In October of 1963 the US Air Force launched the first in a series of satellites inspired by a recently signed nuclear test ban treaty. Signatories of this treaty agreed not to test nuclear devices in the atmosphere or in space. These “Vela” (from the Spanish verb velar, to watch) series satellites were part of an unclassified research and development program whose goal was to develop the technology to monitor nuclear tests from space and give the US a means of verifying the conditions of the treaty. The satellites were launched and operated in pairs with two identical satellites on opposite sides of a circular orbit 250,000 kilometers in diameter (about a 4 day orbit) so that no part of the earth was shielded from direct observation. The Vela satellites carried x-ray, gamma-ray, and neutron detectors as a basic instrumentation complement. They also carried a variety of optical and EMP detectors as well as instruments designed to monitor the space environment. The instruments were designed and built by teams of workers at the Los Alamos Scientific Laboratory (now LANL) and Sandia Laboratories of Albuquerque NM, who were specifically assembled and commissioned for the Vela mission.

The x-ray detectors were intended to directly sense the flash of x-rays from a nuclear blast. Although most of the energy of a bomb blast in space would be directly visible as an x-ray flash, a simultaneous indication by the gamma-ray detectors would provide a confirming signature of a nuclear event. A further confirmation would come from the detection of neutrons. The Vela designers were also aware that detonating a nuclear bomb behind a thick shield or on the far side of the moon would effectively hide the initial flash of x-rays from the satellites’ view. Hence the gamma-ray detectors could also look for hard gamma-radiation resulting from the cloud of radioactive material blown out after the nuclear blast. This blast cloud could not be totally shielded from view and would expand rapidly. It would easily be detected in gamma-rays even if the detonation took place behind the moon, out of direct view of the satellites’ x-ray detectors.

The Vela satellites generally performed well and greatly exceeded their expected operational lifetimes. The satellites’ capabilities were steadily improved with each launch. In particular, Vela 5 a and b (launched in 1969) and Vela 6 a and b had sufficient timing accuracy that they could reasonably determine directions to the triggered events. For these later satellites, the light travel time from one spacecraft to another, across the orbital diameter (around 1 second), was greater than the resolution time of the event’s onset (about 0.2 seconds). The direction angle to the event with respect to the line between a pair of satellites could thus be determined (to about 1/5th of a radian or 10 degrees) based on the difference in trigger times for the two satellites. Direction angles for a single event observed by multiple pairs of satellites could then be combined to determine one or two possible directions for the source of the event.

In 1965, with the construction and launch of the Vela 3 satellites, Ray Klebesadel of Los Alamos Scientific Laboratory assumed the continuing programatic responsibility for the x-ray and gamma-ray instruments. He saw to it that events which triggered the detectors but were clearly not signatures of nuclear detonations were carefully filed away for future study. In 1972, Ian Strong, also at Los Alamos, was asked to look at Ray Klebesadel’s files of Vela gamma-ray events. With the timing accuracy of the later Vela satellites Klebesadel and Strong, along with another Los Alamos colleague, Roy Olsen, were able to deduce the directions to the events with sufficient accuracy to rule out the sun and earth as sources. They concluded that the gamma-ray events were “of cosmic origin”. In 1973, this discovery was announced in Ap.J. letters by Klebesadel, Strong, and Olsen. Their paper discusses 16 cosmic gamma-ray bursts observed by Vela 5a,b and Vela 6a,b between July 1969, and July 1972.

Using a hard x-ray detector on board IMP-6 intended to study solar flares, Tom Cline and Upendra Desai of NASA/GSFC were the first to confirm this finding and provide some spectral information that showed that the burst spectra peaked at gamma-ray energies. Thus the events were not simply the high energy tail of an x-ray phenomenon. A collimated gamma-ray telescope on board OSO-7 (Wheaton et al. 1973) was also able to confirm a direction to one of the events, supporting the original conclusions of cosmic origin. These confirming results, published close on the heels of the original discovery, gave the whole scenario an aura of enhanced mystery. The excitement created in the astronomical community was evidenced by a burst of publications of instrumental and theoretical papers on the newly discovered “cosmic gamma-ray bursts”.

