Starship is the name of the 2019 version of the second stage of the SpaceX reusable super heavy lift vehicle, resting upon the Super Heavy booster. The term Starship may also be used to refer to the complete stack of both stages as well, however.
The origins of Starship were rooted in the Interplanetary Transportation System. This architecture was revealed in a 2016 speech by Elon Musk at the International Astronomical Congress. The concept was conceived to be able to be reused a thousand times and to hold crews of over a hundred people with its primary intent to send people to Mars. The concept would depend upon tanker ships and orbital refueling, and it would extensively utilize in-situ resource utilization to produce the methane fuel required for the return voyage to Earth.
The design was immense and depended upon forty-two methane Raptor engines on the booster alone, allowing it to produce thrust of thirteen-thousand metric tons. Upon stage separation, the booster would return to the launch site, landing propulsively on the launch mounts so that the booster could quickly be refueled and again flown. The second stage, which in some launches would include a cabin, then had nine additional Raptor engines to accelerate the ship to Low Earth Orbit. In order to continue a trip to Mars, the second stage would have to bee refueled by one or more tankers.
The proposal of the carbon fiber launch vehicle came with an estimated necessary cost of investment of ten billion dollars by Elon Musk, who suggested that a massive public-private partnership may be the best option for the vehicle. The original timeline of the proposal called for structures and propulsion development to complete in 2019, when ship testing and orbital testing were to begin.
It was presented by Elon Musk during the announcement of Yusaku Maezawa' Dear Moon project, as an evolution of the BFR/BFS concept and Interplanetary Transportation System (ITS) concepts.
Originally planned to be constructed of carbon fiber composite, it was changed to a Stainless Steel design in January 2019 .
85-120 tonnes mass, 9m diameter, 100-150 tonnes of payload to LEO, 100-150 tonnes to Mars. These are target values, the lower the mass of the vehicle, the higher the payload mass will be. Payload volume of 1000 m3.
3 vacuum Raptor engines with 380s ISP and 3 atmospheric Raptor engines with 330s ISP. Nominal thrust of 2000 kN, (200 tonnes of force per engine) These numbers are subject to change as the engine and the vehicle concepts are under development.
120-160 day transportation time to Mars, using aerocapture at Mars.
Fully reusable, rapid turnover and low maintenance vehicle.
Up to 100 passengers to Mars, although this has not been demonstrated yet by SpaceX.
The fundamental enabling technology of the Starship is supersonic retro propulsive landing on Mars. The use of supersonic retropropulsion in a critical phase of the Mars entry path allows the vehicle to land heavier payloads that previously thought possible. Although the exact details are not public, the current SpaceX Falcon 9 booster rocket has done flight tests that would confirm the flight path. 
A second enabling technology is the capacity of refueling in orbit.
A third enabling technology is the use of methane as fuel, than can be provided by In-situ ressources production systems on Mars, and therefore allow for the re-use of the spaceship.
A fourth technology is a robust heat shield for Mars and Earth entry. This allows for fast re-use and lower costs, but also for faster transit times, reducing the radiation exposure to travellers. The Spaceship is not intended to use low energy Hoffman transfer orbits, but higher velocity orbits. These have lower transit times but leave the vehicle with significant velocity when it reaches Mars or Earth. The Starship must then use direct entry and aerodynamic braking to shed the kinetic energy from the extra velocity.
The NASA Ames research center trajectory browser can be used to explore transit times to Mars and other bodies in the Solar System. Trajectory browser
- "Interplanetary Transport System". n.d. Spaceflight101.Com. Accessed January 4, 2020.
- Musk, Elon. 2016. Making Humans a Multiplanetary Species. Guadalajara, Mexico.
- Popular Mechanics article 
- AEROTHERMAL ANALYSIS OF REUSABLE LAUNCHER SYSTEMS DURING RETRO-PROPULSION REENTRY AND LANDING