Taking the Scenic Route: How to Get Places in the Solar System

In last week’s post about all (well, some) of the ways Mercury is odd, I mentioned that it’s really hard to get to, but didn’t go into a lot of detail why. That killed me a little because I find gravitational dynamics and orbital trajectories fascinating but diving into it there would really have involved going off on a tangent.

So I’m gonna do it here instead! Come join me for a fun swim in the waters of solar system trajectories: why it’s hard to get almost anywhere and how we go about getting our spacecraft places anyway.

Wave As You Go By

The easiest way to get a spacecraft from Earth to somewhere else is to point that spacecraft where your destination is going to be later and shoot it off. And sometimes that’s all you need to do, depending on what your destination and goals are and what kind of rocket you have to work with. But you’re unlikely to be going to stay.

A lot of our early planetary encounters were what we call flybys, where you fire off a spacecraft so that it wings by your target. This is how we got our first up-close views of every planet in our solar system (apart from, you know, Earth). Flybys have the advantage of being fairly easy (as spaceflights go) and you can get pretty close to your target as you go past. 

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Of course, during a flyby you only get a glimpse of your destination. You see it at a single moment in time, which can wind up dramatically biasing how we view that world. So obviously it would be better to go into orbit, if you can, and stay a while. But that’s a whole different kettle of fish.

If you want to go into orbit, you need to slow down. You need your spacecraft to be going below the escape velocity of the world you’re visiting, so that it can be gravitationally caught and move into orbit. But then again traveling through interplanetary space generally involves moving fast—you need to move fast enough to escape Earth orbit and then there’s the Sun’s gravity to contend with. How exactly you contend with that depends on which direction you’re heading. This is where Venus and Mercury get difficult.

Movin’ on In

Let’s say you want to send a spacecraft to one of the innermost planets and put the spacecraft into orbit once you get there. So you point your spacecraft at let’s say Mercury, since that what we were chatting about last week, and you give it a little gas but not too much so that it won’t be going super fast when it arrives at Mercury.

You can get to Mercury pretty quickly this way, but good luck entering orbit. Going directly from Earth to Mercury involves a pretty direct trip down the Sun’s gravity well. Picture a ball at the top of a hill. You give it just a little nudge, providing it with only a little initial speed, but that starts it rolling down the hill. Thanks to gravity it’s going to be going pretty darned fast by the time it hits the bottom, however slow you may have started it.

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More or less the same thing will happen if you try to send your spacecraft directly to Mercury. It will pick up so much speed thanks to the increasing pull of the Sun’s gravity that by the time it gets there it will take a huge amount of energy to slow down enough to go into orbit. That’s not impossible, but it does mean your spacecraft needs to be big enough to hold enough fuel and powerful enough engines to use that fuel to slow down.

Of course now this means launching an enormous spacecraft in the first place. And that means using a bigger rocket with even more fuel to get your enormous spacecraft off of Earth. And big rockets get very expensive very quickly. Suddenly by trying to get into Mercury orbit by going directly there your mission budget has entered a new order of magnitude. Oops.

Headin’ on Out

A similar yet opposite thing can happen on the way out from the Sun. Let’s say this time you want to get your spacecraft to Saturn. You launch it with enough speed to escape Earth’s gravity and then just sit back and let it cruise, right?

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Well, there’s still the Sun to deal with. It’s pulling backwards on your spacecraft, slowing it down more and more. If its initial launch speed was high enough, it would make it to Saturn anyway, but you start encountering the issue of needing a really big rocket again to get that initial push. Frankly we don’t make a lot of rockets that could push a spacecraft of any size all the way to Saturn.

Of course, if you don’t get enough speed from your initial launch to make it all the way out to Saturn before your speed drops to zero, then your spacecraft winds up orbiting the Sun well short of Saturn, and unless you happened to pack a spare rocket engine plus a lot of fuel you’re probably out of luck.

This is where I’ll say that we have sent spacecraft directly to Jupiter before by just pointing and shooting, but those times involved putting a tiny spacecraft on a really big rocket to get that burst of initial speed. Jupiter is about as far out as you could practically manage this. It’s also generally how we send stuff to Mars, which requires far less of an initial shove from a rocket to reach than the outer solar system.

But it also means that if you want to get to the outermost planets, or the innermost for that matter, just aiming at them and going isn’t going to work. You’ll have to try something else.

Spacecraft Stealing from Planets

The solution is to not go directly there at all. Instead we use gravity assists to build up or spill off a spacecraft’s speed to make sure it can get where it needs to be. A gravity assist is actually a momentum transfer between a large body (usually but not exclusively a planet—we’ve used moons before as well) and a spacecraft to change the spacecraft’s velocity.

Let’s look again at our Saturn-bound spacecraft. And let’s say the rocket you launch it on doesn’t have enough power to get it to Saturn, but can get it as far as Jupiter. Well then the answer is to use Jupiter to accelerate your spacecraft back up to a speed that will let it reach Saturn!

If a spacecraft approaches a planet so that it looks like it’s creeping up on it, approaching the planet from the opposite direction the planet is moving, some of the planet’s angular momentum from its orbital movement will be transferred to the spacecraft as it moves through the planet’s gravity well.

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So your spacecraft sneaks up on Jupiter and winds up stealing a tiny bit of Jupiter’s momentum. Jupiter is so huge and moving so fast that the loss of this little bit of momentum makes absolutely no difference to it whatsoever. But for your significantly tinier spacecraft the increase in momentum results in a huge speed boost. You are off to Saturn!

But let’s say your rocket can’t even get you as far as Jupiter. You had to economize and go with the cheapest rocket you could get and it can only get you about halfway to Mars. Well that’s fine…bring your spacecraft back to Earth and let it do a flyby and steal some of Earth’s momentum. With this increased speed it may be able to get the farthest end of its orbit out around the asteroids now—still not far enough.

That’s okay, just dive bomb the inner solar system again and steal some more momentum from Earth. Maybe do it a third time and this time buzz Mars. It may take many years and many gravity assists, but with time you can build up your spacecraft’s speed enough that the outer edge of its solar orbit brings into the vicinity of Saturn. You have arrived! You took the scenic route, but you got there, and didn’t have to break the mint to do it.

Planets Stealing From Spacecraft

And if your target is Mercury? Well this is the beautiful thing about gravity assists, they can work both ways. If you approach the planet from behind your spacecraft steals momentum from the planet and speeds up, but if you come at the planet in a more head-on direction the planet will actually steal momentum from your spacecraft and slow it down. 

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So if you want to get to Mercury it will require taking a very long, looping path that involves a number of flights past inner planets to spill off speed. Getting the MESSENGER spacecraft into orbit around Mercury in 2011 involved launching in 2004 and then performing one gravity assist flyby of Earth, two of Venus, and three of Mercury itself before it finally approached Mercury moving slowly enough to get into orbit.

In the other direction, getting Cassini to Saturn in 2004 involved launching in 1997 and performing two gravity assist flybys of Venus, one of Earth, and one of Jupiter before it built up enough speed to make it out to Saturn’s orbit. Getting JUICE to Jupiter for 2031 required a launch in 2023, then flybys of the Moon, Earth, Venus, Earth, and then Earth again.

Taking the Scenic Route

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The downside of this is that it can take your spacecraft a whole heckuva lot longer to get where it’s going than if you flew there directly. But it makes the mission development cheaper, it means you get to take data on all the places you fly past along the way, both gathering interesting data and giving you a chance to put your spacecraft through its paces, and you do get there eventually. Never underestimate the patience of a planetary scientist with a spacecraft on its way.

As long as you get there in the end, right?