What is the REAL Time it Takes to Reach Every Planet in Our Solar System

Imagine that we have a spaceship. Not a magic sci-fi Millennium Falcon kind of thing, this is an actual spaceship that could be built in our lifetime using technology that we understand today. That means we can't visit every star in the sky, but it does mean we can visit every planet in our own solar system. Think of it like a bucket list of planetary travel destinations. Now, how long would it take to check off every one? Well, the answer is more complicated than you might think, because to navigate through space requires us to battle against the primal forces of the universe and push the limits of what is physically possible with our humble spacecraft. Now, just like any good trip, the first thing we are going to need is a map. Except our map of the solar system is not like any other map you've used before, and the biggest change here is that you can't simply measure a distance between two points. The reason comes down to motion. Everything in the solar system is constantly moving in an orbit around the sun, meaning there are no straight lines, only curves. And right now, Earth is hurtling around the sun at about 107,000 kilometers per hour, while every other planet is also moving in its own unique orbit, traveling at its own speed. So when we launch a spacecraft toward Jupiter, we're not aiming at where Jupiter is, we are aiming at where Jupiter is going to be by the time we arrive. It's like throwing a dart at a moving target from the back of a moving car. And to add an extra layer of complication when it comes to space travel, you only get one shot to hit that dartboard. If you miss, then there goes a decade of work and a few hundred million dollars all down the drain. The reason why we have no margin for error comes down to fuel. This is our most precious resource in space. Imagine you're driving to a destination and your car is filled with the exact amount of gas necessary to arrive by driving only in the most efficient way possible. So if you take just one wrong turn, you don't make it. Even just by driving too fast or slowing down too much, the fuel will run out before you arrive. Sounds stressful, right? And that's what every trip through the solar system is going to be like. So in order to make the most out of what little fuel we have, we need to work in harmony with the most powerful force in the universe, gravity. Unfortunately, before we can work with gravity, we have to fight against it. In order to even get to space, we first have to break free of the Earth's gravity. This is not necessarily the hardest part of the journey, but it is the most energy intensive. Our rocket burns through enough fuel to fill an olympic-sized swimming pool every second of the two-minute-long burn it takes just to rise above the Earth's atmosphere and cross the invisible line into outer space. We are now officially orbiting the Earth. By maintaining a velocity of just over 20,000 kilometers per hour faster than the Earth itself is moving, we can effectively outrun the pull of Earth's gravity and travel in a circular motion around the planet. Now here's where things start to get a little weird. Consider this our introduction to orbital mechanics. From our perspective, we are traveling in a straight line. The ship does not steer itself around the Earth like a NASCAR racetrack. Instead, the Earth's gravity bends space and shapes our path into a curve. The harder we push with our engines against gravity, aka the faster we go and the more fuel we burn, the straighter that curve will become and the further away from the planet we will be able to travel. On the reverse side of the equation, if we were to slow down from here, gravity would curve our path even stronger, and if it curves too much, we end up on a collision course with the surface of the Earth. We don't want to do that, so by doubling our velocity to over 40,000 kilometers per hour faster than the Earth, we are going to finally break free completely of the Earth's gravity and begin our interplanetary voyage. That puts our total speed relative to the Sun at around 150,000 kilometers per hour. Now we have to become yet another object hurtling through the solar system, but we still have some limitations out here. Just like every other planet, moon, asteroid, and comet, we are captured by the gravity of the Sun. This is just like orbiting the Earth, but on a much larger scale. Because the majority of the solar system exists on one flat plane, there are essentially just two directions that we can travel, either toward the Sun or away from it. But no matter which way we go, our path is bound to curve around the gravity well. Remember, no straight lines. And also, just like orbiting the Earth, we control our own path by speeding up and slowing down. Sounds easy enough, and if our goal was just to travel around aimlessly through the solar system, then that would be relatively easy. But if we are going to steer our spacecraft toward a particular destination and then anchor ourselves in orbit when we get there, that is going to require some very serious calculations. Anyway, there's no better way to figure this out than by just going for it, so let's set course for the first planet from the Sun, Mercury. You might be surprised to hear that Mercury is actually the hardest planet for us to reach. That makes it a great choice for our first mission, because if we can figure out how to get there, then everything we learn along the way will set us up easily to conquer the rest of the solar system. In terms of distance, Mercury is relatively close to the Earth, about 80 million kilometers away, but it's even closer to the Sun, orbiting at an average