Monday, April 13, 2009

Cold and Special States of Matter

Although the journey to extreme heat spans many trillion trillion trillions of Kelvin, the journey to extreme cold only goes down a few hundred Kelvin to 0 K, or absolute zero, about -273.15 Celsius and -459.67 Fahrenheit. I will use the Kelvin scale, as it is the most convenient for representing low temperatures (to convert Celsius to Kelvin, simply add 273.15). As we go down in temperature, the movement of particles slows, eventually freezing gases, and creating substances that defy gravity and nearly stop light beams.

The calculation of absolute zero came about in a fairly simple way. Two temperatures were found, and the movement of particles was measured for each temperature. This was done using the boiling and freezing points of water. By plotting these two points on a graph, and then continuing the line until it reached the temperature where there was no movement at all, a very accurate approximation of absolute zero could be found. This, of course, is assuming the function of temperature to particle movement was linear, or a straight line. If it wasn't, more points would be calculated before the zero point could be found.

We will start at room temperature, which is about 293 Kelvin, traveling down on the temperature scale. On this journey, we soon encounter the melting point of the element Mercury, which is a liquid at room temperature. Mercury freezes into a solid at 234 K. As we get lower, we encounter the well known dry ice. Dry ice is frozen carbon dioxide and does not have a "melting point" because on contact with warm air, carbon dioxide sublimes (going directly from solid to gas, without any liquid middle stage). The gas resulting from subliming carbon dioxide is fog. In fact, the liquid form of carbon dioxide cannot occur unless under pressure. Therefore, the "subliming point" of carbon dioxide is 194.65 K.

As the temperatures continue to drop, the gases begin to liquefy. One example is oxygen, which at 90.20 K, becomes a blue liquid. One property of this liquid is its ability to make objects dipped in it very brittle. The classic example of this is dipping a bouncy ball into liquid oxygen and dropping it. The brittle properties of the ball causes it to shatter. Oxygen is supposedly one of the permanent gases, (the term was coined by Michael Faraday) or a gas that cannot be liquefied by pressure alone. These gases are oxygen, nitrogen, and Hydrogen (Helium would be a permanent gas but it wasn't discovered until later).

The second of the so called permanent gases to liquefy is nitrogen at 77 K. Nitrogen then solidifies at 63 K. The next gas is Hydrogen, which liquefies at a very low 20.28 Kelvin. This was the coldest liquefaction point of any gas before Helium was discovered. Hydrogen also becomes a solid at 14.2 K. Finally, the last gas to liquefy is Helium, at an astounding 4.22 Kelvin. Helium solidifies at an even lower temperature, and the solid from of Helium usually requires pressure to keep it stable.

Now, that all the gases are liquefied, various strange phenomena occur, the first of which is superconductivity. Many metals conduct electricity, but it has been discovered that at very low temperatures, metals suddenly have zero resistance to electrical current. Therefore, magnets have the effect of floating on the metal's magnetic field. Until 1986, all metals known had a superconductivity point of less than 30 K. Over recent years, however, metals have been discovered with higher superconductivity points. The first metal to have a point higher than 30 K was LaBaCuO (La=Lanthanum, Ba=Barium, Cu=Copper, O=Oxygen) at 35 K. The next discovery was the metal whose acronym is YBCO, which had a point of 90 K, discovered in 1987. Progress continued over the years, until today, when the highest temperature superconductor is thallium barium calcium copper oxide (Hg (12 atoms) Tl (3 atoms) Ba (30 atoms) Ca (30 atoms) Cu (45 atoms) O (125 atoms)). This remarkable substance has a superconductivity point of 138 K, and possibly up to 164 K under more extreme pressures. If more high temperature superconductors could be discovered, there would be a definite commercial use for superconductivity and electric wires could conduct electricity without any resistance, increasing the efficiency of transporting electricity over long distances. Currently, it is not known why metals reach this curious state at low temperatures or why the temperatures would vary from metal to metal.



A magnet levitating on the magnetic field produced by a superconductor. The superconductor itself is not visible but the wisps of gas are a result of the liquid nitrogen (the coolant to the superconductor) evaporating.

