There are several ways to go to Mars, with Mars at different stages in its orbit relative to Earth. There's also a new idea, ballistic transfer, that could be used to launch to Mars at any time, wherever it is in its orbit.
The usual method is Hohmann transfer, where you launch when Mars is about 45 degrees ahead of us. Happens once every 26 months.
Takes around seven months for the spacecraft to catch up with Mars in a minimum energy transfer (varies depending on the mission). This is the usual method for unmanned spacecraft as they don't need to come back again.
For a crewed mission, in one example from 2008, they would spend 516 days there. Takes 235 days to get there and 191 days to return (these numbers vary depending on the mission).
This is the best one if you want to spend a long time on Mars or in Mars orbit or on Phobos or Deimos before return to Earth.
Arrival and return trajectory:
This is known as conjunction class, because it arrives on Mars when it is in conjunction (the other side of the sun) relative to the position Earth was when the spacecraft was launched.
A variation on this is Robert Zubrin's double Athena, where the first flyby is timed to take you into an orbit almost like Mars, you then fly "parallel" to Mars for half a Mars year gradually drifting away from it, then back again - then the second flyby returns you to Earth. This is a "free return trajectory" that needs no delta v adjustment - if you launch from Earth at the right time and with the right thrust, you will return around 700 days later.
The opposition class route starts with Mars 75 degrees ahead in its orbit.
If you want to return you have to do so within 20 days, and it takes 253 days to go back involving a spiral inside of Earth's orbit, possibly a flyby of Venus with gravity assist. That involves more total delta v but has advantage for crew of less total mission time. In this case 450 days (the numbers here will vary)
This is called opposition class - because Earth and Mars are in opposition (same side of the sun) at some point during the journey.
A variation on this is the free return, as used for Inspiration Mars which gets you back to Earth with no extra delta v needed during the mission, like Robert Zubrin's double athena, but only is available rarely as it depends on Venus being in the right place in its orbit for the return mission.
That lets you go to Mars for a flyby and back with only 500 days total mission time, but the flyby itself gives you only an hour or so when you are really close to Mars. It seems a long journey for such a short time at Mars. Though you do get a flyby of Venus as well.
Though it is a shorter mission, it does have some disadvantages
Advantages of opposition class missions include.
It might be safer than the conjunction class mission. But is the reduced mission time at Mars worth that, and should you launch anyway if there is a significant difference in safety levels between 500 and 700 days? Do we have adequate safety margins if 700 days is significantly more dangerous for the crew than 500 days?
A recent study in 2014 looked at the disadvantages and advantages and suggested that perhaps this type of mission shouldn't be dismissed as readily as it has in the past, and that it has some advantages as stepping stones towards longer duration missions.
See also: Benchmarks: Getting to Mars and Trades Between Opposition and Conjunction Class Trajectories for Early Human Missions to Mars
and A Crewed Mission to Mars...
and Trajectory Options and Abort Scenarios for Human Missions to Mars
There's another very new idea though, called "ballistic transfer" currently being explored. This is work in progress but the preliminary results of the studies so far are promising.
With this method, then according to the paper, it can be used at any time, no matter what position Mars is relative to Earth.
The idea is that you send it to an orbit with aphelion a fair way ahead of Mars in its orbit, and then Mars captures it into a very high orbit around Mars with no fuel at all - it happens automatically - and without the need for an insertion burn to apply thrust at just the right moment of time.
This method has already been used for the Moon several times. Technically what happens is that the v∞ of your spacecraft relative to the Moon changes from positive to negative due to three body interactions (something that would be impossible if you study it as patched together two body interactions).
This was used in 1991 by the Hiten spacecraft from Japan, using a ballistic transfer that takes the spacecraft beyond the Moon first, before it is captured. The same method was used by NASA's Grail mission in 2011. Another type of ballistic transfer was used by ESA's Smart 1 mission in 2004, called an interior transfer, captured without the spacecraft ever going further than the Moon in its orbit - this uses less delta v than the exterior transfer.
So, it turns out, the same approach theoretically probably works for Mars, that if you position it just right, with just the right delta v from Earth, your spacecraft ends up getting captures into a very distant orbit around Mars with no need for an insertion burn there at all.
That may be especially useful if you want to go into orbit around Mars. For a landing on Mars, then you can use the Mars atmosphere to slow down. But for Mars orbit, unless you use risky aerobraking, you need a Mars insertion burn usually. So this avoids that.
Many of the news stories said it is potentially a 25% saving in delta v but if you look at the details it's not actually a fuel saving in total for most missions.
The thing is that you end up in such a high orbit around Mars - so you still need the delta v to get into a low orbit. There's a big saving doing your insertion burn close to Mars because of the Oberth effect.
