astronomy

SELENE

SELENE is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand SELENE rather than just read about it. In short: SELENE (; Selenological and Engineering Explorer), better known in Japan by its nickname Kaguya (かぐや), was the second Japanese lunar orbiter spacecraft following the Hiten probe. Produced by the Institute of Space and Astronautical Science (ISAS) and the National Space Development Agency (NASDA), the spacecraft was launched on September 14, 2007.

Key takeaways

SELENE belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect SELENE to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of SELENE from memory before moving on to harder problems.

Reference excerpt

SELENE (; Selenological and Engineering Explorer), better known in Japan by its nickname Kaguya (かぐや), was the second Japanese lunar orbiter spacecraft following the Hiten probe. Produced by the Institute of Space and Astronautical Science (ISAS) and the National Space Development Agency (NASDA), the spacecraft was launched on September 14, 2007. After orbiting the Moon for a year and eight months, the main orbiter was instructed to impact on the lunar surface near the crater Gill on June 10, 2009.

Nickname

The orbiter's nickname, Kaguya, was selected by the general public. It comes from the name of a lunar princess in the ancient Japanese folktale The Tale of the Bamboo Cutter. After their successful release, its sub-satellites, Rstar and Vstar, were named Okina and Ouna, also derived from characters in the tale.

Mission objectives The main scientific objectives of the mission were to:

Study the origins of the Moon and its geologic evolution Obtain information about the lunar surface environment

Launch

SELENE launched on September 14, 2007, at 01:31:01 UTC on an H-IIA (Model H2A2022) carrier rocket from Tanegashima Space Center into a 281.55-kilometre (174.95 mi) (perigee) / 232,960-kilometre (144,750 mi) (apogee) geocentric parking orbit. The total launch mass was 3,020 kilograms (6,660 lb). The SELENE mission was originally scheduled to launch in 2003, but rocket failures on another mission and technical difficulties delayed the launch until 2007. Launch was planned for August 16, 2007, but was postponed when some electronic components were found to be installed incorrectly.

Lunar operations On October 3, it entered an initial 101-to-11,741-kilometre (63 to 7,296 mi) polar lunar orbit. On October 9, the relay satellite was released into a 100-to-2,400-kilometre (62 to 1,491 mi) orbit, while on October 12 the VLBI satellite was released into a 100-to-800-kilometre (62 to 497 mi) one. Finally, by October 19, the orbiter was in a circular 100-kilometre (62 mi) orbit. The nominal mission duration was one year plus possible extensions. On October 31, 2007, Kaguya deployed its Lunar Magnetometer, Lunar Radar Sounder, Earth-looking Upper Atmosphere and Plasma Imager. On December 21, 2007, Kaguya began regular operations after all fifteen observation experiments had been satisfactorily verified. Kaguya completed the planned operation by the end of October 2008 and began extended operations planned to continue through March 2009. It would then be sent into a circular 50-kilometre (31 mi) orbit, and finally to an elliptical 20-to-100-kilometre (12 to 62 mi) one, with a controlled impact occurring by August 2009. Because of a degraded reaction wheel, the plan was changed so that on February 1, 2009, the orbit was lowered to 50 kilometres (31 mi) ± 20 kilometres (12 mi), and impact with lunar surface occurred at 18:25 UTC on June 10, 2009.

Design The mission featured three separate spacecraft:

Main orbiter Source:

Shape: octagonal prism Mass: 2,914 kilograms (6,424 lb) Size: 2.1 x 2.1 x 4.8 m (6.9 x 6.9 x 16 ft) Attitude control: Three-axis stabilized Power: 3.5 kW (Max.) Mission period: 1 year Mission orbit: Circular, 100-kilometre (62 mi) Inclination: 90 degrees

Okina (small relay satellite) Okina (formerly Rstar) and Ouna (formerly Vstar) were octagonal prisms to support radio science. Okina relayed radio communications between the orbiter and the Earth when the orbiter was behind the Moon. This allowed, for the first time, the direct Doppler shift measurements needed to precisely map the gravitational field of the lunar farside; previously, the farside gravity field could only be inferred by nearside measurements. The relay satellite impacted the lunar farside near the Mineur D crater at 19:46 JST (10:46 UTC) on February 12, 2009.

