Getting to Mars

There have been a lot of spacecraft arriving at Mars in the last few months.

Since February, the US, China and the United Arab Emirates have all arrived there. The US and China have now landed, and the UAE will remain in orbit.

The soviets did it ages ago, of course. Here's a model of their probe.

This is all very cool, but more interesting is how they reached Mars, and why all three arrived at about the same time.

How do you get there?

I probably don't have to say this, but getting to Mars isn't as simple as hopping in your car and driving to Mildura.

One of the complexities of rocket science is that to have fuel available for manoeuvres such as landing, you have to lift that fuel off Earth's surface, and that takes ... well, fuel! How do you get to Mars using the absolute minimum amount?

You figure out the shortest way.

Mars is one planet out from the Sun. Because of this, it orbits slower than Earth. A Mars "year" is about 23 Earth months, and so by the time the Earth gets back to it's start point (wherever that is!) Mars has only gone abut half way round. If you look at points of opposition - where Earth and Mars are closest to each other - these happen only once in about 26 months.

If you want help in visualising this, try https://theskylive.com/3dsolarsystem. With a bit of tweaking you can select what you want to see, set dates and simulate the passage of time. It'll show you the positions of any planets you want.

Launch window

So, enter the "launch window". If you know how fast your craft will be going, you can figure out the best time to leave Earth. Making the journey at other times would be a waste of fuel - and besides, you'd miss Mars entirely. Launch too too late and you'll wind up trailing Mars, but launch too early, and you'll wind up ahead.

And no, in space you can't loiter, and there's no parking. Technically, if you're not in orbit around a planet, you're in orbit around the Sun, so you keep moving.

Hohmann Transfer

So, if you get your launch window right, you've got a chance to encounter Mars. But how do you actually navigate there without wasting that precious fuel? 

I don't know if you've played Kerbal Space Program, but orbital dynamics isn't simple. It's like being given your first equatorial mount. You think you're moving one way and suddenly your find yourself going the other. Whaaa?

You have to think of everything in terms of elliptical orbits, with something big at one focus. Earth and Mars are both in orbit around the Sun, and the Moon and artificial satellites are in orbit around the Earth.

What we need to do is get out of one orbit (the Earth's) and into another (Mars'). To do that, we're going to be in interplanetary space - which is to say, spend time in a third orbit around the Sun.

So how do you change your orbit? Point your craft and light the fuse, Duck Dodgers style!

What does this do? Burning fuel does not change the focus of your ellipse, but does change its eccentricity. Have a look at my scribbled diagram. You're at the red arrow, in orbit around the Earth. If you burn in the direction you're travelling while close to Earth, your orbit grows and gets more elliptical. But the orbit doesn't grow out in front of you, it grows at right angles to where you're pointed!

And if you keep burning, you'll eventually break out of Earth's sphere of influence and enter a solar orbit.

Keep burning until your orbit just reaches Mars' orbit. This is called a Hohmann transfer orbit. Now, shut down the rocket and drift. Once you get to Mars, you'll need to transfer orbits again, doing the whole thing again in reverse.

Get all this right and you'll find yourself in orbit around Mars.

Yes, this is rocket science!

Once you're in orbit around Mars, you've got a heap of alternatives. You can stay there and send images back, you can go check out Phobos (the Soviets did that in 1989), anything is possible.

Landing? Easy, it only takes seven minutes...