Halley and His Comet

Halley and His Comet

The occasional apparitions of the comets disturbingly challenged the notion of an unalterable and divinely ordered Cosmos. It seemed inconceivable that a spectacular streak of milk-white flame, rising and setting with the stars night after night, was not there for a reason, did not hold some portent for human affairs.

So the idea arose that comets were harbingers of disaster, auguries of divine wrath – that they foretold the death of princes, the fall of kingdoms. The Babylonians thought that comets were celestial beards. The Greeks thought of flowing hair, the Arabs of flaming swords. In Ptolemy’s time comets were elaborately classified as “beams”, “trumpets”, “jars” and so on, according to their shapes. Ptolemy thought that comets bring war, hot weather and “disturbed conditions”.

Comet Halley has played an interesting role in human history and may be the target of the first space vehicle probe of a comet, during its next return in 2062. Image: Princess on Purple Magic Horse. Fantasy Art. © Megan Jorgensen (Elena)

Some medieval depictions of comets resemble unidentified flying crucifixes. A Lutheran “Superintendant” or Bishop of Magdebourg named Andreas Celichius published in 1578 a “Theological Reminder of the New Comet”, which offered the inspired view that a comet is “the thick smoke of human sins, rising every day, every hour, every moment, full of stench and horror before the face of God, and becoming gradually so thick as to form a comet, with curled and plaited tresses, which at last is kindled by the hot and fiery anger of the Supreme Heavenly Judge”. But others countered that if comets were the smoke of sin, the skies would be continually ablaze with them.

The most ancient record of an apparition of Halley’s (or any other) Comet appears in the Chinese Book of Prince Huai Nan, attendant to the march of King Wu against Zhou of Yin. The year was 1057 B.C. The approach to Earth of Halley’s Comet in the year 66 is the probable explanation of the account by Josephus of a sword that hung over Jerusalem for a whole year. In 1066 the Normans witnessed another return of Halley’s Comet. Since it must, they thought, presage the fall of some kingdom, the comet encouraged, in some sense precipitated, the invasion of England by William the Conqueror. The comet was duly noted in a newspaper of the time, the Bayeux Tapestry. In 1301, Giotto, one of the founders of modern realistic painting, witnessed another apparition of Comet Halley and inserted it into a nativity scene. The Great Comet of 1466 – yet another return of Halley’s Comet – panicked Christian Europe; the Christian feared that God, who sends comets, might be on the side of the Turks, who had just captured Constantinople.

The leading astronomers of the sixteenth and seventeen centuries were fascinated by comets, and even Newton became a little giddy over them. Kepler described comets as darting through space “as the fishes in the sea”, but being dissipated by sunlight, as the cometry tail always points away from the Sun. David Home, in many cases an uncompromising rationalist, at least toyed with the notion that comets were reproductive cells – the eggs or sperm – of planetary system, that planets are produced by a kind of interstellar sex. As an undergraduate, before his invention of the reflecting telescope, Newton spent many consecutive sleepless nights searching the sky for comets with his naked eye, pursuing them with such fervor that he fell ill from exhaustion. Following Tycho Brahe and Johannes Kepler, Newton concluded that the comets seen from Earth do not move within our atmosphere as Aristotle and others had thought, but rather are more distant from the Moon, although closer than Saturn. Comets shine as the planets do, by reflected sunlight and “they are much mistaken who remove them almost as far as the fixed stars; for if it were so, the comets could receive no more light from our Sun than our planets do from the fixed stars”. He showed that comets, like planets, move in ellipses; “Comets are a sort of planets revolved in very eccentric orbits about the Sun”. This demystification, this prediction of regular cometary orbits, led his friend Edmund Halley in 1707 to calculate that the comets of 1531, 1607 and 1682 were apparitions at 76-year intervals of the same comet, and predict its return inn 1758. The comet duly arrived and was named for him posthumously.

Edmond Halley, the British astronomer Royal whose name is immortalized by a certain popular comet, was born on November 8, 1656.

Before Halley observed his famous namesake, the comet that bears his name visited the inner solar system regularly since at least 240 B.C. Yet not one understood that comets orbits the Sun. It was believed that these mountain-sized chunks of frozen gas and dust appeared only once and then vanished. Many even thought that comets were freakish storms high in Earth’s atmosphere.

Halley, a gentleman adventurer who explored the seas, studied the atmosphere, and charted the skies, developed an interest in comets when he saw the very bright comet of 1680 while in Paris. He got the records of its movements from the director of Paris Observatory and tried – without success – to calculate the comet’s orbit.