Read Full Story

Using radio for astronomical research was still a relatively new idea when Bernard Burke and Kenneth Franklin of the Carnegie Institution in Washington D.C. discovered that the planet Jupiter was a strong source of radio waves.

By this time astronomers knew of several sources of radio waves in the sky. One of them was the Crab Nebula in the constellation of Taurus. Burke and Franklin were going to use the Crab to test how their antenna array was working. The tests seemed to go well and every few weeks they would change the pointing direction progressively towards the south.

During a few of their observations something appeared in their records that they could not identify. They thought at first it was some form of terrestrial interference. At these frequencies you can often get many different types of interference from very down-to-earth things such as car ignitions, power lines etc. The first thing they noticed about this emission was that it appeared to occur at nearly the same time of night each time they heard it. After studying this interference over several more nights they realized that it didn’t quite occur at exactly the same time. It appeared to be occurring about 4 minutes earlier each night. This type of change with time is what they would expect from some celestial object since stars appear to rise 4 minutes earlier each night.

So they knew it was very unlikely to be earth-bound interference. Once they had several months of data they could track more precisely how the timing of this interference changed. They found that it didn’t quite move like the stars moved. This would eliminate any star, nebula or galaxy since they all appear to move across the sky at the same rate. Finally they realized that an object that happened to be near the Crab Nebula at the time they began hearing this interference was Jupiter. Jupiter, like the Earth, orbits the Sun and its orbital motion causes it to appear to move against the background stars. The rate at which Jupiter moved matched the change with time of the strange interference found in the records. On April 6, 1955 at a meeting of the American Astronomical Society, Burke and Franklin announced their discovery of radio emissions from Jupiter.

  As news of this discovery spread other radio astronomers began pouring through their records to see if they had Jupiter in their data. One astronomer from Australia, C.A. Shain, found observations he had taken 5 years earlier that contained Jovian radio bursts that hadn’t been recognized before. Very soon after radio emissions from Jupiter were discovered scientists had a baseline of 5 years of data to work with! The data from long periods of monitoring Jupiter’s radio behavior will prove vital for later discoveries.

Source 

The notion of atoms started from a fundamental problem in chemistry (#1): why did (say) one gram of hydrogen always combine with 8 grams of oxygen, never more, never less? Because each molecule of the resulting compound–water–always contained a fixed number of atoms of each kind. By comparing different reactions, Dalton concluded that (for instance) 2 atoms of hydrogen combined with one of oxygen, to create H2O.

Avogadro (#2, #5), in Italy, meanwhile noted a simple relation between the volume and weight of gases. Quantities which from chemistry seemed to combine naturally–1 gr. of hydrogen, 16 gr. of oxygen, 35.5 gr. of chlorine etc.–had the same volume, and Avogadro proposed that they contained the same number of molecules or atoms.

(He also proposed that atoms in gases often combined in pairs, to create molecules such as H2, O2 and Cl2, explaining some strange factors of two. We thus should talk about the number of molecules in 2 gr. of hydrogen, 32 gram of oxygen, 71 gram of chlorine, etc. That number is now known as Avogadro’s number, and is truly enormous.)

Avogadro’s work was neglected for many years, while chemists struggled to understand the way atoms combined. At a conference of chemists in 1860, Cannizzaro (#5) again drew attention to Avogadro’s results, and after that progress was rapid.

Meanwhile, clues accumulated suggesting that atoms carried electric charges. Humphrey Davy (#3) used an electric current to separate new elements out of molten salts (a process called electrolysis). He obtained sodium and then potassium, soft metals which burned violently.

Faraday, who started as Davy’s assistant, derived in 1833 the laws of electrolysis (#4), which suggested that in a water solution (and also a molten salt), each atom or molecular fragment carried a fixed electric charge.

