Category Physics

Why can t we bounce radar signal off the sun and determine 1 au directly


On April 7, 1959, a three-member team led by Stanford electrical engineer Von R. Eshleman recorded the first distinguishable echo of a radar signal bounced off the sun. A.S.Ganesh tells you more about Eshleman and how his team achieved this success…

When we generally say “reach for the stars,” we use it as a phrase to convey having high or ambitious aims. Some people, however, reach for the stars in the real sense. Stanford electrical engineer Von R. Eshleman was one of them and the star he reached out for was our sun.

Born into a farming community in Ohio, U.S. on September 17, 1924, Eshleman attended the General Motors Institute of Technology in Flint, Michigan, while still being a high school student. Similarly, even before earning his bachelor’s degree in electrical engineering from George Washington University in 1949, he started attending Ohio State University.

Intrigued by wave science

Before this, he had a stint with the navy during World War II, working as an electrical technician from 1943-46. It was during this period that he was drawn towards wave science. Intrigued by both sonar and radar, Eshleman had the idea that he could bounce radio signals off the surfaces of the sun and the moon, in order to study their hidden structures. While his own ship-based experiments of the time weren’t successful, they paved the way for his future research.

Having received his master’s degree from Stanford in 1950, he went on to earn his Ph.D. in 1952. He was recruited by Stanford to be a research professor, a position he held until 1957, when he was promoted to the teaching faculty as an Assistant Professor (Associate Professor back then). By 1962, he had not only managed to bounce radar off the sun, but also became a full professor at Stanford.

The same war that had planted the idea in Eshleman’s mind for bouncing radar off surfaces also saw the rapid development of radar. Bouncing radar off distant surfaces wasn’t an idea exclusive to Eshleman. Radar was successfully bounced off the moon in the 1940s itself and the first attempts to bounce radar off Venus were made in the late 1950s, albeit with mixed results.

16-minute round-trip

Eshleman’s three-member team, including Lt. Col. Robert C. Barthle and Dr. Philip Gallagher, achieved success in bouncing radar off the sun on April 7, 1959. The tests, in fact, were run on April 7, 10 and 12, with an average time of 16 minutes and 32 seconds spent for the signals to travel the 149 million km distance between the Earth and the sun and back again.

The researchers needed many months to confirm that they had indeed succeeded and when they finally made their announcement public with a press conference in February 1960, it was with 99.999% certainty.

Coded pulse

Eshleman had explained to the gathered media persons that the radar antenna consisted of 5 miles of wire that was spread out across over 10 acres of land, and a 40,000 watt transformer.

Every time the test was conducted, a coded pulse was beamed at the sun in 30-second bursts. This was done to enable identification once it returned after bouncing off the sun.

While 40,000 watts were sent out, atmospheric and spatial dissipation meant that only about 100 watts reached the sun. Similar losses during the return journey meant that only a miniscule amount of energy returned, making detection difficult. The task was further complicated by the fact that this small amount of energy was now part of the vast amounts of similar energy that the sun itself radiates. The other wavelengths. By spending over six months with some of the best computers of the time, they were able to conclude that the coded pulse that they sent out was among the radio emissions of the sun.

In 1962, Eshleman, along with Stanford colleagues, founded the Stanford Center for Radar Astronomy to oversee radio experiments. Even though he began his career in radar astronomy, Eshleman is now best remembered for his pioneering work using spacecraft radio signals for precise measurements in planetary exploration. While he briefly served as Deputy Director of the Office of Technology Policy and Space Affairs in the U.S. Department of State, he was most comfortable among academic circles and hence returned to Stanford, where he flourished. Eshleman died in Palo Alto on September 22, 2017, five days after turning 93.

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What makes it snow?

Take a close look at some snowflakes. You can see that they aren’t drops, like rain; or lumps, like hail; or tiny beads, like sleet. They look more like little feathers.

Snow forms when water vapour in clouds freezes. It forms at the top of storm clouds where the air is colder. The frozen water drops grow as more water vapour freezes onto them. They turn into tiny, clear pieces of ice called snow crystals. A snowflake is actually a bunch of snow crystals.

When you look at a snowflake through a magnifying glass, you see a beautiful, lacy shape. Even when they may seem the same, no two snow crystals are exactly alike. Some are flat. Others are shaped like long needles. Most look like pieces of lace. Yet, in one way they are almost all the same. Almost all snow crystals have six sides.

Snow can form high in the sky even in summer. But when snow falls in summer, it melts and becomes rain as soon as it reaches warm air lower down.

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What causes lightning and thunder?

Lightning and Thunder

A flash of light zigs and zags across the sky. Another flash zaps its way to the ground. A loud crack, boom, or rumble sounds soon after. The flash is lightning. The sound is thunder.

The flash we see when snakes through the sky lightning is really a huge electric spark. During a thunderstorm, each tiny drop of water in a cloud becomes electrically charged making the whole cloud charged. When this electrical charge becomes strong enough, it forms a huge electrical spark – lightning.

Lightning can travel in many ways. Sometimes a charge flashes from one place to another within a cloud. Other times, electricity rushes between two clouds with electrical charges. Also, lightning can strike the ground.

A flash of lightning heats up the air around it. The heated air spreads or rushes out in all directions. As it swells, it slams into cooler air, making it shake. This is what causes the sound of thunder.

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What makes a Rainbow?

Long ago, people thought rainbows were magic. Some people believed a rainbow was a bridge that appeared in the sky when the gods wanted to leave heaven and visit the earth. Other people believed that if you could find the end of the rainbow – where it touched the earth – you would find a pot of gold.

