Astronomy

In astronomy, a lunar distance (LD) is a measurement of the distance from the Earth to the Moon. The average distance from the center of the Earth to the center of the Moon is 384,400 km (238,900 mi).One astronomical unit is about 389 LD. The actual distance varies over the course of the orbit of the moon, from 363,104 km (225,622 mi) at the perigee and 405,696 km (252,088 mi) at apogee, resulting in a differential range of 42,592 km (26,465 mi).
Millimeter-precision measurements of the lunar distance are made by measuring the time taken for light to travel between LIDAR stations on the Earth and retroreflectors placed on the Moon. Lunar laser ranging measurement data is available from the Paris Observatory Lunar Analysis Center,^[2] and the active stations.
The Moon is spiraling away from the Earth at an average rate of 3.8 cm (1.5 in) per year, as detected by the Lunar Laser Ranging Experiment.The recession rate is considered anomalously high.By coincidence, the diameter of corner cubes in retroreflectors on the Moon is also 3.8 cm (1.5 in).
The tidal dissipation rate has varied during the Earth's geological history.
The first person to measure the distance to the Moon was the 2nd-century-BC astronomer and geographer Hipparchus, who exploited the lunar parallax using simple trigonometry, measuring the distance as 400,000 kilometers. He was approximately 26,000 km (16,000 mi) off the actual distance, an error of about 6.8%.

Apollo astronauts who flew to the Moon without landing

Besides the twelve people who have walked on the Moon, twelve more have flown to within 0.001 lunar distance of its surface. During each of the six missions with successful lunar landings, one astronaut remained in lunar orbit while the other two landed. In addition, the three-person crews of Apollo 8 and Apollo 10 also entered lunar orbit, and the crew of Apollo 13 looped around the Moon on a free-return trajectory.
All nine manned missions to the Moon took place as part of the Apollo program over a period of just under four years, from 21 December 1968 to 19 December 1972. Of the 24 people who have flown to the Moon, 17 are still living as of November 2015.
John Young, Eugene Cernan and Jim Lovell are the only three people to have flown to the Moon twice. Young and Cernan each set foot on it during their respective second lunar missions, while Lovell is the only person to have flown to the Moon twice without landing.
During Cernan's first lunar mission on Apollo 10, he tied the present record set by Bill Anders on Apollo 8 as the youngest person to fly to the Moon. Each was 35 years and 65 days old on his launch date and 35 years and 68 days old when he entered lunar orbit. (The oldest person to fly to the Moon was Alan Shepard, who walked on its surface during the Apollo 14 mission. Shepard was 47 years and 74 days old on his launch date and 47 years and 78 days old when he entered lunar orbit.)
Because of Apollo 13's free-return trajectory, Lovell, Jack Swigert and Fred Haise flew higher above the Moon's 180° meridian (opposite Earth) than anyone else has flown (254 km/158 mi). Coincidentally, due to the Moon's distance from Earth at the time, they simultaneously set the present record for humans' greatest distance from Earth, reaching an altitude of 400,171 km (248,655 mi) above sea level at 0:21 UTC on 15 April 1970.

	Name	Born	Died	Age on mission	Mission	Mission dates	Service	Notes
1.	Frank Borman	March 14, 1928 (age 87)		40	Apollo 8	December 21–27, 1968	Air Force
2.	Jim Lovell	March 25, 1928 (age 87)		40			Navy	also flew on Apollo 13
3.	Bill Anders	October 17, 1933 (age 82)		35			Air Force
4.	Tom Stafford	September 17, 1930 (age 85)		38	Apollo 10	May 18–26, 1969	Air Force	later flew on Apollo–Soyuz Test Project
	John Young	September 24, 1930 (age 85)		38			Navy	landed on Apollo 16; later flew two space shuttle missions
	Eugene Cernan	March 14, 1934 (age 81)		35			Navy	landed on Apollo 17
5.	Michael Collins	October 31, 1930 (age 85)		38	Apollo 11	July 16–24, 1969	Air Force
6.	Dick Gordon	October 5, 1929 (age 86)		40	Apollo 12	November 14–24, 1969	Navy	trained to land, slated for Apollo 18 (cancelled)[8]
	Jim Lovell	March 25, 1928 (age 87)		42	Apollo 13	April 11–17, 1970	Navy	already flown on Apollo 8; intended to land
7.	Jack Swigert	August 30, 1931	December 27, 1982 (aged 51)	38			NASA
8.	Fred Haise	November 14, 1933 (age 81)		36			NASA[9]	intended to land; later trained to land and slated for Apollo 19 (cancelled);[8] flew shuttle on approach / landing tests
9.	Stu Roosa	August 16, 1933	December 12, 1994 (aged 61)	37	Apollo 14	January 31 – February 9, 1971	Air Force	in rotation to land on Apollo 20 (cancelled)
10.	Al Worden	February 7, 1932 (age 83)		39	Apollo 15	July 26 – August 7, 1971	Air Force
11.	Ken Mattingly	March 17, 1936 (age 79)		36	Apollo 16	April 16–27, 1972	Navy	later flew two space shuttle missions.
12.	Ron Evans	November 10, 1933	April 7, 1990 (aged 56)	39	Apollo 17	December 7–19, 1972	Navy

TYPES OF ORBITS AROUND EARTH

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  Examples

• Earth observation satellites and spy satellites use LEO as they are able to see the surface of the Earth more clearly as they are not so far away. They are also able to traverse the surface of the Earth. A majority of artificial satellites are placed in LEO, making one complete revolution around the Earth in about 90 minutes.