Other researchers studied the flow of electricity in rarefied gases, under the influence of high voltage (fluorescent lamps are one outgrowth of such experiments). It became evident that such currents were carried by positive and negative particles in the gas. Joseph (”J.J.”) Thompson isolated one type, a very light negative particle, measured its properties and named it the electron. (#7).

The conducting gases also contained positive “ions” (wanderers) which J.J. Thompson studied as well. Such ions (”alpha particles,” actually nuclei of helium) were also emitted by heavy radioactive elements, discovered in 1895 (#6).

Starting in 1910, Robert Millikan at the Calif. Inst. of Technology accurately measured the charge of the electron (#9), by spraying tiny droplets of oil from an atomizer into a region of electric forces, between two parallel horizontal plates. Some of the droplets became electrically charged by friction or radioactivity, usually by no more than 1-2 electrons, and fell more slowly in the field of an observing (horizontal) microscope. By adjusting the voltage, Millikan could stop them, so that electric force exactly balanced weight. That force could be derived from the voltage, while the weight was obtained by turning off the voltage and timing the free fall, governed mainly by air resistance.

The one unknown left was the electric charge, which could now be computed. Comparing this to Faraday’s results, one could get “Avogadro’s number”, the number of (say) hydrogen atoms in one gram of hydrogen–an enormously large number. The size of atoms was finally clearly determined! For Millikan’s 1913 report of his discovery, see here.

Ernest Rutherford, born on New Zealand, showed in 1911 that alpha particles were sometimes very strongly scattered by the positive charges of the atom, in a way that could only be explained if such charges were concentrated in a very small volume, practically a point in space. He therefore suggested that every atom had a compact nucleus, with negative electrons floating around it (#8).

Rutherford’s picture suggested that the nucleus was like a miniature Sun, with electrons orbiting it like planets. If Newton’s laws were valid on the atomic scale, that might indeed be so, but as later research showed, on the atomic distance scale Newton’s laws change into other forms. By these new laws of “quantum mechanics,” electrons do not move in precisely defined orbits, but are distributed in space in a way that only allows the probability of finding them anywhere to be calculated. Similarly, energized atoms are only allowed to exist in one of a number of energy levels.

There remained a problem: nuclei were too heavy. Helium nuclei had twice the charge of the proton but 4 times the mass. For a while scientists wondered whether helium nuclei contained 4 protons and 2 electrons. Then in 1932 Chadwick discovered the neutron and it was realized that helium nuclei contained two protons, two neutrons and no electrons. One form of nuclear force (”the weak nuclear force”) controlled the ratio of neutrons to protons.

In 1938 Hans Bethe proposed that the Sun obtained its energy by fusing hydrogen nuclei to form helium. He also showed how fusion could proceed by means of a cyclical process, involving nuclei of carbon and nitrogen; the carbon and nitrogen nuclei are recovered and the only change is that 4 hydrogen atoms combine into helium. Today Bethe’s cycle is believed to operate mainly in stars somewhat hotter than the Sun.

The following is the summary of the events that leads to the Discovery of Atoms & Nuclei:

1.        John Dalton               1803-8  Atomic theory from chemistry
2.       Humphrey Davy       1807       Electric separation of Sodium
3.       Amadeo Avogadro       1811       Atoms and the laws of gases
4.       Michael Faraday       1833       Laws of electrolysis
5.       Stanislao Cannizzaro       1860       Rediscovery of Avogadro’s law
6.       Henri Becquerel       1895       Radioactivity
7.       J.J. Thompson       1897       Discovery of electrons
8.       Ernest Rutherford       1911       Discovery of compact nuclei
9.       Robert Millikan       1913       Measurement of electron charge
10.       James Chadwick       1932       Discovery of the neutron
11.        Hans Bethe               1938       Sun powered by Nuclear Fusion

Source