Questions and Answers about NASA

1. Why NASA should keep spending money on Mars missions after the recent failures?

Space missions are still not routine. There is a lot of risk involved. In fact, one of the riskiest things about Mars Lander missions is that the spacecraft can't see where it's landing! It could land on a rock and tip over, or land on a steep hill and tumble down. This is because we can't see landing sites very well from Earth (or even from Mars orbit), and it's hard to land a spacecraft exactly where we want anyway. Mission planners are lucky to get within several miles of their intended landing spot!

These risks are understood by NASA and by the committees that decide to spend money on a given mission. There are other risks as well. The rocket could explode on liftoff, the solar panels could fail to deploy in space, etc.

What NASA strives for is to minimize risk while knowing that every now and then something will go wrong.

Unfortunately *two* recent missions to Mars failed spectacularly, and although this is within the risk expectation, the public may perceive this as a failure. You can be sure however that NASA learned a lot from these two accidents, which will make the chance of success for the next missions that much greater.

Bottom line: you can't explore space without risk. You do everything you can to minimize it while keeping costs down. If a mission fails now and then, that is to be expected, and isn't a complete loss if we can learn something from it. I for one fully support space exploration. There is so much out there to discover that we must make every effort to find it.

Derek C. Richardson

2. Can you discuss ion rockets and ion engines, and how close NASA is to using them?

Chemical propulsion is still the most common technology for spacecraft use, because it has been used for so long that it is well understood and easily available.  With chemical propulsion, the fuel is burned at high temperatures in a combustion chamber, and then expelled out of a nozzle to provide the thrust.  Although massive amounts of thrust can be obtained (enough to put the space shuttle in orbit) this is a very inefficient process.  It takes a lot of fuel to provide thrust, which means that you need to carry big fuel tanks.

Ion propulsion is a relatively new technology, but it is very much of interest to NASA because it is much more efficient than chemical propulsion. It is very promising for certain missions, though it cannot be used in every situation.

In ion propulsion, atoms (currently xenon) are stripped of an electron giving them a positive charge (creating an ion, which is where the name comes from) The ions are then passed through an electronic grid that accelerates them to a very high speed (much higher than in chemical propulsion). Because of the very high speed that each ion reaches, it takes fewer of them to push the spacecraft, so less fuel needs to be carried along.

The drawback of ion propulsion is that it cannot produce large thrusts (each atom is very light, and only gives a small push to a big spacecraft). Typically, an ion engine can only produce a few grams of thrust.  That's about the weight of a piece of paper.  So this technology can't be used in cases where large thrusts are needed in a very short time - for launching a rocket from Earth's surface into orbit, for example.

However, the promise of ion propulsion comes from the fact that it can be applied for a long time.  The few grams of thrust, when used continuously for weeks or months, can change the speed of a spacecraft by thousands of miles per hour.  So ion propulsion holds a lot of promise for interplanetary missions where high speed is needed, but slow accelerations are acceptable.

Not only is ion propulsion of interest, it has already been developed and used.  The first spacecraft to use it was Deep Space 1 (DS1), which launched in 1998.  DS1 visited asteroid Braille, and then changed its orbit so that it could fly by comet Borrelly. This proved that ion engines could be used get a spacecraft to interplanetary targets.  The ion propulsion in this mission was a big success, and it has since been used in other missions, and is planned for use on others.

3. I was just reading an article about NASA's deep impact space craft and Temple 1 comet and I found out that University of Maryland is the manager of this mission.  I would like to know more about this mission? How does NASA choose who gets the "missions manager"?  What other projects is University of Maryland working on?  What other programs are you planning to work on?

Interesting questions!  For something like Deep Impact, proposals are sent to NASA for various missions to do a variety of things. A panel looks over the proposals and decides which one will most likely produce the best science for a reasonable budget.  The winning proposal then moves on, led by the principal investigator. Therefore, this is an open competition.

If you want to learn more about the University of Maryland astronomy department, check out our webpage:

If you want to know more about our theory group, look at

and for my work in particular, see

Cole Miller

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