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Why are we sending a rocket into the sun? SciShow Space explains the why, what and how of Solar Probe Plus, a mission that’ll give us our closest look yet at our nearest star.

Hosted by: Caitlin Hofmeister

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Sources: [Image of Corona and Solar Wind] [photo from the 1970s showing Corona]
The Solar Corona: Proceedings of International Astronomical Union Symposium No. 16 Held at Cloudcroft, New Mexico, U.S.A., 28–30 August 1961

We've explored a lot in our solar system over the past few decades: Jupiter's moons, Mars, Neptune, even a comet. But one place we haven't gone is into the sun. For kinda obvious reasons, but that's about to change.

In 2018, NASA plans to send a car-sized probe, known as Solar Probe Plus, into the corona, or outer atmosphere of the sun. That's the crown of light you see surrounding the body of the sun during a solar eclipse. A few previous missions, like Helios 2 in the 1970s, orbited the sun at a distance of 43 million km to give us a closer look.

But Solar Probe Plus will get much, much closer, only 6.2 million km away. Scientists hope that this mission will solve a mystery that has plagued solar science for decades. Why is the corona hotter than the surface of the sun? Since the 1940s, scientists have known that the corona generates shorter wavelengths of light, which correspond to higher temperatures, compared to the visible surface of the sun, also known as the photosphere.

The difference in wavelengths is so great that astronomers actually think the corona is 200 times hotter than the sun's surface. Which is confusing, because temperatures normally fall when you move away from a heat source. Which is actually true in the sun's interior.

At the core, it's 16 million degrees Celsius, but further out into the photosphere, it's a balmy 6000 degrees Celsius. But then temperatures rise by a few million degrees when you enter the corona, about 500 km above the photosphere. The corona, which is made up of highly ionized elements, like iron, helium and calcium, is so hot, that the sun's gravity can't hold on to it.

As those atoms get all agitated by the extreme heat, ribbons of plasma called solar winds, peel off from the corona and begin accelerating away from the sun at speeds of up to 900 km per second. These solar winds contain high levels of radiation that can harm satellites, as well as astronauts in deep space.

So, NASA wants Solar Probe Plus to help us understand what's heating the corona and accelerating those solar winds. The craft will use Venus' gravitation to sling it around the sun 24 times over 7 years, getting closer to the corona with each orbit.

On its closest approach, the craft will skirt the outer edge of the corona, where temperatures only reach 1,300 degrees Celsius. Which is still super hot! Similar temperatures were generated when the space shuttles re-entered Earth's atmosphere. That's why Solar Probe will have a thermal protection system, including a heat shield shaped like an enormous dinner plate, 11 centimeters thick and 2.5 meters in diameter. It'll be made of carbon composite and silica foam, which is the same stuff we used on the space shuttle.

The probe's electrical systems, meanwhile, will be powered by two sets of solar arrays, because there's a lot of sunlight when you're going to the sun. The first set of arrays will be bigger, and will be used on the initial leg of its journey. Once the probe enters the corona, however, those panels will retract and two tinier, liquid-cooled arrays will do the rest of the work.

As the probe flies through the corona at 189 kilometers per second, it will begin counting, collecting, and analyzing particles, including electrons, protons, and ions. Meanwhile, a telescope will create intricate 3-D images of the corona and solar winds and send all that data back to Earth. The craft will spend a total of 30 hours within the corona before NASA turns off its transmitter and leaves it to its fate. And it's a fiery one.

Scientists hope that all the data gleaned from this mission will help us understand the corona's dynamics so we can get better at forecasting solar wind events and planning future missions into deep space.

So thanks in advance, Solar Probe Plus, for everything you're going to teach us. And thank you for watching this episode of SciShow Space. If you'd like to learn how you can help us keep exploring the universe together, go to and get some awesome stuff, like a SciShow DVD of possible apocalypses! Yay!