Thursday, February 25, 2010

Those Other Moons of Jupiter

After being periodically surveyed by robotic spacecraft in recent years, did those astronomers that discovered the other moons of Jupiter years before got the accolades that they truly deserve?

By: Ringo Bones

I tend to have a rather rigid definition on the difference between space exploration and astronomy because to me, sending space probes to distant celestial bodies – in my point of view – is space exploration. While peering though an astronomical telescope via the eyepiece or via a high-resolution computer grade monitor in front of me is what I define as true astronomy. The Voyager and Galileo spacecraft flybys of Jupiter and the planet’s retinue of satellites may have gathered data previously unknown to late 19th and early 20th Century astronomers. But the astronomers themselves who discovered those excruciatingly tiny non-Galilean satellites decades before those robotic spacecraft flybys seem to have been largely forgotten in this day and age.

Before the Pioneer X and Voyagers I and II flybys, there are only 12 moons of Jupiter that can be seen from Earth using existing astronomical telescope technology prior to the 1960s. Set side-by-side to the relatively large four Galilean satellites: Io at 2,000 miles, Europa at 1,800 miles, Ganymede at 3,120 miles, and Callisto at 2,800 miles, the other moons of Jupiter look like grains of sands in comparison. With diameters that range from as large as 70 miles to as small as 10 to 12 miles, discovering these Jovian moons without resorting to optical interferometry is probably next to impossible.

The first one of these non-Galilean satellites to be discovered was discovered by an American astronomer named Edward Emerson Barnard of Barnard’s Star fame in 1892 and was called / designated as V. This tiny Jovian moon at just 70 miles in diameter in unique in many ways. V orbits just 110,000 miles from Jupiter allowing Jupiter’s strong gravitation to make the tiny moon hurtle through space at 1,000 miles a minute – 26 times faster than the Earth’s moon.

On average, Jupiter is 629 million kilometers from Earth, which makes Barnard’s discovery of V somewhat of a remarkable feat in astronomy back in 1892 because astronomical telescopes having at least a 4-meter diameter mirror is the minimum needed to see V from Earth due to the Rayleigh Criterion limitations. The smallest celestial object that a 4-meter mirrored astronomical reflecting telescope when the Rayleigh Criterion is taken into account is around 103.5 kilometers – a little under 70 miles - in diameter from 629 million kilometers away using the visible spectrum centered around 550 nanometers. And most important of all, use of optical interferometry in astronomy to bypass the inherent Rayleigh Criterion limitations of existing reflecting telescopes in 1892 was still several years away. Probably a few years after 1910 when Albert A. Michelson used an optical interferometer of his own design to accurately measure the diameters of those newly-discovered tiny satellites or moons of Jupiter.

Even the discovery of the next non-Galilean satellite designated as VI (diameter 50 miles) in 1904 and VII (diameter 20 miles) in 1905 by American astronomer Charles Dillon Perrine is also miraculous given that optical interferometry for astronomical use is still years away. Even back in 1908, when English astronomer Philbert Jaques Mellote discovered Jupiter’s moon designated as VIII with a diameter of 10 miles without an aid of optical interferometry is nothing short of a miracle.

In 1914, an American astronomer named Seth Barnes Nicholson discovered one of the last few moons of Jupiter whose diameter averages between 10 to 12 miles designated as IX, probably with the aid of Albert A. Michelson’s newfangled optical interferometer. Not only will Nicholson discover the last of the tiny moons of Jupiter that can be seen from Earth before the Pioneer X and Voyager spacecraft flybys, he also made remarkable theories that almost accurately predicted the actual climate of the planet Venus. Together with fellow astronomer Charles St. John, their hot and dry Venus climate hypothesis that they proposed back in 1922 was proven to be closer to reality. Compare that to the one proposed in 1918 by Swedish chemist Svante August Arrhenius – who also discovered the mechanism behind the greenhouse effect – depicted planet Venus as covered by hot steamy tropical swamps.

In 1938, Nicholson discovered another two of Jupiter’s very tiny non-Galilean satellites. Designated at the time as X and XI, X because Nicholson declined suggesting names for the new Jovian moons that he discovered orbits in a region 7,400,000 miles away from Jupiter while XI orbits in a region almost 15 million miles away from Jupiter. X is part of the other 4 of Jupiter’s outermost satellites that orbit in a retrograde direction. Two of these outer satellites even have “open” orbits that are never repeated from one circuit to the next. Probably due to the Sun’s much stronger gravitational influence in comparison to Jupiter’s at this distance.