Another strange property of matter at extremely cold temperatures comes about when we reach 2.1768 K. Helium* (see footnote directly below this paragraph) at this point is a liquid, and it is a normal colorless liquid from 4.2 K down to 2.1768 K. Then, a strange thing happens. The liquid switches phases and turns blue. Also, its viscosity becomes zero. The viscosity of a liquid is, in common terms, the "thickness" of the liquid. For example, maple syrup, as you know, takes awhile to flow and clearly has high viscosity compared to water, which is "thin" and flows easily and quickly over a surface. However, even water encounters resistance and barriers, such as rocks or dams, can (temporarily) stop it. However, when a liquid has exactly zero viscosity, it is called a superfluid. The normal Helium 4 atom (which has two electrons, two protons, and two neutrons) becomes a superfluid at its "lambda point" or 2.1768 K, as mentioned above. The amazing properties of superfluid Helium allow it to, without friction, travel up surfaces and defy gravity. For example, if a empty container, devoid of superfluid Helium, was submerged into an area filled with the fluid, a thin film of Helium would travel up the walls of the container and fill it until the level equalizes. In fact, unless sealed, superfluid Helium would flow everywhere until it was heated above its Lambda point or until there was a film of superfluid Helium around the entire Earth! Also, below Helium's freezing point (not exactly calculated, but is probably 1.5 K for pressurized Helium and 0.95 K for regular Helium) Helium is conjectured to become a supersolid. A supersolid is identical to a superfluid, with the exception that a supersolid has solid-like properties that result in an orderly spacing of molecules. Therefore, the solid would be "flowing". Since superfluids move without friction, a superfluid fountain is a perpetual motion device. The fountain continues without any energy at all! The only problem is that superfluids exist at such low temperatures that there is no commercial use.

*The only substance that is capable of being a superfluid is Helium. This is because Helium is the only substance that is a liquid at this extremely low temperature. (Hydrogen freezes at 14.2 K)



A picture showing how superfluids can travel, as a thin film, up the walls of a container. Eventually, the levels will equalize. Also, notice that a thin film circumnavigates the entire structure. If the top was not sealed, the superfluid would creep out and escape.

In 1924, Satyendra Bose sent a paper to Albert Einstein on theories of matter at extremely low temperatures. Einstein applied his own calculations and together they discovered a peculiar property of matter at very low temperature called the Bose-Einstein condensate. The quantum properties of this state of matter are very technical, but it seems that the atoms themselves adopt wave-like properties and grow larger. As the temperature continues to drop, the waves become larger and larger, until they intersect with each other and become one single unit, moving (although very little because the temperature is so low) uniformly. The quantum physics of atoms and particles applies to the larger "atom" and allows events to be seen visibly that usually only occur on very small scales. However, nothing was physically learned about this state of matter until over seventy years later, in 1995, because the temperature needed to attain it was very low (below 0.000001 Kelvin). Before its discovery, it was thought that light atoms would be more useful in producing Bose-Einstein condensates, but the first sample synthesized was of a small sample of Rubidium at 170 nanokelvin (0.000000170 K). Later, another Bose-Einstein condensate was produced, this time with Sodium atoms. This condensate had about one hundred times more atoms, or about two hundred thousand atoms, and the results were very beneficial for seeing how Bose-Einstein condensates interact with each other. Also, the Bose-Einstein condensate has the interesting property of being able to slow down light to observable speeds. The Bose-Einstein condesate is also very fragile, and interaction with even one regular atom could turn the substance back into normal form.



A map of atomic velocities during the production of a Bose-Einstein Condensate. The colors represent how many atoms are moving at a certain velocity. For example, the color red represents that very few atoms are moving at the same velocity while the color white represented thousands of atoms moving at the same rate. The image on the left is just before formation of the condensate, and the atoms are moving to different directions at different speeds. The center and left images are progressions in the life of the Bose-Einstein condensate where the atoms are moving in unison, represented by the white peak.

The temperature at which a Bose-Einstein Condensation is achieved is still above the lowest attained temperature of 0.0000000001 Kelvin, and this is still above absolute zero. The colder you get, the harder the last bit of heat clings to the matter. What happens at absolute zero, and whether it is even attainable, is unknown and may never be known.

