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NASA's Parker Solar Probe will soon fly closer
to the Sun than any spacecraft before it-about 4 million
miles from the visible surface. But getting that close to the Sun
requires some fancy orbital mechanics. It takes 55 times
more energy to go to the Sun than it does to go to Mars.
Why is it so hard to get to the Sun? The answer is related to why
Earth doesn't just fall straight into the Sun, despite the strong gravitational
attraction. Earth, and everything on it, is traveling very
fast-about 67,000 miles per hour-in a direction that is
basically always sideways relative to the Sun. If you launch a
rocket from Earth, straight toward the Sun, it won't lose that sideways speed,
and so it will miss the Sun. The only way to get the rocket
to go right into the Sun is to cancel all that sideways motion. Leave
even a little bit and it will miss the Sun and enter a new orbit.
To cancel Earth's motion, you have to launch the spacecraft backward as fast as Earth
is hurtling forward. But 67,000 miles an hour is really
fast. Spacecraft have to go upward at only 25,000 miles
an hour to escape Earth. Getting to Mars only requires a bit
more speed: 29,000 miles an hour. New Horizons,
which NASA sent rushing out to Pluto, managed 36,000 miles
per hour, or a little more than half what it would have to hit the Sun
instead. Since Parker Solar Probe plans to fly past the Sun,
it doesn't need to cancel out all of Earth's sideways speed, but it does need to remove
53,000 miles per hour of it. That's why it's using one of the most
powerful rockets available and additional gravity assists from Venus
over a period of several years. In this case, rather than speeding up
the spacecraft as in a typical gravity assist, Venus slows down its
sideways motion, so the spacecraft can get close to the Sun.
When it finally does make its closest approach to the Sun, Parker Solar Probe
will have lost much of its sideways speed, but gained a great deal of overall
speed, thanks to the Sun's gravity. Parker Solar Probe will hurtle
past the Sun at 430,000 miles an hour-
the very first human-made object to get that close.


It's Surprisingly Hard to Go to the Sun

116 タグ追加 保存
Huahua 2018 年 8 月 24 日 に公開
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