Casscenery (The Cassini Photos of Saturn)

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Cassini Image of Saturn

As the 20 year long Cassini mission comes to an end, no one wants to miss the exciting events of the last 4.5 months. Cassini is slated to crash into the surface of Saturn on September 15th this year, but not before Cassini swoops progressively lower to the gas giant, giving scientists unprecedentedly close images of the planet and views from within Saturn’s rings. Those hoping to experience the end of Cassini’s mission for themselves should check out the JPL website where NASA will be uploading raw images from Cassini in real time for the general public to look at. You should be cautioned though, Cassini images like the one at the top of this blog are hard to come by amongst the hundreds of thousands of raw images taken by Cassini.

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Raw Cassini Image

Space enthusiasts may also notice that the popular Cassini images are all in color but that the raw images are all in black and white. This is because the Cassini imaging system is sensitive to a wide range of colors, both visible and beyond the human visible range. To filter information from this broad-spectrum imager, Cassini is equipped with a variety of filters it uses with its camera. All raw images from Cassini are grey scale, but by combining the grey scale images of the same scene taken with three different filters, NASA can create the breathtaking pictures we the public often take for granted.

More Cassini Information: Here

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The Arecibo Message

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The Arecibo Observatory

The Arecibo Observatory was constructed in 1963 as the world’s largest and most sensitive radio telescope. Sitting in a naturally spherical valley in Puerto Rico, this telescope looks decidedly different from the pristine optics associated with optical telescopes such as the Keck Observatory or the Hubble Space Telescope. This is because the Arecibo Observatory peers into the Universe with radio waves. These radio waves have large wavelengths, between 3 cm and 1 meter, so the green vegetation stains on the bottom hardly make a difference. In fact, the radio dish isn’t even a smooth round shape; the spherical dish is comprised of 38,778 suspended flat steel plates that approximate the spherical shape used to concentrate radio signals to the moving receiver and transmitter suspended above the dish by three cables.

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Colorized Representation of the Arecibo Message

The unique transmitting feature of the Arecibo Observatory was put to use in 1974, when at the behest of Dr. Frank Drake (author of the Drake equation) and Carl Sagan, a binary message was sent towards the star cluster Messier 13. This message (more information here) contains information about life on Earth, human biology, our place in the solar system, and current Earth technologies. If we are lucky (or potentially unlucky), aliens should be able to pick up this signal from M13 within about 25,000 years, when the light speed signal reaches these distant stars. Now we Earthlings have to patiently wait around 50,000 years for a potential return signal, but in the meantime, the Arecibo Observatory has been busy making radar mappings of the surface of Venus, finding Soviet radar stations by observing radio waves reflected from the Moon, and generally advancing our knowledge of the Universe.

Ida and Dactyl

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Ida and Dactyl

243 Ida is a 56 km long asteroid orbiting in the main asteroid belt with a number of notable features. Ida is an S-type asteroid, or stony asteroid, and is mostly composed of rock and iron from accretion during early solar system formation. Ida was a subject of study by the Galileo spacecraft in 1993, and much of what we thought we knew from telescopic observation has been overturned by the closer look provided by the flyby mission.

Ida orbits with a large number of other asteroids in what is known as the Koronis Asteroid Family. This is a collection of objects with similar orbits, which indicates that they likely were formed from the shattered remains of a single larger asteroid that was recently destroyed by an impact. However, the surface of Ida is heavily cratered, indicating that the asteroid is older than originally anticipated, and older than estimates for the Koronis breakup. It was also discovered that Ida has a natural satellite, Dactyl.

Dactyl is the first natural satellite of an asteroid discovered and scientists have sought to explain how the small asteroid with gravity 0.001 times that of Earth’s gravity could hold onto a satellite. With no atmosphere and such low gravity, it is very unlikely that Dactyl was captured by Ida, so it seems likely that Dactyl may have broken off Ida during a large impact, and then settled in a slow orbit around the asteroid. This theory has gained credibility because of the discovery of a relatively recent crater, named the Azzura crater that may be the result of the impact that created Dactyl. Additionally, ejecta blocks, which are large boulders laying on the surface of Ida, have been found primarily on the leading surfaces of the oblong asteroid as it rotates. This suggests that these boulders were ejected from the surface of Ida and then slowly settled back down to be swept up preferentially by leading surfaces of the quickly spinning asteroid.

 

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Ejecta Blocks on Ida

Much of the information in this blog post came from Cosmic Pinball by Carolyn Somners and Carlton Allen and I would highly recommend reading it for more information about solar system collisions.

Comet Shoemaker-Levy 9

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The “String of Pearls” Pieces of Shoemaker-Levy 9

In 1994, one year after its discovery, the fragmented remains of Comet Shoemaker-Levy 9 crashed into Jupiter’s upper atmosphere in a sequence of 23 large impacts, each releasing the energy equivalent of 25,000 megatons of TNT, more than one million times as much energy as released by the nuclear bomb dropped on Hiroshima. Orbital analysis between its discovery in 1993 and its demise in 1994 indicates that Shoemaker-Levy 9 experienced a close encounter with Jupiter in 1992, and it is thought that tidal forces due to this close encounter broke apart the loose mass of ice and dust.

