Rover radiation data poses manned Mars mission dilemma

Art work of humans on the surface of Mars A single mission to Mars is going to take the astronauts close to or beyond their current career limits for radiation exposure. Scientists say getting to Mars as quickly as possible would lower the risks

Nasa's Curiosity rover has confirmed what everyone has long suspected - that astronauts on a Mars mission would get a big dose of damaging radiation.

The robot counted the number of high-energy space particles striking it on its eight-month journey to the planet.

Based on this data, scientists say a human travelling to and from Mars could well be exposed to a radiation dose that breached current safety limits.

This calculation does not even include time spent on the planet's surface.

When the time devoted to exploring the world is taken into account, the dose rises further still.

This would increase the chances of developing a fatal cancer beyond what is presently deemed acceptable for a career astronaut.

Cary Zeitlin from the Southwest Research Institute in Boulder, Colorado, and colleagues report the Curiosity findings in the latest edition of Science magazine.

Cary Zeitlin discusses the RAD findings with Jonathan Amos

They say engineers will have to give careful consideration to the type of shielding that is built into a Mars-bound crew ship. However, they concede that for some of the most damaging radiation particles, there may be little that can be done to shelter the crew other than to get them to Mars and the partial protection of its thin atmosphere and rocky mass as quickly as possible.

At the moment, given existing chemical propulsion technology, Mars transits take months.

"The situation would be greatly improved if we could only get there quite a bit faster," Dr Zeitlin told BBC News.

"It is not just the dose rate that is the problem; it is the number of days that one accumulates that dose that drives the total towards or beyond the career limits. Improved propulsion would really be the ticket if someone could make that work."

New types of propulsion, such as plasma and nuclear thermal rockets, are in development. These could bring the journey time down to a number of weeks.

Curiosity travelled to Mars inside a capsule similar in size to the one now being developed to take astronauts beyond the space station to destinations such as asteroids and even Mars.

Aeroshell separates from cruise stage The rover travelled to Mars tucked inside a protective capsule. Its RAD instrument was turned on for most of the journey

For most of its 253-day, 560-million-km journey in 2011/2012, the robot had its Radiation Assessment Detector (RAD) instrument switched on inside the cruise vessel, which gave a degree of protection.

RAD counts the numbers of energetic particles - mostly protons - hitting its sensors.

The particles of concern fall into two categories - those that are accelerated away from our dynamic Sun; and those that arrive at high velocity from outside of the Solar System.

Radiation exposures comparison

  • Annual average (all sources, UK) - 2.7mSv
  • Whole-body CT scan - 10mSv
  • Nuclear power worker (annual, UK) - 20mSv
  • 6 months on the space station - 100mSv
  • 6 months in deep space - 330mSv

Source: UK HPA / Nasa

This latter category originates from exploded stars and the environs of black holes.

These galactic cosmic rays (GCRs) impart a lot of energy when they strike the human body and will damage DNA in cells. They are also the most difficult to shield against.

Earth's thick atmosphere, its magnetic field and its huge rock bulk provide protection to people living on its surface, but for astronauts in deep space even an aluminium hull 30cm thick is not going to change their exposure to GCRs very much.

The RAD data revealed an average GCR dose equivalent rate of 1.84 milliSieverts (mSv) per day during the rover's cruise to Mars. (The Sievert is a standard measure of the biological impacts of radiation.) This dose rate is about the same as having a full-body CT scan in a hospital every five days or so.

Number reassessment

Dr Zeitlin and his team used this measurement as a guide to work out what an astronaut could expect on a Mars mission, assuming he or she had a similarly shielded spacecraft, travelled at a time when the Sun's activity was broadly the same and completed the journey in just 180 days - Nasa's "design reference" transit time for a manned mission to Mars.

They calculated the total dose just for the cruise phases to and from Mars to be 660mSv. The team promises to come back with the additional number from surface exposure once Curiosity has taken more measurements at its landing location on the planet's equator.

