Spacecraft propulsion

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A remote camera captures a close-up view of a Space Shuttle Main Engine during a test firing at the John C. Stennis Space Center in Hancock County, Mississippi.

Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the back/rear of the vehicle at very high speed through a supersonic de Laval nozzle. This sort of engine is called a rocket engine.

All current spacecraft use chemical rockets (bipropellant or solid-fuel) for launch, though some (such as the Pegasus rocket and SpaceShipOne) have used air-breathing engines on their first stage. Most satellites have simple reliable chemical thrusters (often monopropellant rockets) or resistojet rockets for orbital station-keeping and some use momentum wheels for attitude control. Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north-south stationkeeping and orbit raising. Interplanetary vehicles mostly use chemical rockets as well, although a few have used ion thrusters and Hall effect thrusters (two different types of electric propulsion) to great success.

Requirements

Artificial satellites must be launched into orbit and once there they must be placed in their nominal orbit. Once in the desired orbit, they often need some form of attitude control so that they are correctly pointed with respect to Earth, the Sun, and possibly some astronomical object of interest.^[1] They are also subject to drag from the thin atmosphere, so that to stay in orbit for a long period of time some form of propulsion is occasionally necessary to make small corrections (orbital stationkeeping).^[2] Many satellites need to be moved from one orbit to another from time to time, and this also requires propulsion.^[3] A satellite's useful life is usually over once it has exhausted its ability to adjust its orbit.
Spacecraft designed to travel further also need propulsion methods. They need to be launched out of the Earth's atmosphere just as satellites do. Once there, they need to leave orbit and move around.
For interplanetary travel, a spacecraft must use its engines to leave Earth orbit. Once it has done so, it must somehow make its way to its destination. Current interplanetary spacecraft do this with a series of short-term trajectory adjustments.^[4] In between these adjustments, the spacecraft simply falls freely along its trajectory. The most fuel-efficient means to move from one circular orbit to another is with a Hohmann transfer orbit: the spacecraft begins in a roughly circular orbit around the Sun. A short period of thrust in the direction of motion accelerates or decelerates the spacecraft into an elliptical orbit around the Sun which is tangential to its previous orbit and also to the orbit of its destination. The spacecraft falls freely along this elliptical orbit until it reaches its destination, where another short period of thrust accelerates or decelerates it to match the orbit of its destination.^[5]
Special methods such as aerobraking or aerocapture are sometimes used for this final orbital adjustment.^[6]

Artist's concept of a solar sail

Some spacecraft propulsion methods such as solar sails provide very low but inexhaustible thrust;^[7] an interplanetary vehicle using one of these methods would follow a rather different trajectory, either constantly thrusting against its direction of motion in order to decrease its distance from the Sun or constantly thrusting along its direction of motion to increase its distance from the Sun. The concept has been successfully tested by the Japanese IKAROS solar sail spacecraft.

Spacecraft for interstellar travel also need propulsion methods. No such spacecraft has yet been built, but many designs have been discussed. Because interstellar distances are very great, a tremendous velocity is needed to get a spacecraft to its destination in a reasonable amount of time. Acquiring such a velocity on launch and getting rid of it on arrival will be a formidable challenge for spacecraft designers.^[8]

Effectiveness

When in space, the purpose of a propulsion system is to change the velocity, or v, of a spacecraft. Because this is more difficult for more massive spacecraft, designers generally discuss momentum, mv. The amount of change in momentum is called impulse.^[9] So the goal of a propulsion method in space is to create an impulse.

When launching a spacecraft from Earth, a propulsion method must overcome a higher gravitational pull to provide a positive net acceleration.^[10] In orbit, any additional impulse, even very tiny, will result in a change in the orbit path.

The rate of change of velocity is called acceleration, and the rate of change of momentum is called force. To reach a given velocity, one can apply a small acceleration over a long period of time, or one can apply a large acceleration over a short time. Similarly, one can achieve a given impulse with a large force over a short time or a small force over a long time. This means that for maneuvering in space, a propulsion method that produces tiny accelerations but runs for a long time can produce the same impulse as a propulsion method that produces large accelerations for a short time. When launching from a planet, tiny accelerations cannot overcome the planet's gravitational pull and so cannot be used.

Earth's surface is situated fairly deep in a gravity well. The escape velocity required to get out of it is 11.2 kilometers/second. As human beings evolved in a gravitational field of 1g (9.8 m/s²), an ideal propulsion system would be one that provides a continuous acceleration of 1g (though human bodies can tolerate much larger accelerations over short periods). The occupants of a rocket or spaceship having such a propulsion system would be free from all the ill effects of free fall, such as nausea, muscular weakness, reduced sense of taste, or leaching of calcium from their bones.

The law of conservation of momentum means that in order for a propulsion method to change the momentum of a space craft it must change the momentum of something else as well. A few designs take advantage of things like magnetic fields or light pressure in order to change the spacecraft's momentum, but in free space the rocket must bring along some mass to accelerate away in order to push itself forward. Such mass is called reaction mass.

In order for a rocket to work, it needs two things: reaction mass and energy. The impulse provided by launching a particle of reaction mass having mass m at velocity v is mv. But this particle has kinetic energy mv²/2, which must come from somewhere. In a conventional solid, liquid, or hybrid rocket, the fuel is burned, providing the energy, and the reaction products are allowed to flow out the back, providing the reaction mass. In an ion thruster, electricity is used to accelerate ions out the back. Here some other source must provide the electrical energy (perhaps a solar panel or a nuclear reactor), whereas the ions provide the reaction mass.^[10]

When discussing the efficiency of a propulsion system, designers often focus on effectively using the reaction mass. Reaction mass must be carried along with the rocket and is irretrievably consumed when used. One way of measuring the amount of impulse that can be obtained from a fixed amount of reaction mass is the specific impulse, the impulse per unit weight-on-Earth (typically designated by ${\displaystyle I_{\text{sp}}}$). The unit for this value is seconds. Because the weight on Earth of the reaction mass is often unimportant when discussing vehicles in space, specific impulse can also be discussed in terms of impulse per unit mass. This alternate form of specific impulse uses the same units as velocity (e.g. m/s), and in fact it is equal to the effective exhaust velocity of the engine (typically designated ${\displaystyle v_{e}}$). Confusingly, both values are sometimes called specific impulse. The two values differ by a factor of g_n, the standard acceleration due to gravity 9.80665 m/s² (${\displaystyle I_{\text{sp}}g_{\mathrm {n} }=v_{e}}$).

A rocket with a high exhaust velocity can achieve the same impulse with less reaction mass. However, the energy required for that impulse is proportional to the exhaust velocity, so that more mass-efficient engines require much more energy, and are typically less energy efficient. This is a problem if the engine is to provide a large amount of thrust. To generate a large amount of impulse per second, it must use a large amount of energy per second. So high-mass-efficient engines require enormous amounts of energy per second to produce high thrusts. As a result, most high-mass-efficient engine designs also provide lower thrust due to the unavailability of high amounts of energy.

Methods

Propulsion methods can be classified based on their means of accelerating the reaction mass. There are also some special methods for launches, planetary arrivals, and landings.

Reaction engines

A reaction engine is an engine which provides propulsion by expelling reaction mass, in accordance with Newton's third law of motion. This law of motion is most commonly paraphrased as: "For every action there is an equal, and opposite, reaction".
Examples include both duct engines and rocket engines, and more uncommon variations such as Hall effect thrusters, ion drives and mass drivers. Duct engines are obviously not used for space propulsion due to the lack of air; however some proposed spacecraft have these kinds of engines to assist takeoff and landing.

Delta-v and propellant

Rocket mass ratios versus final velocity, as calculated from the rocket equation

Exhausting the entire usable propellant of a spacecraft through the engines in a straight line in free space would produce a net velocity change to the vehicle; this number is termed delta-v (${\displaystyle \Delta v}$).

