Category Science

HOW FAR AWAY IS THE MOON?

          The Moon is Earth’s closest neighbour in space. Its orbit around Earth is elliptical, rather than circular, which means that its distance from us varies. At its closest point to Earth (its perigee), the Moon is 384,400km (240,000 miles) away. Incredibly, the Moon’s orbit is slowly carrying it away from Earth at a rate of around 5cm (2in) a year.

          The distance between London, where I live, and Oxford, where I used to live, is about 100 km (or 60 miles). It takes about 90 minutes by car and about 120 minutes by bus. I can easily make sense of that distance.

          Harder to consider: the distance between the Earth and the moon, which is 384,400 km (240,000  miles). It’s a fact we’ve likely all learned in high school. Unlike the distance between London and Oxford, however, it’s not easy to comprehend what 384,400 km means in real terms.

          Luckily, you don’t have to think too hard. A NASA spacecraft has solved that problem for us. In October, OSIRIS-REX, a spacecraft that’s bound to intersect an asteroid in August this year, took the photo above from about 5 million km (3 million miles) away from the Earth. NASA posted the picture on Jan. 2, providing the public with a unique view of our planet and its moon. The angle is great to get a grasp of what the distance between the two celestial bodies really looks like, but it’s not perfect.

          Here’s a back-of-the-envelope calculation to explain why. For ease, we’re going to use round figures. The Earth’s diameter is about 13,000 km (8,000 miles). That means, in the 390,000 km distance between the Earth and the moon.

Picture Credit : Google

WHAT IS THE MOON MADE OF?

          Although the moon’s interior structure is difficult to study, scientists believe that it has a small iron core. Surrounding this is a partially molten zone called the lower mantle. Above this lies the mantle, which is made up of dense rock, and the crust, which is also made of rock. Together, the mantle and the crust form the lithosphere, which can be up to 800km (500 miles) thick. There are only two basic regions on the Moon’s surface — dark plains called mania and lighter highlands. These heavily cratered highlands are the oldest parts of the Moon’s crust, dating back over four billion years. The darker plains are craters that were filled with lava.

          The composition of the Moon is a bit of a mystery. Although we know a lot about what the surface of the Moon is made of, scientists can only guess at what the internal composition of the Moon is. Here’s what we think the Moon is made of.

          Like the Earth, the Moon has layers. The innermost layer is the lunar core. It only accounts for about 20% of the diameter of the Moon. Scientists think that the lunar core is made of metallic iron, with small amounts of sulfur and nickel. Astronomers know that the core of the Moon is probably at least partly molten.

          Outside the core is the largest region of the Moon, called the mantle. The lunar mantle extends up to a distance of only 50 km below the surface of the Moon. Scientists believe that the mantle of the Moon is largely composed of the minerals olivine, orthopyroxene and clinopyroxene. It’s also believed to be more iron-rich than the Earth’s mantle.

          The outermost layer of the Moon is called the crust, which extends down to a depth of 50 km. This is the layer of the Moon that scientists have gathered the most information about. The crust of the Moon is composed mostly of oxygen, silicon, magnesium, iron, calcium, and aluminum. There are also trace elements like titanium, uranium, thorium, potassium and hydrogen.

Picture Credit : Google

WHY ARE THERE SO MANY VOLCANOES ON VENUS?

          Venus is covered by hundreds of thousands of volcanoes. This is because the surface of the planet is a thin skin floating on hot molten rock. This lava is vented wherever possible, meaning that, unlike Earth, Venus has volcanoes everywhere. Most of these volcanoes are around 3km (2 miles) wide and 90m (395ft) high, but there are over 160 much larger than this. Some volcanoes on Venus are over 100km (60 miles) in diameter! The volcanic activity on Venus means that the surface of the planet is always changing.