Take a trip to Mars

Mars Rover

But even this 660mSv figure represents a large proportion of the 1,000mSv for career exposure that several space agencies work to keep their astronauts from approaching. Reaching 1,000mSv is associated with a 5% increase in the risk of developing a fatal cancer. There would likely be neurological impairment and eyesight damage as well. Nasa actually works to keep its astronauts below a 3% excess risk.

"If you extrapolate the daily measurements that were made by RAD to a 500-day mission you would incur exposures that would cause most individuals to exceed that 3% limit," explained Dr Eddie Semones, the spaceflight radiation health officer at Nasa's Johnson Space Center, who added that experts were reviewing the restriction.

"Currently, we're looking at that 3% standard and its applicability for exploration-type missions, and those discussions are going forward on how to handle that and what steps need to be taken to protect the crew."

All this should be set against the dangers associated with space travel in general, such as launching on a rocket or trying to land on another planet. It is a dangerous business.

It also needs to be considered in the context of the risks of contracting cancer during a "normal" lifetime on Earth, which is 26% (for a UK citizen).

Complex calculation

The space agencies have quite deliberately set conservative limits for their astronauts but it seems clear they would have to relax their rules somewhat or mitigate the risks in some other way to authorise a Mars mission.

Does the glory of visiting Mars outweigh the health risks?

However, the scenario for commercial ventures could be very different. Two initiatives - Inspiration Mars and Mars One - have been announced recently that propose getting people to Mars in the next 10 years using existing technologies.

Privateer astronauts that participate in these projects may regard the extra risks associated with radiation to be an acceptable gamble given the extraordinary prize of walking on the Red Planet.

Dr Kevin Fong is director of the Centre for Space Medicine at University College London, UK, and has written about the dangers associated with space exploration. He said that what Dr Zeitlin and colleagues had done was help remove some of the uncertainty in the risk assessment.

"Radiobiology is actually really tricky because how the body will respond to exposure will depend on many factors, such as whether you're old or young, male or female," he told BBC News.

"What's important about this study is that it characterises the deep space radiation environment for the first time in a vehicle whose shielding is not orders of magnitude different from that which you would expect to put a human crew inside."

Position of RAD instrument on Mars The RAD instrument continues to gather data on the surface of Mars
Jonathan Amos Article written by Jonathan Amos Jonathan Amos Science correspondent

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  • rate this

    Comment number 413.

    We knew this back in the 1970's. It was a problem that had to be dealt with. On Mars the suggestions were that the habitation would be buried below the surface to block radiation, during flight we'd use lead or similar. Some talked of solar sails acting as partial screens

  • rate this

    Comment number 412.

    They never mentioned this on Star Trek. It's a problem that needs to be addressed before we attempt a manned trip to Mars. The problem with a private company is that they may be prepared to take a risk that maybe the likes of NASA wouldn't.

  • rate this

    Comment number 411.

    6 Hours ago
    "We have worked on better composite based shielding designs, which increase mass and volume."

    You mean tin foil don't you?


    What are the advantages of tin over other materials?

  • rate this

    Comment number 410.

    Plus considering the times for a single cycle, the shortest being the Aldrin cycler at just over 2 years, the heavy cyclers would have to be in use for a very long time in order to make up for a massive delta-v budget which could otherwise be spent sending several fully equipped habitats on hohmann transfers. I think advanced propulsion will be available before the break even time

  • rate this

    Comment number 409.

    @399 Andy,

    No, they haven't.

  • rate this

    Comment number 408.

    You're talking about a cycler. No need to build it from asteroids though, as long as you can get launch costs down and assemble it from launched sections.
    Problem is the taxi vehicles to the cycler will require a much higher delta-v than would be required for a hohmann transfer, plus you'll need really good recycling of water, food-stuffs and air to make the cycler viable.

  • rate this

    Comment number 407.

    This story is nothing new. NASA published an article on it in 2005

  • rate this

    Comment number 406.