If the exhaust velocity is constant then the total ${\displaystyle \Delta v}$ of a vehicle can be calculated using the rocket equation, where M is the mass of propellant, P is the mass of the payload (including the rocket structure), and ${\displaystyle v_{e}}$ is the velocity of the rocket exhaust. This is known as the Tsiolkovsky rocket equation:

${\displaystyle \Delta v=v_{e}\ln \left({\frac {M+P}{P}}\right).}$

For historical reasons, as discussed above, ${\displaystyle v_{e}}$ is sometimes written as

${\displaystyle v_{e}=I_{\text{sp}}g_{0}}$

where ${\displaystyle I_{\text{sp}}}$ is the specific impulse of the rocket, measured in seconds, and ${\displaystyle g_{0}}$ is the gravitational acceleration at sea level.

For a high delta-v mission, the majority of the spacecraft's mass needs to be reaction mass. Because a rocket must carry all of its reaction mass, most of the initially-expended reaction mass goes towards accelerating reaction mass rather than payload. If the rocket has a payload of mass P, the spacecraft needs to change its velocity by ${\displaystyle \Delta v}$, and the rocket engine has exhaust velocity v_e, then the reaction mass M which is needed can be calculated using the rocket equation and the formula for ${\displaystyle I_{\text{sp}}}$:

${\displaystyle M=P\left(e^{\frac {\Delta v}{v_{e}}}-1\right).}$

For ${\displaystyle \Delta v}$ much smaller than v_e, this equation is roughly linear, and little reaction mass is needed. If ${\displaystyle \Delta v}$ is comparable to v_e, then there needs to be about twice as much fuel as combined payload and structure (which includes engines, fuel tanks, and so on). Beyond this, the growth is exponential; speeds much higher than the exhaust velocity require very high ratios of fuel mass to payload and structural mass.

For a mission, for example, when launching from or landing on a planet, the effects of gravitational attraction and any atmospheric drag must be overcome by using fuel. It is typical to combine the effects of these and other effects into an effective mission delta-v. For example, a launch mission to low Earth orbit requires about 9.3–10 km/s delta-v. These mission delta-vs are typically numerically integrated on a computer.

Some effects such as Oberth effect can only be significantly utilised by high thrust engines such as rockets; i.e., engines that can produce a high g-force (thrust per unit mass, equal to delta-v per unit time).

Power use and propulsive efficiency

For all reaction engines (such as rockets and ion drives) some energy must go into accelerating the reaction mass. Every engine will waste some energy, but even assuming 100% efficiency, to accelerate an exhaust the engine will need energy amounting to

${\displaystyle {\frac {1}{2}}{\dot {m}}v_{e}^{2}}$^[11]

This energy is not necessarily lost- some of it usually ends up as kinetic energy of the vehicle, and the rest is wasted in residual motion of the exhaust.

Due to energy carried away in the exhaust, the energy efficiency of a reaction engine varies with the speed of the exhaust relative to the speed of the vehicle, this is called propulsive efficiency

Comparing the rocket equation (which shows how much energy ends up in the final vehicle) and the above equation (which shows the total energy required) shows that even with 100% engine efficiency, certainly not all energy supplied ends up in the vehicle - some of it, indeed usually most of it, ends up as kinetic energy of the exhaust.

The exact amount depends on the design of the vehicle, and the mission. However, there are some useful fixed points:

• if the ${\displaystyle I_{\text{sp}}}$ is fixed, for a mission delta-v, there is a particular ${\displaystyle I_{\text{sp}}}$ that minimises the overall energy used by the rocket. This comes to an exhaust velocity of about ⅔ of the mission delta-v (see the energy computed from the rocket equation). Drives with a specific impulse that is both high and fixed such as Ion thrusters have exhaust velocities that can be enormously higher than this ideal for many missions.
• if the exhaust velocity can be made to vary so that at each instant it is equal and opposite to the vehicle velocity then the absolute minimum energy usage is achieved. When this is achieved, the exhaust stops in space ^[1] and has no kinetic energy; and the propulsive efficiency is 100%- all the energy ends up in the vehicle (in principle such a drive would be 100% efficient, in practice there would be thermal losses from within the drive system and residual heat in the exhaust). However, in most cases this uses an impractical quantity of propellant, but is a useful theoretical consideration. Anyway, the vehicle has to move before the method can be applied.

Some drives (such as VASIMR or electrodeless plasma thruster) actually can significantly vary their exhaust velocity. This can help reduce propellant usage or improve acceleration at different stages of the flight. However the best energetic performance and acceleration is still obtained when the exhaust velocity is close to the vehicle speed. Proposed ion and plasma drives usually have exhaust velocities enormously higher than that ideal (in the case of VASIMR the lowest quoted speed is around 15000 m/s compared to a mission delta-v from high Earth orbit to Mars of about 4000 m/s).

It might be thought that adding power generation capacity is helpful, and although initially this can improve performance, this inevitably increases the weight of the power source, and eventually the mass of the power source and the associated engines and propellant dominates the weight of the vehicle, and then adding more power gives no significant improvement.

For, although solar power and nuclear power are virtually unlimited sources of energy, the maximum power they can supply is substantially proportional to the mass of the powerplant (i.e. specific power takes a largely constant value which is dependent on the particular powerplant technology). For any given specific power, with a large ${\displaystyle v_{e}}$ which is desirable to save propellant mass, it turns out that the maximum acceleration is inversely proportional to ${\displaystyle v_{e}}$. Hence the time to reach a required delta-v is proportional to ${\displaystyle v_{e}}$. Thus the latter should not be too large.

Energy

Plot of instantaneous propulsive efficiency (blue) and overall efficiency for a vehicle accelerating from rest (red) as percentages of the engine efficiency

In the ideal case ${\displaystyle m_{1}}$ is useful payload and ${\displaystyle m_{0}-m_{1}}$ is reaction mass (this corresponds to empty tanks having no mass, etc.). The energy required can simply be computed as

${\displaystyle {\frac {1}{2}}(m_{0}-m_{1})v_{\text{e}}^{2}}$

This corresponds to the kinetic energy the expelled reaction mass would have at a speed equal to the exhaust speed. If the reaction mass had to be accelerated from zero speed to the exhaust speed, all energy produced would go into the reaction mass and nothing would be left for kinetic energy gain by the rocket and payload. However, if the rocket already moves and accelerates (the reaction mass is expelled in the direction opposite to the direction in which the rocket moves) less kinetic energy is added to the reaction mass. To see this, if, for example, ${\displaystyle v_{e}}$=10 km/s and the speed of the rocket is 3 km/s, then the speed of a small amount of expended reaction mass changes from 3 km/s forwards to 7 km/s rearwards. Thus, although the energy required is 50 MJ per kg reaction mass, only 20 MJ is used for the increase in speed of the reaction mass. The remaining 30 MJ is the increase of the kinetic energy of the rocket and payload.

In general:

${\displaystyle d\left({\frac {1}{2}}v^{2}\right)=vdv=vv_{\text{e}}{\frac {dm}{m}}={\frac {1}{2}}\left[v_{\text{e}}^{2}-\left(v-v_{\text{e}}\right)^{2}+v^{2}\right]{\frac {dm}{m}}}$

Thus the specific energy gain of the rocket in any small time interval is the energy gain of the rocket including the remaining fuel, divided by its mass, where the energy gain is equal to the energy produced by the fuel minus the energy gain of the reaction mass. The larger the speed of the rocket, the smaller the energy gain of the reaction mass; if the rocket speed is more than half of the exhaust speed the reaction mass even loses energy on being expelled, to the benefit of the energy gain of the rocket; the larger the speed of the rocket, the larger the energy loss of the reaction mass.

