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Riding an Ion Drive to the Asteroid Belt
Iron Condor writes "JPL is now close to embarking on another of its trademark, one-of-a-kind missions, this time to the heart of the asteroid belt: The Dawn mission is being prepared for launch this summer from Kennedy Space Center. Dawn will explore Ceres and Vesta, the two largest known asteroids in our solar system, which lie in the vast expanse between Mars and Jupiter. In the process, the mission will make history on several fronts. Besides being the first spacecraft to orbit a main-belt asteroid and the first to ever orbit two targets after leaving Earth, Dawn will be the first science mission powered by electric ion propulsion, the world's most advanced and efficient space propulsion technology."
From the summary: Dawn will be the first science mission powered by electric ion propulsion
s . For example, Deep Space 1 used electric ion thrusters.
No, a quick Wikipedia check says otherwise: http://en.wikipedia.org/wiki/Ion_thruster#Mission
Nasa's Deep Space 1 first used ION Propulsion several years ago. http://nmp.nasa.gov/ds1/tech/sep.html
The world isn't run by weapons anymore, or energy, or money. It's run by little ones and zeroes, little bits of data.
The propulsion is electric, as in the energy source being electricity, although some mass is still needed for the actual thrust, hence the ions.
It's not difficult to do if the two objects are in approximately the same orbital path. So, perhaps, we might soon get a probe looking at Phobos and Deimos.
Of course, most probes orbit the Earth for a while after launch, before injection into their transit path. All the early moon probes orbited both the earth and the moon, and the manned ones then orbited the earth again after return. So you could say that three targets have been orbited before, back in the '60s. This was just the project team looking for a 'first' to impress Americans with.
I suppose the difficulty levels go:
Orbit your base
Orbit your base and your target
Orbit your base, then gravity assist from other object, then orbit your target
Orbit your base, then gravity assist, then swing by one target, then orbit final target (typical multi-planet probe)
Orbit your base, then gravity assist, then orbit one target, then orbit another.
In fact, I was most impressed with the few probes which altered their trajectories in mid-mission to do swing-bys of targets of opportunity. You need flexible fuel provision to do that!
A small correction: The Apollo spacecrafts came straight in from the moon with no orbiting the Earth. See the movie Apollo 13.
Correction: Ceres is now the smallest dwarf planet.
The European Space Agency (ESA) has recently sent a satellite to the moon using ion propulsion. (http://www.esa.int/SPECIALS/SMART-1/SEMLZ36LARE_0 .html)
-- Cave quid dicis, quando, et cui
The Isp (specific impulse) on ion drives varies considerably depending on the type of drive and its operational situation. Chemical rockets are typically 300-450 sec. Ion drives (depending on how broadly you accept the term "ion drive") usually range from 1,000 to 20,000 sec. Of course, Isp isn't the only factor to consider -- you also need to consider thrust, mass (including voltage regulation hardware), and efficiency.
:) The "dusty" part comes from the reactor design. The fuel is nanoscale particles of enriched uranium mixed with graphite. Fission reactions automatically ionize the particles, so to keep them suspended at an even distance from each other, all you have to do is ionize the walls of the reactor. Since the particles are so small, their surface area to volume ratio is incredibly large, making radiative cooling realistic. The radiated heat itself can be turned into extra electricity (and even a bit of extra thrust)
There are some really neat drives on the horizon that combine the best of thermal and ion drives, such as VASIMR. The particles are heated with radio waves to extreme temperatures (like in some fusion apparatuses), but since they're ionized, they're affected by magnetic fields. The fields collaminate them into a spiralling plasma, converting their chaotic energy distribution into a directed flow. A magnetic nozzle then redirects this out the back. Moderate thrust plus high ISP -- a nice combination.
My favorite "long range" design is the dusty fission fragment rocket. Most of the energy of fission reactions is contained in "fission fragments" -- basically, the fission of your fissionable fuel blasts microscopic fragments of the fuel at high speeds. In a normal reactor, these bump into the rest of the fuel or the moderator and are "thermalized"; the heat is then converted into electricity lossily. In a fission fragment reactor, the design is such that the fragments (where are inherently ionized) are allowed to escape the core; they can then be A) decelerated to produce electricity, or B) redirected with a magnetic field and vented out the back to produce a ridiculously high ISP thrust. You can do that with ionized particles.
Nothing says 'welcome to the neighborhood' like a gunny sack full of dead squirrels.
It's all about momentum:
p = m * v
p = momentum, v = velocity, m = mass
Yeah, an ion drive is just spitting out...well...ions. As there's negligible gravity the further your distance from any large body, you don't have (noticeable) friction / angular acceleration to slow you down. Spitting out a single ion might not get you moving much faster, and it might not give you a lot more momentum, but it is still a measurable effect.
Now spit out trillions upon trillions of ions... It's moved from being a measurable effect to cheap (not monetarily, I'm referring to energy expenditure) and effective means of moving in space.
Specific impulse (Isp) is a very real, very important term in rocketry. The standard unit is "seconds" (sec), and is the velocity of the exhaust divided by the acceleration of gravity at the surface of the Earth (9.81m/s^2). Isp and thrust are the two most cited stats for rocket engine performance.
:)
Of course, the velocity of the exhaust itself is more useful in calculations (who needs Earth's gravity factored in?), but that's just how things go.
Nothing says 'welcome to the neighborhood' like a gunny sack full of dead squirrels.
The complaints are either obsolete or misguided.
Misguided: they assume Orion launching from the surface, not in space beyond effective EMP range. Most modern proposals call for a space launch.
Obsolete: We now do have the technology to initiate fusion chain reactions on even a miniscule amount of fusion fuel (antimatter-initialized microfusion, AIM)
Of course, Orion itself is an obsolete design, having been replaced by Medusa. Every part of the design (system mass, ability to scale down, specific impulse, radiation exposure, etc) works better than in the classical Orion design.
Nothing says 'welcome to the neighborhood' like a gunny sack full of dead squirrels.
There's several different types of ion engies. The Dawn mission is using electrostatic ion thrusters of the same design from the Deep Space 1 mission. The Smart-1 mission, and I'm pretty sure the Hayabusa mission as well, used Hall effect ion thrusters. The differences are small and each has a few advantages and disadvantages. So the submitter would have technically been more correct if they said "electrostatic" instead of "electric." The parent's link also has links to descriptions of each type.
Additionally:
Deep Space 1 was primarily a technology demonstrator mission. It carried a few instruments, but these were also prototypes.
Smart-1 was a combined technology demonstrator / science mission.
Hayabusa has both an ion thruster and hypergolic chemical rockets.
Actually having worked for the company http://www.spectrumastro.com/ that built Deep Space 1 http://nmp.nasa.gov/ds1/tech/sep.html. It has been in use as a main propuslion system since 98. There was a 'gas' tank that held xenon that was the source of the propulsion.