page_title: Return of Apollo 15 to Earth
section_title: Last day in lunar orbit and TEI
hierarchical_section_title: Return of Apollo 15 to Earth / Last day in lunar orbit and TEI
caption_attribution_description: English: The Moon as Apollo 15 travels back to Earth. Mare Australe is in the centre. The crater Humboldt is in the lower centre. Near the top on the terminator Vallis Schrödinger is visible, with Sikorsky crater cutting across it.
context_page_description: After the Apollo 15 LM Falcon lifted from the lunar surface on August 2, 1971, it rendezvoused and docked with the CSM Endeavour. After transferring across the lunar samples and other equipment, Falcon was jettisoned. It would fire its rocket engine to cause it to impact the lunar surface.
  Apollo 15 spent one more day in lunar orbit, continuing Worden's observations. After releasing a subsatellite, they ignited their service propulsion system to put them on a trajectory back to Earth. The next day, Worden performed an EVA to retrieve the film cassettes from the scientific instrument module bay cameras.
  The twelfth day in space was uneventful, with Mission Control holding a press conference where the astronauts were asked questions submitted by the news media. On their 13th and final day they prepared for reentry. During descent one of their three parachutes failed, and they splashed down successfully with only two.
context_section_description: Apollo 15 spent one final day in lunar orbit before trans-Earth injection (TEI), the SPS burn that would put them on a trajectory back to Earth. This day was spent mainly on the same tasks that had occupied Worden during the past four days.
  Mission Control decided to make many changes to the flight plan. The Laser Altimeter failed and was declared a lost cause. The crew were asked to use a 250 mm telephoto lens instead of an 80 mm lens on the Hasselblad. They were also instructed to take as many photographs as they liked. There was no point in them returning to Earth carrying unexposed film.
  Among their photographic targets were the terminator region, where the Sun would appear to be either rising or setting for an observer on the ground. These photographs were extremely useful as they showed surface features in strong relief. Now, with three persons on board, the crew were able to acquire many more handheld photographs, with one person always available to run the SIM and receive flight plan updates from Mission Control. The crew were also instructed to just turn on the Panoramic Camera and let it run now that all its primary targets had been imaged.
  As Apollo 15 appeared from behind the Moon on its 73rd orbit, there were only two more orbits left before TEI. During this time the crew had to release the subsatellite and prepare for the rocket burn that would send them home. Like all parts of the CSM and LM, the SPS was highly redundant. The only parts that were not were the engine bell and the combustion chamber.
  The subsatellite was designed to measure the gravitational field of the Moon and investigate the Earth and Moon's magnetospheres. It was 31 inches (79 cm) long, with a 14-inch (36 cm) hexagonal body. Weighing a total of 78.5 pounds (35.6 kg), it was powered by solar cells for daytime operations and a silver-cadmium battery for the nighttime. It had three booms that extended after release, which were 5 ft (1.5 m) long.
  Before releasing the subsatellite, the crew performed another engine burn to raise their orbit. This was designed to allow the subsatellite to last longer. The three-second burn raised their orbit from 65.4 by 52.2 nautical miles (121.1 by 96.7 kilometres) to 76.0 by 54.3 nautical miles (140.8 by 100.6 km). In this new orbit it was expected that the subsatellite would survive for one year.
  As they began their 74th and final orbit of the Moon, the crew positioned the spacecraft for the correct attitude for release. The subsatellite was released on schedule. The release process involved two pyrotechnic bolts being fired to release the satellite which was launched using a spring-loaded mechanism, a pin in a curved groove to impart a spin in the satellite. The spinning helped to stabilise the craft.
  After having the information needed to burn the SPS to send them home radioed from Mission Control, Apollo 15 disappeared behind the Moon for the last time. Igniting on time, the SPS burned for 2 minutes, 21 seconds, adding 3,050 ft/s (930 m/s) to their speed.
  Little was left for the crew to do on their tenth day in space. They continued to photograph the lunar surface as it receded from them, using up the remaining film. And now they were back in the constant sunlight of cis-lunar space, they put the spacecraft into PTC.
caption: The Moon as Apollo 15 travels back to Earth. Mare Australe is in the center. The crater Humboldt is in the lower center. Near the top on the terminator Vallis Schrödinger is visible, with Sikorsky crater cutting across it.
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