We have

${\displaystyle \Delta \epsilon =\int v\,d(\Delta v)}$

where ${\displaystyle \epsilon }$ is the specific energy of the rocket (potential plus kinetic energy) and ${\displaystyle \Delta v}$ is a separate variable, not just the change in ${\displaystyle v}$. In the case of using the rocket for deceleration; i.e., expelling reaction mass in the direction of the velocity, ${\displaystyle v}$ should be taken negative.

The formula is for the ideal case again, with no energy lost on heat, etc. The latter causes a reduction of thrust, so it is a disadvantage even when the objective is to lose energy (deceleration).

If the energy is produced by the mass itself, as in a chemical rocket, the fuel value has to be ${\displaystyle \scriptstyle {v_{\text{e}}^{2}/2}}$, where for the fuel value also the mass of the oxidizer has to be taken into account. A typical value is ${\displaystyle v_{\text{e}}}$ = 4.5 km/s, corresponding to a fuel value of 10.1 MJ/kg. The actual fuel value is higher, but much of the energy is lost as waste heat in the exhaust that the nozzle was unable to extract.

The required energy ${\displaystyle E}$ is

${\displaystyle E={\frac {1}{2}}m_{1}\left(e^{\frac {\Delta v}{v_{\text{e}}}}-1\right)v_{\text{e}}^{2}}$

Conclusions:

• for ${\displaystyle \Delta v\ll v_{e}}$ we have ${\displaystyle E\approx {\frac {1}{2}}m_{1}v_{\text{e}}\Delta v}$
• for a given ${\displaystyle \Delta v}$, the minimum energy is needed if ${\displaystyle v_{\text{e}}=0.6275\Delta v}$, requiring an energy of

${\displaystyle E=0.772m_{1}(\Delta v)^{2}}$.

In the case of acceleration in a fixed direction, and starting from zero speed, and in the absence of other forces, this is 54.4% more than just the final kinetic energy of the payload. In this optimal case the initial mass is 4.92 times the final mass.

These results apply for a fixed exhaust speed.

Due to the Oberth effect and starting from a nonzero speed, the required potential energy needed from the propellant may be less than the increase in energy in the vehicle and payload. This can be the case when the reaction mass has a lower speed after being expelled than before – rockets are able to liberate some or all of the initial kinetic energy of the propellant.

Also, for a given objective such as moving from one orbit to another, the required ${\displaystyle \Delta v}$ may depend greatly on the rate at which the engine can produce ${\displaystyle \Delta v}$ and maneuvers may even be impossible if that rate is too low. For example, a launch to Low Earth orbit (LEO) normally requires a ${\displaystyle \Delta v}$ of ca. 9.5 km/s (mostly for the speed to be acquired), but if the engine could produce ${\displaystyle \Delta v}$ at a rate of only slightly more than g, it would be a slow launch requiring altogether a very large ${\displaystyle \Delta v}$ (think of hovering without making any progress in speed or altitude, it would cost a ${\displaystyle \Delta v}$ of 9.8 m/s each second). If the possible rate is only ${\displaystyle g}$ or less, the maneuver can not be carried out at all with this engine.

The power is given by

${\displaystyle P={\frac {1}{2}}mav_{\text{e}}={\frac {1}{2}}Fv_{\text{e}}}$

where ${\displaystyle F}$ is the thrust and ${\displaystyle a}$ the acceleration due to it. Thus the theoretically possible thrust per unit power is 2 divided by the specific impulse in m/s. The thrust efficiency is the actual thrust as percentage of this.

If, e.g., solar power is used, this restricts ${\displaystyle a}$; in the case of a large ${\displaystyle v_{\text{e}}}$ the possible acceleration is inversely proportional to it, hence the time to reach a required delta-v is proportional to ${\displaystyle v_{\text{e}}}$; with 100% efficiency:

• for ${\displaystyle \Delta v\ll v_{\text{e}}}$ we have ${\displaystyle t\approx {\frac {mv_{\text{e}}\Delta v}{2P}}}$

Examples:

• power, 1000 W; mass, 100 kg; ${\displaystyle \Delta v}$ = 5 km/s, ${\displaystyle v_{\text{e}}}$ = 16 km/s, takes 1.5 months.
• power, 1000 W; mass, 100 kg; ${\displaystyle \Delta v}$ = 5 km/s, ${\displaystyle v_{\text{e}}}$ = 50 km/s, takes 5 months.

Thus ${\displaystyle v_{\text{e}}}$ should not be too large.

Power to thrust ratio

The power to thrust ratio is simply:^[11]

${\displaystyle {\frac {P}{F}}={\frac {{\frac {1}{2}}{{\dot {m}}v^{2}}}{{\dot {m}}v}}={\frac {1}{2}}v}$

Thus for any vehicle power P, the thrust that may be provided is:

${\displaystyle F={\frac {P}{{\frac {1}{2}}v}}={\frac {2P}{v}}}$

Example

Suppose a 10,000 kg space probe will be sent to Mars. The required ${\displaystyle \Delta v}$ from LEO is approximately 3000 m/s, using a Hohmann transfer orbit. For the sake of argument, assume the following thrusters are options to be used:

Engine	Effective exhaust velocity (km/s)	Specific impulse (s)	Mass, propellant (kg)	Energy required (GJ)	Specific energy, propellant (J/kg)	Minimum[a] power/thrust	Power generator mass/thrust[b]
Solid rocket	1	100	190,000	95	500×103	0.5 kW/N	N/A
Bipropellant rocket	5	500	8,200	103	12.6×106	2.5 kW/N	N/A
Ion thruster	50	5,000	620	775	1.25×109	25 kW/N	25 kg/N

Assuming 100% energetic efficiency; 50% is more typical in practice. 

Assumes a specific power of 1 kW/kg

Observe that the more fuel-efficient engines can use far less fuel; their mass is almost negligible (relative to the mass of the payload and the engine itself) for some of the engines. However, note also that these require a large total amount of energy. For Earth launch, engines require a thrust to weight ratio of more than one. To do this with the ion or more theoretical electrical drives, the engine would have to be supplied with one to several gigawatts of power, equivalent to a major metropolitan generating station. From the table it can be seen that this is clearly impractical with current power sources.

Alternative approaches include some forms of laser propulsion, where the reaction mass does not provide the energy required to accelerate it, with the energy instead being provided from an external laser or other beam-powered propulsion system. Small models of some of these concepts have flown, although the engineering problems are complex and the ground based power systems are not a solved problem.

Instead, a much smaller, less powerful generator may be included which will take much longer to generate the total energy needed. This lower power is only sufficient to accelerate a tiny amount of fuel per second, and would be insufficient for launching from Earth. However, over long periods in orbit where there is no friction, the velocity will be finally achieved. For example, it took the SMART-1 more than a year to reach the Moon, whereas with a chemical rocket it takes a few days. Because the ion drive needs much less fuel, the total launched mass is usually lower, which typically results in a lower overall cost, but the journey takes longer.

Mission planning therefore frequently involves adjusting and choosing the propulsion system so as to minimise the total cost of the project, and can involve trading off launch costs and mission duration against payload fraction.

Rocket engines

SpaceX's Kestrel engine is tested

Most rocket engines are internal combustion heat engines (although non combusting forms exist). Rocket engines generally produce a high temperature reaction mass, as a hot gas. This is achieved by combusting a solid, liquid or gaseous fuel with an oxidiser within a combustion chamber. The extremely hot gas is then allowed to escape through a high-expansion ratio nozzle. This bell-shaped nozzle is what gives a rocket engine its characteristic shape. The effect of the nozzle is to dramatically accelerate the mass, converting most of the thermal energy into kinetic energy. Exhaust speed reaching as high as 10 times the speed of sound at sea level are common.
Rocket engines provide essentially the highest specific powers and high specific thrusts of any engine used for spacecraft propulsion.