Dawn

Dawn is a spacecraft launched by the U.S. whose primary mission is to investigate the asteroid Vesta and the asteroid and dwarf planet Ceres. Dawn will investigate these two asteroids in particular, because they are large, and supposedly have remained intact for billions of years. Also, the ways in which Ceres and Vesta were very different, one formed with a "wet" or icy composition, and one farther out and closer to Jupiter, which formed with a "dry" or rocky composition. The contrast of these two asteroids makes the information collected very beneficial to an understanding of the formation of the Solar System.

Dawn was launched on September 27, 2007, after having been delayed several times. Dawn's orbit continued as roughly an outward spiral. The spacecraft completed an orbit around the Sun, and had a flyby of Mars on February 17, 2009 to put in on track to reach Vesta. On May 3, 2011, the first images of Vesta were captured.

The first image of Vesta taken by Dawn at a distance of approximately 750,000 miles. Another image was taken of Vesta on July 9, only about a week before entering orbit (below).

The gravity assist at Mars slowed the spacecraft down enough to orbit Vesta until 2012. The probe successfully entered Vesta orbit on July 15, 2011, and has begun conducting scientific experiments. The probe used an array of spectrometers and detectors to determine the surface composition of Vesta. Further analysis of Vesta's gravitational field also revealed clues concerning the asteroid's inner structure.

After orbital insertion, Dawn continued to decrease its orbital altitude, mapping the surface in broad swaths during the month of August 2011, and later spiraled into an orbit less than 500 miles from Vesta, from where it began more detailed surface analyses.

One significant feature of Vesta is the difference between the northern and southern hemispheres. The northern is littered with craters and the surface is as old as the Solar System itself, over 4 billion years! However, by dating estimates, the southern hemisphere's surface only has 1-2 billion years' worth of craters, suggesting that a very large impact by another asteroid may have changed the surface.



A view of Vesta showing the northern hemisphere (top) and southern (bottom). Many long scores in the surface are present near the equator, further supporting the idea of a large impact on the asteroid. The scores are probably a result of internal fracturing. Dawn also characterized the temperatures of various areas of the surface of Vesta, and the climate was found to be such that there may be frozen water beneath the surface in the colder regions, despite the asteroid's reputation as "dry". Also, later data indicated an unexpected abundance of hydrated minerals, supporting the possibility that asteroid impacts may have fed Earth's oceans.





Observation of the surface of Vesta on different wavelengths records a wider range of emitted radiation. This radiation, in turn, indicates the surface composition and structure. In the final two images above, false color imaging highlights the differences in material along the surface. Much of the surface is composed of iron and magnesium-rich dust, probably from the accumulation of material and not reflecting the internal composition. This point of view is confirmed when observing craters, where an impact has exposed lower layers of the asteroid, and these have been found to be composed of different minerals. In early 2012, Dawn revealed the unexpected intricacy of Vesta's composition, including a many-layered structure and an iron-rich core, a scenario characteristic of much larger bodies, including many moons.

Having spent almost year in orbit, the spacecraft adjusted its orbit outward in June 2012 to record final data before Vesta departure. This data underwent continued analysis, yielding even more insight. For example, the distribution of hydrated (incorporating water) minerals was different than expected and in turn changed our understanding of how planetary bodies, including the earth, gather water. Vesta showed evidence of receiving water from a steady bombardment of small dust particles very early in the history of the solar system, rather than by large impacts. Also, Dawn found evidence that Vesta is in effect a "mini-planet" as far as internal structure is concerned; there are layers corresponding to crust, mantle, and core in its interior. However, the composition of the asteroid suggests that the formation process of Vesta is more complex than previously thought.

The spacecraft propelled itself away from Vesta in early September 2012, beginning its spiral outward to reach Ceres. By December 27, 2013, Dawn was closer to Ceres than Vesta. By early 2015, the probe was beginning its approach towards Ceres. In mid-January, it began to resolve surface features, as in the image below.



On March 6, 2015, Dawn entered orbit around Ceres at a distance of about 30,000 miles. The insertion represented two historic milestones in spaceflight: Dawn became the first spacecraft ever to visit (or orbit) a dwarf planet, and the first spacecraft to successfully orbit two extraterrestrial targets. Most approaches to objects in the solar system by other spacecraft have been flybys, but the use of ion thrusters allowed Dawn to repeatedly accelerate and decelerate and orbit multiple bodies.