The last of Jupiter’s satellites / moons that can be seen by Earth-based telescopes have to await discovery until 1951 when XII – now known as Ananke – was yet again discovered by Seth B. Nicholson. XII or Ananke was a difficult find not only because of the Jovian moon’s small size – 10 miles – but also because it shines no brighter than the light of a burning candle seen from 3,000 miles away at night. This is primarily due to the Jovian moon’s extremely low visual albedo or reflectivity, not to mention the Jovian moon’s relatively small size of 10 miles in diameter. Sadly, these amazing astronomers, especially Seth Barnes Nicholson, are largely forgotten given their amazing feats in the science of astronomy.

Monday, February 22, 2010

Optical Interferometry: A Way Around the Rayleigh Criterion?

Invented by Albert A. Michelson during the 1870s, can optical interferometry be used effectively in circumventing the Rayleigh Criterion limitations of a typical reflecting telescope?

By: Ringo Bones

Albert A. Michelson was more famous for his work with Edward W. Morley in which they won the 1907 Nobel Prize in physics for proving that the mythical medium called the ether wind doesn’t exist, thus paving the way for Einstein’s theory of special relativity. Michelson invented the interferometer primarily as a way of accurately measuring the speed of light back in the 1870s. Albert A. Michelson also managed to earn the fame of being the first one to utilize optical interferometry as a very important astronomical instrument until 1920. It is this time when Michelson became the first ever person to measure a diameter of a star using an optical interferometer of his own design. He determined that Alpha Orion to be 260 million miles in diameter. A few years before, Michelson was also the first person to determine the diameter of Jupiter’s satellites. But how does optical interferometry works?

Optical interferometry depends on the light splitting and summing properties of an interferometer. An interferometer is an instrument that utilizes light interference phenomena for precise determinations of wavelength, fine structure of spectral lines, refractive indices of a given medium and very fine linear displacement of distant objects. By bringing together beams of starlight captured by two or more widely separated telescopes, a typical optical interferometer can achieve the equivalent resolving power of a single instrument equipped with a main mirror as large as the distance between the ganged telescopes.

When the starlight beams are combined, the light waves interfere with one another. Where the peak of one light wave meets the peak of another, they reinforce each other. When the peak of one light wave meets the through of another, they cancel out. An electronic detector or a video digital-to-analog-converter (video DAC) records the resulting pattern of dark and light areas – or interference fringes – which can then be analyzed by computer via digital signal processing to extract detailed information about the object being observed. If at least three telescopes are used, the fringes can be rendered into images hundreds of times crisper than even those obtained by the orbiting Hubble Space Telescope – at a much reduced expense, thus bypassing the Rayleigh Criterion limitations of constructing an ever bigger mirror of a typical astronomical telescope. Given its ability to improve reflecting telescope performance beyond their Rayleigh Criterion limitations, why is it that most astronomers are still mistrustful over optical interferometry?

There had been famous and amazing stargazing feats achieved by optical interferometry since the 1970s. In 1974, Kitt Peak National Observatory astronomers managed to penetrate the Earth’s atmospheric haze for the first time by discerning the features in the atmosphere of a star named Betelgeuse with the computer aided technique called speckle interferometry, while the Mark III Optical Interferometer on Mount Wilson Observatory in California had been in operation since 1986. Astronomers tend to be a conservative bunch and a lot of them consider optical interferometry to be “black magic” because even though they can measure the outlines of celestial objects millions – even billions - of miles away, optical interferometers cannot make true images of these objects.

If you remember the movie version of Tom Clancy’s Patriot Games when Langley analysts got a hard copy of a photo taken by a KH-11 reconnaissance satellite showing the cleavage of the female underwriters of the Ulster Liberation Army standing in the middle of the Libyan Desert. You’ll notice that it is monochromatic – i.e. black and white – yet managed to show features that according to Rayleigh Criterion on the KH-11 reconnaissance satellite’s mirror specifications cannot supposedly resolve that make it appear like a distinct black and white image of a woman’s cleavage seen from 160 miles up. That’s the power of optical interferometry put to use were the video signals are probably processed using the reconnaissance satellite’s built-in 10-bit video DAC that’s probably not more advanced than one’s found in a circa 1998 DVD player.

Image quality-wise, the resulting image is so “abstract” and “clinical” that free-spirited American under-aged teens who are frequent skinny-dippers have no fear having compromising photos taken by computer nerds who know how to re-task the US National Security Agency’s reconnaissance satellites - Largely because its image quality is far inferior in comparison to those photos taken by a typical paparazzi operating in the 90210 area code. Probably due to the fact that optical interferometry – a technique probably utilized by the KH-11 reconnaissance satellite to be able to read Soviet-era Pravda headlines and car license plates from 160 miles up - cannot make true images.

Thursday, February 11, 2010

The Rayleigh Criterion: The Bane of Mirrored Telescopes?

Named after a 19th Century English physicist, is the Rayleigh Criterion the true arbiter over press hype of the true capabilities of large mirrored telescopes and reconnaissance satellites?