Ion propulsion rockets can heat a plasma or charged gas inside a magnetic bottle and release it via a magnetic nozzle, so that no solid matter need come in contact with the plasma. Of course, the machinery to do this is complex, but research into nuclear fusion has developed methods, some of which have been proposed to be used in propulsion systems, and some have been tested in a lab.

See rocket engine for a listing of various kinds of rocket engines using different heating methods, including chemical, electrical, solar, and nuclear.

Electromagnetic propulsion

This test engine accelerates ions using electrostatic forces

Rather than relying on high temperature and fluid dynamics to accelerate the reaction mass to high speeds, there are a variety of methods that use electrostatic or electromagnetic forces to accelerate the reaction mass directly. Usually the reaction mass is a stream of ions. Such an engine typically uses electric power, first to ionize atoms, and then to create a voltage gradient to accelerate the ions to high exhaust velocities.

The idea of electric propulsion dates back to 1906, when Robert Goddard considered the possibility in his personal notebook.^[12] Konstantin Tsiolkovsky published the idea in 1911.

For these drives, at the highest exhaust speeds, energetic efficiency and thrust are all inversely proportional to exhaust velocity. Their very high exhaust velocity means they require huge amounts of energy and thus with practical power sources provide low thrust, but use hardly any fuel.

For some missions, particularly reasonably close to the Sun, solar energy may be sufficient, and has very often been used, but for others further out or at higher power, nuclear energy is necessary; engines drawing their power from a nuclear source are called nuclear electric rockets.

With any current source of electrical power, chemical, nuclear or solar, the maximum amount of power that can be generated limits the amount of thrust that can be produced to a small value. Power generation adds significant mass to the spacecraft, and ultimately the weight of the power source limits the performance of the vehicle.

Current nuclear power generators are approximately half the weight of solar panels per watt of energy supplied, at terrestrial distances from the Sun. Chemical power generators are not used due to the far lower total available energy. Beamed power to the spacecraft shows some potential.

6 kW Hall thruster in operation at the NASA Jet Propulsion Laboratory.

Some electromagnetic methods:

• Ion thrusters (accelerate ions first and later neutralize the ion beam with an electron stream emitted from a cathode called a neutralizer)
  • Electrostatic ion thruster
  • Field-emission electric propulsion
  • Hall effect thruster
  • Colloid thruster
• Electrothermal thrusters (electromagnetic fields are used to generate a plasma to increase the heat of the bulk propellant, the thermal energy imparted to the propellant gas is then converted into kinetic energy by a nozzle of either physical material construction or by magnetic means)
  • DC arcjet
  • microwave arcjet
  • Helicon Double Layer Thruster
• Electromagnetic thrusters (ions are accelerated either by the Lorentz Force or by the effect of electromagnetic fields where the electric field is not in the direction of the acceleration)
  • Plasma propulsion engine
  • Magnetoplasmadynamic thruster
  • Electrodeless plasma thruster
  • Pulsed inductive thruster
  • Pulsed plasma thruster
  • Variable specific impulse magnetoplasma rocket (VASIMR)
• Mass drivers (for propulsion)

In electrothermal and electromagnetic thrusters, both ions and electrons are accelerated simultaneously, no neutralizer is required.

Without internal reaction mass

NASA study of a solar sail. The sail would be half a kilometer wide.

The law of conservation of momentum is usually taken to imply that any engine which uses no reaction mass cannot accelerate the center of mass of a spaceship (changing orientation, on the other hand, is possible). But space is not empty, especially space inside the Solar System; there are gravitation fields, magnetic fields, electromagnetic waves, solar wind and solar radiation. Electromagnetic waves in particular are known to contain momentum, despite being massless; specifically the momentum flux density P of an EM wave is quantitatively 1/c^2 times the Poynting vector S, i.e. P = S/c^2, where c is the velocity of light. Field propulsion methods which do not rely on reaction mass thus must try to take advantage of this fact by coupling to a momentum-bearing field such as an EM wave that exists in the vicinity of the craft. However, because many of these phenomena are diffuse in nature, corresponding propulsion structures need to be proportionately large.^{[original research?]}

There are several different space drives that need little or no reaction mass to function. A tether propulsion system employs a long cable with a high tensile strength to change a spacecraft's orbit, such as by interaction with a planet's magnetic field or through momentum exchange with another object.^[13] Solar sails rely on radiation pressure from electromagnetic energy, but they require a large collection surface to function effectively. The magnetic sail deflects charged particles from the solar wind with a magnetic field, thereby imparting momentum to the spacecraft. A variant is the mini-magnetospheric plasma propulsion system, which uses a small cloud of plasma held in a magnetic field to deflect the Sun's charged particles. An E-sail would use very thin and lightweight wires holding an electric charge to deflect these particles, and may have more controllable directionality.

As a proof of concept, NanoSail-D became the first nanosatellite to orbit Earth.^{[14][full citation needed]} There are plans to add them^{[clarification needed]} to future Earth orbit satellites, enabling them to de-orbit and burn up once they are no longer needed. Cubesail will be the first mission to demonstrate solar sailing in low Earth orbit, and the first mission to demonstrate full three-axis attitude control of a solar sail.^[15]

Japan also launched its own solar sail powered spacecraft IKAROS in May 2010. IKAROS successfully demonstrated propulsion and guidance and is still flying today.

A satellite or other space vehicle is subject to the law of conservation of angular momentum, which constrains a body from a net change in angular velocity. Thus, for a vehicle to change its relative orientation without expending reaction mass, another part of the vehicle may rotate in the opposite direction. Non-conservative external forces, primarily gravitational and atmospheric, can contribute up to several degrees per day to angular momentum,^[16] so secondary systems are designed to "bleed off" undesired rotational energies built up over time. Accordingly, many spacecraft utilize reaction wheels or control moment gyroscopes to control orientation in space.^[17]

A gravitational slingshot can carry a space probe onward to other destinations without the expense of reaction mass. By harnessing the gravitational energy of other celestial objects, the spacecraft can pick up kinetic energy.^[18] However, even more energy can be obtained from the gravity assist if rockets are used.

Planetary and atmospheric propulsion

A successful proof of concept Lightcraft test, a subset of beam-powered propulsion.

Launch-assist mechanisms

The conceptual ocean-located Quicklauncher, a light-gas gun–based space gun

There have been many ideas proposed for launch-assist mechanisms that have the potential of drastically reducing the cost of getting into orbit. Proposed non-rocket spacelaunch launch-assist mechanisms include:

• Skyhook (requires reusable suborbital launch vehicle, not engineeringly feasible using presently available materials)
• Space elevator (tether from Earth's surface to geostationary orbit, cannot be built with existing materials)
• Launch loop (a very fast enclosed rotating loop about 80 km tall)
• Space fountain (a very tall building held up by a stream of masses fired from its base)
• Orbital ring (a ring around Earth with spokes hanging down off bearings)
• Electromagnetic catapult (railgun, coilgun) (an electric gun)
• Rocket sled launch
• Space gun (Project HARP, ram accelerator) (a chemically powered gun)
• Beam-powered propulsion rockets and jets powered from the ground via a beam
• High-altitude platforms to assist initial stage

Airbreathing engines

Studies generally show that conventional air-breathing engines, such as ramjets or turbojets are basically too heavy (have too low a thrust/weight ratio) to give any significant performance improvement when installed on a launch vehicle itself. However, launch vehicles can be air launched from separate lift vehicles (e.g. B-29, Pegasus Rocket and White Knight) which do use such propulsion systems. Jet engines mounted on a launch rail could also be so used.
On the other hand, very lightweight or very high speed engines have been proposed that take advantage of the air during ascent:

• SABRE - a lightweight hydrogen fuelled turbojet with precooler^[19]
• ATREX - a lightweight hydrogen fuelled turbojet with precooler^[20]
• Liquid air cycle engine - a hydrogen fuelled jet engine that liquifies the air before burning it in a rocket engine
• Scramjet - jet engines that use supersonic combustion
• Shcramjet - similar to a scramjet engine, however it takes advantage of shockwaves produced from the aircraft in the combustion chamber to assist in increasing overall efficiency.

