformation is spread out over a much larger area than the plate boundary, so earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace. Also, all tectonic plates have internal stress fields caused by their interactions with neighboring plates and sedimentary loading or unloading. These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes.

The other type of earthquake is the artificial or man-made quake. This type of quake has been felt all over the world after the detonation of a nuclear weapon. There is very little actual data that is readily available on this type of quake, but, of the two types of of earthquakes it is the only type that can be easily predicted and controlled.

We have written many articles about earthquakes for Universe Today. Here’s an article about how earthquakes happen, and here’s an article about famous earthquakes.

If you’d like more info on earthquakes, check out the U.S. Geological Survey Website. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded related episodes of Astronomy Cast about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.

Sources:
Types of Earthquakes
http://en.wikipedia.org/wiki/Earthquake
http://earthquake.usgs.gov/learn/topics/plate_tectonics/rift_man.php

Posted on by Steve Nerlich

Astronomy Without A Telescope – Through A Lens Darkly

Gravitational lensing in action - faint hints of an "Einstein ring' forming about an area of space which has been 'lensed' by the warping of space-time. If the galactic cluster has been orientated in aplane the lay faceon directly towards earth - the ring would be much more apparent. Credit: HST, NASA.

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Massive galactic clusters – which are roughly orientated in a plane that is roughly face-on to Earth – can generate strong gravitational lensing. However, several surveys of such clusters have reached the conclusion that these clusters have a tendency towards lensing too much – at least more than is predicted based on their expected mass.

Known (to some researchers working in the area) as the ‘over-concentration problem’, it does seem to be a prima facie case of missing mass. But rather than just playing the dark matter card, researchers are pursuing more detailed observations – if only to eliminate other possible causes.

The Sunyaev-Zel’dovich (SZ) effect is a novel way of scanning the sky for massive objects like galactic clusters – which distort the Cosmic Microwave Background (CMB) via inverse Compton scattering – where photons (in this case, CMB photons) interact with very energized electrons which impart energy to the photons during a collision, shifting the protons to a shorter wavelength frequency.

The SZ effect is largely independent of red-shift – since you start with the most consistently red-shifted light in the universe and are looking for a one-off event that will have the same effect on that light whether it happens close by or far away. So, with equipment sensitive to CMB wavelengths, you can scan the whole sky – detecting both close objects, which might be directly observable in optical, as well as very distant objects which may have been red-shifted into the radio spectrum.

The SZ effect causes CMB distortions in the order of one thousandth of a Kelvin and the effect does require really massive structures – a single galaxy is not sufficient to generate the SZ effect on its own. But, when it works – the SZ effect offers a method to measure the mass of a galactic cluster – and does it in a way that is quite different to gravitational lensing.

The SZ effect is thought to be mediated by electrons in the inter-cluster medium. This means that the SZ effect is solely the result of baryonic matter, since it is a consequence of the inverse Compton effect. However, gravitational lensing is the result of the warping of space-time – which is partly due to the presence of baryonic matter, but also of dark (i.e. non-baryonic) matter.

Gralla et al used the Sunyaev-Zel’dovich Array, an array of eight 3.5 meter radio telescopes in California, to survey 10 strongly lensing galactic clusters. They found a consistent tendency for the Einstein radius of each gravitational lens to be around twice the value expected for the mass, determined from the SZ effect, of each cluster.

A distant, actually double, Einstein ring captured by the Hubble Space Telescope. Many more Einstein rings are visible from distant galactic clusters - although they are generally only 'visible' in radio wavelengths.

The Einstein radius is a measure of the size of the Einstein ring that would be formed if a cluster was exactly orientated in a plane that was exactly face-on to Earth – and where you, the lens and the distant light source being magnified, are all in a straight line of sight. Strongly lensing galaxies are generally only in close approximation to this geometry, but their Einstein ring and radius (and hence their mass) can be inferred easily enough.

Gralla et al note that this is work in progress, for now just confirming the over-concentration problem found in other surveys. They suggest one possibility is that the amount of inter-cluster medium may be less than expected – meaning that the SZ effect is underestimating the real mass of the cluster.

If, alternatively, it is a dark matter effect, there would be more dark matter in these clusters than the current ‘standard model’ for cosmology (Lambda-Cold Dark Matter) predicts. The researchers seem intent on undertaking further observations before they go there.

