The revolutionary ion engine that took spacecraft to Ceres

March 9, 2015 by Steve Gabriel, The Conversation
Inside an ion engine. Credit: NASA

The NASA spacecraft Dawn has spent more than seven years travelling across the Solar System to intercept the asteroid Vesta and the dwarf planet Ceres. Now in orbit around Ceres, the probe has returned the first images and data from these distant objects. But inside Dawn itself is another first – the spacecraft is the first exploratory space mission to use an electrically-powered ion engine rather than conventional rockets.

The ion engine will propel the next generation of spacecraft. Electric power is used to create charged particles of the fuel, usually the gas xenon, and accelerate them to extremely high velocities. The exhaust velocity of conventional rockets is limited by the chemical energy stored in the fuel's molecular bonds, which limits the thrust to about 5km/s. Ion engines are in principle limited only by the electrical power available on the spacecraft, but typically the exhaust speed of the charged particles range from 15km/s to 35km/s.

What this means in practice is that electrically powered thrusters are much more fuel efficient than chemical ones, so an enormous amount of mass can be saved through the need for less fuel onboard. With the cost to launch a single kilogramme of mass into Earth orbit of around US$20,000, this can make spacecraft significantly cheaper.

This can be of great benefit to commercial manufacturers of geostationary satellites, where electric propulsion can allow them to manoeuvre adding new capabilities to the satellite during its mission. However, for scientific missions such as interplanetary travel to the outer regions of the Solar System, electric propulsion is the only means to carry useful scientific payload quickly across the enormous distances involved.

Electric space power

There are three broad types of electric propulsion, depending on the method used to accelerate the fuel.

The Dawn spacecraft, equipped with large solar panels to power its electrical engine. Credit: NASA

Electrothermal engines use electric power to heat the propellant either by passing a current through a heating element, a configuration known as a resistojets, or by passing a current through the hot ionized gas or plasma itself, an arcjet.

Electromagnetic engines ionise the propellant by turning it into an electrically conductive plasma, which is accelerated via the interaction of a high electrical current and a magnetic field. Known as pulsed plasma thrusters, this technique is in fact quite similar to how an electric motor works.

Electrostatic engines use an electric field generated by applying a high voltage to two grids perforated with many tiny holes to accelerate the propellant, called a gridded ion engine, which is what powers Dawn. Another electrostatic design is the Hall effect thruster, which operates in a similar fashion but instead of high voltage grids generates an electric field at the thruster's exit plane by trapping electrons in a magnetic field.

Half a century in the making

The concept of electric propulsion has been around for 50 years or more, but was deemed too experimental to commit to major projects. Only now is it beginning to find real applications. For example, keeping geostationary satellites in their correct orbit, to counteract the aerodynamic drag from the very tenuous atmosphere 200km above the Earth. Or interplanetary missions such as Deep Space 1 – the first experimental mission to use , it was originally intended as a technology demonstrator but performed a successful fly-past of the asteroid 9969 Braille and the comet Borrelly 15 years ago.

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Another very successful mission using ion engines was the ESA Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite which for four years until 2013 was able to map in unprecedented detail the Earth's gravity field.

Future designs

Now that electric spacecraft engines have entered mainstream use, they look set to reduce the cost of deploying satellites. With compact ion engines onboard, satellites can raise themselves from low Earth orbit to their final geostationary orbit under their own power. This will save enormous amounts of fuel required to lift the satellite through conventional chemical rockets, and allow the use of much smaller launch vehicles which will save a lot of money. Boeing was the first off the blocks in 2012 with an all-electric version of their 702 platform satellite fitted with xenon-powered gridded ion engines, and other satellite manufacturers are following suit.

Currently all designs use xenon gas as the propellant, but the search is on for alternative propellants since xenon is enormously expensive and in limited supply. But electrical power is here to stay, and over the longer term, space tugs and even manned missions to Mars based on nuclear will be the next on the drawing board.

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5 / 5 (7) Mar 09, 2015
keeping geostationary satellites in their correct orbit, to counteract the aerodynamic drag from the very tenuous atmosphere 200km above the Earth.