Normal rocket launch vehicles fly almost vertically before rolling over at an altitude of some tens of kilometers before burning sideways for orbit; this initial vertical climb wastes propellant but is optimal as it greatly reduces airdrag. Airbreathing engines burn propellant much more efficiently and this would permit a far flatter launch trajectory, the vehicles would typically fly approximately tangentially to Earth's surface until leaving the atmosphere then perform a rocket burn to bridge the final delta-v to orbital velocity.

Planetary arrival and landing

A test version of the MARS Pathfinder airbag system

When a vehicle is to enter orbit around its destination planet, or when it is to land, it must adjust its velocity. This can be done using all the methods listed above (provided they can generate a high enough thrust), but there are a few methods that can take advantage of planetary atmospheres and/or surfaces.

• Aerobraking allows a spacecraft to reduce the high point of an elliptical orbit by repeated brushes with the atmosphere at the low point of the orbit. This can save a considerable amount of fuel because it takes much less delta-V to enter an elliptical orbit compared to a low circular orbit. Because the braking is done over the course of many orbits, heating is comparatively minor, and a heat shield is not required. This has been done on several Mars missions such as Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter, and at least one Venus mission, Magellan.
• Aerocapture is a much more aggressive manoeuver, converting an incoming hyperbolic orbit to an elliptical orbit in one pass. This requires a heat shield and much trickier navigation, because it must be completed in one pass through the atmosphere, and unlike aerobraking no preview of the atmosphere is possible. If the intent is to remain in orbit, then at least one more propulsive maneuver is required after aerocapture—otherwise the low point of the resulting orbit will remain in the atmosphere, resulting in eventual re-entry. Aerocapture has not yet been tried on a planetary mission, but the re-entry skip by Zond 6 and Zond 7 upon lunar return were aerocapture maneuvers, because they turned a hyperbolic orbit into an elliptical orbit. On these missions, because there was no attempt to raise the perigee after the aerocapture, the resulting orbit still intersected the atmosphere, and re-entry occurred at the next perigee.
• A ballute is an inflatable drag device.
• Parachutes can land a probe on a planet or moon with an atmosphere, usually after the atmosphere has scrubbed off most of the velocity, using a heat shield.
• Airbags can soften the final landing.
• Lithobraking, or stopping by impacting the surface, is usually done by accident. However, it may be done deliberately with the probe expected to survive (see, for example, Deep Impact (spacecraft)), in which case very sturdy probes are required.

Table of methods

Below is a summary of some of the more popular, proven technologies, followed by increasingly speculative methods.

Four numbers are shown. The first is the effective exhaust velocity: the equivalent speed that the propellant leaves the vehicle. This is not necessarily the most important characteristic of the propulsion method; thrust and power consumption and other factors can be. However:

• if the delta-v is much more than the exhaust velocity, then exorbitant amounts of fuel are necessary (see the section on calculations, above)
• if it is much more than the delta-v, then, proportionally more energy is needed; if the power is limited, as with solar energy, this means that the journey takes a proportionally longer time

The second and third are the typical amounts of thrust and the typical burn times of the method. Outside a gravitational potential small amounts of thrust applied over a long period will give the same effect as large amounts of thrust over a short period. (This result does not apply when the object is significantly influenced by gravity.)

The fourth is the maximum delta-v this technique can give (without staging). For rocket-like propulsion systems this is a function of mass fraction and exhaust velocity. Mass fraction for rocket-like systems is usually limited by propulsion system weight and tankage weight. For a system to achieve this limit, typically the payload may need to be a negligible percentage of the vehicle, and so the practical limit on some systems can be much lower.

Propulsion methods

Method	Effective exhaust velocity (km/s)	Thrust (N)	Firing duration	Maximum delta-v (km/s)	Technology readiness level
Solid-fuel rocket	<2 .5="" td="">	<10 sup="">7

Minutes 7 9: Flight proven Hybrid rocket

Minutes >3 9: Flight proven Monopropellant rocket 1 – 3^{[citation needed]} 0.1 – 100^{[citation needed]} Milliseconds – minutes 3 9: Flight proven Liquid-fuel rocket <4 .4="" td=""> <10 sup="">7 Minutes 9 9: Flight proven Electrostatic ion thruster 15 – 210^{[21][full citation needed]}
Months – years >100 9: Flight proven Hall-effect thruster (HET) 8 – 50^{[citation needed]}
Months – years >100 9: Flight proven^[22] Resistojet rocket 2 – 6 10⁻² – 10 Minutes ? 8: Flight qualified^[23] Arcjet rocket 4 – 16 10⁻² – 10 Minutes ? 8: Flight qualified^{[citation needed]} Field emission
electric propulsion (FEEP) 100^[24] – 130 10⁻⁶ – 10^−3[24] Months – years ? 8: Flight qualified^[24] Pulsed plasma thruster (PPT) 20 0.1 80 – 400 days ? 7: Prototype demonstrated in space Dual-mode propulsion rocket 1 – 4.7 0.1 – 10⁷ Milliseconds – minutes 3 – 9 7: Prototype demonstrated in space Solar sails 299792, light 9/km² at 1 AU
230/km² at 0.2 AU
10⁻¹⁰/km² at 4 ly Indefinite >40

• 9: Light pressure attitude-control flight proven
• 6: Deploy-only demonstrated in space
• 5: Light-sail validated in medium vacuum

Tripropellant rocket 2.5 – 5.3^{[citation needed]} 0.1 – 10^{7[citation needed]} Minutes 9 6: Prototype demonstrated on ground^[25] Magnetoplasmadynamic
thruster (MPD) 20 – 100 100 Weeks ? 6: Model, 1 kW demonstrated in space^[26] Nuclear–thermal rocket 9^[27] 10^7[27] Minutes^[27] >20 6: Prototype demonstrated on ground Propulsive mass drivers 0 – 30 10⁴ – 10⁸ Months ? 6: Model, 32 MJ demonstrated on ground Tether propulsion N/A 1 – 10¹² Minutes 7 6: Model, 31.7 km demonstrated in space^[28] Air-augmented rocket 5 – 6 0.1 – 10⁷ Seconds – minutes >7? 6: Prototype demonstrated on ground^[29][30] Liquid-air-cycle engine 4.5 10³ – 10⁷ Seconds – minutes ? 6: Prototype demonstrated on ground Pulsed-inductive thruster (PIT) 10 – 80^[31] 20 Months ? 5: Component validated in vacuum^[31] Variable-specific-impulse
magnetoplasma rocket
(VASIMR) 10 – 300^{[citation needed]} 40 – 1,200^{[citation needed]} Days – months >100 5: Component, 200 kW validated in vacuum Magnetic-field oscillating
amplified thruster 10 – 130 0.1 – 1 Days – months >100 5: Component validated in vacuum Solar–thermal rocket 7 – 12 1 – 100 Weeks >20 4: Component validated in lab^[32] Radioisotope rocket 7 – 8^{[citation needed]} 1.3 – 1.5 Months ? 4: Component validated in lab Nuclear–electric rocket As electric propulsion method used 4: Component, 400 kW validated in lab Orion Project (near-term
nuclear pulse propulsion) 20 – 100 10⁹ – 10¹² Days 30 – 60 3: Validated, 900 kg proof-of-concept^[33][34] Space elevator N/A N/A Indefinite >12 3: Validated proof-of-concept Reaction Engines SABRE^[19] 30/4.5 0.1 – 10⁷ Minutes 9.4 3: Validated proof-of-concept Electric sails 145 – 750, solar wind ? Indefinite >40 3: Validated proof-of-concept Magnetic sails 145 – 750, solar wind 2/t^[35] Indefinite ? 3: Validated proof-of-concept Mini-magnetospheric
plasma propulsion 200 1/kW Months ? 3: Validated proof-of-concept^[36] Beam-powered/laser As propulsion method powered by beam 3: Validated, 71 m proof-of-concept Launch loop/orbital ring N/A 10⁴ Minutes 11 – 30 2: Technology concept formulated Nuclear pulse propulsion
(Project Daedalus' drive) 20 – 1,000 10⁹ – 10¹² Years 15,000 2: Technology concept formulated Gas-core reactor rocket 10 – 20 10³ – 10⁶ ? ? 2: Technology concept formulated Nuclear salt-water rocket 100 10³ – 10⁷ Half-hour ? 2: Technology concept formulated Fission sail ? ? ? ? 2: Technology concept formulated Fission-fragment rocket 15,000 ? ? ? 2: Technology concept formulated Nuclear–photonic rocket 299,792 10⁻⁵ – 1 Years – decades ? 2: Technology concept formulated Fusion rocket 100 – 1,000^{[citation needed]} ? ? ? 2: Technology concept formulated Antimatter-catalyzed
nuclear pulse propulsion 200 – 4,000 ? Days – weeks ? 2: Technology concept formulated Antimatter rocket 10,000 – 100,000^{[citation needed]} ? ? ? 2: Technology concept formulated Bussard ramjet 2.2 – 20,000 ? Indefinite 30,000 2: Technology concept formulated Method Effective exhaust
velocity (km/s) Thrust (N) Firing
duration Maximum
delta-v (km/s) Technology
readiness level