Further reading: Gralla et al. Sunyaev Zel’dovich Effect Observations of Strong Lensing Galaxy Clusters: Probing the Over-Concentration Problem.

And just for interest, Einstein’s letter on lensing and rings: Einstein, A (1936) Lens-like Action of a Star by the Deviation of Light in the Gravitational Field. Science 84 (2188): 506–507.

Posted on by Jerry Coffey

How Do Magnets Work

How Do Magnets Work
Bar Magnet

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We have all played with magnets from time to time. Every time you do, you have asked yourself ‘how do magnets work?’ Many of us understand that magnets have two different charges and that like charges repel each other, but that still does not explain how a magnet works. Below is an attempt to explain the basics behind the secret inner workings of the mysterious magnet.

A magnet is any material or object that produces a magnetic field. This magnetic field is responsible for the property of a magnet: a force that pulls on other ferromagnetic materials and attracts or repels other magnets. A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. Materials that can be magnetized, which are strongly attracted to a magnet, are called ferromagnetic. Although ferromagnetic materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field.

Some facts about magnets include:

  • the north pole of the magnet points to the geomagnetic north pole (a south magnetic pole) located in Canada above the Arctic Circle.
  • north poles repel north poles
  • south poles repel south poles
  • north poles attract south poles
  • south poles attract north poles
  • the force of attraction or repulsion varies inversely with the distance squared
  • the strength of a magnet varies at different locations on the magnet
  • magnets are strongest at their poles
  • magnets strongly attract steel, iron, nickel, cobalt, gadolinium
  • magnets slightly attract liquid oxygen and other materials
  • magnets slightly repel water, carbon and boron

The mechanics of how do magnets work really breaks right down to the atomic level. When current flows in a wire a magnetic field is created around the wire. Current is simply a bunch of moving electrons, and moving electrons make a magnetic field. This is how electromagnets are made to work.

Around the nucleus of the atom there are electrons. Scientists used to think that they had circular orbits, but have discovered that things are much more complicated. Actually, the patterns of the electron within one of these orbitals takes into account Schroedinger’s wave equations. Electrons occupy certain shells that surround the nucleus of the atom. These shells have been given letter names K,L,M,N,O,P,Q. They have also been given number names, such as 1,2,3,4,5,6,7(think quantum mechanics). Within the shell, there may exist subshells or orbitals, with letter names such as s,p,d,f. Some of these orbitals look like spheres, some like an hourglass, still others like beads. The K shell contains an s orbital called a 1s orbital. The L shell contains an s and p orbital called a 2s and 2p orbital. The M shell contains an s, p and d orbital called a 3s, 3p and 3d orbital. The N, O, P and Q shells each contain an s, p, d and f orbital called a 4s, 4p, 4d, 4f, 5s, 5p, 5d, 5f, 6s, 6p, 6d, 6f, 7s, 7p, 7d and 7f orbital. These orbitals also have various sub-orbitals. Each can only contain a certain number of electrons. A maximum of 2 electrons can occupy a sub-orbital where one has a spin of up, the other has a spin of down. There can not be two electrons with spin up in the same sub-orbital(the Pauli exclusion principal). Also, when you have a pair of electrons in a sub-orbital, their combined magnetic fields will cancel each other out. If you are confuse, you are not alone. Many people get lost here and just wonder about magnets instead of researching further.

When you look at the ferromagnetic metals it is hard to see why they are so different form the elements next to them on the periodic table. It is generally accepted that ferromagnetic elements have large magnetic moments because of un-paired electrons in their outer orbitals. The spin of the electron is also thought to create a minute magnetic field. These fields have a compounding effect, so when you get a bunch of these fields together, they add up to bigger fields.

To wrap things up on ‘how do magnets work?’, the atoms of ferromagnetic materials tend to have their own magnetic field created by the electrons that orbit them. Small groups of atoms tend to orient themselves in the same direction. Each of these groups is called a magnetic domain. Each domain has its own north pole and south pole. When a piece of iron is not magnetized the domains will not be pointing in the same direction, but will be pointing in random directions canceling each other out and preventing the iron from having a north or south pole or being a magnet. If you introduce current(magnetic field), the domains will start to line up with the external magnetic field. The more current applied, the higher the number of aligned domains. As the external magnetic field becomes stronger, more and more of the domains will line up with it. There will be a point where all of the domains within the iron are aligned with the external magnetic field(saturation), no matter how much stronger the magnetic field is made. After the external magnetic field is removed, soft magnetic materials will revert to randomly oriented domains; however, hard magnetic materials will keep most of their domains aligned, creating a strong permanent magnet. So, there you have it.