The South Polar Cap of Mars. Photo by NASA of public domain

Later, Halley befriended Isaac Newton, the physicist and mathematician who devised the three laws of motion and the law of universal gravitation. Halley later convinced Newton to record his work in a volume known as the Principia, and he even paid for the book`s publication.

Newton told Halley that he believed comets circle the Sun in elliptical orbits. Using Newton`s laws, Halley computed the orbits of twenty-four comets. He found that the comets of 1456, 1531, 1607, and 1682 had very similar orbits. Halley decided they represented just one object, and he predicted that the comet would return in 1758.

And it did. On Christmas night 1758, sixteen years after Halley`s death, amateur astronomer George Palitzsch saw the comet on its way toward a rendezvous with the Sun in early 1759.

Interstellar Ship

Interstellar Ship

Let us spend a moment thinking about an interstellar ship. The Earth gravitationally attracts us with a certain force, which if we are falling we experience as an acceleration. Where we to fall out of a tree – and many of our proto-human ancestors must have done so – we would plummet faster and faster, increasing our fall speed by ten meters (thirty-two feet) per second, every second. This acceleration, which characterizes the force of gravity holding us to the Earth surface, is called 1g, g. for Earth gravity. We are comfortable with accelerations of 1 g; we have grown up with 1 g. If we lived in an interstellar spacecraft that could accelerate at1 g., we would find ourselves in a perfectly natural environment. In fact, the equivalence between gravitational forces and the forces we would feel in an accelerating spaceship is a major feature of Einstein’s later general theory of relativity. With a continuous 1 g acceleration, after one year in space we would be traveling very close to the speed of light (0,01 km/sec2) x (3×10(7) sec) = 3 x 10(5) km/sec).

Suppose that such a spacecraft accelerates at 1g, approaching more and more closely to the speed of light until the midpoint of the journey; and then is turned around and decelerates at 1 g until arriving at its destination. For most of the trip the velocity would be very close to the speed of light and time would slow down enormously. A nearby mission objective, a sun that may have planets, is Barnard’s Star, about six light-years away. It could be reached in about eight years as measured by clocks aboard the ship; the center of the Milky Way, in twenty-one years; M31, the Andromeda galaxy, in twenty-eight years.

Of course, people left behind on Earth would see thing differently. Instead of twenty-one years to the center of the Galaxy, they would measure an elapsed time of 30,000 years. When we got home, few of our friends would be left to greet us. In principle, such a journey, mounting the decimal points ever closer to the speed of light, would even permit us to circumnavigate the known universe in some fifty-six years ship time.

We would return tens of billions of years in our future = to find the Earth a charred cinder and the Sun dead. Relativistic spaceflight makes the universe accessible to advanced civilisations, but only to those who go on the journey. There seems to be no way for information to travel back to those left behind any faster that the speed of light.

The designs for Orion, Daedalus and the Bussard Ramjet are probably farther from the actual interstellar spacecraft we will one day build than Leonardo’s models are from today’s supersonic transports. But if we do not destroy ourselves, Carl Sagan believed that we would one day venture to the stars.

When our solar system is all explored, the planets of other stars will beckon. Image: As Space Art meets Science by © Megan Jorgensen (Elena)

Is it possible for the Humans to reach another galaxy? Well, the answer is not simple.

If the Humans lived in an interstellar spacecraft that could accelerate at 1 g., they would find themselves in a perfectly natural environment. In fact, the equivalence between gravitational forces and the forces the Earthlings would feel in an accelerating spaceship is a major feature of Einstein’s later general theory of relativity. With a continuous 1 g acceleration, after one year in space an interstellar ship would be traveling very close to the speed of light.

Well, now suppose that such a spacecraft accelerates at 1 g, approaching gradually to the speed of light until the midpoint of the journey. Then the spacecraft turns around and decelerates at 1 g until arriving at its destination.

We can see thus that for most of the trip the velocity would be close to the speed of light. According to the Einstein’s theory, time would slow down enormously.

Interstellar ship. When the Solar system is all explored, the other stars and galaxies will beckon. Image: Spaceship traveling near star by © Elena

In this case, a nearby mission objective, f.i. the Barnard’s Star, located about six light-years away, could be reached in about eight years as measured by clocks aboard the spaceship.

The Humans thus could reach the center of the Milky Way, in twenty one years and the Andromeda Galaxy (M31) in twenty-eight years only! In principle, such a journey, mounting the decimal points ever closer to the speed of light, would even permit a crew to circumnavigate the known universe in some fifty-six years ship time.

Unfortunately, people left behind on Earth would see things differently. Instead of twenty-one years to the center of the Galaxy, they would measure an elapsed time of 30,000 years. When the interstellar ship got home, Humanity would change for the better or for the worse and even the mission would be obliterated.