Testing

Spacecraft propulsion systems are often first statically tested on Earth's surface, within the atmosphere but many systems require a vacuum chamber to test fully. Rockets are usually tested at a rocket engine test facility well away from habitation and other buildings for safety reasons. Ion drives are far less dangerous and require much less stringent safety, usually only a large-ish vacuum chamber is needed.

Famous static test locations can be found at Rocket Ground Test Facilities

Some systems cannot be adequately tested on the ground and test launches may be employed at a Rocket Launch Site.

Speculative methods

Artist's conception of a warp drive design

A variety of hypothetical propulsion techniques have been considered that mostly require a deeper understanding of the properties of space, particularly inertial frames and the quantum vacuum. To date, such methods are highly speculative and include:

• Black hole starship
• Differential sail
• Faster-than-light
• Field propulsion
  • RF resonant cavity thruster
• Gravitational shielding
  • Diametric drive
  • Disjunction drive
  • Pitch drive & bias drive
• Photon rocket
• Photonic laser thruster
• Quantum vacuum thruster
• Reactionless drive
  • Abraham—Minkowski drive
  • Alcubierre drive
  • Heim drive
  • Woodward effect

A NASA assessment of its Breakthrough Propulsion Physics Program divides such proposals into those that are non-viable for propulsion purposes, those that are of uncertain potential, and those that are theoretically not impossible.^[37]

Climate engineering

From Wikipedia, the free encyclopedia

Climate engineering, commonly referred to as geoengineering, also known as climate intervention,^[1] is the deliberate and large-scale intervention in the Earth’s climate system with the aim of affecting adverse global warming.^[2][3][4] Climate engineering is an umbrella term for measures that mainly fall into two categories: greenhouse gas removal and solar radiation management. Greenhouse gas removal approaches, of which carbon dioxide removal represents the most prominent subcategory addresses the cause of global warming by removing greenhouse gases from the atmosphere. Solar radiation management attempts to offset effects of greenhouse gases by causing the Earth to absorb less solar radiation.

Climate engineering approaches are sometimes viewed as additional potential options for limiting climate change or its impacts, alongside mitigation and adaptation.^[5][6] There is substantial agreement among scientists that climate engineering cannot substitute for climate change mitigation. Some approaches might be used as accompanying measures to sharp cuts in greenhouse gas emissions.^[7] Given that all types of measures for addressing climate change have economic, political, or physical limitations,^[8][9] some climate engineering approaches might eventually be used as part of an ensemble of measures, which can be referred to as climate restoration.^[10] Research on costs, benefits, and various types of risks of most climate engineering approaches is at an early stage and their understanding needs to improve to judge their adequacy and feasibility.^[2]

Almost all research into solar radiation management has to date consisted of computer modelling or laboratory tests, and an attempt to move to outdoor experimentation has proven controversial.^[11] Some carbon dioxide removal practices, such as afforestation^[12], ecosystem restoration and bio-energy with carbon capture and storage projects, are underway to a limited extent. Their scalability to effectively affect global climate is, however, debated. Ocean iron fertilization has been investigated in small-scale research trials. These experiments have proven controversial.^[13] The World Wildlife Fund has criticized these activities.^[14]

Most experts and major reports advise against relying on climate engineering techniques as a simple solution to global warming, in part due to the large uncertainties over effectiveness and side effects. However, most experts also argue that the risks of such interventions must be seen in the context of risks of dangerous global warming.^[15][16] Interventions at large scale may run a greater risk of disrupting natural systems resulting in a dilemma that those approaches that could prove highly (cost-) effective in addressing extreme climate risk, might themselves cause substantial risk.^[15] Some have suggested that the concept of engineering the climate presents a so-called "moral hazard" because it could reduce political and public pressure for emissions reduction, which could exacerbate overall climate risks; others assert that the threat of climate engineering could spur emissions cuts.^[17][18][19] Some are in favour of a moratorium on out-of-doors testing and deployment of solar radiation management (SRM).^[20][21]

General

With respect to climate, geoengineering is defined by the Royal Society as "... the deliberate large-scale intervention in the Earth’s climate system, in order to moderate global warming."^[22] Several organizations have investigated climate engineering with a view to evaluating its potential, including the US Congress,^[23] the National Academy of Sciences,^[24] the Royal Society,^[25] and the UK Parliament.^[26] The Asilomar International Conference on Climate Intervention Technologies was convened to identify and develop risk reduction guidelines for climate intervention experimentation.^[27]

Some environmental organisations (such as Friends of the Earth^[28] and Greenpeace^[29]) have been reluctant to endorse solar radiation management, but are often more supportive of some carbon dioxide removal projects, such as afforestation and peatland restoration. Some authors have argued that any public support for climate engineering may weaken the fragile political consensus to reduce greenhouse gas emissions.^[30]

History

The 1965 landmark report, "Restoring the Quality of Our Environment" by U.S. President Lyndon B. Johnson’s Science Advisory Committee warned of the harmful effects of fossil fuel emissions, the report also mentioned "deliberately bringing about countervailing climatic changes," including by "raising the albedo, or reflectivity, of the Earth."^[31] Teller et al. 1997 suggested to research and deploy reflective particles, to reduce incoming solar radiation, and thus to cancel the effects of fossil fuel burning.^[32]

Proposed strategies

Several climate engineering strategies have been proposed. IPCC documents detail several notable proposals.^[33] These fall into two main categories: solar radiation management and carbon dioxide removal.

Solar radiation management

Solar radiation management (SRM)^[4][34] techniques would seek to reduce sunlight absorbed (ultra-violet, near infra-red and visible). This would be achieved by deflecting sunlight away from the Earth, or by increasing the reflectivity (albedo) of the atmosphere or the Earth's surface. These methods would not reduce greenhouse gas concentrations in the atmosphere, and thus would not seek to address problems such as the ocean acidification caused by CO₂. In general, solar radiation management projects presently appear to be able to take effect rapidly and to have very low direct implementation costs relative to greenhouse gas emissions cuts and carbon dioxide removal. Furthermore, many proposed SRM methods would be reversible in their direct climatic effects. While greenhouse gas remediation offers a more comprehensive possible solution to global warming, it does not give instantaneous results; for that, solar radiation management is required.^{[dubious – discuss]} Solar radiation management methods^[4] may include:

• Surface-based: for example, using pale-colored roofing materials, attempting to change the oceans' brightness, or growing high-albedo crops.
• Troposphere-based: for example, marine cloud brightening, which would spray fine sea water to whiten clouds and thus increase cloud reflectivity.
• Upper atmosphere-based: creating reflective aerosols, such as stratospheric sulfate aerosols, specifically designed self-levitating aerosols,^[35] or other substances.
• Space-based: space sunshade—obstructing solar radiation with space-based mirrors, dust,^[36] etc.