          The surface of Venus is dominated by volcanic features and has more volcanoes than any other planet in the Solar System. It has a surface that is 90% basalt, and about 65% of the planet consists of a mosaic of volcanic lava plains, indicating that volcanism played a major role in shaping its surface. There are more than 1,000 volcanic structures and possible periodic resurfacing of Venus by floods of lava. The planet may have had a major global resurfacing event about 500 million years ago, from what scientists can tell from the density of impact craters on the surface. Venus has an atmosphere rich in carbon dioxide, with a density that is 90 times greater than Earth’s atmosphere.

          Even though there are over 1,600 major volcanoes on Venus, none are known to be erupting at present and most are probably long extinct. However, radar sounding by the Magellan probe revealed evidence for comparatively recent volcanic activity at Venus’s highest volcano Maat Mons, in the form of ash flows near the summit and on the northern flank. Although many lines of evidence suggest that Venus is likely to be volcanically active, present-day eruptions at Maat Mons have not been confirmed.

Picture Credit : Google

WHY IS VENUS LIKE A GREENHOUSE?

          Less than 20% of sunlight falling on Venus breaks through the clouds. Despite this, Venus has the hottest surface temperature of any planet in the Solar System. This is because infrared radiation (heat) released from the planet cannot escape back into space. The atmosphere traps heat inside, like the glass in a green-house, meaning that the temperature is over 400°C (750°F), greater than it would he if Venus had no atmosphere.

          Greenhouse involving carbon dioxide and water vapor may have occurred on Venus. In this scenario, early Venus may have had a global ocean if the outgoing thermal radiation was below the Simpson-Nakajima limit but above the moist greenhouse limit. As the brightness of the early Sun increased, the amount of water vapor in the atmosphere increased, increasing the temperature and consequently increasing the evaporation of the ocean, leading eventually to the situation in which the oceans boiled, and all of the water vapor entered the atmosphere. This scenario helps to explain why there is little water vapor in the atmosphere of Venus today. If Venus initially formed with water, the greenhouse would have hydrated Venus’ stratosphere, and the water would have escaped to space. Some evidence for this scenario comes from the extremely high deuterium to hydrogen ratio in Venus’ atmosphere, roughly 150 times that of Earth, since light hydrogen would escape from the atmosphere more readily than its heavier isotope, deuterium. Venus is sufficiently strongly heated by the Sun that water vapor can rise much higher in the atmosphere and be split into hydrogen and oxygen by ultraviolet light. The hydrogen can then escape from the atmosphere while the oxygen recombines or bonds to iron on the planet’s surface. The deficit of water on Venus due to the runaway greenhouse effect is thought to explain why Venus does not exhibit surface features consistent with plate tectonics, meaning it would be a stagnant lid planet. Carbon dioxide, the dominant greenhouse gas in the current Venusian atmosphere, owes its larger concentration to the weakness of carbon recycling as compared to Earth, where the carbon dioxide emitted from volcanoes is efficiently sub ducted into the Earth by plate tectonics on geologic time scales through the carbonate-silicate cycle, which requires precipitation to function.

Picture Credit : Google

HOW DOES RADAR TECHNOLOGY WORK?

          Radar works in the same way as an echo. When you shout loudly at a distant wall, you will hear the echo of your voice a few seconds later. This is because the sound waves hit the solid wall and bounce back towards you. Radar uses high-frequency waves that travel much faster and much further. The radar sends out a short burst of radio waves and then listens for an echo, which tells it how far away the target is, and what it is made of.

          Airplane pilots get around this difficulty using radar, a way of “seeing” that uses high-frequency radio waves. Radar was originally developed to detect enemy aircraft during World War II, but it is now widely used in everything from police speed-detector guns to weather forecasting. Let’s take a closer look at how it works!

          We can see objects in the world around us because light (usually from the Sun) reflects off them into our eyes. If you want to walk at night, you can shine a torch in front to see where you’re going. The light beam travels out from the torch, reflects off objects in front of you, and bounces back into your eyes. Your brain instantly computes what this means: it tells you how far away objects are and makes your body move so you don’t trip over things.

