Oppressive China's Chang'e enters Lunar orbit

China's Chang'e 3 moon mission, the country's first flight to land a rover on the moon, is depicted in this graphic released by the China Aerospace Science and Technology Corporation. 

The mission launched on Dec. 2, 2013 Beijing Time and arrived in lunar orbit less than five days later. Credit: China Aerospace Science and Technology Corporation

Less than five days after leaving Earth atop a blazing Long March launcher, China's Chang'e 3 spacecraft reached lunar orbit Friday to prepare for an historic rocket-assisted touchdown in the moon's Bay of Rainbows later this month.

Outfitted with a six-wheeled robotic rover and smarts to avoid hazards in the landing zone, Chang'e 3 is China's boldest unmanned space mission to date, extending feats achieved by a pair of lunar orbiters launched in 2007 and 2010.

The four-legged lander fired its propulsion system for six minutes and braked into orbit around the moon at 0953 GMT (4:53 a.m. EST) Friday, according to China's state-run Xinhua news agency.

NASA ARTEMIS Spacecraft Prepares for Lunar Orbit

They've almost arrived. It took one and a half years, over 90 orbit maneuvers, and - wonderfully - many gravitational boosts and only the barest bit of fuel to move two spacecraft from their orbit around Earth to their new home around the moon.

Along their travels, the spacecraft have been through orbits never before attempted and made lovely curlicue leaps from one orbit to the next. This summer, the two ARTEMIS spacecraft - which began their lives as part of the five-craft THEMIS mission studying Earth's aurora - will begin to orbit the moon instead. THEMIS is an acronym for the Time History of Events and Macroscale Interaction during Substorms spacecraft.

The view from above of the ARTEMIS orbits as they make the transition from the kidney-shaped Lissajous orbits on either side of the moon to orbiting around the moon. 

Credit: NASA/Goddard Space Flight Center

Even with NASA's decades of orbital mechanics experience, this journey was no easy feat.

The trip required several maneuvers never before attempted, including several months when each craft moved in a kidney-shaped path on each side of the moon around, well, nothing but a gravitational point in space marked by no physical planet or object.

"No one has ever tried this orbit before, it's an Earth-moon libration orbit," says David Folta a flight dynamics engineer at NASA's Goddard Space Flight Center in Greenbelt, Md. "It's a very unstable orbit that requires daily attention and constant adjustments."

The journey for ARTEMIS - short for Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun - began in 2009, after THEMIS had completed some two years of science data collection on the magnetic environment around Earth, the aurora, and how these are affected by the sun.

The spacecraft are solar-powered, but orbits for the two outermost THEMIS spacecraft had slipped over time and were going to be subjected to regular eight-hour periods of darkness. These spacecraft could withstand up to three hours without sunlight, but this much darkness would soon leave the batteries completely discharged.

Chang'e-2 enters Lunar Orbit

China's second lunar probe entered the moon's orbit on Saturday, eight days after the Long March 3C rocket carrying the Chang'e-2 blasted off from the launch centre, state media reported.

The unmanned probe will start "scientific exploration activities soon," said the official Xinhua news agency, citing the Beijing Aerospace Control Center.

The Chang'e-2 will first circle the moon at a distance of 100 kilometres (62 miles). The plan is for it to drop into orbit 15 kilometres from the moon's surface.

It will conduct various tests over a six-month period in preparation for the expected launch in 2013 of the Chang'e-3, which China hopes will be its first unmanned landing on the moon.

The Chang'e programme, named after a mythical Chinese goddess who flew to the moon, is seen as an effort to put China's space exploration programme on a par with those of the United States and Russia.

The first lunar probe, launched in October 2007, was in orbit for 16 months.

Beijing hopes to bring a moon rock sample back to earth in 2017, with a manned mission pencilled in for around 2020, according to state media.

Lunar Orbit: Perigee and Apogee Moon(s)

Properties of the lunar orbit
The orbit of the Moon is distinctly elliptical with an average eccentricity of 0.0549.

The non-circular form of the lunar orbit causes variations in the Moon's angular speed and apparent size as it moves towards and away from an observer on Earth.

The mean angular daily movement relative to an imaginary observer at the barycenter is 13.176358° to the east. The orientation of the orbit is not fixed in space, but precesses over time.

Elongation
The Moon's elongation is its angular distance east of the Sun at any time. At new moon it is zero and the Moon is said to be in conjunction. At full moon the elongation is 180° and it is said to be in opposition. In both cases the moon is in syzygy, that is, the Sun, Moon and Earth are nearly aligned. When elongation is either 90° or 270° the Moon is said to be in quadrature.

Nodes
The line of nodes of the lunar orbit (the line of the intersection of the plane of the lunar orbit with the plane of the ecliptic) has a retrograde motion, that is, it rotates towards the west (along the ecliptic) at a rate of 19°21′ per year, that is, with a period of 6,793 days or 18.60 years (nutation period).

Thus the node, or point at which the Moon's orbit crosses the ecliptic, moves steadily clockwise (viewed from celestial north), closely aligned with Earth's orbital path but in the opposite direction. The interval of time between the passage of the Moon through the same node is called the draconitic month, and is 27.2122 days. The times of the eclipses depend upon these factors.

Line of Apsides
Perigee is the distance of closest approach, whereas apogee is its farthest recession. The line joining these two points (the line of apsides) has a progressive motion or advances, that is, it slowly rotates counterclockwise in space in the plane of the Moon's orbit, that is, direct motion, making one complete revolution in 3232.575 days or about 8.85 years.

Lunar Standstill
When the ascending node of the moon's orbit coincides with the vernal equinox in the northern hemisphere, the declination of the moon in the sky reaches a maximum at 23°29′ + 5°9′ or 28°36′.

This is called the major standstill. Nine and a half years later, when the descending node has come to the same point, the angle is only 23°28′ − 5°8′ or 18°19′, and the declination of the moon is a minimum. This is the minor standstill.