Dawn's arrival trajectory brought it around the side of Ceres facing away from the Sun. The first images taken in orbit (two are shown above) reveal crescents of Ceres from a distance of 30,000 miles. Over the following months, the spacecraft performed several more maneuvers to spiral in towards Ceres in preparation for entering its science orbit in late April.



During the month of May, Dawn returned numerous images of Ceres from its first mapping orbit. In particular, it captured in great resolution the mysterious "bright spots" on Ceres (see below).



The unusually reflective spots are suspected to be ice, but the spacecraft's data had not yet established this definitively. In late May, Dawn began to spiral inward to an altitude of 2,700 miles where it will enter its second mapping orbit. Further thrusts subsequently brought the orbiter to its final science orbit at an altitude of only 235 miles in October of that year. This allowed images to be taken with resolutions as high as 120 ft/pixel.



Further study revealed that the bright spots were primarily due to the presence of a salt compound, sodium carbonate. Its presence had paradigm-shifting ramifications for our understanding of Ceres's interior, namely that this material must have reached the surface due to hydrothermal activity underneath it. This in turn implies that the asteroid's interior is warmer and more dynamic than previously anticipated.

As the analysis of Ceres continued into 2016, the Dawn mission pursued other techniques of analyzing its interior, including through its gravitational field. More orbital maneuvers were making Dawn's orbit about Ceres larger over time, offering global views. By using radio signals to measure precisely how the spacecraft was responding to Ceres's gravitational pull, scientists could infer the distribution of mass in the asteroid. They concluded that its interior was fairly low in density and was differentiated into layers, as with other large Solar System bodies such as planets.

Another significant discovery occurred in February 2017, when Dawn detected organic molecules on Ceres near a crater known as Ernutet. This was the first discovery of its kind for a main belt asteroid and bolstered theories that meteorites on Earth harboring such materials could trace their origins to these objects.
Dawn's trajectory followed an outward spiral from Earth to Mars, Vesta, and Ceres. For more information, see the NASA page on Dawn.

Images from wikipedia, and Dawn website, at http://dawn.jpl.nasa.gov/

Sunday, April 5, 2009

Kepler

Kepler, named after Johannes Kepler, is a spacecraft launched by the U.S. The objective of Kepler's mission is to detect exosolar planets, or planets outside our system.

The Kepler spacecraft consists of a large telescope, equipped only for observing subtle signs of planets. The telescope will detect transits, or slight eclipses of light from the star when a planet passes in front. Some of these light changes are so slight that the difference in brightness is equal to that of a fly on a windshield, but Kepler will detect them all the same.

The Kepler spacecraft was launched on March 7, 2009 from Cape Canaveral, Florida. It escaped Earth's orbit and settled into its orbit around the Sun, which causes Kepler to follow Earth around its orbit. Kepler went through a commissioning phase and began observation on May 13, 2009. Then, the first information was transmitted to Earth in June. NASA will sorts through the thousands of images to find signs of exosolar planets. In September, Kepler verified the existence of an exosolar planet. The planet's orbital period is just over two days, so Kepler took under a week to detect its transit three times.

On January 12, 2010, the first five new planets were discovered from an analysis of the results obtained in November of the previous year. On August 26, 2010, three additional planets orbiting the same star were announced. Another major discovery occurred in early 2011, when a system of six planets was announced, along with the smallest extrasolar planet yet discovered. Planet discoveries continued to trickle in as the year went on, including another planet in May 2011.

Another interesting discovery was that of a planet, named Kepler-16b, orbiting a binary star system. Discovered in September 2011, it is the first definitively confirmed circumbinary (circum = around, binary = two [stars]) planet. Also, in November 2011, Kepler-21b was discovered. It is a rocky planet only 60% massive than Earth. Unfortunately, it is so close to its parent star that it orbits in less than three days.