    Energy is key. Also, not building stuff on Earth. Launching something with > 30 cm thick shielding from earth is silly. Building it in orbit from asteroids, then blasting into a slingshot orbit for a few years to build up speed. Then getting astronauts on to it as it zips past (Probe Giotto did it!) and firing it off towards Mars might work (and having a similar setup to return them afterwards)

  • rate this

    Comment number 405.

    I got marked down for mentioning this little "problem" with the very silly notion of travelling to Mars before on HYS. :-D

  • rate this

    Comment number 404.

    I have read the science fiction book you mention, may I remind you that it is fiction. Have you actually seen shielding materials, read about the moon race, Van Allen Belts, etc.?
    Most proposed mars missions have a "safe room" for a reason, as properly shielding the entire human habit area will double its mass, requiring substantially more fuel to send to mars and back.

  • rate this

    Comment number 403.

    Wow these comments have been mostly unscientific.
    I have seen an example light weight shielding material, which could double the empty mass of a mars capsule.
    Nasa takes radiation and dust impacts seriously, but it is not something they discuss publicly much, out of fear of losing funding.
    Astronauts who are past the age of having children tend to be the ones to do multiple missions for a reason.

  • rate this

    Comment number 402.

    Occasional solar flares pointed in the astronauts' direction represent the only lethal levels of radiation, but fortunately it is possible to shield against them.
    Cosmic rays are not practical to shield against but represent a much smaller cumulative dose, but a few space station astronauts have already received similar cosmic rays doses with no radiological health effects.

  • rate this

    Comment number 401.

    396.Nigelpwsmith - "Solar radiation is not as serious a problem as the media suggest.............."

    This is a science thread, ergo would care to:

    A/. Put forward an arguement against solar radiation being so bad for life

    B/. Proivde the evidence to back your arguement up.


  • rate this

    Comment number 400.

    @398 David
    You miss the point, the problem is orbiting a craft with sufficient shielding to protect against high speed particles. The amount of rocket to orbit heavy shielding from Earth makes it prohibitive. You ought to read Rendezvous With Rama by Arthur C Clarke. Radiation is no longer a problem if the vehicle has very thick rock/composite walls & is big enough for a flight taking decades.

  • rate this

    Comment number 399.

    "It is about overcoming the huge challenges of space travel and proving that certain things are possible, not just pride."

    Nasa have already sent a dog to Mars so it can't be that hard to follow it up with a human.

  • rate this

    Comment number 398.

    The international space station is in low earth orbit. Please read about the Van Allen Belts. Proper shielding for any manned space mission is critical, as the farther away we go from earth, the greater the risks from radiation and small particles like dust traveling at high speeds.
    [ i.e. Damaged glass of the space shuttle, + lethal solar storm in between Apollo missions. ]

  • rate this

    Comment number 397.

    @395 Andy

    I hope you are joking, seriously. If you are, it is damn hilarious, otherwise I weep for humanity.

    But yeah, to the person who said there is no need to send humans to mars because probes do just fine, why not just not bother sending probes either?

    It is about overcoming the huge challenges of space travel and proving that certain things are possible, not just pride.

  • rate this

    Comment number 396.

    Solar radiation is not as serious a problem as the media suggest. The biggest problem is life support for a duration mission. The ISS uses solar cells to split the oxygen from water, but they still need supplies, spare parts & food. Mars expedition vehicles with heavy shielding would be far easier to make on the Moon, using mass accelerators to orbit them from lunar surface manufacturing plants.

  • rate this

    Comment number 395.

    "I have seen lightweight examples made of high density foam with layers of lead foil.."

    This is total overkill man! One layer of aluminium foil (I recommend a good brand like Alcan) is suffice. Nothing will get through coz it is metal. Lead is heavy - don't use that - it will make it harder to get you're spaceship orf the ground. Also, lead is not shiny so won't look as nice.

  • rate this

    Comment number 394.

    The lunar lander was shielded with an inadequate layer of foil.
    We have designed and tested on earth, composite shielding for both radiation and high speed particles. I have seen lightweight examples made of high density foam with layers of lead foil that are over a foot thick. But this light weight solution substantially increases the size and mass of any manned space craft.


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