Mooncrete is an idea first proposed by Larry A. Beyer of the University of Pittsburgh in 1985. It is a hypothetical aggregate building material, similar to concrete formed from lunar regolith, that could cut the construction costs of building on the moon.

Basic ingredients for mooncrete would be the same as those for terrestrial concrete: aggregate, water, and cement. In the case of mooncrete, the aggregate would be lunar regolith. The cement would be manufactured by beneficiating lunar rock that had a high calcium content. Water would either be supplied from the moon, or by combining oxygen with hydrogen produced from lunar soil.

The casting of mooncrete would require a pressurized environment, because attempting to cast in a vacuum would simply result in the water, required for the chemical reaction that forms the curing process, evaporating, and the mooncrete failing to harden. Two solutions to this problem have been proposed: premixing the aggregate and the cement and then using a steam injection process to add the water, or the use of a pressurized concrete fabrication plant that produces pre-cast concrete blocks.

Mooncrete shares the same lack of tensile strength as terrestrial concrete. One suggested lunar equivalent tensioning material for creating pre-stressed concrete is lunar glass, also formed from regolith, much as fibreglass is already sometimes used as a terrestrial concrete reinforcement material. Another tensioning material, suggested by David Bennett, is Kevlar, imported from Earth, which would be cheaper, in terms of mass, to import from Earth than conventional steel.

Usonia is a word used by American architect Frank Lloyd Wright to refer to his vision for the landscape of the United States, including the planning of cities and the architecture of buildings. Wright proposed the use of the adjective Usonian in place of American to describe the particular New World character of the American landscape as distinct and free of previous architectural conventions.

Usonian is a term usually referring to a group of approximately fifty middle-income family homes designed by Frank Lloyd Wright beginning in 1936 with the Jacobs House. The Usonian Homes were typically small, single-story dwellings without a garage or much storage, L-shaped to fit around a garden terrace on odd (and cheap) lots.

They were environmentally conscious with native materials, flat roofs and large cantilevered overhangs for passive solar heating and natural cooling, natural lighting with clerestory windows, and radiant-floor heating. A strong visual connection between the interior and exterior spaces is an important characteristic of all Usonian homes.

Variants of the Jacobs House design are still in existence today and do not look overly dated. The Usonian design is considered among the aesthetic origins of the popular ranch tract home popular in the American west of the 1950s.

Tides are the rising of Earth’s ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans.Tides cause changes in the depth of the marine and estuarine water bodies and produce oscillating currents known as tidal streams, making prediction of tides important for coastal navigation. The strip of seashore that is submerged at high tide and exposed at low tide, the intertidal zone, is an important ecological product of ocean tides.

The changing tide produced at a given location is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the effects of Earth rotation and the bathymetry of oceans, seas and estuaries. Besides the ocean, tidal phenomena can occur in other systems whenever a gravitational field that varies in time and space is present.

In addition to oceanic tides, there are atmospheric tides as well as earth tides. All of these are continuum mechanical phenomena, the first two being fluids and the third being essentially the thin solid Earth’s crust on top of the semi-liquid Earth’s interior.

Atmospheric tides are negligible from ground level and aviation altitudes, drowned by the much more important effects of weather. Atmospheric tides are both gravitational and thermal in origin and are the dominant dynamics from about 80 km to 120 km where the molecular density becomes too small to behave as a fluid.

Earth tides or terrestrial tides affect the entire mass of the Earth, which can be viewed as a liquid gyro with a very thin crust. The Earth’s crust shifts in response to the Moon’s and Sun’s gravitation, ocean tides, and atmospheric loading. While negligible for most human activities, the semidiurnal amplitude of terrestrial tides can reach about 55 cm at the equator which is important in GPS calibration.

When oscillating tidal currents in the stratified ocean flow over uneven bottom topography, they generate internal waves with tidal frequencies. Such waves are called internal tides. The galactic tide is the tidal force exerted by galaxies on stars within them and satellite galaxies orbiting them. The effects of the galactic tide on the Solar System’s Oort cloud are believed to be the cause of 90 percent of all observed long-period comets.

Space burial is a burial procedure in which a small sample of the cremated ashes of the deceased are placed in a capsule the size of a tube of lipstick and are launched into space using a rocket. As of 2004, samples of about 150 people have been “buried” in space.

