Why can't the Dawn Probe's engines run on hydrogen instead of xenon?

Question

Just curious.
Well to be honest, I was wondering if NASA can't build a robot space tug to go out to the asteroid belt to pick up a suitable Icy Ice asteroid to mine/process into rocket fuel and a Metal Element asteroid to mine/process-*experiemental ore processing and raw materiel fabrication.

Because, once the robot space tugs have a viable re-fueling depot (in orbit) they can go back out to get more asteroids for exploration & processing for building materials.
We would only have to send up the exotic essentials for building our future space station/platforms.

Well, then it gets more complicated of course, but I just had to ask, we gotta start somewhere,...

Thanks to those who Do know better,... cheers!!!

Answer 1

They use xenon because it has high atomic weight (~131), ionizes easily, and is inert and easy to handle. Hydrogen, being the lightest element (atomic weight ~1), wouldn't be suitable. The ion engine works by pushing out ionized atoms, the propulsive force coming from the momentum of the ion stream. So higher mass atoms give you more momentum (momentum =mass x velocity).

The idea of building and fueling spacecraft in the asteroid belt has been around for a long time. The advantage of course launching from the much lower gravity well of the asteroid belt instead of Earth. We're probably a long way from developing the technology and infrastructure to do that, but if travel to the outer planets and beyond becomes profitable, expect it to happen. A somewhat easier version of the same idea is to mine materials on the moon. The big problem there is the current lack of a source of water, though the asteroid belt may be no better.

Answer 2

You can get propellant gas from the asteroids perhaps but you can't get energy, unless of course you discover one asteroid with lots of free oxygen and another with lots of free methane. You need energy to heat the propellant and that means a chemical rocket, which sort of defeats the purpose of heading for the asteroids, or a nuclear rocket engine.

Nuclear rocket engines were tested in the 1960s and were successful, but at the moment are politically just about impossible. People scream about the small nuclear energy sources in space probes that put out a few hundred watts, if they heard a 5 megawatt reactor was to go into orbit the yelling could be heard from the Moon.

The other thing is that it is uncertain what most of the asteroids are made of. It seems probable that most of them are rocky, though some could be ices - carbon dioxide, water, methane, ammonia are likely. These all would make good rocket propellants if squirted therough a nuclear rocket engine. The idea is a good one though and has been round since the 1950s or a bit later.

Don't know why Dawn Probe uses xenon not hydrogen. Possibly has to do with the physical properties of xenon being more favorable than hydrogen. Hydrogen is a small molecule and tends to find the smallest leaks. Xenon is monatomic and larger, and may not leak so readily. It might also have to do with the ability to get it into liquid form, which may be easier with xenon as it has a much higher boiling point. Rocket engineers like nice dense propellants.

Answer 3

Well NASA is going back to the moon and the moon is a great place to start building "real" spacecraft. Without an atmosphere and gravity 16,5% of earths a spacecraft taking off from the surface of the moon could have just about any shape or size. And there is plenty of construction materials, energy and even lots and lots of oxygen. Just supplying the ISS with oxygen from the moon instead of earth would make going back to the moon a good investment for NASA. But there is that little snag of achieving propulsion on the moon. Just oxygen wouldn´t do. The hydrogen that might exist as water ice would be far too valuable to spend on using it as rocket fuel.

Ion props can´t be used on the moon at all. They generate far too little thrust to get off the surface. In space that is irrevelant as any thrust can yield enormus speed if the thrust is just achieved for long enough. As stated it uses as heavy ions as possible but the have to be inert. Any other element, such as hydrogen, would become very reactive in a superheated inonized state and could possibly corrode the engine destroying it.

Answer 4

Because we don't have metallic hydrogen.

Remember the ion engine isn't burning fuel it is charging the metallic particles and throwing them out the rear. The drive has to add extra ions to the reaction mass and it must be conductive to do that. We don't have the degenerate matter that would be metallic hydrogen or any way to create it so we only can burn hydrogen as rocket fuel. The Ion Engine doesn't burn it charges.

According to Wikipedia: http://en.wikipedia.org/wiki/Ion_thruster
"An ion thruster (more specifically an electrostatic ion thruster) is one of several types of spacecraft propulsion, specifically electric propulsion. It uses beams of ions — electrically charged atoms or molecules — for propulsion. The precise method for accelerating the ions may vary, but all designs take advantage of the charge-to-mass ratio of ions to accelerate them to very high velocities using a high electric field. Ion thrusters are therefore able to achieve high specific impulse, reducing the amount of reaction mass required, but increasing the amount of specific power required compared to chemical rockets. Ion thrusters can generally deliver one order of magnitude greater propellant efficiency than traditional liquid fuel rocket engines, but are constrained to very low accelerations by the power/mass ratios of available power sources....

In an electrostatic ion thruster, atoms of argon, mercury or xenon are ionized by exposure to electrons provided by a cathode filament. The ions are accelerated by passing them through highly charged grids. Electrons are also fired into the ion beam downstream of the grids as the positively charged ions leave the thruster. This keeps the spacecraft and the thruster beams neutral electrically. The acceleration uses up very little reaction mass (i.e., the specific impulse, or Isp, is very high). In the 1970s and 1980s, research of ion propulsion first began with cesium, but this was found to erode the grid. After that, the mid-noble gases were mainly used as a propulsion source."

According to Wikipedia: http://en.wikipedia.org/wiki/Metallic_hydrogen
"Metallic hydrogen results when hydrogen is sufficiently compressed and undergoes a phase change; it is an example of degenerate matter. It consists of a crystal lattice of atomic nuclei (namely, protons), with a spacing which is significantly smaller than a Bohr radius. Indeed, the spacing is more comparable with an electron wavelength (see De Broglie wavelength). The electrons are unbound and behave like the conduction electrons in a metal."

Why go to the asteroid belt when asteroids and comets are coming to us. In the year 2029 the metallic asteroid Apohos will approach so close to the earth that it will come in UNDER our weather satellites. In the year 2039 it will come back again with a 1:45,000 chance of hitting the earth. We could go get it and put a breaking system on it to park it into a stable earth orbit or into L-5 (where it is much more likely to stay put). Once there we can access it and add to our collection to build a space colony.

Most of the asteroids in the asteroids zone are metallic, to find an icy one you either need to go past Uranus, to Saturn's rings or raid a comet.