Carbon dioxide removal

An oceanic phytoplankton bloom in the South Atlantic Ocean, off the coast of Argentina. The aim of ocean iron fertilization in theory is to increase such blooms by adding some iron, which would then draw carbon from the atmosphere and fix it on the seabed.

Significant reduction in ice volume in the Arctic Ocean in the range between 1979 and 2007 years

Carbon dioxide removal (sometimes known as negative emissions technologies or greenhouse gas removal) projects seek to remove carbon dioxide from the atmosphere. Proposed methods include those that directly remove such gases from the atmosphere, as well as indirect methods that seek to promote natural processes that draw down and sequester CO₂ (e.g. tree planting). Many projects overlap with carbon capture and storage projects, and may not be considered to be climate engineering by all commentators. Techniques in this category include:

• Creating biochar, which can be mixed with soil to create terra preta
• Bio-energy with carbon capture and storage to sequester carbon and simultaneously provide energy
• Carbon air capture to remove carbon dioxide from ambient air
• Afforestation, reforestation and forest restoration to absorb carbon dioxide
• Ocean fertilization including iron fertilization of the oceans

Many of the IPCC model projections to keep global mean temperature below 2C, are based on scenarios assuming deployment of negative emissions technologies.^[37]

Justification

Tipping points and positive feedback

Climate change during the last 65 million years. The Paleocene–Eocene Thermal Maximum is labelled PETM.

It is argued that climate change may cross tipping points^[38] where elements of the climate system may 'tip' from one stable state to another stable state, much like a glass tipping over. When the new state is reached, further warming may be caused by positive feedback effects,.^[39] An example of a proposed causal chain leading to more warming is the decline of Arctic sea ice, potentially triggering subsequent release of ocean methane.^[40] Evidence suggests a gradual and prolonged release of greenhouse gases from thawing permafrost.^[41]

The precise identity of such "tipping points" is not clear, with scientists taking differing views on whether specific systems are capable of "tipping" and the point at which this "tipping" will occur.^[42] An example of a previous tipping point is that which preceded the rapid warming leading up to the Paleocene–Eocene Thermal Maximum. Once a tipping point is crossed, cuts in anthropogenic greenhouse gas emissions will not be able to reverse the change. Conservation of resources and reduction of greenhouse emissions, used in conjunction with climate engineering, are therefore considered a viable option by some commentators.^[43][44][45]

Buying time

Climate engineering offers the hope of temporarily reversing some aspects of global warming and allowing the natural climate to be substantially preserved whilst greenhouse gas emissions are brought under control and removed from the atmosphere by natural or artificial processes.^[46]

Costs

Estimates of direct costs for climate engineering implementation vary widely. In general, carbon dioxide removal methods are more expensive than the solar radiation management ones. In their 2009 report Geoengineering the Climate the Royal Society judged afforestation and stratospheric aerosol injection as the methods with the "highest affordability" (lowest costs). More recently, research into costs of solar radiation management have been published.^[47] This suggests that "well designed systems" might be available for costs in the order of a few hundred million to tens of billions of dollars per year.^[48] These are much lower than costs to achieve comprehensive reductions in CO₂ emissions. Such costs would be within the budget of most nations, and even some wealthy individuals.^[49]

Ethics and responsibility

Climate engineering would represent a large-scale, intentional effort to modify the climate. It would differ from activities such as burning fossil fuels, as they change the climate inadvertently. Intentional climate change is often viewed differently from a moral standpoint.^[50] It raises questions of whether humans have the right to change the climate deliberately, and under what conditions. For example, there may be an ethical distinction between climate engineering to minimize global warming and doing so to optimize the climate. Furthermore, ethical arguments often confront larger considerations of worldview, including individual and social religious commitments. This may imply that discussions of climate engineering should reflect on how religious commitments might influence the discourse.^[51] For many people, religious beliefs are pivotal in defining the role of human beings in the wider world. Some religious communities might claim that humans have no responsibility in managing the climate, instead seeing such world systems as the exclusive domain of a Creator. In contrast, other religious communities might see the human role as one of "stewardship" or benevolent management of the world.^[52] The question of ethics also relates to issues of policy decision-making. For example, the selection of a globally agreed target temperature is a significant problem in any climate engineering governance regime, as different countries or interest groups may seek different global temperatures.^[53]

Politics

It has been argued that regardless of the economic, scientific and technical aspects, the difficulty of achieving concerted political action on global warming requires other approaches.^[54] Those arguing political expediency say the difficulty of achieving meaningful emissions cuts^[55] and the effective failure of the Kyoto Protocol demonstrate the practical difficulties of achieving carbon dioxide emissions reduction by the agreement of the international community.^[56] However, others point to support for climate engineering proposals among think tanks with a history of global warming skepticism and opposition to emissions reductions as evidence that the prospect of climate engineering is itself already politicized and being promoted as part of an argument against the need for (and viability of) emissions reductions; that, rather than climate engineering being a solution to the difficulties of emissions reductions, the prospect of climate engineering is being used as part of an argument to stall emissions reductions in the first place.^[57]

Risks and criticisms

Change in sea surface pH caused by anthropogenic CO₂ between the 1700s and the 1990s. This ocean acidification will still be a major problem unless atmospheric CO₂ is reduced.

Various criticisms have been made of climate engineering,^[58] particularly solar radiation management (SRM) methods.^[59] Decision making suffers from intransitivity of policy choice.^[60] Some commentators appear fundamentally opposed. Groups such as ETC Group^[21] and individuals such as Raymond Pierrehumbert have called for a moratorium on climate engineering techniques.^[20][61]

Ineffectiveness

The effectiveness of the techniques proposed may fall short of predictions. In ocean iron fertilization, for example, the amount of carbon dioxide removed from the atmosphere may be much lower than predicted, as carbon taken up by plankton may be released back into the atmosphere from dead plankton, rather than being carried to the bottom of the sea and sequestered.^[62] Model results from a 2016 study, suggest that blooming algae could even accelerate Arctic warming.^[63]

Moral hazard or risk compensation

The existence of such techniques may reduce the political and social impetus to reduce carbon emissions.^[64] This has generally been called a potential moral hazard, although risk compensation may be a more accurate term. This concern causes many environmental groups and campaigners to be reluctant to advocate or discuss climate engineering for fear of reducing the imperative to cut greenhouse gas emissions.^[65] However, several public opinion surveys and focus groups have found evidence of either assertions of a desire to increase emission cuts in the face of climate engineering, or of no effect.^{[66][67][68][69][70][71][72]} Other modelling work suggests that the threat of climate engineering may in fact increase the likelihood of emissions reduction.^{[73][74][75][76]}

Governance

Climate engineering opens up various political and economic issues. The governance issues characterizing carbon dioxide removal compared to solar radiation management tend to be distinct. Carbon dioxide removal techniques are typically slow to act, expensive, and entail risks that are relatively familiar, such as the risk of carbon dioxide leakage from underground storage formations. In contrast, solar radiation management methods are fast-acting, comparatively cheap, and involve novel and more significant risks such as regional climate disruptions. As a result of these differing characteristics, the key governance problem for carbon dioxide removal (as with emissions reductions) is making sure actors do enough of it (the so-called "free rider problem"), whereas the key governance issue for solar radiation management is making sure actors do not do too much (the "free driver" problem).^[77]

Domestic and international governance vary by the proposed climate engineering method. There is presently a lack of a universally agreed framework for the regulation of either climate engineering activity or research. The London Convention addresses some aspects of the law in relation to biomass ocean storage and ocean fertilization. Scientists at the Oxford Martin School at Oxford University have proposed a set of voluntary principles, which may guide climate engineering research. The short version of the 'Oxford Principles'^[78] is:

• Principle 1: Geoengineering to be regulated as a public good.
• Principle 2: Public participation in geoengineering decision-making
• Principle 3: Disclosure of geoengineering research and open publication of results
• Principle 4: Independent assessment of impacts
• Principle 5: Governance before deployment

These principles have been endorsed by the House of Commons of the United Kingdom Science and Technology Select Committee on “The Regulation of Geoengineering”,^[79] and have been referred to by authors discussing the issue of governance.^[80]

The Asilomar conference was replicated to deal with the issue of climate engineering governance,^[80] and covered in a TV documentary, broadcast in Canada.