Geostationary is at about 36000km, not 200km. It's low earth orbit sattelites that experience atmospheric drag.
1.3 / 5 (6) Mar 09, 2015
Antimatter Propulsion with Ablation

A spacecraft using a linear particle accelerator to create matter antimatter pairs having the
highest energy density of any know materials will be used as propulsion to propel a
vehicle through the Sun's Solar system and beyond. Quantum electrodynamics (QED)
critical field strength of an electron rest mass of 2 times can easily be reached by the use
of a terawatt chirped pulse amplified (CPA) laser causing a dramatic effect in the
conversion of light into matter antimatter via electron / multiphoton reaction. Under current proposals, the annihilation of matter with antimatter is a BILLION TIMES more efficient
that the oxygen-hydrogen combustion in the Space Shuttle's main engines, and about 100 times more than fission or fusion reactions. Antimatter offers the greatest specific impulse of any propellant currently available or in development, and its thrust-to-weight ratio is still comparable with that of chemical propulsion.
5 / 5 (9) Mar 09, 2015
actually, holoman, matter/antimatter is wildly _inefficient_, particularly when the antimatter is created onboard. The creation of antimatter by necessity requires more energy than is released in its annihilation. (due to efficiency losses in the system creating them)

Then, when they annihilate, the ship must convert, at best, two extremely high energy photons, emitted back-to-back into forward momentum. This means some thick radiation absorbing medium to absorb the "forward" travelling photon, and letting the other escape to space. But there's no guarantee that the "forward" photon will be fully "forward." The directions they're emitted are random, so you lose more momentum for every off-axis annihilation.

Finally, if you use something like hydrogen/anti-hydrogen reactions, the energy can be carried away by neutrinos which don't assist your ship in propulsion in any way.
5 / 5 (5) Mar 09, 2015
They will still need chemical rockets to get above the atmosphere, so we can still get the light, sound, and shaking. My first night at Edwards AFB was interrupted by heavy shaking, and I got out of bed thinking "earthquake!", ran out the door, down the fire escape, and into the desert in my skivvies as the ground shook and my insides as well.

I then saw the glow across the lakebed to Rocket Site, and realized it was our first full-powered test of the 1,500,000 pound-thrust F-1, which was to take us to the Moon.

I was the only one out there, so nobody noticed.
1.1 / 5 (7) Mar 09, 2015

I disagree with your comment.

5 / 5 (3) Mar 09, 2015
The main problem with electrical engines is not the fuel, but where to get the energy that they need. Dawn is using giant solar panels, and that formula can not be scaled up due to excessive weight. With the current technology, only nuclear energy could solve the problem.
5 / 5 (4) Mar 09, 2015

Where did he go wrong? Be precise.

5 / 5 (2) Mar 09, 2015
My understanding of ion propulsion is that takes a long time to accelerate and equally long to decelerate. Did they need to decelerate here or was Ceres' gravity enough to capture it at speed?
5 / 5 (3) Mar 09, 2015
Adam, I understand they decelerate over a long period to get captured, then to get the orbit down.
Mar 09, 2015
This comment has been removed by a moderator.
4.5 / 5 (8) Mar 09, 2015
Joe_Chang: Do you fit the definition of 'troll'? Save the harangue for a different sort of site.
4.2 / 5 (10) Mar 09, 2015
Joe_Chang, keep your self promotion out of here please. Thank You.
Mar 09, 2015
This comment has been removed by a moderator.
Mar 09, 2015
This comment has been removed by a moderator.
not rated yet Mar 09, 2015
It may be possible to use another kind of linear accelerator technology to power the next generation of ion thrusters.

Perhaps a reaction motor based on plasma wakefield acceleration using lasers may offer extremely high specific impulses.
5 / 5 (1) Mar 09, 2015
Ion engines or not they're still using good old Newton law and momentum conservation. Nothing really new, along lines of GR or something similar yet on horizon. The energy problem was solved with mini nuclear reactors installed on Voyager I and II in 1970-ies later banned.
3.7 / 5 (3) Mar 10, 2015
"the force we use to propulsion is em force, it has a speed limit c. ftl traveling is impossible." So whose talking about FTL, Joe? Not the article, and not the comments as far as I can see. I guess we should be thankful that you finally made a post that was at least *somewhat* relevant to the topic under discussion.
Keep spaming your same links in every thread and you'll soon go away. See that "Report" button there? That gets pushed every time you do that, and pretty soon you go the way of the movementiseternal moron who couldn't figure that out. He got axed a week or two ago, and you're well on your way. You don't make friends by repeatedly spamming your website, even if it *isn't* total bullshit.

Mar 10, 2015
This comment has been removed by a moderator.
not rated yet Mar 10, 2015

Nice !
5 / 5 (1) Mar 14, 2015

I would point out that if you're generating an electron beam with 1.022 Mev of energy (minimum required for antimatter), you'd get far better results if you simply directed the beam out the back of your spacecraft. Using the beam to generate, and then re-annihilate positron-electron pairs wouldn't give you any extra energy, it would in fact waste almost 100% of your energy.

In any case, there's nothing fundamentally wrong with using particle accelerators as engines. That's what an ion engine essentially is after all.

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