There’s a good reason we say the Andromeda Galaxy is 2.5 million light years away
A light-year is a measurement of distance, often used in astronomy to express how far light travels across space in a standard Earth year of 365.25 days. Because light travels so fast – 300,000 kilometres (186,000 miles) per second – numbers get too messy when dealing with galaxies and stars beyond the solar system.
One light-year is equal to 9,500,000,000,000 kilometres (about 6 trillion miles). Or, to be more precise, one light-year is 9.460730472580800 trillion kilometres (5.878625 trillion miles).
When considering Proxima Centauri, the nearest star, it’s a lot easier to express the distance as 4.243 light years as opposed to 40,018,889,899,016.78 kilometers. Would you rather say the Andromeda Galaxy is 2.5 million light years away or 23,651,826,181,452,000,000 kilometres away?
Short answer: A light-year is a measurement of the distance light travels through space in a year. It is often used to express distances beyond the solar system.
The Starfish Prime test could be seen from Honolulu well over 1000km away.
During the Cold War, the United States and the Soviet Union made a habit of testing their nuclear weapons on land, sea, in the air and under the ground. While these tests were justified as having scientific purpose, the two nations benefitted from the propaganda value of testing bombs and missiles that could obliterate their enemies (ie. each other).
One of the more bizarre chapters of this rivalry was that of nuclear weapons tested in space. Once the United States and Soviet Union proved they could fire nuclear missiles at each other from thousands of kilometres away, they found themselves having to experiment with ways of shooting their enemies’ missiles down. Precision guidance systems didn’t exist, so the solution was to simply use massive explosions to knock the missiles out of the game.
Between 1958 and 1962, there were 17 nuclear tests ranging from an altitude of 22.7 kilometres (14.1 miles) to 540 kilometres (335.5 miles) above the Earth. The largest explosion was that of the Starfish Prime test. A W49 thermonuclear warhead detonated at an altitude of 400 kilometres (250 miles) with a force of 1.4 megatons of TNT. The result was the creation of an artificial radiation belt in space which destroyed three satellites, while the electromagnetic pulse of the explosion damaged electronic systems over a thousand kilometres away.
The Soviets experienced similar problems with their space testing program, with power and communcations systems being crippled.
Consequently, the tests ended. They worked too effectively and posed a threat to manned space exploration.
Short answer: Nuclear weapon tests in space were conducted in the 1950s and 1960s, but were discontinued because they caused too much damage.
Here’s a picture I took of the moon.
At an approximate distance of 400,000 kilometres (250,000 miles) from Earth, the Moon is the fifth largest moon in the Solar System. Its gravitational pull creates ocean tides, influences the length of the day, and has played a role in the development of the calendar. But where does the moon come from?
There are competing theories as to the origin of the Moon but the leading candidate is that of a cosmic collision. It is believed that the early Earth collided with a Mars-sized body called Theia about 4.527 billion years ago.
During the early stages of the solar system collisions between planets, asteroids and proto-planets are believed to have occurred on a regular basis. The sun was surrounded by an accretion disc of rock, dust and ice. The gravitational influence of large bodies within the disc scooped up the smaller bodies which, causing the large bodies to increase in size while clearing their orbits of anything significant in their paths.
The collision between Earth and Theia is resulted in an enormous explosion that transformed the surface of both into magma.
The impact was so immense that material from the two was ejected into space, forming an accretion disc around the Earth. Within a hundred years the Moon was formed from the debris.
Short answer: The Moon was formed when an object roughly the size of Mars smashed into Earth. A huge amount of rock was sent into space, and gravity caused it to group together into what we call the Moon.