distance of 58 million kilometers. And because Mercury is so far down inside the Sun's gravity well, it has to move very fast to avoid getting pulled in, so the planet is orbiting at around 170,000 kilometers per hour. At our current velocity, we have two challenges that need to be overcome. One is slowing down enough to begin falling into the Sun, and then the second is controlling that fall in a way that we arrive safely in orbit around Mercury and don't end up going way too far. So from our position, orbiting just beyond the Earth, we need to slow down by quite a lot, taking our velocity down by almost half, so we're undoing all of the gain that we made to escape Earth's gravity, and then some. In this way, we are intentionally losing the battle against the gravity of the Sun and allowing it to begin curving our path down until it begins to intersect with the orbit of Mercury. And that's actually the easy part. It means burning another portion of our limited fuel supply, but it also sets us up for an even bigger problem to solve. So falling into the gravity of the Sun is kind of like rolling down a hill. You tumble slowly at first, but the further you fall, the faster you go. In the context of the Sun, the closer you get to it, the harder its gravity pulls on you. So even though we have to slow down at first, we're actually going to end up going a lot faster as we approach the Sun. In theory, this will help us catch up to Mercury, but the real challenge is that we're actually going to end up going too fast by the time we get down that far. Our fall down the hill of gravity will accelerate the ship to around 210,000 kilometers per hour. This is where we meet our challenge. The ship needs to slow down for a second time, or we'll just blow straight past Mercury. This is kind of what happened to an old NASA mission called Mariner 10. It was launched in 1973, and it used the falling down the hill method to arrive at Mercury in just about five months time. The only downside was that by arriving so fast, the probe was totally unable to slow down enough to orbit the planet. So it ended up gathering as much data as it could while flying straight past Mercury. Now, in theory, we could burn our engines yet again to bleed off all the excess velocity, but that would be pushing our fuel supply far beyond anything that might be realistically possible, hence the fate of Mariner 10. And in reality, there has been just one NASA mission that has successfully made it all the way down to orbit Mercury. That is the Messenger probe launched in 2004, and it took six and a half years to complete the journey from Earth. The reason why it took so long is because that probe used the gravity of Earth, Venus, and Mercury to achieve the same result as burning an impossible amount of fuel. The big downside is that it's going to require multiple trips around the Sun in order to make all of those planetary encounters work for us, hence the multi-year journey. So following the path laid out by the Messenger probe, our first target on the journey to Mercury will actually be the planet Venus. The idea is that we're falling down toward the Sun in such a way that we're going to fall right past Venus, and the goal will be to come close enough to that planet that its own gravity begins to have an effect on the direction of our spacecraft. We call that maneuver a gravity assist because we're basically using it to assist us with steering our ship, and if gravity is responsible for changing our direction, that means the engines don't have to do as much of the work, and that means we preserve our fuel. In this case, we are going to approach in such a way that the gravity of Venus will pull our line up a little bit. So we're still falling toward the Sun, but now the hill isn't quite as steep as it was before. This is going to allow us to fly all the way around the Sun and catch up with Venus again for a second gravity assist, but this time we're coming much closer to the planet itself within just a few hundred kilometers. So that is going to allow gravity to bend our curve even more, and this change in direction sets us up for our first encounter with Mercury. The Messenger probe that we're basing our flight plan on had to fly past Mercury three times over the course of nearly two years. With each encounter, the gravity of Mercury bends the path of our spaceship just a little bit every time, bringing us closer and closer to matching the orbit of the planet itself. And after the third gravity assist from Mercury, we are finally close enough to make one last course correction and insert our spacecraft into a stable orbit around the planet itself. Now we'll need to fire up the main engines again for the first time since we began our plummet into the Sun, and we're going to time this burn so that it slows us down right as we pass close to Mercury. The small planet has a relatively weak gravity, only about one-third the strength of Earth, but if we can slow down enough at just the right time, then the gravity of Mercury will be able to pull harder on our ship than the gravity of the Sun, and the path of our ship will be curved all the way around into a stable orbit of the planet. All in, assuming that we have a bigger and more powerful ship than the old messenger probe, but also accepting that we will need to perform a similar round of gravity assist to lock in our final trajectory and complete many trips around the Sun before we arrive, the journey from Earth to Mercury is going to take us about five years to complete. Now the good news is that not every interplanetary trip has to be so complicated. When we look at Venus, we see our closest planetary neighbor and our shortest trip through the solar system, but