How Close Can We Get To The Sun?

How Close Can We Get to the Sun?

In 1926, Robert Goddard created the first liquid-fueled rocket. In ‘57, the Soviet Union launched Sputnik. Less than five years later, Yuri Gagarin was the first human in space, and by the end of the 1960s, the world was watching Neil Armstrong and Buzz Aldrin walk on the moon.

Next, came Voyager 1, launched in 1977. It reached Jupiter, Saturn, and that planet's largest moon; Titan. And then it kept on going. And going. Travelling further and further away from the sun, and reaching interstellar space more than three decades after it had originally set sail. More often than not, our cosmic gaze is set outwards. Out to the furthest reaches of the Solar System, and possibly beyond. But now, NASA has its sights set inwards, granting unprecedented focus on that hot ball of burning plasma which enables Earth to harbor life. So, how close can we truly get to the Sun?

In terms of distance required, touching the star might seem like a more achievable goal than escaping the solar system, but the reverse actually holds true. And, it’s a matter of gravity. The fundamental force drags all objects towards the sun's core; but Earth (and every other planet) doesn’t fall in to a fiery grave because it orbits the sun fast enough to withstand the drag. In theory (and practice) this makes it far easier for vehicles to travel outwards, because the orbital speed is on their side. But, to jet off towards the sun, a probe needs to match the Earth's orbital speed (of approximately 19 miles per second) while traveling in the opposite direction… It’s a feat almost impossible to currently perform, which makes things rather complicated.

Clearly, it’s tricky to get a solar mission off the ground and on its way. But, things don’t get any easier once the journey has started. Scientists and engineers have to devise state-of-the-art systems capable of withstanding the sun's enormous energy and traversing the considerable distance separating our planet from the solar system's center. On average, Earth is just shy of 150 million kilometers away from the Sun, which is also known as one astronomical unit. It’s by no means an insignificant length, and the route required definitely isn’t direct, but we have already gone further. Much further. With Voyager 1 travelling more than 140 astronomical units during its record-breaking flight.

However, while Voyager 1 has travelled into the ‘unknown’, any sun-bound spaceship is travelling toward something we already know quite a bit about. And most of what we do know doesn’t make for a particularly hospitable destination…

First, the burning issue; exactly how hot is the Sun? With a radius of nearly 700,000 kilometers, and multiple distinct layers, the temperature varies greatly from one location to the next. The core is unsurprisingly the hottest region, though, at around 28 million degrees Fahrenheit. But the temperature plummets the closer you get to the surface. The photosphere – the lowest layer visible during the average day – reaches a relatively refreshing 10,000 degrees.

But, things take an unexpected turn at this point. While logic says that the outermost layer of the sun's atmosphere should be the coolest, it isn’t. Known as the Corona, its temperature can readily rise to with the millions of degrees Fahrenheit. Researchers theorize this discrepancy might be due to nanoflares – tiny but remarkably hot explosions which discharge heat into the atmosphere.

Crucially, the Corona is also responsible for releasing charged particles as solar wind, a phenomenon that messes with Earth's magnetic fields and damages electrical systems. It all means that travelling toward the sun isn’t a linear experience wherein the danger steadily increases. In fact, there are potentially mission-melting problems at various points. So, due to the multiple mysteries surrounding the Sun's upper atmosphere and its tangible impact on everyday life, NASA reckons it’s about time a couple of these riddles are solved. Enter, the Parker Solar Probe.

Prior to 2018, 1976's Helios-B held the record for the closest flyby of the Sun, peaking at around 0.29 astronomical units (or 43 million kilometers) away from its target. While this operation helped shed some light on solar wind and cosmic rays, NASA's recently launched mission – the Parker Solar Probe – has already broken Helios-B's record. Ultimately, the aim is that it will be the first human-made device to ‘touch the Sun’.

Parker's mission is pretty simple: enter the Sun's upper atmosphere, and collect data. Specifically, the focus is on the mechanisms behind solar wind. It’s a noble quest, but how does Parker plan to achieve its goal?