• The International Space Station is in a LEO about 400 km (250 mi) above the Earth's surface.

• Since it requires less energy to place a satellite into a LEO and the LEO satellite needs less powerful amplifiers for successful transmission, LEO is still used for many communication applications. Because these LEO orbits are not geostationary, a network (or "constellation") of satellites is required to provide continuous coverage. (Many communication satellites require geostationary orbits, and move at the same angular velocity as the Earth. Some communications satellites including the Iridium phone system use LEO.)

• Lower orbits also aid remote sensing satellites because of the added detail that can be gained. Remote sensing satellites can also take advantage of sun-synchronous LEO orbits at an altitude of about 800 km (500 mi) and near polar inclination. Envisat is one example of an Earth observation satellite that makes use of this particular type of LEO.

Space debris

The LEO environment is becoming congested with space debris due to the frequency of object launches. This has caused growing concern in recent years, since collisions at orbital velocities can easily be dangerous, and even deadly. Collisions can produce even more space debris in the process, creating a domino effect, something known as Kessler Syndrome. The Joint Space Operations Center, part of United States Strategic Command (formerly the United States Space Command), currently tracks more than 8,500 objects larger than 10 cm in LEO. However, a limited Arecibo Observatory study suggested there could be approximately one million objects larger than 2 millimeters, which are too small to be visible from Earth-based observatories.

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• List of orbits
• Medium Earth orbit (MEO)
• High Earth orbit (HEO)
• Highly elliptical orbit (HEO)
• Geostationary orbit (GEO)
• Specific orbital energy examples
• Suborbital spaceflight
• Atmosphere of Earth

low earth orbit(LEO)

A low Earth orbit (LEO) is an orbit around Earth with an altitude between 160 kilometers (99 mi) (orbital period of about 88 minutes), and 2,000 kilometers (1,200 mi) (about 127 minutes). Objects below approximately 160 kilometers (99 mi) will experience very rapid orbital decay and altitude loss.^[1][2] The orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s, but reduces with increased orbital altitude.
With the exception of the manned lunar flights of the Apollo program, all human spaceflights have taken place in LEO (or were suborbital). The altitude record for a human spaceflight in LEO was Gemini 11 with an apogee of 1,374.1 kilometers (853.8 mi). All manned space stations to date, as well as the majority of satellites, have been in LEO.

Comparison of GPS, GLONASS, Galileo and Compass (medium earth orbit) satellite navigation system orbits with the International Space Station, Hubble Space Telescope and Iridium constellation orbits, Geostationary Earth Orbit, and the nominal size of the Earth.^[a] The Moon's orbit is around 9 times larger (in radius and length) than geostationary orbit.

Orbital characteristics

Objects in LEO encounter atmospheric drag in the form of gases in the thermosphere (approximately 80–500 km up) or exosphere (approximately 500 km and up), depending on orbit height. Objects in LEO orbit Earth between the atmosphere and below the inner Van Allen radiation belt. The altitude is usually not less than 300 km for satellites, as that would be impractical due to atmospheric drag.
The orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s, but reduces with increased orbital altitude. Calculated for circular orbit of 200 km it is 7.79 km/s and for 1500 km it is 7.12 km/s.^[3] The delta-v needed to achieve low Earth orbit starts around 9.4 km/s. Atmospheric and gravity drag associated with launch typically adds 1.5–2.0 km/s to the launch vehicle delta-v required to reach normal LEO orbital velocity of around 7.8 km/s (28,080 km/h).

Equatorial low Earth orbits (ELEO) are a subset of LEO. These orbits, with low inclination to the Equator, allow rapid revisit times and have the lowest delta-v requirement (i.e., fuel spend) of any orbit. Orbits with a high inclination angle to the equator are usually called polar orbits.
Higher orbits include medium Earth orbit (MEO), sometimes called intermediate circular orbit (ICO), and further above, geostationary orbit (GEO). Orbits higher than low orbit can lead to early failure of electronic components due to intense radiation and charge accumulation.

Use of LEO

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Roughly half an orbit of the ISS.