that doesn't mean this journey will be boring. From our starting point, just beyond the gravity of Earth, we know that we need to go down toward the Sun, which means firing up our main engine to begin killing off some orbital momentum and start falling down the hill of gravity. That is going to get us to Venus pretty quick. In just about four months, the Sun's gravity is going to pull us down to the orbit of our next planet, but just like with Mercury, we're now traveling too fast to be captured by the gravity of Venus, so if we don't find a way to reduce our speed, we'll just sail right by. Fortunately, we have more options at our disposal this time. For one, Venus is a much larger planet than Mercury, so its gravity has nearly three times more power to bend our path and pull us into an orbit, and for two, Venus has an atmosphere. It's very thick, almost 100 times more dense than Earth's, so by flying very close to the planet and touching the upper edge of the atmosphere, we can actually slow down a lot just by using aerodynamic drag. And by combining these two natural forces with a powerful burn from our main engine, we can insert our spacecraft into orbit around Venus less than five months after leaving the Earth. When it comes to reaching our next destination, Mars, this is where everything gets flipped, because now we have to fly away from the Sun for the first time, and that means it's all about accelerating up into the solar system, climbing up the hill of Sun's gravity instead of falling down it. And yet, slowing down will now be even harder. Now, spacecraft from Earth have flown to Mars many times over the past four decades. It's by far our most visited planetary neighbor, so we've developed a method for getting there that prioritizes efficiency over maximum speed, and it's called the Hohmann Transfer Orbit. We wait until Mars and Earth line up on their closest approach around the Sun, an occasion that comes only once every two years. But during that window of time, all we need to do is launch our spacecraft from the Earth and accelerate to a just a little bit faster than our own planet is moving, and that's enough to get us rising up and away from the Sun. In theory, we could accelerate a lot more and get to Mars really fast, but that would only make things more complicated when we actually arrive. Because Mars is further from the Sun than the Earth, it travels at a much slower speed, so if we are accelerating away from Earth right now, then we're going to need to slow down a lot by the time we reach Mars, from around 150,000 kilometers per hour to 86,000. Unlike Mercury, there are no planets we can use along the way for a gravity assist, and unlike Venus, there is no dense atmosphere around Mars to slow us down. Mars even has a much weaker force of gravity, so it's all about using the engines on our spacecraft and burning fuel to get our velocity low enough to achieve Martian orbit. This is why it takes as long as eight months to reach Mars. We could get there a lot faster, but we'd never be able to slow down enough to stick around. Now, Jupiter is where things start to get really interesting, because we are going deep out into the solar system for the first time. You see, Jupiter is five times further away from the Sun than the Earth, which means that we are in for another long haul. That distance means that we can't simply hop from one orbit to the next like we did for Mars. Instead, we return to the gravity assist technique that we used to reach Mercury. On that flight, we leveraged the gravity of Venus to curve our path down towards the Sun, but all we need to do now is switch up our approach, and we can actually use the planet's gravity to fling us out on a path straight to Jupiter. Once we are on our way, we now have two challenges to consider. One is crossing the asteroid belt. This is a region of space between Mars and Jupiter that contains hundreds of thousands of rocky objects, ranging from grains of dust to asteroids the size of small moons. And at the velocity we're traveling right now, even getting hit by a very small rock would have catastrophic results. If we were in a scientific fiction movie, then this would really be something to worry about. But in reality, it's mostly empty out there. The rocks are so spread apart that you could fly through the belt a hundred times and statistically never come close to hitting one. So, after crossing through the asteroid belt, we meet challenge number two. That's Jupiter itself. This planet is humongous. It's two and a half times more massive than every other planet in the solar system combined. So, the force of gravity around Jupiter is equally as powerful, almost like a small sun. Which means, just like the sun, Jupiter's gravity starts pulling us in as we get closer. So, we transition from climbing up the hill of the sun's gravity to falling down the hill of Jupiter's gravity. That definitely helps us get where we're going faster, but it also brings back an old nemesis. You guessed it. Slowing down. Out here, in deep space, far from the intense pull of the sun's everything moves at a much more relaxed pace. Remember Mercury racing around at 170,000 kilometers per hour? Well, Jupiter cruises at about 47,000. Now, our spaceship is traveling over five times that speed, so we've got some work to do. The power of Jupiter's gravity is a blessing and a curse. On the one hand, it pulls us in and makes us go even faster, but because the force of gravity is so powerful, it can bend our path into an orbit even when we are outrunning the planet itself. So, we do not have to take our speed all the way down to match the slow progression of Jupiter. We only have to bleed off enough velocity to get