          Radar works in much the same way. The word “radar” stands for radio detection and ranging—and that gives a pretty big clue as to what it does and how it works. Imagine an airplane flying at night through thick fog. The pilots can’t see where they’re going, so they use the radar to help them.

          An airplane’s radar is a bit like a torch that uses radio waves instead of light. The plane transmits an intermittent radar beam (so it sends a signal only part of the time) and, for the rest of the time, “listens” out for any reflections of that beam from nearby objects. If reflections are detected, the plane knows something is nearby—and it can use the time taken for the reflections to arrive to figure out how far away it is. In other words, radar is a bit like the echolocation system that “blind” bats use to see and fly in the dark.

Picture Credit : Google

WHAT WAS THE MAGELLAN MISSION?

          The most detailed information about Venus was acquired by a space probe called Magellan. Launched in 1989, Magellan travelled to Earth’s neighbour and spent three years building a complete map of the planet. Flying as low as 294km (183 miles) above the surface, Magellan bounced radar pulses off the solid ground beneath and sent the data back to Earth to he analyzed. It measured strips of land 24km (14 miles) wide and 10,000km (6000 miles) long each time it circled the planet, while its altimeter measured its height above the surface.

          The Magellan spacecraft was the first planetary explorer to be launched by a space shuttle when it was carried aloft by the shuttle Atlantis from Kennedy Space Center in Florida on May 4, 1989. Atlantis took Magellan into low Earth orbit, where it was released from the shuttle’s cargo bay and fired by a solid-fuel motor called the Inertial Upper Stage (IUS) on its way to Venus. Magellan looped around the Sun one-and-a-half times before arriving at Venus on August 10, 1990. A solid-fuel motor on the spacecraft then fired, placing Magellan into a near-polar elliptical orbit around Venus.

          Spacecraft carried a sophisticated imaging radar, which was used to make the most highly detailed map of Venus ever captured during its four years in orbit around Venus from 1990 to 1994. After concluding its radar mapping, Magellan also made global maps of Venus’s gravity field. Flight controllers then tested a new maneuvering technique called aero braking, which uses a planet’s atmosphere to slow or steer a spacecraft. The spacecraft made a dramatic plunge into the thick, hot Venusian atmosphere on October 12, 1994, and was crushed by the pressure of Venus’s atmosphere. Magellan’s signal was lost at 10:02 Universal Time (3:02 a.m. Pacific Daylight Time) that day.

          The Magellan mission was divided up into “cycles” with each cycle lasting 243 days (the time necessary for Venus to rotate once under the Magellan orbit). The mission proceeded as follows:

Magellan Assembly

          On May 4, 1989, the Magellan spacecraft was deployed from the shuttle. The spacecraft is topped by a 3.7-meter (12-foot) diameter dish-shaped antenna that was a spare part left over from the Voyager program. The long, white, horn-shaped antenna, attached just to the left of the dish antenna, is the altimeter antenna that gathers data concerning the surface height of features on Venus. Most of the spacecraft is wrapped in reflective white thermal blankets that protect its sensitive instruments from solar radiation. 

Deployment

          The Magellan spacecraft’s deployment from the shuttle Atlantis’ cargo bay was captured by an astronaut with a hand-held camera pointed through the shuttle’s aft flight deck windows. Deployment occurred in the early evening of May 4, 1989, after Atlantis had carried Magellan and its Inertial Upper Stage (IUS) booster rocket, into low Earth orbit. Once the shuttle was safely away from the spacecraft, the IUS ignited and placed Magellan on course for its 15-month journey to Venus.

Magellan Orbiting Venus

          On August 10, 1990, Magellan entered into orbit about Venus, as depicted in this artist’s view. During its 243-day primary mission, referred to as Cycle 1, the spacecraft mapped well over 80 percent of the planet with its high-resolution Synthetic Aperture Radar (SAR). The spacecraft returned more digital data in the first cycle than all previous U.S. planetary missions combined.

Picture Credit : Google