The ballistic transfer method is only lower delta v than a normal Hohmann transfer + insertion burn in low orbit around Mars if you want to target a high orbit there. If you want to get even to its outermost moon Deimos, then the insertion burn uses slightly less total delta v because of the Oberth effect (for Deimos, the difference is negligible, but for low mapping orbits or Phobos then the Oberth effect makes Hohmann transfer significantly better).
But it is safer because as soon as it leaves Earth on the right trajectory, you know it will get captured by Mars, even if you don't do anything more. So if there is any issue with your rocket motors when you get to Mars, you can deal with them at your leisure, don't have to have them functional for one particular date for a time critical insertion burn. If there is some last minute issue and you aren't ready to fire it yet, no problem.
It's also good because you can use ion thrusters so need to take less reaction mass. You don't need high thrust because you don't have to do an insertion burn. Instead you can gradually spiral down to the lower orbit over periods of weeks, or months. So, though there is more total delta v, it may actually use less fuel in total (because ion thrusters need less fuel for same amount of thrust).
It takes a few months longer than a Hohmann transfer orbit - but unlike Hohmann transfer you can also do it any time, not limited to a launch window once every two years.
That also seems a lot safer for planetary protection. Which is a top priority for scientists, especially exobiologists, and is mandated under the Outer Space Treaty. When you get to Mars, you wouldn't want to find life there, potentially one of the greatest discoveries in biology of all time, only to find that it is just life that you brought there yourself.
So, if a human crew okays the impulse approach are willing to take the extra risk to get there faster, you still have the extra question - is whatever level of risk is involved acceptable for planetary protection? And this is a risk taken on behalf of all the world, anyone interested in the potential discovery of life on Mars.
The way that you send orbital missions to Mars at present involves a trajectory biased away from Mars, then a single burn to put the spacecraft into orbit around Mars. That burn could continue for too long, and then put the rocket into a collision course with the surface.
That actually happened with the Mars Climate Orbiter impacting Mars (due to mix up of SI and imperial units). Which had planetary protection implications as it was only sterilized at the level appropriate for an orbital mission.
So as someone who thinks a lot about planetary protection, then this orbit seems especially appealing. Especially for human missions, where a hard impact on Mars is not only a risk for the crew (who can choose whether or not to accept that risk - at present every spaceflight has a measure of risk that is surely higher than an airplane flight or car trip even on our safest spacecraft the Soyuz) - it could potentially lead to the planet being irreversibly contaminated with Earth life (if there are habitats for Earth life there).
With a ballistic transfer, then there is no chance at all of an aerobraking maneuver going wrong or an insertion thrust leading to your spacecraft impacting Mars. Especially if your aim is Mars orbit or Phobos or Deimos this seems the safest approach from point of view of planetary protection as well as for the crew.
For details see A New Way to Reach Mars Safely, Anytime and on the Cheap (Scientific American). And Making the Trip to Mars Cheaper and Easier: The Case for Ballistic Capture (Universe Today). The technical article is Earth--Mars Transfers with Ballistic Capture
See also To Explore Mars With Likes Of Occulus Rift & Virtuix Omni - From Mars Capture Orbit, Phobos Or Deimos where I talk a bit about the advantages of ballistic transfer for planetary protection.
Thanks to Hop David for discussion back in May / June when the ballistic mission idea story broke in the news, pointing out that ballistic transfer is not a delta v saving in total for low orbits around Mars, see Hop's Blog: EML2 and discussion on Will NASA's Sample Return Answer Mars Life Questions? Need For Comparison With In Situ Search
As usual if you see anything to fix in this answer, don't hesitate to say in the comments.
HOHMANN TRANSFER - CONJUNCTION CLASS
The usual method is Hohmann transfer, where you launch when Mars is about 45 degrees ahead of us. Happens once every 26 months.
For a crewed mission, in one example from 2008, they would spend 516 days there. Takes 235 days to get there and 191 days to return (these numbers vary depending on the mission).
This is the best one if you want to spend a long time on Mars or in Mars orbit or on Phobos or Deimos before return to Earth.
Arrival and return trajectory:
This is known as conjunction class, because it arrives on Mars when it is in conjunction (the other side of the sun) relative to the position Earth was when the spacecraft was launched.
ZUBRIN'S FREE RETURN DOUBLE ATHENA - 700 DAYS MISSION
A variation on this is Robert Zubrin's double Athena, where the first flyby is timed to take you into an orbit almost like Mars, you then fly "parallel" to Mars for half a Mars year gradually drifting away from it, then back again - then the second flyby returns you to Earth. This is a "free return trajectory" that needs no delta v adjustment - if you launch from Earth at the right time and with the right thrust, you will return around 700 days later.
OPPOSITION CLASS - LESS TIME AT MARS BUT LESS TOTAL MISSION TIME
The opposition class route starts with Mars 75 degrees ahead in its orbit.
