Deep-space propulsion compared

How far has each way of propelling a spacecraft in space been shown to work, and what limits it?

This table compares four ways to propel spacecraft in space on flight record, thrust, best use and main limitation. Chemical rockets and solar sails have flown. Nuclear thermal engines have been ground-tested but have not flown, and laser-driven light sails have been tested in laboratory experiments but not flown.

Deep-space propulsion compared. Each cell shows a value, how well it is evidenced, and its sources.
Option	What has flown or been testedFlights and tests, with year and scale (lab, ground or in orbit). Not a measured quantity.	Thrust or accelerationThrust in newtons (N) or millinewtons (mN), or specific impulse in seconds (s). Says whether a figure is vacuum or sea-level, and whether it was measured or estimated.	Best use caseThe mission type each approach is suited to, as demonstrated or proposed in the sources. Not a measured quantity.	Main limitationThe most important constraint on use. Any figure is given in its own units.
Chemical rockets	NASA names chemical propulsion as the type used for Apollo, Space Shuttle and Artemis, and expects it to stay in use for payload launches.Demonstrated[1]	Specific impulse of about 350 seconds for a liquid hydrogen and liquid oxygen rocket, a rough planning value. Vacuum or sea-level basis not stated.Reported, unverified[2]	Launching payloads, including crewed Apollo, Space Shuttle and Artemis missions, according to NASA.Demonstrated[1]	Lower propellant efficiency. NASA says nuclear thermal propulsion gives twice the propellant efficiency of chemical rockets.Reported, unverified[1]
Nuclear thermal and electric	No nuclear thermal or electric propulsion system has flown in the sources reviewed. Nuclear thermal engines were ground-tested at the Nevada Test Site and the Nuclear Rocket Engine Development Station. NASA says the NERVA programme ran between 1961 and 1973.Demonstrated[1][4]	No thrust figure in the sources reviewed. NASA calls nuclear thermal high thrust and nuclear electric low thrust.Reported, unverified[1]	Proposed: high-thrust nuclear thermal propulsion, and low-thrust nuclear electric propulsion, which NASA describes as efficient for accelerating spacecraft over extended periods.Proposed[1]	No nuclear propulsion system has flown in the sources reviewed, so in-space performance is not shown. NASA plans to launch SR-1 Freedom, which carries a nuclear reactor, in late 2028.Demonstrated[3][1]
Laser-driven light sails	Laboratory tests in 2000 used lasers of 7.9 to 13.9 kW directed at sails mounted on pendulums. No laser-pushed sail has flown in the sources reviewed.Demonstrated[5]	No reliable public figure	Proposed: flexible lightsails for interstellar exploration, driven by radiation pressure.Proposed[6]	Stability results for flexible lightsails come from a computational study. The Caltech record mentions no physical lightsail experiments.Demonstrated[6][5]
Solar sails	Flown: IKAROS, launched by JAXA in May 2010, and LightSail 2, which raised its orbit around Earth in 2019.Demonstrated[7][8][9]	IKAROS: 1.12 mN of solar light pressure thrust, which JAXA called the expected value and estimated from orbit determination. LightSail 2: apogee rose about 2 km in four days.Reported, unverified[7][8][9]	Proposed: slow orbit changes driven by solar light pressure. IKAROS achieved a total velocity change of 100 m/s within about half a year.Proposed[8][9]	Thrust is only 1.12 mN, so changes build slowly. LightSail 2's team expected atmospheric drag to eventually overcome the sail's thrust.Reported, unverified[8][9]

Deep-space propulsion compared. Each cell shows a value, how well it is evidenced, and its sources.

Option

What has flown or been testedFlights and tests, with year and scale (lab, ground or in orbit). Not a measured quantity.

Thrust or accelerationThrust in newtons (N) or millinewtons (mN), or specific impulse in seconds (s). Says whether a figure is vacuum or sea-level, and whether it was measured or estimated.

Best use caseThe mission type each approach is suited to, as demonstrated or proposed in the sources. Not a measured quantity.

Main limitationThe most important constraint on use. Any figure is given in its own units.