The effort and cost of launching an object into space is very high. Furthermore, the cost is directly related to the payload, i.e. the mass of the object. Therefore various measures are taken to reduce the mass of the burial. The corpse is cremated, reducing the mass of the remains to about 5% of the initial mass. Also, only a small sample of the ashes is included, typically only about 5 grams. The remainder of the ashes can be buried conventionally in the earth or in the sea.

The second factor greatly influencing the cost includes the target location of the payload. Most burials do not actually leave the gravitational field of the earth but only achieve an orbit around earth. The capsules containing the samples of the remains circle the earth, until the upper layers of the Earth’s atmosphere have slowed down the capsules, and they reenter the atmosphere. The capsules burn up upon reentry similar to a shooting star, and the ashes are scattered in the atmosphere. The time between launch and reentry depends on the orbit of the satellite, and can vary widely. The first burial reentered after only 5 years, but other burials are not expected to reenter in less than 250 years.

There are a number of alternative options if a reentry into the earth atmosphere is not desired. All of them are more complex and expensive than a burial in earth orbit. If an object leaves the gravitational field of the earth, it enters the gravitational field of another body in space. The closest object near the earth for that purpose is the moon. Although the moon is technically also in the gravitational field of the earth, it will not hit the earth within any human timeframe. A service is available for space burial on the moon. As of 2005, the only person buried this way is Dr. Eugene Shoemaker, best known for co-discovering the Comet Shoemaker-Levy 9.

The practice of space burials began at the end of the 20th century as the technical difficulties and costs involved in launching an object into space previously made it unfeasible. The first space burial Earthview 01: The Founders Flight was launched on April 21, 1997. An aircraft carried a modified Pegasus rocket containing samples of the remains of 24 people to an altitude of 38,000 ft above the Canary Islands. Famous people buried on this flight were Gene Roddenberry and Timothy Leary.

The second space burial was the burial of a sample of the remains of Dr. Eugene Shoemaker on the moon by the Lunar Prospector probe, launched on January 7, 1999 by a three-stage Athena rocket. The probe containing scientific instruments and the ashes of Dr. Shoemaker impacted the moon near the lunar south pole on 4:52 a.m. Central Daylight Time, July 31, 1999.

Currently, only one company, Space Services Inc., offers space burials. Space Services took over the assets of Celestis, Inc., which launched four flights from 1997 to 2001. As science progresses it is expected that the cost and difficulties of space burials will be reduced, and other companies may enter the market.

Time travel is the concept of moving between different moments in time in a manner analogous to moving between different points in space, either sending objects or information backwards in time to a moment before the present, or sending objects forward from the present to the future without the need to experience the intervening period at the normal rate. 

Time travel has not been proven to be impossible nor possible. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a time machine.

Some interpretations of time travel also suggest that an attempt to travel backwards in time might take one to a parallel universe whose history would begin to diverge from the traveler’s original history after the moment the traveler arrived in the past. Although time travel has been a common plot device in fiction since the 19th century, and one-way travel into the future is arguably possible given the phenomenon of time dilation based on velocity in the theory of special relativity, it is currently unknown whether the laws of physics would allow backwards time travel.

Several experiments have been carried out to try to entice future humans, who might invent time travel technology, to come back and demonstrate it to people of the present time. Events such as Perth’s Destination Day or MIT’s Time Traveler Convention heavily publicized permanent advertisements of a meeting time and place for future time travelers to meet.

In 1982, a group in Baltimore, identifying itself as the Krononauts, hosted an event of this type welcoming Visitors from the Futures. These experiments only stood the possibility of generating a positive result demonstrating the existence of time travel, but have failed so far. No time travelers are known to have attended these events.

It is theoretically possible that future humans have traveled back in time, but have traveled back to the meeting time and place in a parallel universe. Another factor is that for all the time travel devices considered under current physics, it is impossible to travel back to before the time machine was actually made.

A planetesimal is a solid object arising during the accumulation of planets whose internal strength is dominated by self gravity and whose orbital dynamic is not significantly affected by gas drag. This corresponds to objects with a diameter larger than approximately 1 mile in the solar nebula. The word planetesimal comes from the mathematical concept infinitesimal and literally means an ultimately small fraction of a planet.

While the name is always applied to small bodies during the process of planet formation, some scientists also use the term planetesimal as a general term to refer to many small solar system bodies such as asteroids which are left over from the formation process.

In the current Solar System, these small bodies are usually also classified by dynamics and composition, and may have subsequently evolved to become comets or Kuiper belt objects. In other words, some planetesimals became other populations once planetary formation had finished, and may be referred to by either or both names.

