© 1989, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112
The Earth, too, must have experienced a wealth of such impacts, but our dynamic planet has several characteristics that erase craters relatively quickly (on the geologic time scale) — or even prevent them from forming in the first place:
However, the impacts that formed most craters on the Moon (and elsewhere) did so by literally exploding, not just by gouging up the ground. Craters formed by such detonations — similar to those made by artillery shells — are circular regardless of what direction the projectiles came from.
The energy source for an impact's explosion is the raw speed of the impacting object. An interplanetary chunk headed toward the Moon is accellerated by the Moon's gravity; astronomers' calculations indicate that an object falling from deep space will have a minimum speed of about 5,000 mph on impact. At that speed, the sudden collision of a meteoroid (as these cosmic pieces of debris are called) with the Moon's surface generates enough heat to vaporize much or all of itself and some of the ground it penetrates. The vaporized material violently expands — explodes — forming a circular crater.
Craters larger than about 10 kilometers across often have central peaks, which are hills or mountains pushed up by pressure within the Moon when the weight of the rocks that were blasted away was removed.
The very largest impact features on the Moon are the enormous impact basins: great circular plains from 300 to more than a thousand kilometers across. There are only about two or three dozen of these — and all of the largest ones are on the Moon's "nearside" (the hemisphere that faces Earth). Astronomers believe that many of these great basins formed about four billion years ago in a relatively short period time — only two-tenths of a billion years or so. Why the largest impacts took place only then is not certain, but it is clear that after that only smaller impacts took place.
The highlands make up about 80% of the Moon's surface (including virtually all of the farside) and are saturated with ancient impact craters.
Much more recently, there were a few, brief visits by "objects" that left footprints instead of craters — twelve American astronauts.
From 1959 (when the Luna 1 probe made the first flyby of the Moon) until 1976 (when Luna 24 returned the last lunar sample to Earth), 32 unmanned probes and 9 manned missions to the Moon or near-Moon space were successfully carried out. It was an intensely exciting time to be alive, as we human beings first exercised our ability to navigate the void of space. The accompanying tables list those 41 early missions to the Moon.
Notice that all of the Moon missions listed in this section's tables were launched by the Soviet Union or the United States. Much — if not all — of the impetus behind the 1959-76 rush of spacecraft to the Moon was political: space (and the Moon in particular) was looked at as a great proving-ground of the technological might of the "superpowers." In a sense, the United States "won" the race to the Moon; the Apollo program produced the only manned lunar landings so far. However, it can be argued that the Soviets were more consistent and persistent in their Moon program (their series of automated Luna spacecraft continued for four years after the Apollo program).
Among the genuine scientific benefits that came about as a result of this largely political contest was a much more detailed knowledge of our neighbor world's composition and structure, a clearer view of its geological history and future — and hundreds of thousands of breathtaking photographs. The photographs returned by Apollo crews, Surveyor robots, Luna orbiters and landers, and by all the others in these data tables constitute a landmark in the history of our species. They are our collective memory of our first visits to another world.
|Luna 1||01/02/59||First flyby of Moon|
|Luna 2||09/12/59||Impact on 09/14; first manmade object on Moon|
|Luna 3||10/04/59||Flyby; first photographs of Moon's farside|
|Ranger 7||07/28/64||First close-ups of Moon; impacted in Mare Nubium|
|Ranger 8||02/17/65||Impacted in Mare Tranquillitatis|
|Ranger 9||03/21/65||Impacted in crater Alphonsus|
|Zond 3||07/18/65||Farside photographs|
soft-landing on Moon
|Luna 10||03/31/66||First probe to go into orbit around Moon|
|Surveyor 1||05/30/66||Soft-landed in Oceanus Procellarum|
|Lunar Orbiter 1||08/10/66||Photographed proposed Apollo landing sites from orbit|
|Luna 12||10/22/66||First Soviet Orbiter to return photographs|
|Lunar Orbiter 2||11/06/66||Photographs of proposed landing sites|
|Luna 13||12/21/66||Soft-landed in Oceanus Procellarum|
|Lunar Orbiter 3||02/05/67||Photographs of proposed landing sites|
|Surveyor 3||04/17/67||Soft-landed in Oceanus Procellarum, visited by Apollo 12 astronauts in 1969|
|Lunar Orbiter 4||05/04/67||Photographs for mapping the nearside|
|Explorer 35||07/19/67||Magnetic field studies from lunar orbit|
|Lunar Orbiter 5||08/01/67||Photographs for mapping the farside|
|Surveyor 5||09/08/67||Soft-landed in Mare Tranquillitatis|
|Surveyor 6||11/07/67||Soft-landed in Sinus Medii|
|Surveyor 7||01/07/68||Landed near crater Tycho|
|Luna 16||09/12/70||Lander; first automated return of soil sample to Earth (101 grams)|
|Luna 17||11/10/70||Automated Lunokhod (rover) traveled 10.5 km on surface|
|Luna 20||02/14/72||Lander; returned about 100 gram soil sample to Earth|
|Luna 21||01/08/73||Lunokhod traveled more than 35 km on surface|
|Luna 23||10/28/74||Lander; no sample returned|
|Luna 24||08/09/76||Lander; returned about 150 gram soil sample to Earth|
[Spacecraft named Luna or Zond were Soviet missions; all others were U.S.]
The Apollo Project: Manned Lunar Landing Missions
|Apollo 11||07/16/69||Neil A. Armstrong
Edwin E. Aldrin, Jr.
|First manned Landing, 07/20/69, in Mare Tranquillitatis|
|Apollo 12||11/14/69||Charles Conrad,
Richard F. Gordon
Alan L. Bean
|Landed in Oceanus Procellarum|
|Apollo 13||04/11/70||James A. Lovell,
John L. Swigert, Jr.
Fred W. Haise, Jr.
|Accident en route required return after one swing around farside|
|Apollo 14||01/31/71||Alan B. Shepard,
Stuart A. Roosa
Edgar D. Mitchell
|Landed in Fra Mauro|
|Apollo 15||07/26/71||David R. Scott
Alfred M. Worden
James B. Irwin
|Landed at edge of Imbrium Basin near Apennine Mountains|
|Apollo 16||04/16/72||John W. Young
Thomas K. Mattingley, II
Charles M. Duke, Jr.
|Landed in highlands near Crater Descartes|
|Apollo 17||12/07/72||Eugene A.
Ronald E. Evans
Harrison H. Schmitt
|Landed in Taurus Littrow Valley|
It's especially instructive to get students to go back and survey their hometown newspaper or read the articles from major newsmagazines from 1968 to 1972. Many libraries today can also provide sound and video recordings from that era. NASA makes films available to schools on loan (for more information, contact the NASA center nearest you).
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