Although the Earth's pull due to gravity in LEO is not much less than on the surface of the Earth, people and objects in orbit experience weightlessness because they are in free fall.
A low Earth orbit is simplest and cheapest for satellite placement. It provides high bandwidth and low communication time lag (latency), but satellites in LEO will not be visible from any given point on the Earth at all times.

persons to step on moon

Thirty-two astronauts were assigned to fly in the Apollo manned lunar landing program. Twenty-four of these left Earth’s orbit and flew around the Moon (Apollo 1 never launched and Apollo 7 and Apollo 9 were low Earth orbit spacecraft testing missions). In addition, nine astronauts flew Apollo spacecraft in the Apollo Applications Programs Skylab and Apollo–Soyuz Test Project.
Twelve of these astronauts walked on the Moon’s surface, and six of those drove Lunar Roving Vehicles on the Moon. While three astronauts flew to the Moon twice, none of them landed on the Moon more than once. The nine Apollo missions to the Moon occurred between December 1968 and December 1972.
Apart from these 24 people who visited the Moon, no human being has gone beyond low Earth orbit. They have, therefore, been farther from the Earth than anyone else. They are also the only people to have directly viewed the far side of the Moon. The twelve who walked on the Moon are the only people ever to have set foot on an astronomical object other than the Earth.
Of the 24 astronauts who flew to the Moon, two went on to command a Skylab mission, one commanded Apollo–Soyuz, one flew as commander for approach and landing tests of the Space Shuttle, and two went on to command orbital Space Shuttle missions. A total of 24 NASA astronauts from the Apollo era flew on the Space Shuttle.

Apollo astronauts who walked on the Moon

"Moonwalkers" redirects here. For the upcoming film, see Moonwalkers (film).

Twelve people have walked on the Moon. Eight of them are still living as of November 2015. All manned lunar landings took place between July 1969 and December 1972 as part of the Apollo program.

	Name	Born	Died	Age at first step	Mission	Lunar EVA dates	Service	Alma Mater
01.	Neil Armstrong	August 5, 1930	August 25, 2012 (aged 82)	38y 11m 15d	Apollo 11	July 21, 1969[6]	NASA[7]	Purdue University, University of Southern California
02.	Buzz Aldrin	January 20, 1930 (age 85)		39y 6m 0d			Air Force	United States Military Academy, MIT
03.	Pete Conrad	June 2, 1930	July 8, 1999 (aged 69)	39y 5m 17d	Apollo 12	November 19–20, 1969	Navy	Princeton University
04.	Alan Bean	March 15, 1932 (age 83)		37y 8m 4d			Navy	University of Texas, Austin
05.	Alan Shepard	November 18, 1923	July 21, 1998 (aged 74)	47y 2m 18d	Apollo 14	February 5–6, 1971	Navy	United States Naval Academy, Naval War College
06.	Edgar Mitchell	September 17, 1930 (age 85)		40y 4m 19d			Navy	Carnegie Mellon University, Naval Postgraduate School, MIT
07.	David Scott	June 6, 1932 (age 83)		39y 1m 25d	Apollo 15	July 31 – August 2, 1971	Air Force	University of Michigan (freshman year, and later, an honorary doctorate), United States Military Academy, MIT
08.	James Irwin	March 17, 1930	August 8, 1991 (aged 61)	41y 4m 14d			Air Force	United States Naval Academy, University of Michigan
09.	John W. Young	September 24, 1930 (age 85)		41y 6m 28d	Apollo 16	April 21–23, 1972	Navy	Georgia Institute of Technology
10.	Charles Duke	October 3, 1935 (age 80)		36y 6m 18d			Air Force	United States Naval Academy, MIT
11.	Eugene Cernan	March 14, 1934 (age 81)		38y 9m 7d	Apollo 17	December 11–14, 1972	Navy	Purdue University, Naval Postgraduate School
12.	Harrison Schmitt	July 3, 1935 (age 80)		37y 5m 8d			NASA	Caltech, University of Oslo (exchange), Harvard University

On each of the Apollo 17 extravehicular activities (EVAs), Harrison Schmitt was the second person out of, and the first person back into, the lunar module. Schmitt is thus the twelfth and last person to have stepped onto the Moon. Eugene Cernan, as the second person to enter the lunar module on the final EVA, is the last person to have walked on the Moon.
Alan Shepard was the oldest person to walk on the Moon, at age 47 years and 79 days. Charlie Duke was the youngest, at age 36 years and 201 days.
Jim Lovell and Fred Haise were scheduled to walk on the Moon during the Apollo 13 mission, but the lunar landing was aborted following a major malfunction en route to the Moon. Haise was again scheduled to walk on the Moon as commander of Apollo 19, but Apollo 18 and Apollo 19 were canceled on September 2, 1970.
Joe Engle had trained on the backup crew for Apollo 14 to explore the Moon with Cernan, but Engle was replaced by Schmitt on the primary crew for Apollo 17. Schmitt had previously been crewed with Apollo 12 command module pilot Dick Gordon in anticipation of Apollo 18, but Schmitt replaced Engle on Apollo 17 after the cancellation of Apollo 18 and Apollo 19, leaving Gordon as the last Apollo astronaut to train extensively for lunar exploration without ever landing on the Moon.