After the trip to Andromeda Galaxy, the ship would return tens of billions of years in the future, to find the Earth a charred cinder and the Sun dead.

The conclusion is thus obvious: Relativistic spaceflight makes the universe accessible to advanced civilizations, but only to those who go on the journey. There seems to be no way for information to travel back to those left behind any faster that the speed of light.

Is there a way to break the rule? No! But… But there exists a perfect and incredible easy way to bend it! Technically, Human civilization can start using it today. Financially… well, financially it is another matter.

Supernova and Rare Elements

Supernova and Rare Elements

Some of the rare chemical elements are generated in the supernova explosion. The origin and evolution of life are connected in the most intimate way with the origin and evolution of the stars.

We have relatively abundant gold and uranium on Earth only because many supernova explosions had occurred just before the solar system formed. Would our lives be improved if gold and uranium were as obscure and unimportant on Earth as praseodymium?

The Sun is a second – or third – generation star. All the matter in it, all the matter you see around you, has been through one or two previous cycles of stellar alchemy.

Other planetary systems may have somewhat different amounts of our rare elements. There are planets where the inhabitants proudly display pendants of niobium and bracelets of protactinium, while gold is a laboratory curiosity.

The evolution of life on Earth is driven by the spectacular death of distant, massive suns. Image: © Elena

The very matter of which we are composed, the atoms that make life possible, were generated long ago and far away in giant red stars. In fact the relative abundance of the chemical elements found in the Cosmos matches the relative abundance of atoms generated in stars. No doubt, red giants and supernovae are the ovens and crucibles in which matter has been forged.

The existence of certain varieties of heavy atoms on the Earth suggests that there was a nearby supernova explosion shortly before the solar system was formed. But this is unlikely before the solar system was formed and this is unlikely to be a mere coincidence; more likely, the shock wave produced by the supernova compressed interstellar gas and dust and triggered the condensation of the solar system.

When the Sun turned on, its ultraviolet radiation poured into the atmosphere of the Earth; its warmth generated lighting; and these energy sources sparked the complex organic molecules that led to the origin of life.

Life on Earth runs almost exclusively on sunlight. Plants gather the photons and convert solar to chemical energy. Animals parasitize the plants. Farming is simply the methodical harvesting of sunlight, using plants as grudging intermediaries.  We are, almost all of us, solar-powered.

Finally, the hereditary changes called mutations provide the raw material for evolution. Mutations, from which nature selects its new inventory of life forms, are produced in part by cosmic rays – high-energy particles ejected almost at the speed of light in supernovae explosions.

Proving Einstein Right

Proving Einstein Right

Sir Arthur Stanley Eddington, the British astronomer whose observations during a solar eclipse confirmed part of Albert Einstein`s general theory of relativity was born on December 28, 1882. Sir Stanley Eddington who served as Director of Cambridge University Observatory for almost a third of a century, also significantly advanced our understanding of the internal workings of stars.

Eddington was an outstanding theoretical and observational astronomer. In 1909, while working at the Royal Observatory at Greenwich, he was dispatched to Malta to determine the longitude of an observing station there. In 1912, he led an eclipse expedition to Brazil.

Eddington distinguished himself in his studies of stars. He determined that the reason stars do not contract under the influence of gravity is because outward pressure from heat and radiation counterbalances gravity`s inward pull. He was also the first to show that the interior temperatures of stars must be several million degrees and that a star`s luminosity depends on its mass.

The Milky Way Galaxy Viewed from Afar

One of his greatest areas of interest, however, was relativity. In 1919, Eddington led an expedition that confirmed a basic point of general relativity. According to the theory, relativity should bend light rays; the stronger the gravitational field, the greater the angle of deflection. Eddington realized that it would be possible to test this point during a total solar eclipse, when stars that appear near the Sun in our sky become visible.

On May 29, 1919, Eddington observed a total solar eclipse from the island of Principe in West Africa. True to Einstein`s predictions, the Sun`s gravity bent the light coming from stars near the solar limb, deflecting their apparent positions slightly away from the solar disk.

Lunar Science: Light Amid the Heat

Lunar Science: Light Amid the Heat

Before man’s first Lunar landing, most scientists thought of the Moon as a Rosetta stone: an untouched repository of precious clues that would help reveal its origin and history, to say nothing of providing new insights about the evolution of the Earth and other planets. After first successful landings, most of these fondest hopes have been realized. The Apollo missions brought back 594 lbs or lunar rocks and soil, thousands of photographs and a flood of data that have changed some of man’s basic concepts about the Moon. But many of the mysteries remain. Indeed, the very act of exploration has created new lunar puzzles. “The Moon”, said once Geophysicist Gerald Wasserburg, whose laboratory at Caltech dated many of the lunar rocks, “is giving us answers that we don’t even questions for”.

