Getting to Mars

Ken Tapping, May 1, 2013

In the sky this week…

  • Jupiter is still unmissable, but is getting low in the western sky in the evening.
  • Saturn rises before sunset and is well up in the east by dark.
  • The Moon will reach Last Quarter on 2nd May.

Hopefully, before too long, astronauts will be on their way to Mars, the Red Planet, to see first hand what the robot explorers are telling us. Having people on the spot will make it possible to change our research programmes depending upon what they encounter. One manned mission will do the work of a large number of rovers.

When we set out to drive somewhere, we follow the planned route, making speed and direction adjustments as we go, until we arrive. Getting there by plane or ship is not very different. However, space travel, at least using technologies now available, is something else entirely.

Imagine a friend has thrown a ball. After a second or so we throw a small stone so that it gently touches the ball in mid-flight. This is very difficult; the timing, speed and direction have to be just right, with very little room for error. Because errors are inevitable, we can improve the situation by putting a tiny rocket motor on the stone, so that small course corrections are possible, and to minimize the speed with which the objects make contact. However, we cannot make the stone too heavy to throw, so that rocket motor cannot be too big and not much fuel can be carried. Therefore we cannot afford large launch errors. How we navigate the Solar System is very much like this, with the stone representing our spacecraft and the ball our destination planet.

A possible mission to Mars could be like this. Large rockets lift into orbit the components for possibly three spacecraft. These would be parked near the International Space Station (ISS), where they will be assembled. Multiple spacecraft will reduce the chance of astronauts being stranded in space or on the surface of Mars by equipment failures during a possibly multi-year mission. There would be astronauts on each spacecraft, but extra seats and accommodation will be included, just in case everyone has to come home in two or perhaps one of the vehicles. Because everything has to be lifted into orbit from the surface of the Earth keeping down the total weight of the spacecraft is important. Similarly, the heavier the spacecraft, the more fuel we need for propulsion, course corrections and landing, and inevitably that adds more weight.

At exactly the right time the spacecraft engines are started, throwing the flotilla onto its trajectory to Mars. We accelerate to the right speed, in the right direction. Then we shut down the engines and for some months we “fall to Mars”.

As we approach the Red Planet, its gravity starts to tug at our spacecraft, accelerating us towards our destination. We have to get rid of this speed, or crash. Fortunately Mars has an atmosphere, so we can use “air drag” to slow us down. Then we can use either parachutes or wings to control our descent. However, Mars’ atmosphere is too thin for these to give us a sufficiently gentle landing, so we use rocket motors and fuel to set us down softly on the Martian Surface,. Then we can argue among ourselves as to who gets to make the first footprints. That is, other than any that might have been made beforehand by the locals.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mailken.tapping@nrc-cnrc.gc.ca

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