Function: two-way radio science relay, orbiter-earth Mass: 53 kilograms (117 lb) Size: 1.0 x 1.0 x 0.65 m (3.3 x 3.3 x 2.1 ft) Attitude control: spin-stabilized Power: 70 W Initial orbit: 100 to 2,400 kilometres (62 to 1,491 mi) Inclination: 90 degrees

Ouna (VLBI satellite) Ouna used Very Long Baseline Interferometry as a second way to map the Moon's gravity field. It was especially useful at the lunar limb, where the gravitational acceleration is perpendicular to the line of sight to Earth, making Doppler measurements unsuitable.

Function: VLBI radio science Mass: 53 kilograms (117 lb) Size: 1.0 x 1.0 x 0.65 m (3.3 x 3.3 x 2.1 ft) Attitude control: spin-stabilized Power: 70 W Initial orbit: 100 to 800 kilometres (62 to 497 mi) Inclination: 90 degrees

Instruments

SELENE carried 13 scientific instruments "to obtain scientific data of the lunar origin and evolution and to develop the technology for the future lunar exploration":

Terrain camera (TC) (resolution 10 meters [33 ft] per pixel) X-ray fluorescence spectrometer (XRS) Lunar magnetometer (LMAG) Spectral profiler (SP) (resolution per pixel: 562 x 400 m [1840 x 1310 ft]) Multi-band imager (MI) (resolution of visible light 20 [66 ft] meters per pixel, near-infrared 62 meters [200 ft] per pixel) Laser altimeter (LALT) Lunar radar sounder (LRS) Gamma ray spectrometer (GRS) Charged particle spectrometer (CPS) Plasma analyzer (PACE) Upper atmosphere and plasma imager (UPI) Radio wave repeater (RSAT) aboard Okina Radio wave source for VLBI (VRAD) aboard Okina and Ouna Two 2.2 megapixel CCD HDTV cameras, one wide-angle and one telephoto, were also on board, primarily for public outreach. The HDTV system, developed by NHK (Japan Broadcasting Corporation), produced over 1.3 TB of video and stills over 19 months. JAXA collected names and messages that were carried on SELENE through their "Wish Upon the Moon" campaign. 412,627 names and messages were printed on a sheet measuring 280 mm x 160 mm (11 x 6.3 in) at 70 μm (0.0003 in) per character. The sheet was installed under the photovoltaic modules and cooling panels beneath the multi-layered insulation.

Results Major results include:

Improved lunar global topography maps. This detailed altitude and geological data was provided to Google for free to make Google Moon 3-D. Detailed gravity map of the far side of the Moon. First optical observation of the permanently shadowed interior of the crater Shackleton at the lunar south pole. Evidence of Earth's oxygen being transported to the Moon via magnetospheric ions.

… excerpt ends here. Continue reading the full article.

Illustrations

SELENE: First optical still captured by the onboard HDTV camera. Earth is seen at a distance of 11,000 km.[17] — First optical still captured by the onboard HDTV camera. Earth is seen at a distance of 11,000 km.[17]

Worked examples

Example 1 — a first encounter with SELENE

Start with the simplest possible case. Write down what SELENE claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to SELENE before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about SELENE ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of SELENE

In research: SELENE appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses SELENE in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: SELENE is common in secondary-school and first-year university syllabi. It links to neighbouring topics 2009 on the Moon, Japanese Lunar Exploration Program, Japanese missions to the Moon, so understanding it makes those chapters shorter.
In everyday life: Look for SELENE outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study SELENE in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what SELENE means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain SELENE out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is SELENE in simple terms?

Why does SELENE matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study SELENE?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on SELENE.