Editing Galileo project (section)

==Earth encounters==
===Flybys===
''Galileo'' made two course corrections on April 9 to 12 and May 11 to 12, 1990, to alter its velocity by {{convert|35|m/s|sp=us}}.{{sfn|Meltzer|2007|p=157}} The spacecraft flew by Earth twice; the first time at a range of {{convert|960|km|mi|abbr=on}} at 20:34:34&nbsp;UTC on December 8, 1990.<ref name="PDS"/> This was {{convert|5|mi|order=flip|0|abbr=on}} higher than predicted, and the time of the closest approach was within a second of the prediction. It was the first time that a deep space probe had returned to Earth from interplanetary space.{{sfn|Meltzer|2007|p=157}} A second flyby of Earth was at {{convert|304|km|mi|abbr=on}} at 15:09:25&nbsp;UTC on December 8, 1992.<ref name="PDS"/> This time the spacecraft passed within a kilometer of its aiming point over the South Atlantic. This was so accurate that a scheduled course correction was cancelled, thereby saving {{convert|5|kg|sp=us}} of propellant.{{sfn|Meltzer|2007|p=164}}

===Earth's bow shock and the solar wind===
[[File:Galileo Earth - PIA00114.jpg|thumb|right|''Galileo'' image of Earth, taken in December 1990|alt=refer to caption]]
The Earth encounters provided an opportunity for a series of experiments. A study of Earth's bow shock was conducted as ''Galileo'' passed by Earth's day side. The solar wind travels at {{convert|200|to|800|km/s|sp=us}} and is deflected by [[Earth's magnetic field]], creating a [[magnetic tail]] on Earth's dark side over a thousand times the radius of the planet. Observations were made by ''Galileo'' when it passed through the magnetic tail on Earth's dark side at a distance of {{convert|56000|km|sp=us}} from the planet. The magnetosphere was quite active at the time, and ''Galileo'' detected magnetic storms and [[whistler (radio)|whistlers]] caused by lightning strikes.<ref>{{cite web |title=Collaborative Study of Earth's Bow Shock |publisher=NASA |url=https://spdf.gsfc.nasa.gov/bowshock/ |access-date=14 November 2020 |archive-date=November 16, 2020 |archive-url=https://web.archive.org/web/20201116112941/https://spdf.gsfc.nasa.gov/bowshock/ |url-status=live }}</ref>{{sfn|Meltzer|2007|pp=158–159}}

The NIMS was employed to look for [[mesospheric clouds]], which were thought to be caused by [[methane]] released by industrial processes. The water vapor in the clouds breaks down the [[ozone]] in the upper atmosphere. Normally the clouds are only seen in September or October, but ''Galileo'' was able to detect them in December, an indication of possible damage to Earth's ozone layer.{{sfn|Meltzer|2007|pp=158–159}}

===Remote detection of life on Earth===
Carl Sagan, pondering the question of whether [[detecting Earth from distant star-based systems|life on Earth could be easily detected from space]], devised a set of experiments in the late 1980s using ''Galileo''{{'s}} remote sensing instruments during the mission's first Earth flyby in December 1990. After data acquisition and processing, Sagan published a paper in ''[[Nature (journal)|Nature]]'' in 1993 detailing the results of the experiment. ''Galileo'' had indeed found what are now referred to as the "Sagan criteria for life". These included strong absorption of light at the red end of the visible spectrum (especially over [[continents]]) by chlorophyll in photosynthesizing plants; absorption bands of molecular oxygen as a result of plant activity; infrared bands caused by the approximately 1 micromole per [[mole (unit)|mole]] of methane (a gas which must be replenished by volcanic or biological activity) in the atmosphere; and modulated narrowband radio wave transmissions uncharacteristic of any known natural source. ''Galileo''{{'s}} experiments were thus the first [[scientific control]]s in the newborn science of [[astrobiological]] remote sensing.{{sfn|Sagan et al.|1993|pp=715–721}}

===Lunar observations===
<gallery class="center" widths="220px" heights="220px">
File:Moon-galileo-color.jpg|[[Mare Orientale]] on the [[Moon]]|alt=The maria are large areas with less cratering
File:The Moon from Galileo - GPN-2000-000473.jpg|''Galileo'' shot of the [[lunar north pole]]|alt=The far side is cratered; maria on the near side
File:Moon Crescent - False Color Mosaic.jpg|[[False-color]] mosaic by ''Galileo'' showing [[Geology of the Moon|compositional variations]] of the Moon's surface |alt=refer to caption
</gallery>

