The long cruise

We now wait until the Earth is left well behind. From now on, we  no longer want to be concerned about Earth or its gravity, and so it's worth waiting until Earth is far enough away to be ignored. Once it's gravity is only 1% of the Sun's, that is close enough to being true.

The picture above is about 12 days after the ejection burn from Earth, which is now well over 2 million kilometres away. The orbit shown on the left is no longer accurate, but shows that Earth's gravity is only 1% of the total. Our velocity relative to Earth is 2.511k - which is about 100 metres per second faster than planned.

The transfer MFD window on the right is rather reminiscent of the earlier hypothetical orbit, but now Earth's orbit is no longer part of the picture. The source is now the spacecraft ('x') rather than Earth, and the HTO is off. Since the transfer MFD does not take Earth's gravity into account, we had to wait until Earth's influence was gone before we could believe what it said. Now it tells us that the ejection burn has gone according to plan, and we are pretty much on target.

If you are flying this route too and your craft is not on target, now is a good time to correct it. This tends to be somewhat ad hoc - at least it does for me. If your orbit is too large, you can fire in the retrograde direction to reduce it. If the ellipse seems to be pointing in the wrong direction, you can point the nose of the spacecraft at the Sun and use forward or retro thrusters to correct it. Whatever size of correction you need, now is the time to do it, as correcting it later only costs more fuel. There's no need to get it to exactly the same state as the one you planned earlier - as long as the lines match on the transfer MFD, that's close enough for now.

One word of warning - on my transfer MFD, Mars and my spacecraft intersect at quite a narrow angle. It is possible to arrange for much large angles between their flight paths. Don't do this unless you want to encounter Mars at a very, very high relative speed.

Aligning orbits

The next step on our journey is aligning our orbital plane with that of Mars again. The reason we need to do this again is that we are now orbiting the Sun rather than the Earth, so our orbit now has different characteristics. Switching on the alignment MFD again, we will see something like this.

This shows us several things. Our current orbit is very similar in its inclination to Earth's orbit, which is defined as having an inclination of 0.00'. Mars's figures are a bit different, which means that our instrument is predicting a one minute burn to sort things out.

However, it is also predicting that the right time for this manoevre is some three million seconds in the future - just over a month. This is when our current orbit crosses the plane of Mars's orbit.

There are some more advanced flying techniques which can get around the need for most of this manoevre. NASA scarcely ever flies like this. But we will, for simplicity's sake.

We now need to time accelerate ourselves through the month. When it's up, the display will look like this.


Now our position is right over the ascending node - the place where our orbit crosses that of Mars in the ascending direction. Because we don't really want it to actually cross, we have selected Orbit Normal(-) autopilot. We will thrust in the descending direction, and therefore will not ascend through the plane of Mars's orbit, but align with it. That's the plan.

At this point, all that's needed is to follow the align MFD's instructions.


After the manoevre is over, the MFD will look something like this.

One of the bigger advantages of this manoevre is that it allows us to use the Sync Orbit MFD for the first time. The target is Mars, needless to say. What happens on subsequent orbits isn't very interesting. The most interesting thing is the value of DTmin. At the moment we will miss Mars by about 40 thousand seconds - about eight hours of travelling time. That's not too bad, but it could be better.

At the moment we are arriving 40 thousand seconds earlier than Mars. We can try to change that in various ways - rotating our ellipse by thrusting towards or away from the Sun, or thrusting prograde or retrograde. I tend to find that none are as useful as I might like. Keep an eye on DTmin whilst you try potential corrections and see what helps. Keep the corrections as small as possible whilst still doing the job.

It is also possible to use the transfer MFD for this manoevre - line up TLi for both yourself (source) and the target.

This is what my Sync MFD looked like after I'd finished with it. 14 seconds of error is only a few tens of kilometres - Mars is much bigger than that.

The other story that this MFD is telling us is that we won't arrive at Mars for another 10 million seconds - 116 days. Time for some more time acceleration. But we're now accurately on target - nothing will need to be done for some time.

Final approach