Home > Physics, Uncategorized > Neptune was discovered only this year

Neptune was discovered only this year

Do you know that Neptune was first discovered by us, less than a year ago. It is shocking, is it not, considering what we have been taught in our school, but it is true. Let me explain.

Neptune was discovered in 1846. As of today, it is the outermost planet of the Solar System after Pluto was declared not to be a planet. It is, on average, 30 times as far away from the Sun, as the Earth is from the Sun. Since the Earth is, about 150 million kilometres from the Sun, that means Neptune is roughly 4.5 billion kilometres away.

A picture of Neptune taken from Voyager 2 mission

A picture of Neptune taken from Voyager 2 mission. The original can be downloaded from http://photojournal.jpl.nasa.gov/catalog/PIA02210

The interesting thing is that since it is so far away, it takes about 165 Earth years for it to complete one revolution around the Sun. Now you might wonder that, if the planet is 30 times as far away from the Sun as the Earth is, then it should take about 30 Earth years to complete its revolution. Then, why is Neptune taking 165 years to go around the Sun?

The answer lies in that, our expectation is valid, only if both Earth and Neptune are travelling at the same speed. But, that is not the case. Earth is going around the Sun at a speed of 30 kms/second (pretty good speed, isn’t it?) while Neptune is orbiting the Sun at 5.5 Kms/second. Since it is about 5.5  times slower (30 / 5.5  kmps = 5.5) than the Earth, its time to make one orbit would be 5.5 times longer than our usual expectation of 30 Earth years which means it would take about 165 (30 x 5.5 = 165) years, which is what is its correct orbital duration.

But its distance has not deterred us from reaching out to it. NASA’s Voyager 2 mission, has gone closest to Neptune on August 25, 1989 and it took about 12 years to reach Neptune from Earth. But that spacecraft will eventually go out of the solar system and into interstellar space. The FAQ section, tells me that this could happen in about 2020, but by then, it will exhaust all its supply of energy to power its instruments. After that point, neither will it be able to move on its own, nor will we be able to communicate with it. It will be lost forever. These spacecrafts will be the first human-made objects to go out of the Solar System. You can track the progress of these spacecrafts here.

Now let us come back to our topic. On July 12th 2011, 5 days from now, Neptune will complete one revolution around the Sun, counting from the day we discovered it in 1846. We count one year as the time it takes us to go around the Sun once. Measuring the Neptunian year the same way, not even one Neptunian year has passed since we discovered it. Now, tell me. Is the title justified or not?

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