Apollo samples showed, for example, that the Moon and Earth have significantly different chemical compositions. That finding challenged the old idea that the Moon was ripped from the Earth. Yet scientists are still at a loss to explain how – or when – it was formed. Paleomagnetic studies of lunar rock indicate that the Moon once had an unexpectedly strong magnetic field – and thus a large molten iron core. Yet equally valid data suggest that a core of significant size could not have existed. Even the ages of the rock present new problems.  The oldest specimens show that the Moon’s surface underwent a violent event about 3, 9 billion years ago that remelted them, but scientists are still debating what might have caused that cataclysm.

For all the heat, Apollo’s missions shed considerable light on the Moon. It revealed that the Moon – and presumably the Earth – was under incredibly intense bombardment by great chunks of space debris in the first 600 million to 800 million years after its formation 4, 6 billion years ago. But 3, 1 billion years ago this bombardment stopped. The evidence returned by Apollo shows that the Moon’s surface has remained virtually unchanged through those eons of time. Perhaps most important of all, exploration of the Moon has shown that it is not a simple, uncomplicated sphere but a true planetary body with a complex history and evolution of its own. Like the Earth, the Moon was once at least partially molten, and thus became differentiated (many heavier elements sank toward its center, while lighter elements floated to the surface to form a crust). In the words of Apollo’s chief scientist in the 1970s, Noel Hinners “It is a piece of the solar pot from which all the inner planets are made. We had no idea of that before we went there.” Indeed, it is the rich load of Moon data already brought back by Apollo that makes the premature conclusion of the program such a bitter disappointment to many lunar scientists.

Nonetheless, all the missions were scientifically very productive and scientist-astronaut Harrison Schmitt was the first professional geologist on the Moon. The Taurus-Littrow landing site contained what may be small volcanically created cinder cones; they seem to be miniature versions of earthly features like Honolulu’s Diamond Head. The cones may well be remnants of what NASA Geochemist Robin Brett called “some of the last belches of lunar activity before the Moon turned off”. Apollo 17 planners scheduled a program of experiments and observation far more sophisticated than any of the earlier scientific efforts on the Moon. For Apollo 17 four wholly new instruments were included in the ALSEP package: a mass spectrometer to measure the Moon’s tenuous atmosphere; a detector that let earthbound scientists monitor the bombardment of cosmic dust particles and micrometeorites on the Moon’s surface; an array of four listening devices – geophones – that can pick up shock waves from explosive charges that were detonated after the astronauts had left and told much about the substructure of the landing site; an extremely sensitive gravimeter that was designed to pick up minuscule variations in lunar surface gravity.

The Moon

Gravity Waves

Recording those tiny variations on the Moon went a long way toward settling an argument raging among physicists. In the 1960s, University of Maryland Physicist Joseph Weber astonished his colleagues with the announcement that he had detected gravity waves. Predicted by Einstein’s 1916 general theory of relativity, such waves are the vehicles presumed to transmit gravitational energy across space. Critics contended that Weber’s detectors probably sensed some of the Earth’s own rumblings. But if sudden variations in gravity are simultaneously picked up by a detector on the Moon and a comparable device on Earth, the sceptics may well be silenced.

During their travels across Taurus-Littrow, astronauts Cernan and Schmitt also performed several new “traverse” experiments. They took on the spot measurements to determine local fluctuations in the Moon’s gravitational field in hope of learning something about the density and structure of the material under the site. With data from a device called a “neutron probe”, scientists were able to calculate how long a particular sample has been lying on or near the lunar surface. The astronauts also sent penetrating microwaves into the lunar surface with a radio transmitting-receiving system. The pattern of the reflected signals could indicate, among other things, whether water is present up to a mile under the surface.

While his buddies worked on the Moon, Ron Evens marked his scientific contributions from high above in America. Besides intensive picture taking with both hands-held and automatic cameras, he examined the moon with a battery of experiments, including an ultraviolet spectrometer that made measurements of the thin lunar atmosphere that was used for comparison with those from the ground-based ASLEP spectrometer, and an infra-red scanner that took continuous temperature readings of the moon’s surface (its margin of accuracy within 2 degrees F). Evans did also his own detailed description of what he saw below him. Besides, it was just such “eyeball” observations by Apollo 15’s Al Worden that discovered the tantalizing cinder-cone-like features in the Taurus-Littrow region and played a key role in its selection as the landing site for the final lunar mission in the XXth century.