Implementation issues

There is general consensus^[who?] that no climate engineering technique is currently sufficiently safe or effective to greatly reduce climate change risks, for the reasons listed above. However, some may be able to contribute to reducing climate risks within relatively short times.

All proposed solar radiation management techniques require implementation on a relatively large scale, in order to impact the Earth's climate. The least costly proposals are budgeted at tens of billions of US dollars annually.^[81] Space sunshades would cost far more. Who was to bear the substantial costs of some climate engineering techniques may be hard to agree. However, the more effective solar radiation management proposals currently appear to have low enough direct implementation costs that it would be in the interests of several single countries to implement them unilaterally.

In contrast, carbon dioxide removal, like greenhouse gas emissions reductions, have impacts proportional to their scale. These techniques would not be "implemented" in the same sense as solar radiation management ones.The problem structure of carbon dioxide removal resembles that of emissions cuts, in that both are somewhat expensive public goods, whose provision presents a collective action problem.

Before they are ready to be used, most techniques would require technical development processes that are not yet in place. As a result, many promising proposed climate engineering do not yet have the engineering development or experimental evidence to determine their feasibility or efficacy.

Public perception

In a 2017 focus group study conducted by the Cooperative Institute for Research in Environmental Sciences (CIRES) in the United States, Japan, New Zealand and Sweden, participants were asked about carbon sequestration options, reflection proposals such as with space mirrors, or brightening of clouds, and their majority responses could be summed up as follows:

• What happens if the technologies backfire with unintended consequences?
• Are these solutions treating the symptoms of climate change rather than the cause?
• Shouldn’t we just change our lifestyle and consumption patterns to fight climate change, making climate engineering a last resort?
• Isn’t there a greater need to address political solutions to reduce our emissions?

Moderators floated then the idea of a future "climate emergency" such as rapid environmental change. The participants felt that mitigation and adaptation to climate change were strongly preferred options in such a situation, and climate engineering was seen as a last resort.^[82]

Evaluation of climate engineering

Most of what is known about the suggested techniques is based on laboratory experiments, observations of natural phenomena, and on computer modelling techniques. Some proposed climate engineering methods employ methods that have analogues in natural phenomena such as stratospheric sulfur aerosols and cloud condensation nuclei. As such, studies about the efficacy of these methods can draw on information already available from other research, such as that following the 1991 eruption of Mount Pinatubo. However, comparative evaluation of the relative merits of each technology is complicated, especially given modelling uncertainties and the early stage of engineering development of many proposed climate engineering methods.^[83]

Reports into climate engineering have also been published in the United Kingdom by the Institution of Mechanical Engineers^[9] and the Royal Society.^[10] The IMechE report examined a small subset of proposed methods (air capture, urban albedo and algal-based CO₂ capture techniques), and its main conclusions were that climate engineering should be researched and trialled at the small scale alongside a wider decarbonisation of the economy.^[9]

The Royal Society review examined a wide range of proposed climate engineering methods and evaluated them in terms of effectiveness, affordability, timeliness and safety (assigning qualitative estimates in each assessment). The report divided proposed methods into "carbon dioxide removal" (CDR) and "solar radiation management" (SRM) approaches that respectively address longwave and shortwave radiation. The key recommendations of the report were that "Parties to the UNFCCC should make increased efforts towards mitigating and adapting to climate change, and in particular to agreeing to global emissions reductions", and that "[nothing] now known about climate engineering options gives any reason to diminish these efforts".^[10] Nonetheless, the report also recommended that "research and development of climate engineering options should be undertaken to investigate whether low risk methods can be made available if it becomes necessary to reduce the rate of warming this century".^[10]

In a 2009 review study, Lenton and Vaughan evaluated a range of proposed climate engineering techniques from those that sequester CO₂ from the atmosphere and decrease longwave radiation trapping, to those that decrease the Earth's receipt of shortwave radiation.^[8] In order to permit a comparison of disparate techniques, they used a common evaluation for each technique based on its effect on net radiative forcing. As such, the review examined the scientific plausibility of proposed methods rather than the practical considerations such as engineering feasibility or economic cost. Lenton and Vaughan found that "[air] capture and storage shows the greatest potential, combined with afforestation, reforestation and bio-char production", and noted that "other suggestions that have received considerable media attention, in particular "ocean pipes" appear to be ineffective".^[8] They concluded that "[climate] geoengineering is best considered as a potential complement to the mitigation of CO₂ emissions, rather than as an alternative to it".^[8]

In October 2011, a Bipartisan Policy Center panel issued a report urging immediate researching and testing in case "the climate system reaches a 'tipping point' and swift remedial action is required".^[84]

National Academy of Sciences

The National Academy of Sciences conducted a 21-month project to study the potential impacts, benefits, and costs of two different types of climate engineering: carbon dioxide removal and albedo modification (solar radiation management). The differences between these two classes of climate engineering "led the committee to evaluate the two types of approaches separately in companion reports, a distinction it hopes carries over to future scientific and policy discussions."^[85]

According to the two-volume study released in February 2015:

Climate intervention is no substitute for reductions in carbon dioxide emissions and adaptation efforts aimed at reducing the negative consequences of climate change. However, as our planet enters a period of changing climate never before experienced in recorded human history, interest is growing in the potential for deliberate intervention in the climate system to counter climate change. ...Carbon dioxide removal strategies address a key driver of climate change, but research is needed to fully assess if any of these technologies could be appropriate for large-scale deployment. Albedo modification strategies could rapidly cool the planet’s surface but pose environmental and other risks that are not well understood and therefore should not be deployed at climate-altering scales; more research is needed to determine if albedo modification approaches could be viable in the future.^[86]

The project was sponsored by the National Academy of Sciences, U.S. Intelligence Community, National Oceanic and Atmospheric Administration, NASA, and U.S. Department of Energy.^[85][87]

Intergovernmental Panel on Climate Change

The Intergovernmental Panel on Climate Change (IPCC) assessed the scientific literature on climate engineering (referred to as "geoengineering" in its reports), in which it considered carbon dioxide removal and solar radiation separately. Its Fifth Assessment Report states:^[88]

Models consistently suggest that SRM would generally reduce climate differences compared to a world with elevated GHG concentrations and no SRM; however, there would also be residual regional differences in climate (e.g., temperature and rainfall) when compared to a climate without elevated GHGs....

Models suggest that if SRM methods were realizable they would be effective in countering increasing temperatures, and would be less, but still, effective in countering some other climate changes. SRM would not counter all effects of climate change, and all proposed geoengineering methods also carry risks and side effects. Additional consequences cannot yet be anticipated as the level of scientific understanding about both SRM and CDR is low. There are also many (political, ethical, and practical) issues involving geoengineering that are beyond the scope of this report.