caught in its deep gravity well. When NASA's Juno spacecraft approached Jupiter in 2016 on a very similar path that we're taking today, the probe had to burn its main engine for 35 minutes straight just to lose enough speed for an orbital capture. So, after falling down toward the sun, speeding up, looping around Venus, getting flung out into deep space, crossing the asteroid belt, falling into Jupiter, and then slowing down again, it's taken us about three years to arrive at this point. Now, here's a really fun question. What happens if we don't slow down at Jupiter? Well, the answer is how we unlock the rest of the solar system. So, in order to reach Saturn, we do it all again, but this time we use the gravity of Jupiter as our slingshot. If we maintain our velocity, we'll be going too fast for Jupiter's gravity to pull us all the way in, but it will still have enough power to bend our path as we fly by. It's kind of like falling down a hill, getting looped around a right-hand turn, and then falling up the hill again. So, all of the extra speed that we gained by approaching Jupiter we lose as we fly away. The hills kind of cancel each other out, but the curve in our path has now pointed us directly at the planet Saturn. Our biggest challenge now is just distance. When you look at a typical map of the solar system, Jupiter and Saturn might appear to be pretty close together, but that's only done for the sake of making an illustration that's easy to read on a single page, because in reality, Saturn is almost twice as far away from the Sun as Jupiter. So, we end up several years just coasting between the two gas giants, but eventually the rings of Saturn begin coming into our view. Now, even though Saturn is really big, similar to Jupiter in size, it's actually a lot less massive, with only about one-third the weight. That means only about one-third the gravity. So, we don't have anywhere near as much pulling force that we need to push against with our engines. That makes for a relatively easy maneuver to slow our vehicle down and become captured in the planet's orbit. That extra coast time extends our journey from Earth to Saturn out to nearly seven years, and now the time has come to set our sights on Uranus. As you might expect by now, we're going to repeat our gravity hopping procedure from the inner planet to the outer planets and use Jupiter as our big kicking off point to deep space. We can also add Saturn to that list and use its gravity to curve our path once again toward the outer reaches of the solar system. But there's a catch here. In order to use multiple planets as slingshots in a sequence like this, they'll all have to be in very specific positions when you launch from Earth, lined up in a way that lets you swing from one to the next. Unfortunately, that alignment doesn't happen often. The Voyager probes launched in 1977 took advantage of this rare planetary alignment that made the grand tour of the solar system possible, but that opportunity won't come again until the year 2152. And by that time, we'll probably have a warp drive that can take you anywhere in an instant, so it won't even matter. But there's still hope for Uranus in our lifetime. There is a launch window opening in 2031 that would allow a spacecraft to use a gravity assist around Venus and Earth to launch itself out on a path to intersect Uranus in about 13 years. That's not too bad, considering that it took the Voyager 2 probe about 9 years to get there, using the very special planetary alignment. Now, Neptune is the furthest planet from the sun. At least, that's what they want you to believe. And the distance we're looking at now is kind of unfathomable to the human mind. It's 30 times further away from the sun than the Earth. It's 6 times further than Jupiter. Out here, light from the sun barely even registers. After using a gravity assist from Uranus, it took the Voyager probe another 3 years of coasting to reach Neptune. About 12 years in total flight time. But those probes didn't have powerful engines on board. They relied entirely on gravity to steer them out this deep into space. So, over such a long distance, our spacecraft could probably start catching up, even without a perfect planetary alignment. And that makes our travel time more like 15 years at the most. Now, technically, Pluto is not a planet. Depends on who you ask. But let's go there anyways, because it's cool. And because there actually has been one successful NASA mission to Pluto. It was a probe called New Horizons. The only problem was that by the time it got all the way out here to the very edge of the solar system, it had nowhere near enough fuel remaining to slow down and get caught in the orbit of such a small object moving at such a low velocity. See, Pluto travels around the sun at about just 17,000 kilometers per hour, while New Horizons was traveling at about 83,000 kilometers per hour. By the time it reached the orbit of Pluto, that is. By using this strategy, the probe was able to fly from the Earth to Pluto in just about 10 years. If we wanted to continue our streak of arriving and orbiting every planet and non-planet, then it would be closer to 20 years before we actually arrive at Pluto to stay. Okay, so we've just visited eight planets plus Pluto. How long does that all take? Well, if you add it all up, we've clocked just over 64 years traveling to each destination of the solar system. Probably the kind of endeavor that you'd want to start young, or choose your favorite destination strategically. But if you want to start learning about every planet in detail, then there's no better place to start than Mercury. See the shocking discovery NASA found there in this video on the screen right now.