As we know, the technology to fly directly at the sun just doesn’t exist. So, touching its literal surface is out of the question. But, the probe is set to enter the Corona and fly as close as possible, aided by a helping hand or two. Using a planet or astronomical object as a slingshot, gravity assists allow it to change direction and speed, while also cutting down on fuel. So, to get to the sun, Parker’s route regularly passes by one of the Solar System’s other great inhabitants. Venus.

During seven scheduled flybys of Venus, the probe will routinely steal a bit of the planet's momentum, resulting in a substantial speed boost and a redirection towards the Sun. While the first flyby (in October 2018) was enough to break Helios-B's record and skip past Mercury, by the mission end in 2025, Parker will’ve more than doubled its speed to travel to within 6.5 million kilometers of our central star.

These gravity assists solve NASA's momentum and distance conundrums, but there’s still the question about the temperature. Given that the Corona is so unimaginably hot, what’s stopping Parker from melting quicker than an ice cream sundae on a midsummer day?

Well, firstly there is a difference between ‘temperature’ and ‘heat’. ‘Temperature’ refers to a particle's rate of movement, but this means little if there are barely any particles around. ‘Heat’ relates to the energy transferred between whatever particles are present. So, even though the Corona's temperature is absurdly high, the atmosphere's low density means that less energy is shifted, and less heat is created. Back here on Earth, this principle explains why you can comfortably sit in a sauna with a temperature equal to (or higher than) boiling water.

So, while the solar atmosphere's temperature might register somewhere in the millions, Parker only needs to withstand a few thousand degrees of heat. Clearly, though, the Corona is still extremely hot, and the probe needs to survive a ceaseless onslaught for almost seven years… So, if it does succeed, it’d be an incredible achievement, regardless. But, Parker cost a whopping $1.5 billion to make and, thanks to its innovative Thermal Protection System (or TPS), it’s hoped that it’ll hold up under heat blasts of up to 2,500 degrees.

Without this protective technology, it’s no exaggeration to say that NASA's lofty ambitions would still be the stuff of sci-fi. The fabled TPS is exceptionally resilient, it limits internal temperatures to around 85 degrees Fahrenheit, and it weighs less than the average U.S. man – at 160 pounds. The shield itself consists of a 4.5-inch-thick carbon foam sandwiched between two bendable carbon sheets, which A) stops Parker from melting, and B) prevents the heat from reaching the spacecraft's core. Lastly, NASA sprayed a white aluminum oxide layer across the shield, to best reflect the sun’s energy away from the probe.

But, wait. How about fuel? A seven-year trip can hardly be carried out on an empty stomach, can it. Here’s where Parker’s destination is actually a huge help. Because, after the initial launch, the probe has used (and will use) the Sun to generate energy. Protected by a cutting edge cooling system, solar panels should keep the spacecraft speeding through the solar system for the 24 planned orbits around the star itself. Given its ultimate goal, solar power shouldn’t be too hard to come by!

How close is the Parker Solar Probe expected to reach? A ‘perihelion’ refers to the point where an object comes closest to the Sun, and the spacecraft is set to experience 24 such moments before its mission ends in 2025. Most passing orbits will reduce the distance between Parker and the Sun, with NASA predicting the closest approach to happen during the 22nd spin, with an expected distance of approximately 6.1 million kilometers – that’s just 0.04 astronomical units away from the target. At that time, Parker should be traveling at about 430,000 miles per hour.

December 24th, 2024. Mark it in your diaries, because that’s the day that – all being well – humanity ‘touches the Sun’. (Or, at least, gets to within just a few million kilometers of it.) From there, the mission will wind down until Parker performs its final solar orbit, and eventually succumbs to the very thing it has been sent to study. Ultimately, the Solar Probe will break up and become part of the sun’s atmosphere. A fitting end for a fantastic piece of technology.