We have written many articles about magnets for Universe Today. Here’s an article about bar magnets, and here’s an article about super magnets.

If you’d like more info on magnets, check out some cool experiments with magnets, and here’s a link to an article about super magnets by Wise Geek.

We’ve also recorded an entire episode of Astronomy Cast all about Magnetism. Listen here, Episode 42: Magnetism Everywhere.

Sources:
Wise Geek
Wikipedia: Magnet
Wikipedia: Ferromagnetism

Posted on by Matt Williams

Charles Law

Charles's Law
Charles's Law. Image Credit: NASA GRC

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For most people, the words “ideal gas” might conjure up the image of some kind of super fuel, perhaps a near-inexhaustible kind that creates zero air pollution! Sadly, this is not what is meant by ideal gas. In reality, an ideal gas is a theoretical gas composed of a set of randomly-moving, non-interacting point particles. At normal conditions such as standard temperature and pressure, most real gases such as air, nitrogen, oxygen, hydrogen, noble gases, and some heavier gases like carbon dioxide behave like an ideal gas and can be treated as such within reasonable tolerances. It is only when they are treated with higher temperatures and lower pressure that they deviate from this trend. Once they get into this territory, experimental gas laws, such as Charles’s Law, come into play.

Also known as the law of volumes, Charles’s Law is an experimental gas law which describes how gases tend to expand when heated. It was first published by French natural philosopher Joseph Louis Gay-Lussac in 1802, although he credited the discovery to unpublished work from the 1780s by Jacques Charles, hence the name. This law applies generally to all gases, and also to the vapours of volatile liquids if the temperature is more than a few degrees above the boiling point. Given the interest in hot air balloons at the time, it is certainly understandable why Gay-Lussac, Charles and other scientists around the globe were so interested in the relationship between volume, pressure and temperature when it came to gasses.

In lay terms, the law states that: at constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature on the absolute temperature scale (i.e. the gas expands as the temperature increases). This can be written as: V? T, where V is the volume of the gas; and T is the absolute temperature. In mathematical terms, the law can also be expressed as: V100 – V0 = kV0, where V100 is the volume occupied by a given sample of gas at 100 °C; V0 is the volume occupied by the same sample of gas at 0 °C; and k is a constant which is the same for all gases at constant pressure. Gay-Lussac’s value for k was ½.6666, remarkably close to the present-day value of ½.7315.

Combined with Boyle’s law, these laws make up what is known as the “Ideal Gas Law” which was first stated by ÉmileClapeyron in 1834.

We have written many articles about Charles’s Law for Universe Today. Here’s an article about the Combined Gas Law, and here’s an article about Boyle’s Law.

If you’d like more info on Charles’s Law, check out a discussion about Charles’s Law, and here’s a link to an article about Charles’s Law by the Glenn Research Center.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.

Sources:
http://en.wikipedia.org/wiki/Charles%27s_law
http://en.wikipedia.org/wiki/Ideal_gas
http://www.chm.davidson.edu/vce/gaslaws/charleslaw.html
http://www.grc.nasa.gov/WWW/K-12/airplane/glussac.html
http://en.wikipedia.org/wiki/Ideal_gas_law

Posted on by Mark Mortimer

Exploring the Solar System with Binoculars

Exploring the Solar System with Binoculars A Beginner's Guide to the Sun, Moon, and Planets
Exploring the Solar System with Binoculars A Beginner's Guide to the Sun, Moon, and Planets

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Passion drives us to do things beyond mere instinctive survival. Varied and distinct, these pastimes can absorb hours and days. The night sky beckons many even though, or perhaps, because they will never be able to visit. Stephen James O’Meara’s book “Exploring the Solar System with Binoculars, A Beginner’s Guide to the Sun, Moon, and Planets” is testament to one man’s affliction with the shapes and colours that continually transcend the velvet backdrop of space. Through his passion, the book draws the reader into an ever changing, lively night time display.