Today we know that a rainbow is made by sunlight shining through drops of water. Sunlight looks white, but it is really made up of many colours. When sunlight enters a raindrop, it breaks up into lots and lots of colours, including violet, blue, green, yellow, orange, and red. We see these colours in the rainbow. But because the colours blend, we usually see only four or five of them.

For you to see a rainbow during a rain shower, the sun must be behind you, and the rain must be somewhere in front of you. Rays of sunlight break up into colours as they reflect off many drops of falling rain. Together they make a shimmering, curved, colourful rainbow. If the rain is heavy, one or both ends of the rainbow may appear to touch the earth, many kilometres apart.

Some rainbows form when it isn’t raining. Sometimes small rainbows appear in waterfalls, in sprays of water from the sea, or in fountains where the water shoots high.

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Is rain recycled water?

Rain is recycled

Rain falls from clouds, but the water in the clouds comes from water on the earth. How does that happen?

The earth is covered with water – in lakes, streams, ponds, rivers, puddles, and huge oceans. Even the earth’s plants and animals have water in them.

Every day, the sun dries up huge amounts of this water, turning it into vapour. The warmth makes the vapour rise. It rises so high that it cools and forms clouds made of water and ice drops. As the drops get larger, they become too heavy to stay in the air. They fall as rain or snow.

Some of the rain and snow soaks into the soil and is used by plants. Some collects in streams or rivers that flow into the ocean. The sun warms it up and turns it into water vapour again. This water cycle happens over and over. The earth is always recycling its water!

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What is Weather?


Somewhere on the earth right now, it is cloudy and rainy. Somewhere it is sunny. Somewhere it is dark, windy, and snowing.

What is the weather like today where you are? Is it raining? Does it look as if it’s about to snow? Is the sun shining?

Do you ever talk about the weather? Many people do. Almost everyone cares about the weather.

The weather affects us in many ways. Day-to-day changes in weather can influence how we feel and the way we look at the world. Severe weather, such as tornadoes, hurricanes, and blizzards, can disrupt many people’s lives because of the destruction they cause.

Weather doesn’t just stay in one place. It moves, and changes from hour to hour or day to day. Over many years, certain conditions become familiar weather in an area. The average weather in a specific region, as well as its variations and extremes over many years, is called climate.

There are six main components, or parts, of weather. They are temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness. Together, these components describe the weather at any given time. These changing components, along with the knowledge of atmospheric processes, help meteorologists – scientists who study weather – forecast what the weather will be in the near future.

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What is Fog?

Eeee-rump! Eeee-rump! The sound of a foghorn echoes in the night. A thick, grey fog creeps in from the ocean and settles over the waterfront. You can hardly see your hand in front of your face. It seems as if you are in the middle of a cloud.

As a matter of fact, that’s just what fog is. It’s a cloud of tiny water drops that touches the earth, instead of floating high in the sky.

Like every other kind of cloud, fog forms when warm, moist air meets cool air. Fog often forms when warm, moist air passes over the cold water or an ocean, lake, or river. The warm air quickly cools. Then the water vapour in it becomes millions of tiny drops – a cloud that rolls in from the water and spreads out over the land.

Fog forms over the land in much the same way when ground that has been warm all day begins to cool off. As the warm air above the ground cools, the water vapour in the air turns into tiny drops of water. A fog hugs the ground.

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What are Clouds?

Clouds sometimes look like gobs of whipped cream in the sky. Sometimes they look like soft feathers. But what are clouds?

Clouds are billions and billions of tiny drops of water or ice crystals clustered together. Some clouds are all water, some are all ice, and some are a mixture.

You might think the water drops and ice crystals would be so heavy they would fall to the ground. But they are so tiny that the air holds them up, and breezes keep them floating in the air.

The water that makes clouds comes from the earth. Every day, the heat of the sun dries up tonnes of water from everywhere on the earth. All this water is turned into water vapour, which is water in the form of gas. This gas floats up into the air. But as it rises higher and higher, it begins to cool. When it cools enough, it turns back into water or ice. Then, tiny drops form around tiny, tiny bits of dust in the air. These drops make up the clouds.

Why don’t all clouds look alike?

There are many different kinds of clouds, and each kind has a name. Most clouds are named after their shape.

The clouds that look like great sheets pulled across the sky are called stratus clouds. These are the kinds of clouds that are closest to the ground. They form when a layer of warm air rolls over a layer of cooler air. Together they form a thick sheet-like layer.

The clouds that look like fluffy balls of cotton or scoops of ice cream are called cumulus clouds. Cumulus clouds that rise high into the air and grow dark and heavy with rain are the kinds of clouds that cause thunderstorms.

The highest clouds of all look like thin, wispy streaks or curls. They are so high up in the air, where the air is cold, that they are made of ice droplets. These clouds are called cirrus clouds.

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What are Twisters?

A thick, dark cloud forms in the sky. Warm air from near the earth rises rapidly towards the cloud. At the bottom of the cloud, the air starts spinning. This wind spins and twists and drops down into the shape of a long tube, or funnel. This wind is called a tornado, or twister.

The funnel may touch the earth’s surface. When a tornado touches ground, it can be dangerous. It can pick up heavy machinery and toss it far. The winds of a tornado rip roofs from houses and uproot large trees.

Tornadoes can happen in many parts of the world. But most tornadoes happen over the central U.S.A. People have recorded whirlwinds about 480 kilometres per hour there.

A tornado is a whirling wind that reaches down from the clouds and touches the ground. But dust devils and sand pillars are whirling winds that go from the ground up into the sky. They are much smaller than a tornado.

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