# Theoretical: How might we land on a planet/moon that has an orbit going in the reverse direction of Earth?

Imagine that Mars was orbiting around the sun in the opposite direction that it is now. Assuming both Mars and Earth are now travelling in opposite directions, how might we land on Mars? More specifically, what would our launch trajectory and path look like?

Translating this to more simple concepts, we can look at Earth and Mars as 2 cars travelling down a straight highway. As they exist now, Earth and Mars would be travelling on the same side of the highway at relatively similar speeds. Jumping from one moving car (while unadvised), would be relatively simple.

With this theoretical question, Earth and Mars would be travelling on opposite sides of the highway and would have greatly different relative speeds. Jumping from one moving car to the other would be extremely difficult. Is the only way to make this jump, to reduce the relative speed, that is to bring the car to a stop and then start reversing until it is reversing at the same speed?

• I assume you want to somehow orbit or land? Crashing into it should be not too difficult.
– gerrit
Apr 28 '14 at 18:04
• I would accept answers about orbiting but I am looking for answers about landing. I asked about landing in the body of my question but I added it to the title to be more clear. Apr 30 '14 at 13:13
• This doesn't answer your question, but the first few Mars missions were flybys. A flyby of your hypothetical backwards Mars would be no more difficult than the ones done in real life in the 1960s (but they would have had less time to take photos). Apr 30 '14 at 21:32

A good heat shield. Simply aim for the planet and use aerobraking. You'll hit the atmosphere at a velocity similar to the probe Galileo dropped into Jupiter--we did it then, we can do it again.

You can get home the same way.

As for the request for numbers: The Galileo probe hit Jupiter at 47km/sec. If you hit Mars at it's farthest from the sun it's orbital velocity is 22 km/sec, doubled to 44 km/sec as it's going the wrong way. It's actually a little bit less than that as it won't be going quite the 22 km/sec of a body in Mars' orbit.

The aerobrake will be tricky given how thin the Martian atmosphere is but that's not the same thing as saying it's impossible. You can pass through the atmosphere twice--make your approach on a line that will be tangent to the surface after considering the aerobraking effects. You also don't need to shed all your velocity--so long as you exit with less than 5 km/sec gravity will bring you back.

I don't believe this is possible for a manned mission but the original didn't specify that.

• Won't work for Mars. The atmosphere is too thin. You would have an extremely brief aerobraking phase with little effect, followed by a severe lithobraking phase. Apr 28 '14 at 18:52
• The ballistic coefficient you'd need would be ridiculously low. Apr 28 '14 at 18:59
• Lithobraking phase: I will reuse this word. Nice! Apr 30 '14 at 18:24
• The bounce phase of the Spirit and Opportunity rovers could be described as moderate lithobraking. Severe versions of same would work for many vibration resistant items.... albeit with more of a roll and less of a drop. May 2 '14 at 9:03
• @Loren Pechtel, which is why lithobraking can be a crash landing. It slows down the craft on impact and if that impact is too great, it will crash. May 2 '14 at 15:28

The only practical way I can think of to maybe slow down enough to land on a retrograde Mars would be to do a Jupiter flyby to mostly reverse your solar orbit direction.

• Why not approach Mars head-on, and then aerocapture into orbit? Do we have heatshields that strong? Apr 29 '14 at 9:35
• Aerocaputing into the Martian orbit may not be possible with the very high velocity and very thin Martian atmosphere Apr 29 '14 at 9:51
• Why Jupiter? Won't Earth work?
– gerrit
Apr 30 '14 at 13:15
• Earth doesn't provide any where near as much bend as Jupiter. Maybe you could do it with several Earth and/or Venus flybys. Apr 30 '14 at 13:41

The basic process of getting from Earth to normal-Mars, without any slingshot manoeuvers, is a Hohmann transfer orbit: give the probe enough velocity to put it in an elliptical orbit with perihelion at Earth's orbit, and aphelion at Mars' orbit, timing the start so that Mars is there when the probe arrives. A second burn at aphelion to circularize the orbit, matching speed with Mars, and you're done.

