It’s Perseid Time!

The Perseids are one of the most watched meteor showers, partly because they generally put on a fantastic show and partly because they hit their maximum intensity every year in mid-August, when staying up and partying all night in the middle of the desert is relatively comfortable (the Leonids can put on a much bigger display but they peak in mid-November, so -- come on!).

I was going to do a post about where to go to watch them, when the peak times are (tonight and tomorrow are best, but it should be great this weekend), what to drink while you are watching, etc., but I’ll leave that to the actual astronomers and others...

The Perseids are most commonly associated with comet Swift-Tuttle, a short-period comet that reaches perihelion every 133 years. As the comet approaches the sun, sublimating gases erupt from the surface in huge geysers and carrying small grains of solids with them. These silicate particles can range in size from silt and sand size up to (in the rarest cases) pebbles and small cobbles. This “atmosphere” of gas and grains, though quite tenuous, can puff up to the diameter of a planet (or even the sun) around the comet’s nucleus, which may be as small as a few kilometers across. This can make some comets, under ideal conditions, among the brightest objects visible in the sky.

The anatomy of a comet

A tiny object like a comet cannot hold on to an atmosphere at all, much less one this size. As the solar wind pushes the material radially away from the sun, it breaks up into two components: the gases stream radially away from the sun creating the ion tail, which under ideal circumstances can exceed 1 AU in length. “Tail” is somewhat of a misnomer, as the generally more prominent ion tail always points away from the sun -- when the comet is moving away from the sun, its tail can lead the way!

Comet Hale-Bopp (1997) The blue tail is the ion (gas) tail and points radially away from the sun. The white tail is the dust tail and it curves to indicate the general direction of motion (right to left, tilted slightly toward the horizon.

The silicate particles of the dust tail, also under pressure from the solar wind, are affected more strongly by the sun’s gravity. The largest of these particles will fall into a solar orbit that more or less mimics that of the comet. These particles, known as meteoroids, can range in size from sand grains to pebbles to medium-sized cobbles, and the stream of meteoroids is known as a “dust trail” (as opposed to a “dust tail”).

The lifespan of any short period comet is limited -- each time it approaches the inner solar system it boils away a bit more, leaving fewer volatiles and smearing more of these meteoroids along its orbital path. If we are lucky enough, the Earth will cross this orbital path once or twice per year.

Earth moves at close to 30 km/s around the sun, and the portion of the dust trail in our path is moving at close to this speed (15 km/s to 45 km/s). So, depending on whether a meteoroid approaches us in retrograde or in direct it may hit the upper atmosphere at anywhere from 15-75 km/s. Interestingly, since we are moving counter-clockwise around the sun (viewed from the north pole) and rotating in the same direction, meteoroids we encounter closer to sunrise typically move faster than those we encounter near sunset.

It is a common (and logical) assumption that friction with the atmosphere will heat up the meteoroid. However, the heat is largely generated by the rapid compression of air molecules in front of the speeding particle. This process is called “ram pressure,” and this heat is largely responsible for heating the meteoroid, causing it to glow, melt and evaporate.

Perseid meteors

The grain becomes a meteor -- a “shooting star” -- when it is hot enough to become incandescent, usually at between 60 and 100 kilometers. As it streaks across the sky it evaporates, and the length of the trail depends on the size of the grain. Most meteors are no larger than a sand grain: pebble- and cobble-sized meteors, called “fireballs,” put on a much more impressive show, tumbling and shedding visible mass as they streak across the sky and are brighter than any object in the sky other than the sun or moon.

A fireball meteor

The Perseids are so called because they seem to radiate from a point located in the constellation Perseus. This is, of course, because we encounter the dust trail as we are moving in our orbit in the general direction of this constellation. This is the same effect as driving down a dark desert highway at night; the bugs generally are scattered and moving in random directions, but from the point of view of someone in a fast moving car, those bugs you encounter seem to radiate from a single point in front of your car.

At the peak of the shower (in the last few hours approaching sunrise) you may see up to a meteor per minute (in much more rare meteor storms you can see several or even dozens of meteors per minute). In the first few hours after sunset you have the best chance of seeing a few glowing fireballs, since Perseus is low in the sky and visible meteors will be hitting the atmosphere at a very low angle.

Most people are aware that you don’t need a telescope to watch a meteor shower, but a decent telescope (or, in a pinch, a good pair of binoculars) will be useful since Jupiter is close to opposition now and will be visible all night. Also, the other four naked-eye planets (Mercury, Saturn, Venus and Mars) are clustered together near the crescent moon. Tonight (August 11) Mercury and the moon will set simultaneously -- tomorrow it will set between Saturn and Venus, and on Friday night it will set after Mars.

Little Boy

Note: I originally wrote this post in 2010, the 65th anniversary of the bombing of Hiroshima -- I have updated it here to fix some broken links.

"We knew the world would not be the same. A few people laughed, a few people cried. Most people were silent. I remembered the line from the Hindu scripture, the Bhagavad-Gita; Vishnu is trying to persuade the Prince that he should do his duty, and to impress him, takes on his multi-armed form and says, 'Now I am become Death, the destroyer of worlds.' I suppose we all thought that, one way or another."

