We've been to the Moon, landed spacecraft on Mars and had close encounters with other planets — and Pluto. Next stop, the Sun.
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Dreams of sending a spacecraft to explore the big ball of seething energy that warms our planet have been on NASA's bucket list for 60 years, and now the ambitious mission to touch the Sun is in its final phase before launch.
Originally called the Solar Probe Plus, the mission was renamed in honour of astrophysicist Professor Eugene Parker, who predicted the existence of high-speed solar winds — the mass of particles that are spewed into space from the Sun.
Set to kick off next July, the plan is to plunge the Parker Solar Probe into the Sun's corona — the hazy bit you can see around the edges of the Sun during a total solar eclipse — to study this phenomenon.
"We describe this as a mission of extremes," said Dr Nicola Fox, mission project scientist at the Johns Hopkins University Applied Physics Laboratory.
The car-sized spacecraft will get closer to the Sun than any other mission ever has.
Travelling at the dizzying speed of more than 720,000 kilometres per hour, the probe will eventually come within less than 6.4 million kilometres of the Sun's surface.
"This may not sound particularly close — but if you think of the Sun and Earth as being 1 metre apart, then our spacecraft would be located just 4 centimetres from the Sun," said Dr Fox, who is overseeing the build of the spacecraft.
And it will be hot. Very hot.
"The probe will be in regions of the corona where temperatures exceed 1,400 degrees Celsius," Dr Fox said.
What are we trying to find out?
We've been studying the Sun for thousands of years, and even though we now have remote sensing observatories and spacecraft that examine it in spectacular detail, many questions still remain.
The two big ones are:
- Why is the corona on the outside of the Sun at least 300 times hotter than the surface?
- Why does the solar wind speed up?
"These questions are important because we literally live in the atmosphere of the Sun," Dr Fox said.
"This outer region gets accelerated and moves away from the Sun, bathing all of the planets."
When large events such as sunspots or coronal mass ejections happen, they can have dramatic effects on our planet, causing spectacular aurorae but also disrupting communication systems.
By understanding how this solar wind is generated and modified in this region close to the Sun, we can better predict what could impact our planet.
Astronomer Dr Brad Tucker of the Australian National University said understanding how the solar wind works also has important implications for space exploration.
"Once you're in space there's no gravity and no atmosphere so you just need that push, that gust of wind to get you going and then you'll keep going," Dr Tucker said.
In fact, he said, spacecraft such as the Kepler Space Telescope already use this technique.
Dr Tucker said understanding solar winds is also integral to understanding how much radiation we could be exposed to in space.
"Luckily here on Earth we're protected by a magnetic field. But when we talk about going to the moon or Mars where the atmosphere is little or none ... we don't have that protection," he said.
"So if we want to live and work and study and have fun on these places we need to understand how the Sun will impact that."
What will stop the spacecraft melting if it gets too close?
The probe will have a heat shield like nothing we've seen before.
"The mission was first proposed in 1958 and it has taken this long to be develop the technologies to make the mission a reality," Dr Fox said.
The front of the spacecraft will have a shield, 2.3 metres wide and 11 centimetres thick, made up of a special carbon foam sandwiched between two thin sheets. The front face will be covered in aluminium oxide to reflect light and heat.
So while the shield will get up to a toasty 1,400C, the instruments inside will stay at room temperature.
Dr Tucker said materials made out of carbon are not only lightweight, they are able to withstand great heat and energy.
"It's this revolution of technology from completely different fields of science that has led to this [mission] and will enable this probe to get close to the Sun, which will then help us explore space and Mars and other places," he said.
So what's the plan?
NASA plans to launch the spacecraft sometime between July 31 and August 20 next year.
It's bringing out the big guns to make sure it has the speed needed to surf around the Sun without getting pulled in.
"The spacecraft will be launching aboard a Delta IV Heavy — the largest launch vehicle currently in the NASA fleet — and it will have three stages," Dr Fox said.
The spacecraft will orbit the Sun 24 times and pass by Venus seven times, using the gravity of the planet to slow the spacecraft down a little bit each time so it can pass closer to the Sun.
It will come closest to the Sun on December 19, 2024.
Dr Fox said the team is now in the final push leading up to the launch.
"The mission team is assembling the spacecraft and heat shield, testing the software and systems, putting everything through the incredible rigorous simulations and environmental tests that will make sure it's ready to launch in 2018," she said.
"We've installed many of the instruments, and almost all of them will be completely integrated by the end of the [northern] summer."
It will be moved to Goddard Flight Centre for four more months of testing before going to Cape Canaveral.
And then the Sun.