If you want to return you have to do so within 20 days, and it takes 253 days to go back involving a spiral inside of Earth's orbit, possibly a flyby of Venus with gravity assist. That involves more total delta v but has advantage for crew of less total mission time. In this case 450 days (the numbers here will vary)
FREE RETURN INSPIRATION MARS - CROCCO GRAND TOUR
A variation on this is the free return, as used for Inspiration Mars which gets you back to Earth with no extra delta v needed during the mission, like Robert Zubrin's double athena, but only is available rarely as it depends on Venus being in the right place in its orbit for the return mission.
That lets you go to Mars for a flyby and back with only 500 days total mission time, but the flyby itself gives you only an hour or so when you are really close to Mars. It seems a long journey for such a short time at Mars. Though you do get a flyby of Venus as well.
Gaetano Crocco who first suggested the idea of a very short opposition class human mission to Mars using flybys of Mars and Venus in 1956. Crocco Grand Tour See also wikipedia entry: Gaetano Crocco Grand Tour. A similar idea is the basis for the Inspiration Mars proposal.
IS THIS TRADE OFF OF A SHORTER MISSION TIME WORTH THE REDUCTION IN TIME AT MARS?
Though it is a shorter mission, it does have some disadvantages
- Less science return (much less time spent at Mars)
- it uses more delta v.
- it has a far higher velocity relative to Earth when it returns to Earth making Earth capture using aerobraking more difficult - you hit the atmosphere too fast for a normal aerocapture and would need to look into other methods.
- Also the closer approach to the Sun, as close as Venus is an additional slight increase in hazard over the conjunction class mission
Advantages of opposition class missions include.
- Less time spent in zero g (unless artificial gravity is used) for health
- Less exposure to radiation
- Fewer supplies needed for the crew
It might be safer than the conjunction class mission. But is the reduced mission time at Mars worth that, and should you launch anyway if there is a significant difference in safety levels between 500 and 700 days? Do we have adequate safety margins if 700 days is significantly more dangerous for the crew than 500 days?
A recent study in 2014 looked at the disadvantages and advantages and suggested that perhaps this type of mission shouldn't be dismissed as readily as it has in the past, and that it has some advantages as stepping stones towards longer duration missions.
"Short duration opposition class trajectories have largely been dismissed as a viable alternative for human mission to Mars due to concerns over the increased propulsive requirements, short destination durations, and architecture extensibility. However, a more comprehensive analysis indicates that the relative penalties associated with opposition concepts may not be as severe as originally anticipated. Furthermore, short-stay missions could represent a stepping-stone to long-duration missions, reducing overall campaign risk and allowing for a more progressive build-up of capabilities. The investigation of propulsive requirements, element delivery, and mission risk provides a deeper understanding of the advantages and trade-offs associated with opposition trajectories, presenting new arguments towards their viability as part of a larger exploration campaign."
Trades Between Opposition and Conjunction Class Trajectories for Early Human Missions to Mars
See also: Benchmarks: Getting to Mars and Trades Between Opposition and Conjunction Class Trajectories for Early Human Missions to Mars
and A Crewed Mission to Mars...
and Trajectory Options and Abort Scenarios for Human Missions to Mars
BALLISTIC TRANSFER
There's another very new idea though, called "ballistic transfer" currently being explored. This is work in progress but the preliminary results of the studies so far are promising.
With this method, then according to the paper, it can be used at any time, no matter what position Mars is relative to Earth.
The idea is that you send it to an orbit with aphelion a fair way ahead of Mars in its orbit, and then Mars captures it into a very high orbit around Mars with no fuel at all - it happens automatically - and without the need for an insertion burn to apply thrust at just the right moment of time.
This method has already been used for the Moon several times. Technically what happens is that the v∞ of your spacecraft relative to the Moon changes from positive to negative due to three body interactions (something that would be impossible if you study it as patched together two body interactions).
This was used in 1991 by the Hiten spacecraft from Japan, using a ballistic transfer that takes the spacecraft beyond the Moon first, before it is captured. The same method was used by NASA's Grail mission in 2011. Another type of ballistic transfer was used by ESA's Smart 1 mission in 2004, called an interior transfer, captured without the spacecraft ever going further than the Moon in its orbit - this uses less delta v than the exterior transfer.
So, it turns out, the same approach theoretically probably works for Mars, that if you position it just right, with just the right delta v from Earth, your spacecraft ends up getting captures into a very distant orbit around Mars with no need for an insertion burn there at all.
That may be especially useful if you want to go into orbit around Mars. For a landing on Mars, then you can use the Mars atmosphere to slow down. But for Mars orbit, unless you use risky aerobraking, you need a Mars insertion burn usually. So this avoids that.
