Manned Venus Flyby
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A manned Venus flyby was considered by NASA in the mid-1960s as part of the Apollo Applications Program, using hardware derived from the Apollo program. Launch would take place on October 31, 1973, with a Venus flyby on March 3, 1974 and return to Earth on December 1, 1974.
The proposed mission would use a Saturn V to send three men to fly past Venus in a flight which would last approximately one year. The S-IVB stage would be a 'wet workshop' similar to Skylab, first using the S-IVB engine to launch the mission on course to Venus, and then venting of any remaining fuel to serve as home for the crew for the duration of the mission. The Apollo SM engine would be used for course corrections on the way to Venus and back to Earth, and for a braking burn before the Command Module re-entered Earth's atmosphere. In order to free up more space in the Spacecraft Lunar Module Adapter for the docking tunnel connecting the CSM to the S-IVB, the SPS engine on the Service Module would be replaced by two LM engines. These would provide similar thrust with shorter nozzles, and would also give the mission the added safety of redundant engines.
Precursors to the Venus flyby would include an initial orbital test flight with an S-IVB 'wet workshop' and basic docking adapter, and a year-long test flight taking the S-IVB to a near-geostationary orbit around the Earth.
One oddity of the Venus flyby mission is that, unlike trips to the Moon, the CSM would separate and dock with the S-IVB stage before the S-IVB burn, so the astronauts would fly 'eyeballs-out', the thrust of the engine pushing them out of their seats rather than into them. This was required because there was only a short window for an abort burn by the CSM to return to Earth after a failure in the S-IVB, so all spacecraft systems needed to be operational and checked out before leaving the parking orbit around Earth to fly to Venus.
The mission would measure:
- Atmospheric density, temperature and pressure as functions of altitude, latitude and time.
- Definition of the planetary surface and its properties.
- Chemical composition of the low atmosphere and the planetary surface.
- Ionospheric data such as radio reflectivity and electron density and properties of cloud layers.
- Optical astronomy - UV and IR measurements above the Earth's atmosphere to aid in the determination of the spatial distribution of hydrogen.
- Solar astronomy - UV, X-ray and possible infrared measurements of the solar spectrum and space monitoring of solar events.
- Radio and radar astronomy - radio observations to map the brightness of the radio sky and to investigate solar, stellar and planetary radio emissions; radar measurements of the surface of Venus and Mercury
- X-ray astronomy - measurements to identify new X-ray sources in the galactic system and to obtain additional information on sources previously identified.
- Data on the Earth-Venus interplanetary environment, including particulate radiation, magnetic fields and meteoroids.
- Data on the planet Mercury, which will be in mutual planetary alignment with Venus approximately two weeks after the Venus flyby
Phase A 
Phase A of the plan would have launched a 'wet workshop' S-IVB and a standard Block II Apollo CSM into orbit on a Saturn V. The crew would separate the CSM from the S-IVB by blowing off the SLA panels, then perform a Transposition and Docking maneuver similar to that conducted on the lunar flights, in order to dock with the docking module attached to the front of the S-IVB. Optionally they could then use the S-IVB engine to launch them into a high orbit before they vented any remaining fuel into space and entered the S-IVB fuel tanks to conduct experiments for a few weeks. After evaluating the use of the S-IVB as a long-term habitat for astronauts, they would separate the CSM from the S-IVB and return to Earth.
Phase B 
Phase B would test the Venus flyby spacecraft in a long duration mission in high orbit. A Saturn V would launch a Block III CSM designed for long-term spaceflight and a modified S-IVB with the Environmental Support Module required for the real Venus flyby, and following the transposition and docking maneuver the S-IVB engine would carry the spacecraft to a circular orbit at an altitude of about 25,000 miles around the Earth. This altitude would be high enough to be clear of Earth's radiation belts while exposing the spacecraft to an environment similar to that of a trip to Venus, yet close enough to Earth that the astronauts could use the CSM to return in a few hours in an emergency.
Power would probably be provided by solar panels similar to those used on Skylab, as fuel cells would require a very large amount of fuel to operate for a year. Similarly the fuel cells in the SM used to provide power on lunar flights would be replaced by batteries which would provide enough power for the duration of launch and re-entry operations.
Phase C 
Phase C would be the actual manned flyby, using a Block IV CSM and an updated version of the Venus flyby S-IVB which would carry a large radio antenna for communication with Earth and two or more small probes which would be released shortly before the flyby to enter the atmosphere of Venus. The Block IV CSM has LM engines replacing the Service Propulsion System engines, batteries to replace the fuel cells, and other modifications to support long-range communication with Earth and the higher re-entry velocities required for the return trajectory compared to a return from lunar orbit.
The Phase C mission was planned to launch in late October or early November 1973, when the velocity requirements required to reach Venus and the duration of the resulting mission would be at their lowest. After a brief stay in Earth parking orbit to check out the spacecraft the crew would head for Venus: in the event of a major problem during the Trans-Venus Injection burn, they would have roughly an hour to separate the CSM from the S-IVB and use the SM engine to cancel out most of the velocity they gained from the burn. This would put them into a highly elliptical orbit which would typically bring them back to Earth for a re-entry two to three days later. Beyond that time the SM engine would not have enough fuel to bring the CSM back to Earth before the SM batteries ran out of power: it would literally be 'Venus or Bust'.
After a successful S-IVB burn, the spacecraft would pass approximately 3000 miles from the surface of Venus about four months later. The flyby velocity would be so high that the crew would only have a few hours for detailed study of the planet. At this point, one or more unmanned probe landers would separate from the main craft and land on Venus.
During the rest of the mission the crew would perform astronomical studies of the Sun, the sky and Mercury, which they would approach within 0.3 astronomical units.
See also 
- Colonization of Venus
- Manned mission to Mars
- Orion Asteroid Mission – Proposed Project Constellation mission similar in nature, but to a Near-Earth Asteroid
- TMK (Soviet flyby plan)
- Manned Venus Flyby study, Feb. 1, 1967
- Preliminary considerations of Venus exploration via manned flyby, Nov 30, 1967
- A Venus lander probe for manned flyby missions, Feb 23, 1968
- A survey of manned Mars and Venus flyby missions in the 1970s May 17, 1966
- Manned Venus flyby meteorological balloon system, July 29, 1968
- Experiment payload for manned venus encounter mission - venus tracking and data orbiter, Jun 13, 1968
- Drop sonde and photo sinker probes for a manned venus flyby mission , May 7, 1968
- Human Venus Exploration Architecture Studies
- Manned Venus Orbiting Mission
- NASA Technical Memorandum - Manned Venus Orbiting Mission
- Overview of a related Apollo to Venus mission.
- Youtube video of the mission, simulated using the Orbiter Spaceflight Simulator
- Manned Venus Flyby: Apollo’s Hail Mary Pass, Article at the Failed Steps: The Space Race As It Might Have Been Blog.