Chemical rockets

NASA names chemical propulsion as the type used for Apollo, Space Shuttle and Artemis, and expects it to stay in use for payload launches.Demonstrated[1]

Specific impulse of about 350 seconds for a liquid hydrogen and liquid oxygen rocket, a rough planning value. Vacuum or sea-level basis not stated.Reported, unverified[2]

Launching payloads, including crewed Apollo, Space Shuttle and Artemis missions, according to NASA.Demonstrated[1]

Lower propellant efficiency. NASA says nuclear thermal propulsion gives twice the propellant efficiency of chemical rockets.Reported, unverified[1]

Nuclear thermal and electric

No nuclear thermal or electric propulsion system has flown in the sources reviewed. Nuclear thermal engines were ground-tested at the Nevada Test Site and the Nuclear Rocket Engine Development Station. NASA says the NERVA programme ran between 1961 and 1973.Demonstrated[1][4]

No thrust figure in the sources reviewed. NASA calls nuclear thermal high thrust and nuclear electric low thrust.Reported, unverified[1]

Proposed: high-thrust nuclear thermal propulsion, and low-thrust nuclear electric propulsion, which NASA describes as efficient for accelerating spacecraft over extended periods.Proposed[1]

No nuclear propulsion system has flown in the sources reviewed, so in-space performance is not shown. NASA plans to launch SR-1 Freedom, which carries a nuclear reactor, in late 2028.Demonstrated[3][1]

Laser-driven light sails

Laboratory tests in 2000 used lasers of 7.9 to 13.9 kW directed at sails mounted on pendulums. No laser-pushed sail has flown in the sources reviewed.Demonstrated[5]

No reliable public figure

Proposed: flexible lightsails for interstellar exploration, driven by radiation pressure.Proposed[6]

Stability results for flexible lightsails come from a computational study. The Caltech record mentions no physical lightsail experiments.Demonstrated[6][5]

Solar sails

Flown: IKAROS, launched by JAXA in May 2010, and LightSail 2, which raised its orbit around Earth in 2019.Demonstrated[7][8][9]

IKAROS: 1.12 mN of solar light pressure thrust, which JAXA called the expected value and estimated from orbit determination. LightSail 2: apogee rose about 2 km in four days.Reported, unverified[7][8][9]

Proposed: slow orbit changes driven by solar light pressure. IKAROS achieved a total velocity change of 100 m/s within about half a year.Proposed[8][9]

Thrust is only 1.12 mN, so changes build slowly. LightSail 2's team expected atmospheric drag to eventually overcome the sail's thrust.Reported, unverified[8][9]

What the table shows

NASA names chemical rockets as the propulsion used for Apollo, Space Shuttle and Artemis. A planning value for a liquid hydrogen and liquid oxygen rocket is about 350 seconds specific impulse.
Solar sails have flown twice in the sources reviewed: IKAROS in 2010 gave an estimated 1.12 mN of thrust, and LightSail 2 raised its orbit in 2019.
No nuclear thermal or nuclear electric propulsion system has flown in the sources reviewed, and the sources give no thrust figure for either.
Laser-driven light sails were tested in 2000 laboratory experiments with lasers of 7.9 to 13.9 kW, and no laser-pushed sail has flown in the sources reviewed.

Read with care

The figures come from different test settings: laboratory experiments, ground engine tests and in-orbit flights. They are not directly comparable.
The chemical figure is a planning value from NASA Glenn, and its vacuum or sea-level basis is not stated, so it is not a measured engine result.
The sources reviewed show no flight of nuclear thermal or nuclear electric propulsion. That does not prove that no such test has been run.
SR-1 Freedom's late 2028 launch is NASA's plan, not a result.

Sources

government report
Space Nuclear Propulsion
NASA Space Technology Mission Directorate, 21 Jul 2026. Accessed 21 Oct 2026.
other
Ideal Rocket Equation
NASA Glenn Research Center, Beginner's Guide to Aeronautics, 21 Nov 2023. Accessed 21 Oct 2026.
government report
SR-1 Freedom Advances with Progress on Nuclear Power Module
NASA, 20 Oct 2026. Accessed 21 Oct 2026.
conference paper
Nuclear Thermal Propulsion Ground Test History
Harold P. Gerrish. NASA Technical Reports Server, 24 Feb 2014. Accessed 21 Oct 2026.
news
Sail Technology Beamed to Future Space Exploration
NASA Jet Propulsion Laboratory, 5 Jul 2000. Accessed 21 Oct 2026.
peer reviewed paper
Dynamically stable radiation pressure propulsion of flexible lightsails for interstellar exploration
Ramon Gao, Michael D. Kelzenberg, Harry A. Atwater. Nature Communications, 17 May 2024. Accessed 21 Oct 2026.
government report
IKAROS press release
JAXA, 9 Jul 2010. Accessed 21 Oct 2026.
other
IKAROS
eoPortal, 21 Oct 2026. Accessed 21 Oct 2026.
news
LightSail 2 spacecraft successfully demonstrates flight by light
Phys.org, 1 Aug 2019. Accessed 21 Oct 2026.