On January 26, 2012, 26 new planet discoveries were released, including two more instances of circumbinary planets, and numerous systems containing two or more planets, leading to higher estimates of planets per star in the Milky Way. In response to this unexpected bounty of planets, NASA extended the mission of Kepler through 2016 in April 2012. This allowed the confirmation of orbiting bodies with longer periods of revolution. Other significant discoveries of 2012 include the first known occurrence of two planets orbiting two stars and also the discovery of a planet that orbits one of the two stars in a binary system.

By 2013, Kepler had discovered over 100 planets. In April of that year, planets in the habitable zone of two stars were discovered, and they were also Earth-like in size, being less than twice the size of the Earth. Unfortunately, a failure of the orienting mechanism of the telescope on the spacecraft in May halted observation. The spacecraft was then put into hibernation while NASA planned maneuvers to restore Kepler's mobility. Over the next several months, tests revealed that the failure could not be corrected.

Despite these difficulties, Kepler turned to other observations with its remaining capabilities, including the study of supernovae and small solar system bodies. Meanwhile, analysis of the data that Kepler had already provided continued to reveal many new extrasolar planets. Using a data analysis method called verification by multiplicity, many planets in multiple-planet systems were verified in early 2014, culminating in an announcement on 2014 that an astounding 715 new planets had been confirmed! Several of these planets were also small (smaller than Neptune) and a few were Earth-sized and in their stars' habitable zones.

In April 2014, data analysis revealed the most Earth-like planet yet known: an Earth-sized body orbiting in the habitable zone of its star, a red dwarf. Known as Kepler-186f, this planet is about 492 light-years away, and is potentially habitable.

On May 16, 2014, NASA approved a new mission for the Kepler telescope itself, after scientists developed a method to keep the spacecraft sufficiently steady with only two reaction wheels to observe an area of the sky continuously for over 80 days, enough to detect transiting planets. Using the radiation pressure from the Sun as a counterforce, the telescope can balance the force from the remaining orientation mechanism. The image below illustrates the so-called K2 mission.



Instead of fixing the gaze of the telescope at a small area of sky for a number of years, as in the first scientific campaign, the K2 mission explored several different fields of view, spending a few months on each phase or "campaign". The new mission detected its first confirmed exoplanets in January 2015. That same month, the number of confirmed planets from the Kepler mission surpassed 1000 with the further analysis of previous data.

In July 2015, one of Kepler's more notable planet candidates was confirmed. Known as Kepler-452b, the planet was the most similar to Earth of any yet discovered: it is 60% larger than the Earth in diameter (and therefore has a good chance of being rocky), orbits a sun-like star in an orbit only 5% larger than Earth's, and has an orbital period of 385 Earth days, very close to our own. In addition, the planet is estimated to have existed for 6 billion years, even longer than the Earth, giving it a better chance of harboring life.

Mission operations continued normally until April 7, 2016, at which time it was discovered that the spacecraft had entered emergency mode. NASA immediately made efforts to return the telescope to normal operations in order to make the scheduled maneuver. These effort were successful and the spacecraft was able to resume normal operations on April 22. It was then able to begin Campaign 9 (abbreviated C9) of its mission. This involved using gravitational microlensing to detect planets farther away from their host stars. This works as follows: when a planet passes in front of its star, the mass of the planet causes starlight to bend (very slightly) around it, causing a temporary increase in brightness, as illustrated in a graphic below from Kepler's NASA website.


The scale of the bending is exaggerated here for illustration. Note that for planets close to their host stars, Kepler looked for a decrease in brightness that would indicate starlight being blocked. However, for sufficiently massive and distant planets, the gravitational microlensing effect is larger, leading to a net increase in brightness.

Meanwhile, continuing data analysis continued to yield new planet confirmations from among Kepler's earlier candidates. On May 10, 2016, NASA announced that an additional 1,284 planets had been confirmed, more than doubling the total that Kepler had verified. Among these, almost 550 were of a size that they could be rocky, and nine of these were in the habitable zones of their parent stars.

In June 2016, the mission was officially extended through the anticipated end of Kepler's fuel resources.

Sources: http://www.nasa.gov/mission_pages/kepler/main/, http://keplerscience.arc.nasa.gov/K2/, http://www.theverge.com/2016/4/8/11395796/nasa-kepler-spacecraft-mission-emergency-mode,https://keplerscience.arc.nasa.gov/k2-mission-officially-extended-through-end-of-mission.html