The nucleus of a comet is a dirty snowball formed by accretion of hydrogen compounds in the outer solar system early in its formation. These comets are primarily composed of water ice and carbon dioxide ice, and generally are less dense than astronomers would expect, at about 0.6 g/mL. Scientists think that this lower density may be due to comets’ outgassing of material as they partially sublimate during their passes at the inner solar system. The selective melting of material may leave holes and crevices that lower the density of the comet below what we would expect. It is not known how the tidal forces of Jupiter were able to so completely break apart comet Shoemaker-Levy 9, but it is possible that such crevices weakened the internal structure of the comet nucleus.

Regardless of its structure, the energy released by the ice fragments as they plunged into Jupiter’s upper atmosphere at 63 kilometers-per-second was formidable, and the Earth size purple bruises left on Jupiter’s surface served as a reminder of the dangers of solar system objects to any inhabitants of its larger worlds.

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Impact “Bruises” on Jupiter

Much of the information in this blog post came from Cosmic Pinball by Carolyn Somners and Carlton Allen and I would highly recommend reading it for more information about solar system collisions.

Read more about Shoemaker-Levy 9 here and here.

Find more information about comet structure and discovery here.

Ballooning on Venus

After their launch in 1984, the identical spacecraft Vega 1 and Vega 2 launched from a Russian Proton Rocket for their double mission of flying through the tail of Halley’s Comet and landing scientific payloads on the surface of Venus. In addition to a regular parachuted lander, the Vega spacecraft each carried a 22-kilogram balloon assembly that detached from the main lander during descent and deployed about 50 km above the surface of Venus. Both of these balloons landed on the night side of Venus. With only 60 hours of battery life on the balloons, and with days on Venus clocking in at a lingering 243 Earth days, it would seem that the Vega balloons would never see the daylight side of Venus. However, over the next 60 hours, the balloons were carried to the daylight side of Venus by the hurricane speed winds found in the middle of the three layers of the Venusian atmosphere. According to NASA, the probes measured “the local atmospheric dynamics, pressure, temperature, lightning, illumination levels, and cloud properties over a period of about 46 hours in both the night- and day-side.”

ESA Balloon

Pictured above is a formerly proposed mission by the ESA to launch another Venus balloon to follow up the Vega missions, but this project has not yet received funding.

Solar Probe Plus

In 1976, NASA’s Helios 2 spacecraft set the current distance record by orbiting the Sun with a closest approach of 43.4 million kilometers. Even though this is barely inside the orbit of Mercury, the intense heat close to the Sun has previously prevented any closer observation. The Goddard Space flight Center “Living with a Star Program” hopes to change this. In 2018, NASA’s Solar Probe Plus is expected to launch from Cape Canaveral and begin a six-year journey to its closest approach to the Sun. Using seven Venus flybys to bleed off orbital energy and fall closer to the Sun, the spacecraft will enter a highly elliptical orbit with a record setting distance of closest approach of 6 million kilometers.

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Solar Probe Plus Orbit

At this distance, the Sun is 500 times brighter than at Earth. To protect the sensitive instruments, the probe will be protected with a 4.5 inch thick carbon composite heat shield and will be actively cooled with large radiator arrays.

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Solar Probe Plus Heatshield

Using the data collected at close range astronomers hope to learn about the energy flow on the surface of the Sun, why the solar corona is hotter than the surface of the sun, and what accelerates the solar wind. Historically, the Sun has only been observed from around the equator, because the orbit of the Earth and other planets in the Solar System is approximately in the plane of the Sun’s equator. Solar Probe Plus will use the Venus flybys to increase its orbital inclination and better observe the solar poles.

Sources:

Solar Probe Plus

Living With a Star

The Era of Long Refractors

Telescopes focus light down to a point to increase the light gathering capacity of the astronomer’s eye. The optimal shape for such focus is a parabola, either a parabolic mirror, or a refracting lens of parabolic shape. Unfortunately, parabolic lenses do not have the same curvature everywhere the way spherical lenses do, making their construction without computer controlled equipment very difficult. Seventeenth century astronomers avoided this issue by constructing very shallow spherical lenses because at small angles, the spherical lens approximates the shape of a parabolic lens.

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Spherical vs Parabolic Optics

The lack of a single focal point from a spherical lense is called “spherical aberration.”

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Spherical Aberration

The problem with this arrangement is that the extremely shallow lenses had extremely long focal lengths, and therefore the telescopes during this era were extremely long.

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Hevelius’ Refractor

While Christiaan Huygens was able to discover Titan with a 12-foot long refractor, astronomers of the time saw no reason to stop there. Such thinking led to the 150-foot long telescope constructed by Johannes Hevelius pictured in the drawing. Unfortunately, at these extreme lengths, the telescope was very difficult to move and susceptible minor changes in weather, but it was these limitations that led to the improvement of reflector telescopes that are primarily in use today.

Sources:

Optical Aberration Information

Historical Long Telescopes