En route to ''Galileo''{{'s}} second gravity-assist flyby of Earth, the spacecraft flew over the [[lunar north pole]] on December 8, 1992, at an altitude of {{convert|110,000|km|sp=us}}. The north pole had been photographed before, by ''[[Mariner 10]]'' in 1973, but ''Galileo''{{'s}} cameras, with their {{convert|1.1|km|sp=us}} per [[pixel]] imagery, provided new information about a region that still held some scientific mysteries. The infrared spectromer surveyed the surface minerals and revealed that the region was more minerallogically diverse than expected. There was evidence that the [[Moon]] had been volcanically active earlier than originally thought, and the spectrometer clearly distinguished different lava flows on the [[Mare Serenitatis]]. Areas where titanium-rich material had been blasted from vents, like the one sampled by [[Apollo 17]], showed up clearly.{{sfn|Harland|2000|pp=65–67}}

===''Galileo'' Optical Experiment===
During the second Earth flyby, another experiment was performed. Optical communications in space were assessed by detecting light pulses from powerful lasers with ''Galileo''{{'s}} CCD. The experiment, dubbed ''Galileo'' Optical Experiment or GOPEX,<ref name="GOPEX">{{cite web |url=http://lasers.jpl.nasa.gov/PAPERS/GOPEX/gopex_s2.pdf |title=GOPEX SPIE 1993 (Edited) |publisher=NASA/Jet Propulsion Laboratory |access-date=May 15, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110721050447/http://lasers.jpl.nasa.gov/PAPERS/GOPEX/gopex_s2.pdf |archive-date=July 21, 2011}}</ref> used two separate sites to beam laser pulses to the spacecraft, one at [[Table Mountain Observatory]] in California and the other at the [[Starfire Optical Range]] in [[New Mexico]]. The Table Mountain site used a [[Nd:YAG laser]] operating at a [[nonlinear optics#Frequency doubling|frequency-doubled]] wavelength of 532&nbsp;nm, with a repetition rate of 15 to 30&nbsp;Hertz and a pulse power [[full width at half maximum]] (FWHM) in the tens of megawatts range, which was coupled to a {{convert|0.6|m|ft|abbr=on}} [[Cassegrain reflector]] telescope for transmission to ''Galileo''. The Starfire range site used a similar setup with a larger {{convert|1.5|m|ft|abbr=on}} transmitting telescope. Long-exposure (~0.1 to 0.8&nbsp;s) images using ''Galileo''{{'s}} 560&nbsp;nm centered green filter produced images of Earth clearly showing the laser pulses even at distances of up to {{convert|6|e6km|e6mi|abbr=unit}}.<ref name="GOPEX" />

Adverse weather conditions, restrictions placed on laser transmissions by the U.S. [[Space Defense Operations Center]] ([[Cheyenne Mountain Complex|SPADOC]]) and a pointing error caused by the scan platform on the spacecraft not being able to change direction and speed as quickly as expected (which prevented laser detection on all frames with less than 400&nbsp;ms exposure times) contributed to a reduction in the number of successful detections of the laser transmission to 48 of the total 159 frames taken.<ref name="GOPEX" /> Nonetheless, the experiment was considered a resounding success and the data acquired were used to design laser downlinks to send large volumes of data very quickly from spacecraft to Earth. The scheme was studied in 2004 for a data link to a future Mars-orbiting spacecraft.<ref name="lasers">{{cite news |url=https://www.space.com/534-nasa-test-laser-communications-mars-spacecraft.html |title=NASA To Test Laser Communications With Mars Spacecraft |publisher=[[Space.com]] |date=November 15, 2004 |access-date=January 19, 2021 |archive-date=November 8, 2021 |archive-url=https://web.archive.org/web/20211108200952/https://www.space.com/534-nasa-test-laser-communications-mars-spacecraft.html |url-status=live }}</ref> On December 5, 2023, NASA's [[Deep Space Optical Communications]] experiment on the ''[[Psyche (spacecraft)|Psyche]]'' spacecraft used infrared lasers for two-way communication between Earth and the spacecraft.<ref>{{cite press release |title=NASA's Deep Space Optical Comm Demo Sends, Receives First Data |date= |publisher=NASA |url=https://www.jpl.nasa.gov/news/nasas-deep-space-optical-comm-demo-sends-receives-first-data |access-date=April 2, 2024 |archive-date=April 2, 2024 |archive-url=https://web.archive.org/web/20240402195233/https://www.jpl.nasa.gov/news/nasas-deep-space-optical-comm-demo-sends-receives-first-data |url-status=live }}</ref><ref>{{cite news |title=Pew! Pew! Pew! NASA's 1st successful two-way laser experiment is a giant leap for moon and Mars communications |first=Andrew |last=Jones |date=December 20, 2023 |publisher=[[Space.com]] |url=https://www.space.com/nasa-laser-communication-1st-two-way-link-iss |access-date=2 April 2024 |archive-date=April 2, 2024 |archive-url=https://web.archive.org/web/20240402195233/https://www.space.com/nasa-laser-communication-1st-two-way-link-iss |url-status=live }}</ref>