Where no spacecraft has gone before

The list of bodies in the solar system unvisited by human devices is tantalizingly short. But there’s one object that many people might be surprised to find out we haven’t sent probes to; the sun.

On 12 August 2018, NASA’s Parker Solar Probe began to make its way closer to the sun than any man-made object before it, and there are a couple reasons why its journey was challenging to design. The obvious one is that the sun is hot; it took decades to invent and perfect insulators, sensors and solar panels that could handle the heat. But the other reason has to do with orbital mechanics; it is significantly harder to go straight to the sun than it is to leave the solar system.

How could this be? Right away we can tell that it’s not about distance. Jupiter is five times further than the sun, yet we’ve gone there nine times. The key is that “going places” in space is effortful in a totally different way than going places on the surface of the earth. Rocket scientists don’t think in terms of distance.

On earth, getting some place is hard because there’s friction everywhere, and stuff in the way. When you walk or run, every time you put your foot down, the ground reabsorbs much of the energy from the last step. Cars in neutral roll to a stop after a few hundred feet. Even jet engines have to burn huge amounts of fuel just to push through the drag of the air. Since there is resistance along every meter or mile, travel is effortful per unit distance; per unit of stuff you have to push past.

In space there is nothing resisting motion, so distance is in some sense free. Once you get up to speed, you just stay that speed. Of course it does still take time to go that distance, but it doesn’t cost any more “effort” (which is usually fuel, for a spacecraft).

I just said “you stay that speed”, but more accurately, you stay at that “energy level”. Travelling through a gravitational field changes your trajectory. Things also get more complicated because those sources of gravity are points in space, which means that travellers will end up rotating around them. Rotation always makes things less intuitive.

I find it very useful to imagine the physical analogy of the sun “sinking” down into space, causing a big gravity well. When other bodies are near the sun, they feel the sloped edges of the gravity well, and want to move toward it. But if those bodies are also moving sideways, then they can just roll around the sides of the well like marbles without ever falling in. This video gives a real-world demonstration.

The Earth orbits the sun in a circular path, and therefore stays the same speed all the way around. But comets, for example, orbit the sun in a path called an ellipse. For part of their path they are a small distance from the sun and moving quickly, and for another part of their path, they are a large distance from the sun and moving slowly. They whip around the gravity well in bursts. But at any given time, the “kinetic” energy from its speed and the “potential” energy (the energy an apple has before falling from a tree) adds up to the same energy level. Travelling the path of the orbit will shift the balance of the two kinds, but the total remains the same.

So if your current path takes you to where you want to go, you can just wait; but if your destination is at a different “energy level”, you have to change your energy to match it. (And then wait.) There’s also the question of what we measure our energy level with respect to; for our purposes, we’ll use the most dominant body in the system, namely the sun.

If you want to orbit the sun close to its surface, you need a small energy level with respect to the sun. The earth, since it is relatively far from the sun, has a large energy level with respect to the sun. If you are on the earth, you share the same energy level.

Consider the gravity well analogy above. If the earth is a marble rolling around the well endlessly, and the space probe is a tiny micro-marble, and the probe separates from earth, it will also just roll around the gravity well at the same distance. The sun’s pull doesn’t imply that the probe can just “give up” and drop into the well; it has to lose the kinetic energy it has, and then the sun’s pull will drop its orbit closer.

Therefore, if you are coming from earth like a newly built space probe and you want to get to the sun, you have to get rid of a lot of the energy of your current level. In space, getting rid of energy is as hard as gaining it. There’s no friction to drain it from you. So to change your energy level, you have to push or pull on other stuff. To change your energy level a lot, you can either push or pull really hard, push or pull on a lot of stuff, or just not weigh very much to start with. The Parker Solar Probe will use all three strategies.

The Delta IV series. The Parker Solar Probe launched on the rightmost rocket. Source: Wikimedia Commons

To push really hard, you want a big, big rocket. Parker launched on the Delta IV Heavy, which was the most capable rocket in operation from 2011 (when the Space Shuttle retired) until this year (with the launch of the Falcon Heavy). The Delta IV is a hefty machine; it could haul a few cars into orbit in one go. Parker, weighing less than a smart car, is going a lot farther away (or “faster away”) than earth orbit, so it requires the Heavy variant. This rocket is three Delta IV boosters bolted together, with the second stage on top of the middle booster. It could take up a school bus and the Hubble at the same time. But Parker still isn’t satisfied. It has a need; a need for speed. So it uses that unallocated payload capacity to bring up its own personal third-stage rocket. The Parker Solar Probe is the only mission that has needed a third stage on top of the Delta IV Heavy.

But in truth, this isn’t nearly enough. So after all the rockets burn, the Probe is headed toward Venus to pull on a lot of stuff. Parker will be performing seven gravity assists from Venus during its mission. When any two bodies move past each other in space, they pull on each other through gravity. Depending on your reference frame, this causes one of them to speed up, and the other to slow down. In our case, Parker will be using Venus to lose energy with respect to the sun, therefore falling deeper into the sun’s gravity well. Since planets have an enormous amount of mass, Venus will only speed up an infinitesimal amount, but Parker will feel a huge change.

Ultimately, the Probe will manage to lose enough energy to get within 4.5 solar diameters from the sun. This course takes it directly through the corona, a thin layer of physical sun stuff. This stuff doesn’t act like we expect it to; it’s hundreds of times hotter than the sun’s surface below. Heliophysicists have been unable to account for this fact using remote observations. So to solve this mystery, we will have to go to the heart of it.

You can follow the Parker Solar Probe’s progress toward the sun by following NASA Sun & Space on twitter.

The Parker Solar Probe being mounted inside the payload fairing of the Delta IV Heavy. On the top of the probe is the sunshield; the grey sphere in the middle is the fuel tank for the third stage, which detached after firing. Source: Goddard Media Studios