Why a Mission to The Sun is Really Hard

Because Gravity! Hi! Hiyu here! Sending stuff to the Sun is a lot harder than you might think, and it’s all because of how orbits work, so let’s learn about those first. Say you want to stay in space above the Earth, without falling back down. We’ve already figured out that going straight up isn’t a good solution, because unless you go stupid far - and I mean, way past the moon - you’re just gonna fall right back down. What you have to do is to go to an orbit, where you’re going sideways around the Earth so fast that you never hit it, even though you’re always falling toward it. Orbital mechanics is pretty unintuitive because it isn't like normal movement - it involves circling around a lot of stuff, so here’s some basic fundamentals. The idea of an orbit is to stay circling something, like the Earth. If your orbit is closer to a planet or a star, then the pull of gravity is also stronger, so you have to be going faster sideways in order to not fall down into it. This is exactly what the Earth does - it’s going just fast enough, at 150 Million kilometres above the Sun, to be orbiting in a circle. Venus and Mercury are much closer to the sun, so they have much shorter years than we do - meanwhile, Mars’s orbit is much further out, so it has to have a longer orbital period, meaning its year is almost twice as long as ours! This knowledge is something we can use to our advantage. Say you’re a Satellite TV operator. You don’t want everyone who uses your TV service to have to buy an expensive satellite dish that spins around to track your satellites as they pass overhead, right? Using orbital mechanics, you can figure out that if you put your satellite in a particular, much higher orbit around the Earth, it can have an orbital period of 24 hours! For comparison, here on the International Space Station, I’m orbiting the Earth about once every 90 minutes. (That also means night and day get a bit messed-up up here!) If your satellite has a day-long orbit around the Earth’s equator, that means that from the Earth, it'll look like it's fixed in the sky. This is called a Geostationary Orbit, and it means your TV customers only have to buy a cheaper, fixed satellite receiver, that can stay pointed in one direction facing the sky. A lot of weather satellites use this orbit too, because it’s great for taking a lot of pictures of the Earth from a fixed position, so you can compare them over time. So! How do you launch something up to an orbit like that? Well, initially, you have to get to space. ¯\_(ツ)_/¯ This is typically done with a rocket that jumps up out of the thickest part of the atmosphere, then starts accelerating sideways in order to reach a low orbit. Then, at just the right time, opposite of where you want your satellite to end up, the booster points forward in its orbit and thrusts again, in order to push the highest point of its orbit higher, until that point reaches geostationary altitude at 35,786 kilometres above the Earth. This orbit is elliptical, meaning one side is bigger than the other, and in this case it’s called a Geostationary transfer orbit, or GTO. Also, fun fact! When the rocket makes this burn, it speeds up considerably, but as we mentioned before, as it goes around in its orbit and it goes higher up, it actually slows down! The spacecraft then coasts for 5 hours, until it reaches its highest point, or apogee, then makes a final burn to raise the lowest point, or its perigee, up to the same altitude, to circularise its orbit. Now these egg-shaped transfer orbits to get from one orbit to another are called Hohmann Transfer Orbits, named after a smart German dude. They’re the most fuel-efficient way of getting from orbit A to orbit B, but they also take the most time as a result. Achieving a Hohmann transfer to Mars takes relatively little effort, but takes a whole 9 months, so for travelling back-and-forth quickly, it’s not ideal. But, if you have a lot of time to kill, you can use it in some really cheeky ways. Say you're going to Jupiter, and you’re not worried about how long it’ll take, but you need to use as little fuel as possible. Instead of adding to the Earth’s speed around the Sun and going out into the solar system, instead, you can start by going inward. A transfer to Venus takes much less fuel, and if you approach it just right, its gravity can give you a huge boost to get into the outer solar system. Now, when you get a gravity assist from a planet, it pulls you along and speeds you up as your approach it - but equally, it pulls on you and slows you down as you move away. The major point here is that while your speed ends up staying the same, your direction changes - and that makes all the difference. Lots of interplanetary probes shop around for gravity assists from the inner planets, and then one day bolt out into the outer solar system to where they need to go. A great example is the European Space Agency’s Rosetta probe, which had a mission to go to a comet. The comet was really far away, and travelling super fast, so to reach it, Rosetta had to get a gravity assist from Earth a year after its launch, Then Mars two years later, Then Earth again later that year, Then Earth again two years later, after finding an asteroid that it took pictures of on the way. This got it past the orbit of Jupiter, while using basically no fuel, in order to reach the comet! So, why’s it hard to get to the Sun then? Well, the Earth’s already going around the Sun super fast! So to get down into the solar system, to negate that speed, you need a buttload of fuel, or lots of gravity assists. And very soon, there’s a mission that’s gonna do just that! At the [beginning of August] this year, the Parker Solar Probe will start on its mission to the Sun, going closer than anything else we’ve ever gone before, deep into the sun’s atmosphere, called the Corona. For a sense of just how close it’s gonna go, here’s what the Sun’s gonna look like from its perspective! Ugh, it's getting a bit toasty in here- By the way, for this video, I tried looking up where the corona ends - and it practically doesn't! Like, we're all inside of it! So that’s why studying it matters so much. So to get down that close to the Sun, the probe is going to fly by Venus seven times over the course of six years. It'll become the fastest vehicle in history, travelling at 700,000 km/h at its lowest point around the Sun. That’s 200 kilometres every second! It’ll be going so fast that during one of those years, it’ll be flying by Venus twice! With every gravity assist, it’ll be lowering its orbit around the Sun, getting closer and closer to the thing that it's studying, and returning juicy data back to Earth about the solar wind and how it's formed. Solar activity plays a much bigger role in our lives than we typically think about, as electrical and communications companies have to really consider it carefully when designing their systems. So while it’ll take six years, and several trips around the inner solar system for this probe to reach its destination, it’s a really important one to go to and study. I mean, what would we be without the Sun, y’know? Thank you for watching this video on orbits! Subscribe button, bell for notifications, Patreon, Twitter! Also! Go watch Scott Manley’s videos if you don’t already - they’re way more in-depth than mine, and they're really good, if that’s what you you’re looking for! I actually finished writing my first draft of this video just before he uploaded his video on exactly the same thing, it’s kinda silly! Anyway, this has been Hiyu. Bye!