The only thing different to reach retrograde-Mars is that first, you need to kill the Earth's $30$ km/sec velocity, then add $30$ km/sec back in in the opposite direction, and then do all the Hohmann burns. So, you've just added a Delta-V of $60$ km/sec to your job...

• Wouldn't it be more efficient to do a normal Hohmann transfer and possibly apply the change in velocity at Mars, since its orbital velocity is significantly lower than that of Earth (according to the Loren Pechtel's answer 22 km/s). May 12 '14 at 0:14

Getting to a contra-orbiting 'Mars' can be done using a solar sail that uses the Sun to 'crank' its heliocentric orbit into a reverse direction. As mentioned, Jupiter can also be used this way so just take your pick. In either case your approach trajectory relative to the planet would be in the same direction as the planet is orbiting. The dynamics of going into orbit and landing are the same as for the real Mars.

For a solar sail mission, the time of flight would be a matter of payload mass related to sail mass and sail area with characteristic accelerations based on how close you wanted to get to the Sun during the cranking phase of the flight.

Such missions are no more challenging to a solar sail than for the real case.

The problem is similar to the never-flown Halley’s Comet rendezvous. The mission plan there involved using a close-in heliocentric orbit (.25 AU) to crank the orbit into a 145-degree inclination. Another 35 degrees and it would have been completely reversed. This cranking would have taken about 440 days. Had it been done at I AU, with only about 1/6th the acceleration available, it would have taken many years. For the present problem, the inclination would have to be 180 degrees and the sail would then have to fly a more or less ordinary, but reversed, trajectory out to the counter-Mars. Its a good thing the problem did not include a return to Earth.

• The best references for this are Dr. Robert Forward's (JPL) book 'Space Sailing' or 'Solar Sails' by Vulpetti, Johnson & Maytloff. Both are authoritative and comprehensive works. Apr 30 '14 at 16:54

Firstly current tech and widely accepted theories:

Unfortunately the key point in this question is:

Is the only way to make this jump, to reduce the relative speed

The answer is yes. It really comes down to conservation of momentum, or more specifically conservation of angular momentum.

Your spacecraft at Earth has a fixed angular momentum around the sun. A few points people often think can get around this:

1) It doesn't matter how you change the attitude of your spacecraft the angular moment stays the same.

2) If you give the spacecraft angular momentum along a different axis so that it goes around the sun in a polar orbit it still won't have the right angular momentum when it gets to 180deg from it's starting position because it still has its original angular momentum.

In essence if you wish to match velocity with retrograde Mars then you will need to reverse your angular momentum vector (and increase the magnitude).

Now the more interesting concepts that may be able to short circuit the problem:

The first unusual method that springs to mind is an Alcubierre drive, mainly due to that question being asked recently. It seems to me that you would be able to shift your spacecraft to the opposite side of the sun using such a drive. At which point the spacecraft would be moving in the same direction as retrograde Mars IF the Alcubierre drive can transport an object in one dimension (eg X direction) whilst the object has a velocity perpendicular. I don't seem any immediate reason why this type of drive wouldn't work here (or at least no more reason than it working anywhere else) so this is a possibility.

The second unusual method that springs to mind is the very sci-fi idea of wormholes. In theory a wormhole could be exist connecting opposite sides of the sub where the relative velocity of the two objects would be much closer. Similar to the Alcubierre drive in a way. A problem here is that some current thinking on wormholes is that they are very small. We're talking the same scale as the Planck length for the neck of the wormhole. Also good luck trying to create one; this patent should get you started!

• +1 for getting to the nub of the problem, the angular momentum. Now, let me get back to making my St. Clair wormhole generator: wemustknow.wordpress.com/2010/09/22/… May 2 '14 at 16:14