BALLISTIC TRANSFER NOT NECESSARILY A SAVING IN DELTA V
Many of the news stories said it is potentially a 25% saving in delta v but if you look at the details it's not actually a fuel saving in total for most missions.
The thing is that you end up in such a high orbit around Mars - so you still need the delta v to get into a low orbit. There's a big saving doing your insertion burn close to Mars because of the Oberth effect.
The ballistic transfer method is only lower delta v than a normal Hohmann transfer + insertion burn in low orbit around Mars if you want to target a high orbit there. If you want to get even to its outermost moon Deimos, then the insertion burn uses slightly less total delta v because of the Oberth effect (for Deimos, the difference is negligible, but for low mapping orbits or Phobos then the Oberth effect makes Hohmann transfer significantly better).
In detail, in section 6, (of Earth--Mars Transfers with Ballistic Capture), they find that ballistic transfer is better than Hohmann transfer for orbits down to a periapsis radius of a little under 30,000 km. That's twice the distance of Deimos.
For the actual savings summarized in table 3, they say saving is about 25% if you want to transfer to an orbit that has closest point to Mars of 200,000 km (about ten times further away than Deimos) of course rare that you want to do that.
The news stories have summarized this as saying that the ballistic transfer gives a 25% saving over Hohmann, but for a close orbit, the Hohmann gives a saving over ballistic.
BUT SAFER, AND MAY NEED LESS FUEL
But it is safer because as soon as it leaves Earth on the right trajectory, you know it will get captured by Mars, even if you don't do anything more. So if there is any issue with your rocket motors when you get to Mars, you can deal with them at your leisure, don't have to have them functional for one particular date for a time critical insertion burn. If there is some last minute issue and you aren't ready to fire it yet, no problem.
It's also good because you can use ion thrusters so need to take less reaction mass. You don't need high thrust because you don't have to do an insertion burn. Instead you can gradually spiral down to the lower orbit over periods of weeks, or months. So, though there is more total delta v, it may actually use less fuel in total (because ion thrusters need less fuel for same amount of thrust).
It takes a few months longer than a Hohmann transfer orbit - but unlike Hohmann transfer you can also do it any time, not limited to a launch window once every two years.
PLANETARY PROTECTION ADVANTAGES OF BALLISTIC TRANSFER
That also seems a lot safer for planetary protection. Which is a top priority for scientists, especially exobiologists, and is mandated under the Outer Space Treaty. When you get to Mars, you wouldn't want to find life there, potentially one of the greatest discoveries in biology of all time, only to find that it is just life that you brought there yourself.
So, if a human crew okays the impulse approach are willing to take the extra risk to get there faster, you still have the extra question - is whatever level of risk is involved acceptable for planetary protection? And this is a risk taken on behalf of all the world, anyone interested in the potential discovery of life on Mars.
The way that you send orbital missions to Mars at present involves a trajectory biased away from Mars, then a single burn to put the spacecraft into orbit around Mars. That burn could continue for too long, and then put the rocket into a collision course with the surface.
That actually happened with the Mars Climate Orbiter impacting Mars (due to mix up of SI and imperial units). Which had planetary protection implications as it was only sterilized at the level appropriate for an orbital mission.
So as someone who thinks a lot about planetary protection, then this orbit seems especially appealing. Especially for human missions, where a hard impact on Mars is not only a risk for the crew (who can choose whether or not to accept that risk - at present every spaceflight has a measure of risk that is surely higher than an airplane flight or car trip even on our safest spacecraft the Soyuz) - it could potentially lead to the planet being irreversibly contaminated with Earth life (if there are habitats for Earth life there).
With a ballistic transfer, then there is no chance at all of an aerobraking maneuver going wrong or an insertion thrust leading to your spacecraft impacting Mars. Especially if your aim is Mars orbit or Phobos or Deimos this seems the safest approach from point of view of planetary protection as well as for the crew.
For details see A New Way to Reach Mars Safely, Anytime and on the Cheap (Scientific American). And Making the Trip to Mars Cheaper and Easier: The Case for Ballistic Capture (Universe Today). The technical article is Earth--Mars Transfers with Ballistic Capture
See also To Explore Mars With Likes Of Occulus Rift & Virtuix Omni - From Mars Capture Orbit, Phobos Or Deimos where I talk a bit about the advantages of ballistic transfer for planetary protection.
Thanks to Hop David for discussion back in May / June when the ballistic mission idea story broke in the news, pointing out that ballistic transfer is not a delta v saving in total for low orbits around Mars, see Hop's Blog: EML2 and discussion on Will NASA's Sample Return Answer Mars Life Questions? Need For Comparison With In Situ Search
As usual if you see anything to fix in this answer, don't hesitate to say in the comments.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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