It is generally believed that about 3.8 billion years ago, after a period known as the Late Heavy Bombardment, most of the planetesimals within the solar system had either been ejected from the solar system entirely, into distant eccentric orbits such as the Oort cloud, or had collided with larger objects due to the regular gravitational nudges from the planets between Jupiter and Neptune. A few planetesimals may have been captured as moons, such as the moons of Mars, Phobos and Deimos, and many of the small high-inclination moons of the Jovian planets.

A widely accepted theory of planet formation states that planets form out of dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometer, then they can attract each other directly through their mutual gravity, aiding further growth into moon-sized protoplanets enormously. This is how planetesimals are often defined.

Bodies that are smaller than planetesimals must rely on Brownian motion or turbulent motions in the gas to cause the collisions that can lead to sticking. Alternatively, planetesimals can form in a very dense layer of dust grains that undergoes a collective gravitational instability in the mid-plane of a protoplanetary disk. Many planetesimals may eventually break apart during violent collisions, but a few of the largest planetesimals can survive such encounters and continue to grow into protoplanets and later, planets.

Planetesimals that have survived to the current day are valuable to scientists because they contain information about the birth of our solar system. Although their exteriors are subjected to intense solar radiation that can alter their chemistry, their interiors contain pristine material essentially untouched since the planetesimal was formed. This makes each planetesimal a “time capsule”, and their composition can tell us of the conditions in the Solar Nebula from which our planetary system was formed.

A star is a massive, luminous ball of plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star’s interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars.

Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere, and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.

The motion of the Sun against the background stars and the horizon was used to create calendars, which could be used to regulate agricultural practices. The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth’s rotational axis relative to the nearest star, the Sun.

The oldest accurately dated star chart appeared in Ancient Egypt. Islamic astronomers gave names to many stars which are still used today, and they invented numerous astronomical instruments which could compute the positions of the stars.

William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he performed a series of measurements in 600 directions, and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. He is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.

The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. It was discovered that the color of a star, and hence its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed very precise measurements of magnitude at multiple wavelength intervals.

Important conceptual work on the physical basis of stars occurred during the first decades of the twentieth century. Successful models were developed to explain the interiors of stars and stellar evolution. The spectra of stars were also successfully explained through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.

With the exception of supernovae, individual stars have primarily been observed in our local group of galaxies, and especially in the visible part of the Milky Way. In the local supercluster it is possible to see star clusters. The most distant stars resolved have been up to hundred million light years away. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located one billion light years away, ten times the distance of the most distant star cluster previously observed.

The Kessler Syndrome is a scenario, proposed by NASA consultant Donald J. Kessler, in which the volume of space debris in Low Earth orbit is so high that objects in orbit are frequently struck by debris, creating even more debris and a greater risk of further impacts. The implication of this scenario is that the escalating amount of debris in orbit could eventually render space exploration, and even the use of satellites, unfeasible for many generations.

Every satellite, space probe and manned mission has the potential to create space debris. As the number of satellites in orbit grow and old satellites become obsolete, the risk of a cascading Kessler Syndrome becomes greater.

Fortunately, at the most commonly used Low Earth Orbits residual air drag helps keep the zones clear. Altitudes under around 300 miles will be swept clear in a matter of months. Collisions that occur under this altitude are also less of an issue, since the resulting orbits of the fragments inherently have perigee below this altitude.

At altitudes above this level lifetimes are much greater, but drag gradually brings debris down to lower altitudes where it finally re-enters. At very high altitudes this can take millennia.

The Kessler Syndrome is especially insidious because of the “domino effect” and “feedback runaway”. Any impact between two objects of sizable mass spalls off shrapnel debris from the force of collision. Each piece of shrapnel now has the potential to cause further damage, creating even more space debris. With a large enough collision (such as one between a space station and a defunct satellite), the amount of cascading debris could be enough to render Low Earth Orbit essentially impassable.

The Kessler Syndrome presents a unique problem to human space travel. Space debris is very difficult to deal with directly, as the small size and high velocities of most debris would make retrieval and disposal impractically difficult. Given thousands of years, most debris in Low Earth Orbit would eventually succumb to air resistance in the rarefied atmosphere and plunge to the Earth. If magnetically susceptible, the debris could fall in a few decades due to the drag of the Earth’s magnetic field.

To minimize the chances of damage to other vehicles, designers of a new vehicle or satellite are frequently required to demonstrate that it can be safely disposed of at the end of its life, for example by use of a controlled atmospheric reentry system or a boost into a graveyard orbit.