Nuclear Rockets and Project Breakthrough Starshot.

"Modular Assembled Radiators for Nuclear Electric Propulsion Vehicles, or MARVL, aims to take a critical element of nuclear electric propulsion, its heat dissipation system, and divide it into smaller components that can be assembled robotically and autonomously in space. This is an artist’s rendering of what the fully assembled system might look like. NASA/Tim Marvel" (NASA,Nuclear Electric Propulsion Technology Could Make Missions to Mars Faster)

Nuclear rockets can have football-field-size heat conductors to control the thermal effect of a nuclear reactor. 

The new nuclear systems can journey to Mars faster than regular chemical rockets. The problem with nuclear thermal engines is the heat of the system. Nuclear reactors can transfer their thermal energy to water. Then, water transports heat to the larger water systems like lakes. In space nuclear reactors must shine that thermal energy to space. In the NASA construction reactor, the football-field size element conducts the heat from the reactor to space. The reactor itself can use highly enriched plutonium or neptunium as the nuclear fuel. 

The nuclear chemical reactors can use almost all types of liquids as propellant. In the simplest models, the nuclear reactor simply boils liquid that travels through it. The system called Nuclear Engine for Rocket Vehicle Applications (NERVA), is one of the most promising systems that NASA created. The rocket itself is simply a nuclear reactor and liquid hydrogen flows through that reactor. NERVA is a suitable tool for Jupiter and Saturn missions because it can use almost any liquid as fuel. 

In more complicated models the nuclear reactor can use things like laser rays to increase the expansion of the propellant. There are two ways to make sure. Nuclear reactors can also offer an energy supply to spacecraft. The first case is that the nuclear thermal engine can use propellant to rotate turbo generators. 

In other cases, the nuclear reactor can use different separated cycles to rotate the generator. The NERVA system can refill its tanks from the giant planet's atmospheres, their water moons, or from the hydrocarbon oceans of Titan. The system can use water or it can split water electrolytically. And then drive hydrogen to its tanks. The nuclear reactor forms particle flow that can also push spacecraft ahead. 

"An artist’s rendering that shows the different components of a fully assembled nuclear electric propulsion system. NASA/Tim Marvel" (NASA,Nuclear Electric Propulsion Technology Could Make Missions to Mars Faster)

The reactor itself is quite similar to a miniature nuclear reactor. 

The new system is more complicated and powerful than NERVA. The system is called Modular Assembled Radiators for Nuclear Electric Propulsion Vehicles, or MARVL. The system is a modular version of NERVA. The idea is that the modular nuclear rocket can be shot into orbiter modules. That makes it more comfortable and safer to handle. The nuclear rocket will shoot the orbiter by using chemical rockets. There the support system can load highly radioactive nuclear fuel to the reactor. 

In some models, the Space X Falcon heavy transports large components to the Earth orbiter. Then the system will push the large Mars craft to a higher stationar orbiter or moon orbiter. Then miniature shuttles will transport the highly radioactive nuclear material to the reactor and astronauts and robots will push those nuclear elements to the reactor. 

The system can also include 3D printers that can help to build and assemble large components in space. The 3D components can make the repairing part production more flexible. 3D printers can make miniature nuclear reactors in space. The only problem is nuclear fuel. The fuel must be shared and packed into small parts so that in the case of accidents the pollution is minimal. 

By the way...

Spaceborne nuclear reactors can also be used to create neutron beams. The system can use neutrons from a nuclear reactor and stress them with electromagnetic radiation. The system can have a neutron source at the bottom. And then it shoots neutrons through a channel that travels through the nuclear reactor. Nuclear rockets can half the flight time to Mars, Jupiter, and Saturn. The flight time to Mars is maybe 45 days. The length of the mission is about 2 years. 

But the stars are too far away. 

"A solar sail concept" (Wikipedia, Breakthrough Starshot)

Project Breakthrough Starshot.

Project Breakthrough Starshot is one of the first serious study projects to send space probes to Alpha Centauri, or accurately saying, the Proxima Centauri system in 40 years.  The system would use the solar sail to travel to the other solar system. 

The system can involve the hydrogen bomb detonations that happen at the Starshot craft trajectory. Those bombs make the Starshot operate like Medusa drive or improved Orion. In the original Project Orion, the nuclear bombs give speed to interplanetary spacecraft. In Medusa drive, the Orion spacecraft will be equipped with a solar sail. That gives more thrust to the nuclear bombs. The system uses nuclear pulse propulsion (NPP) or a series of nuclear bombs to travel outside the solar system. 

It's possible. That regular solar sail will get more power to its drive from nuclear bombs. The other space systems like other solar sails or nuclear rockets will send those nuclear bombs to the solar sail's route. And when solar sail makes the flyby that hydrogen bomb will detonate. That gives more thrust to the solar sail. But 40 years is a long time for waiting results. 

https://www1.grc.nasa.gov/historic-facilities/rockets-systems-area/7911-2/

https://www.nasa.gov/general/nuclear-electric-propulsion-technology-could-make-missions-to-mars-faster/

https://www.nasa.gov/space-technology-mission-directorate/tdm/space-nuclear-propulsion/

https://www.space.com/nasa-darpa-nuclear-thermal-rocket-2027

https://en.wikipedia.org/wiki/Breakthrough_Starshot

https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion

https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)