Unit 7 : Manned or Unmanned? : Round Trip or Not? : How Long? : Landing on Mars? : Plan a Mission

Manned or Unmanned?

Why haven’t we sent anyone to Mars yet? Why are all the missions unmanned? You probably already know some of the answers to this question.

First, space travel is physically hard on the human body. There are episodes of high acceleration, such as during liftoff from Earth, that cause people to feel as if they have suddenly gotten much heavier, or as if the gravity they experience has suddenly increased. (That is why we describe spaceship acceleration in units of “g”s (pronounced ‘gees’), g for gravity. One ‘gee,’ written as “1 g” is equivalent to the force of gravity at sea level here on Earth.)

While during liftoff astronauts feel very strong forces, once in space they experience microgravity – which is typically called “zero-g” or free-fall. This is where astronauts feel exactly as though they were falling and falling and falling, even though they stay in the same place inside their spacecraft. The terms “microgravity” and “zero-g” are actually a bit misleading, although they describe what the astronaut feels rather vividly. The astronaut isn’t really in zero-gravity, because Earth is pulling on the astronaut just about as much as it is on you. More accurately, the astronaut is in a circular orbit around Earth and the spaceship is in the same orbit, so they stay together and it feels as though there is no gravity. As you saw in Unit 4, they are both falling around the Earth together. So “free-fall” is a better name for the phenomenon.

Although you might not expect zero-g to be a problem, many astronauts feel sick at the start of a flight and not everyone gets over that feeling as the days pass. Also, in zero-g or free-fall, the human body changes in subtle ways. Astronauts who are up for a couple of weeks tend to get taller, to the tune of three inches. The extra height disappears after they land again. A more serious health threat is that over an extended time period (months and years), bones get weaker. While the details are not yet understood, this is probably related to the fact that people who do a lot of weight-bearing exercise are less likely to get osteoporosis (fragile bones) when they get old.

A trip to Mars that takes several months will have a significant effect on the astronauts’ bones, which might mean that seemingly minor accidents could result in broken bones. Or, they could possibly even break bones during the high-gee episodes when landing on the surface of Mars or when returning to Earth.

Another risk for humans is the exposure to radiation during the trip, and even to some extent while on Mars. There are two kinds of radiation that are damaging to humans: High energy electromagnetic radiation, and particle radiation. Electromagnetic radiation includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma-rays. Radiowaves, microwaves, infrared and visible light are low-energy forms of radiation. Microwaves heat our food by jiggling water atoms, but do not mess with our genes. Infrared radiation we feel as heat. Visible light we use to see. Ultraviolet radiation tans our skin, but also sets us up for skin cancer. X-rays allow for medical imaging but high doses also can set us up for cancer. Gamma rays can really mess with our genes or, in high doses, be pretty deadly. The Sun produces a lot of IR/visible/UV light and also some X-rays and gamma rays. The Earth's atmosphere stops much of the UV and essentially all of the X-rays and gamma rays, as well as most of the high-energy particles from space. Only on an airplane do you get a measurable amount of these forms of radiation. However, in space there is no atmosphere to protect us, and to make a spacecraft with enough shielding that is still light enough to send to Mars is not really practical. So our astronauts would be exposed to radiation in excess of what we consider safe, making cancer a possibility and also possibly interfering with them having children later. You can read a detailed report for free online at the National Academy Press.

A second problem with sending people to Mars is the cost. People are heavy (compared with modern electronics) and people require heavy support equipment, including water and air, along with some equipment to recycle water and air. Humans also need a lot of food for an extended expedition, and some means of recycling solid waste.

A third “problem” with sending people to Mars is that there are more and more tasks that robots and computers can handle as well as, if not better than, astronauts. For instance, if NASA scientists want to grind a sample of rock and conduct experiments on it, they can build a robot that lands on the surface of Mars, approaches the rock, conducts tests, and transmits those results back to Earth. This is much easier than flying astronauts to Mars with all of the needed equipment and having them conduct the same experiment. However, if the robot encounters mobile martian life (unlikely) or the results of the first experiments suggest that it would be more useful to dig for deeply buried rocks, the robot may not be equipped to handle these new tasks. Humans are really useful for handling surprises – situations that were not anticipated at the time of launch. Humans are good at improvising and are able to carry out a wide range of tasks; robots tend to be designed for more specific purposes.

Question to ponder: Can you think of a specific situation where having humans present would be an advantage over just having robots? How about a situation where it is an advantage to just have robots? Possible answers: What if there is a situation that poses a danger to the mission. Would you rather have humans, who might cope and might get killed, or robots, who are less likely to cope but a lot easier to “write off”?

A final thought: Despite many engineers’ and scientists’ desire for the space exploration process to be non-political, the group that will ultimately give approval for U.S. exploration of Mars is the U.S. Congress. If the exploration is carried out jointly with other nations, other political bodies will also be involved. These groups have different agendas than just scientific exploration. For example, having “flags and footprints” on the surface of Mars might do more to satisfy the political needs of Washington than missions that better serve the needs of the research community.


Why do we need to use "intelligent" robots rather than remote-controlled exploration devices for Mars? Answer: The time it takes radio waves to travel between Earth and Mars is several minutes at best, and up to half an hour one-way at times. A remote-controlled vehicle can get into a lot of trouble in ten minutes of travel! Hint: How far does a mars-rover move in 10 minutes, if it is moving at the speed of a slow walk (2mph)?