The book’s title and subtitle succinctly frame the book’s contents. Between the covers, the reader will learn of methods to accurately and safely see features of our solar system. Whether sunspots on the Sun, mares on the Moon or fireballs from nowhere, there are subjects galore to entice the beginner to spend just another five minutes looking upwards. In addition, the book details both methods and tricks to get the most out of the time spent viewing. In particular though, it lists distinguishing characteristics of the subject whether colour, shape or sound. A diamond ring from an eclipse, a crescent of Venus or a sword slicing as from a comet are just some of the many vibrant distinctions brought to the reader’s attention throughout this book.

While the descriptions and facts should ably answer the many questions of the beginner, the book’s anecdotal passages make this publication shine. The author shares his passion through selections describing his emotions such as ‘I saw the spirit of the fireball dancing on its grave’ when describing an aerial explosion. The mood is continually heightened such a Tolstoy character who in ‘rapture and his eyes wet with tears, contemplated the radiant stare’ for the comet of 1812 or Agesinax’s ‘all round about environed with fire she is illumined’ to describe the Moon. These historical connections and the many references to ongoing research tells the reader that they share the wonder of the grandeur and complexity of Earth’s immediate neighbourhood.

A passion to explore the night sky burns in the hearts of many. Not knowing where to start or how to share this longing is no impediment. With bare eye or inexpensive binoculars, Stephen James O’Meara’s book “Exploring the Solar System with Binoculars” will guide you to satisfy your feelings.

Click here to read more reviews or buy this book from Amazon.com.

Written by Mark Mortimer

Posted on by Nancy Atkinson

SpaceX Test Fire Aborted

The Falcon 9 rocket makes its way to the launchpad on Thursday in preparation for the test firing. Credit: SpaceX

A static test firing of the SpaceX Falcon 9 rocket was cut short as computer systems shut down the first-stage engines before the test was complete. The firing was only going to last two seconds, but the engines ran for 1.1 sec due to high engine chamber pressure, according to SpaceX. Space News reported that engineers are analyzing the data and that a second attempt is likely to occur tomorrow, Dec. 4. This abort occurred just four days before SpaceX is schedule to conduct the maiden launch of its Dragon space capsule on board the medium-class Falcon 9.

This video is from SpaceX’s webcast of the firing and unfortunately is a bit jumpy.

The first-stage firing was part of a dress rehearsal conducted in preparation for the planned Dec. 7 launch, the first of three increasingly complicated flight demonstrations of Falcon 9 and Dragon under the company’s Commercial Orbital Transportation Services (COTS) agreement with NASA.

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In a press release from SpaceX from Dec. 2, the company said the rehearsal would “exercise the countdown processes and end after the engines fire at full power for two seconds, with only the hold-down system restraining the rocket from flight.”

After the test, SpaceX said they would conduct a thorough review of all data as engineers make final preparations for the upcoming launch.

The rockets uses kerosene and liquid oxygen, and the nine Merlin engines generate one million pounds of thrust in vacuum.

The $278 million COTS agreement has SpaceX developing and demonstrating hardware capable of ferrying cargo to and from the International Space Station.

We’ll post more information about the abort as it becomes available.

Posted on by Nancy Atkinson

Gallery: X-37B Space Plane Returns to Earth

The X-37B after landing. Credit: 30th Space Wing (Vandenberg Air Force Base, Calif.

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The secret military space plane has returned home, and while the mission was classified, the Air Force and Boeing have supplied pictures of the craft after landing. With this mission appearing to be a success, the Air Force is preparing to launch the next X-37B, OTV-2, in Spring 2011 aboard an Atlas V booster.

See more images below.

X-37B is shown here after landing at 1:16 a.m. Pacific time today, concluding its more than 220-day experimental test mission. Credit: Boeing
The X-37B after landing. Credit: 30th Space Wing (Vandenberg Air Force Base, Calif.
X-37B on the runway at Vandenburg Air Force Base. Credit: Boeing.
X-37B after landing. Credit: 30th Wing, Vandenberg Air Force Base.
X-37B is shown here after landing at 1:16 a.m. Pacific time today, concluding its more than 220-day experimental test mission. Credit: Boeing
X-37B Landing by 30th Space Wing (Vandenberg Air Force Base, Calif.)

Here’s a video which includes the landing (which we showed on our previous article) plus post landing activities.