By: Ringo Bones

I could have titled this blog entry as; “Is the KH-Series of American Reconnaissance Satellites’ Capability of Seeing Soviet-era Pravda headlines from 160 kilometers up Somewhat Overly Optimistic?” but that would only answer a part of a little understood concept of optics. And if the “hype” over the runaway success of adaptive optics were to be believed, would this imply that the best astronomical telescopes could soon be found here on Earth as opposed to orbital space? Though that too is only part of the story, but the point of this discussion is about how an oft-ignored, yet vital aspect, of advanced telescope design. Whether using a Hubble Space Telescope-sized 2.4-meter primary mirror or a 10-meter primary mirrored Earth-based astronomical telescope is ultimately diffraction limited. This means that the resolution of a large reflecting telescope – whether those on the KH-series reconnaissance satellites, the Hubble Space Telescope, or those very large astronomical telescopes found on top of Mauna Kea or in the Chilean desert – is limited by diffraction.

The first person to study the diffraction limitation problem of reflecting or mirrored astronomical – or other large - telescopes was a 19th Century English physicist / gentleman-scientist named John William Strutt, but he’s better known to the rest of the world as the 3rd Baron Rayleigh or Lord Rayleigh. Also more famous for his 1904 Nobel prize for Physics for the discovery of the element argon – which he isolated in cooperation with Sir William Ramsay – than his work / investigations in optics that lead to the criterion named after him. The Rayleigh Criterion defines the resolution capabilities of a reflecting telescope where two points are just resolved when their angular separation is equal to an angle designated as theta at which the first diffraction minimum occurs. As given in the equation theta = 1.22 multiplied by the wavelength of light of interest divided by the mirror diameter of the telescope. The number 1.22 is a constant derived by Lord Rayleigh when he used differential equations in tackling this problem. The angle theta can also be designated as dividing the linear separation of the characters of interest – like the headline of a Soviet-era Pravda newspaper at around an inch or 2.5 cm – with the orbital altitude of the reconnaissance satellite – often at 160 kilometers up.

If anyone – besides me – is really curious if a KH-11 like reconnaissance satellite is really capable of seeing clearly a Pravda newspaper headline from 160 kilometers from the Earth’s surface. One can use the Rayleigh Criterion to test to test whether the KH series of reconnaissance satellite’s capabilities are nothing more than cold War era media hype. Let’s just assume that the KH-11 reconnaissance satellite has a main mirror similar in size to that of the Hubble Space Telescope at 2.4 meters since the two are almost of similar dimensions. Assuming that it works in the visible light spectrum at 550 nanometers (smack down in the middle or the green portion of the visible light spectrum) as the wavelength of interest.

So to get the resolution limit of your typical reconnaissance satellite, just multiply 1.22 with the orbital altitude of the reconnaissance satellite – usually around 160 kilometers or 160,000 meters. Multiply this number with the quotient that resulted when the wavelength of interest – 550 nanometers (550 times 10 to the negative 9 meters) – is divided by the reconnaissance satellite’s main mirror diameter of 2.4 meters. The resulting figure is 4.47 centimeters, which makes the reflecting telescope of your typical reconnaissance satellite really have a hard time seeing a Pravda newspaper headline – even license plates – from 160 kilometers up. The letters and numbers may be 4.47 centimeters tall but if they are spaced closer than 4.47 centimeters, the resulting image will be a blur if taken from 160 kilometers up. Shifting to the longer infrared wavelengths worsen the resolution when looking at “small” objects, while the shorter ultraviolet wavelengths could face problems of increasing atmospheric opacity from going through more than 100 kilometers of air.

There might be truth to the rumors of the gripes of “civilian” optical technicians working on the Hubble Space Telescopes main mirror during the Reagan Administration being denied access to the mirror testing equipment primarily used to test the main mirrors on the KH-11 series of reconnaissance satellites. Citing national security concerns back when America was still engaged in a Cold War with the Soviet Union. Sadly, this resulted in the Hubble Space Telescope’s technicians not knowing that the telescope’s main mirror is ground a few millionths of an inch too much while still on the ground in the NASA clean room. Worse still, the astronomical community only knew of the Hubble’s misshapen mirror only after it has been launched 350 kilometers into orbital space when they tested it, hence the then famous press headline of the Hubble Space Telescope being a 1.2 billion dollar blunder.

Maybe, the end of the Cold War proved to be a blessing in disguise to the Hubble Space Telescope because there are rumors too that the potato chip-shaped “corrective lenses” that are retrofitted to the Hubble back in 1993 were borrowed from the KH series of reconnaissance satellites. Maybe this specially shaped lenses are the “magic wand” that allowed the NSA reconnaissance satellites to beat around the Rayleigh Criterion / diffraction limitations of space-based reflecting telescopes. Regardless whether they are used to look up and out into space or look down on Earth’s surface in search of WMDs.