--J. Robert Oppenheimer, on witnessing the Trinity test

"Little Boy" was detonated above the city of Hiroshima on August 6, 65 years ago; the fission reactions took less than a microsecond to convert about 64 kilograms of uranium (enriched to 80% Uranium-235) into 63.5 kilograms of uranium plus 15-20 kilotons of energy.

It was only the second nuclear device detonated, after the Trinity test on July 15, and was unique in several respects: it was the first uranium weapon ever exploded; and the only gun-type" mechanism ever used -- both the Trinity device and the "Fat Man" bomb dropped several days later on Nagasaki were implosion-type weapons that used plutonium-239 as their fission target.

The bomb had several arming/switching mechanisms -- the first was a 15 second timer triggered by a wire when the bomb was released from the B-29 at an altitude of 31,000 feet, preventing it from accidentally detonating at too high an altitude; the second was a barometric switch that was triggered at an altitude of about 7000 feet; this triggered a radar altimeter, which sent out a weak signal toward the ground and set off the firing mechanism at an altitude of about 1900 feet (determined to be the altitude that would cause the greatest damage at the surface).

The firing mechanism was a converted artillery gun that fired an 18 centimeter long hollow cylinder, 16 centimeters in diameter with a 10 centimeter diameter opening, into an 18 x 10 centimeter round spike. The hollow cylinder contained just over 1.5 critical masses of enriched uranium (the hollow opening kept it below critical density) and the target spike contained just less than one critical mass. When the gun fired, the hollow cylinder traveled a few feet in 1/250th of a second, swallowing the solid spike and creating a single supercritical mass.

This type of firing mechanism had not been tested in a nuclear device, but it was such a simple mechanism that failure was considered a very low probability. It would only work with uranium though -- The fissile plutonium that was being manufactured in Tennessee and in Washington state fissioned too quickly to be combined over 0.004 seconds, so plutonium bombs required a much more complex implosion mechanism. This is the design that was tested three weeks earlier in the New Mexico desert, and was detonated on August 8th over Nagasaki. It was nicknamed "fat man" because of its spherical shape.

Spontaneous fission in uranium is rare -- a mass of this size will produce only a few hundred neutrons per second spontaneously; most of these neutrons will go flying out of the mass with no effect, but a few will managed to bounce around the interior long enough to slow themselves down to thermal velocities ("slow" neutrons). ²³⁵U is somewhat unique among naturally occurring elements in that is can be induced to fission by the absorption of a slow neutron.

The extra neutrons are absorbed by a few ²³⁵U nuclei, causing them to wobble, destabilize, and split apart, creating two nuclear fragments roughly half the size of the original nucleus plus a couple of extra neutrons. These two neutrons are absorbed by two other uranium nuclei, causing them to split and generating four more neutrons, and so on... the reaction expands geometrically at incredible speeds, going through eighty generations of induced fissions in a microsecond. While this is occurring the bomb falls only an additional few inches.

Though the sum of the nuclear particles post reaction is the same, the sum of their masses is smaller by just over 1% -- about 600 grams of the material in the reaction is converted into energy by Einstein's equation, E=mc². Some of this energy goes to heating the bomb components, but most is released in the form of gamma rays and X-rays.

There is a blinding flash as the air immediately around the explosion is heated to millions of degrees. The expanding fireball reached a diameter of just over 1000 feet, cooling as it expanded, dropping to a temperature of only 4000°C at its outer edge. It is as if a new sun appeared in the sky, hundreds of times larger than our own sun. The superheated air causes materials at ground level to spontaneously burst into flame -- people, animals and plants are vaporized, leaving behind nothing more than carbon residue within a mile of ground zero.

The rapidly expanding air also generated a pressure/shock wave, a blast of wind moving outward at greater than the speed of sound. Within 2 miles around ground zero structural damage is near total. As this shock wave passed, pressure dropped momentarily to values close to the vacuum of space. The debris generated by the destruction also served to add fuel to the fires within this zone.

Though the secondary radioactive products of the fission reaction are dangerous, an air blast such as this one causes much of the physical debris of the bomb to dissipate through the atmosphere. There is however, a significant flux of gamma and neutron radiation at ground level. Immediate casualties due to radiation exposure were few; more victims died soon after of radiation sickness; the highest rates of casualties came later from increased rates of leukemia as well as other forms of cancer associated with radiation exposure.

Hiroshima (left) and Nagasaki (right)

By some accounts, Hiroshima was chosen as a target at least partly because it had been spared conventional bombing up to this point -- making it a "clean slate" for the study of the effects of the bombing, both during and after the event. Most of the data about the explosion itself was collected by a radio transmitter dropped by a plane flying in formation with the Enola Gay.

It was estimated at the time that 70,000 people died in the initial blast from the atomic bomb. By the end of 1945, deaths due to injuries, burns, radiation and other causes increased the death toll to 120-150 thousand. Some estimates put the number at 200 thousand by 1950. At least one study claims that 1 in 10 cancer deaths in victims since 1950 can be attributed to the bombing.

Though it has been 65 years since devices like these have been used as weapons, the ability to refine fuel (either enriched uranium or plutonium-239) has come within the technological capabilities of just about any country on Earth, and the design and manufacture of a working device is well within the abilities of a group of engineering grad students.

This video "1945-1998" represents the work of Japanese artist Isao Hashimoto and is a document of all 2053 known nuclear detonations on Earth during those years, shown by location at a rate of 1 month = 1 second.