Editing Galileo project (section)

==Asteroid encounters==
===951 Gaspra===
[[File:951 Gaspra.jpg|[[951 Gaspra]] (enhanced colorization)|thumb|right|alt=Potato-shaped asteroid]]
Two months after entering the [[asteroid belt]], ''Galileo'' performed the first asteroid encounter by a spacecraft.{{sfn|Meltzer|2007|pp=161–164}} ''Galileo'' passed [[951 Gaspra]], an [[S-type asteroid]], at a distance of {{convert|1604|km|mi|abbr=on}} at 22:37&nbsp;UTC on October 29, 1991, at a relative speed of about {{convert|8|km/s|mi/s|sp=us}}.<ref name="PDS"/> Fifty-seven images of Gaspra were taken with the SSI, covering about 80 percent of the asteroid.{{sfn|Veverka|Belton|Klaasen|Chapman|1994|p=2}} Without the HGA, the bit rate was only about 40 bit/s, so an image took up to 60 hours to transmit back to Earth. The ''Galileo'' project was able to secure 80 hours of Canberra's 70-meter dish time between 7 and 14 November 1991,{{sfn|Veverka|Belton|Klaasen|Chapman|1994|p=7}} but most of images taken, including low-resolution images of more of the surface, were not transmitted to Earth until November 1992.{{sfn|Meltzer|2007|pp=161–164}}

The imagery revealed a cratered and irregular body, measuring about {{convert|19|by|12|by|11|km|mi|sp=us}}.{{sfn|Veverka|Belton|Klaasen|Chapman|1994|p=2}} Its shape was not remarkable for an asteroid of its size.{{sfn|Veverka|Belton|Klaasen|Chapman|1994|p=8}} Measurements were taken using the NIMS to indicate the asteroid's composition and physical properties.{{sfn|Granahan|2011|pp=265–272}} While Gaspra has plenty of small craters—over 600 of them ranging in size from {{convert|100|to|500|m|sp=us}}—it lacks large ones, hinting at a relatively recent origin,{{sfn|Meltzer|2007|pp=161–164}} although it is possible that some of the depressions were eroded craters. Several relatively flat planar areas were found, suggesting that Gaspra was formed from another body by a collision.{{sfn|Veverka|Belton|Klaasen|Chapman|1994|p=8}} Measurements of the solar wind in the vicinity of the asteroid showed it changing direction a few hundred kilometers from Gaspra, which hinted that Gaspra might have a magnetic field, but this was not certain.{{sfn|Meltzer|2007|pp=161–164}}

===243 Ida and Dactyl===
{{Main|243 Ida#Galileo flyby|l1=243 Ida}}
Following the second Earth encounter, ''Galileo'' performed close observations of another asteroid, [[243 Ida]]. A slight trajectory correction was made to enable this on August 26, 1993. With four hours to go before the encounter with Ida, ''Galileo'' spontaneously abandoned the observation configuration and resumed its cruise configuration. Engineers were able to correct the problem and have the instruments ready by 16:52:04&nbsp;UTC on August 28, 1993, when ''Galileo'' flew past Ida at a range of {{convert|2410|km|mi|abbr=on}}. High-resolution images were taken to create a color mosaic of one side of the asteroid, with the highest resolution image taken at a range of {{convert|10,500|mi|abbr=on}}.{{sfn|Harland|2000|pp=72–77}} Measurements were taken using SSI and NIMS.{{sfn|Belton et al|1996|pp=2–3}}{{sfn|Chapman|Veverka|Thomas|Klaasen|1995|pp=783–785}}

[[File:243 ida.jpg|[[243 Ida]], with its moon Dactyl to the right|thumb|left|alt=Another potato-shaped asteroid; Dactyl is just a small dot]]
Transmission was still limited to the 40 bit/s data rate available during the Gaspra flyby. At that rate, it took thirty hours to send each of the five frames. In September, the line of sight between ''Galileo'' and Earth was close to the Sun, so there was only time to send one mosaic before it was blocked by the Sun on September 29, 1993; the rest of the mosaics were transmitted in February and March, after Earth had come around the Sun. ''Galileo''{{'s}} tape recorder was used to store the images, but tape space was also required for the primary Jupiter mission. A technique was developed whereby only image fragments of two or three lines out of every 330 were initially sent. A determination could then be made as to whether the image was of 243 Ida or of empty space. Ultimately, only about 16 percent of the SSI data recorded could be sent back to Earth.{{sfn|Harland|2000|pp=72–77}}{{sfn|Belton et al|1996|p=7}}

When astronomer Ann Harch examined the images on February 17, 1994, she found that Ida had a small moon measuring around {{convert|1.6|km|mi|sp=us|sigfig=1}} in diameter, which appeared in 47 images.{{sfn|Harland|2000|pp=72–77}} A competition was held among ''Galileo'' project members to select a name for the moon, which was ultimately dubbed [[Dactyl (moon)|Dactyl]] after the legendary [[Dactyls (mythology)|Dactyls]], mythical beings which lived on [[Mount Ida (Crete)|Mount Ida]], the geographical feature on [[Crete]] the asteroid was named for. Craters on Dactyl were named after individual dactyloi. Regions on 243 Ida were named after cities where [[Johann Palisa]], the discover of 243 Ida, made his observations, while ridges on 243 Ida were named in honor of deceased ''Galileo'' team members.{{sfn|Belton et al|1996|p=10}}<ref>{{cite web |title=243 Ida |publisher=NASA |url=https://science.nasa.gov/solar-system/asteroids/243-ida/ |access-date=April 3, 2024 |archive-date=April 3, 2024 |archive-url=https://web.archive.org/web/20240403200221/https://science.nasa.gov/solar-system/asteroids/243-ida/ |url-status=live }}</ref>

Dactyl was the first [[asteroid moon]] to be discovered. Moons of asteroids had been assumed to be rare, but the discovery of Dactyl hinted that they might in fact be quite common. From subsequent analysis of this data, Dactyl appeared to be an [[S-type asteroid]], and spectrally different from 243 Ida, although Ida is also an S-type asteroid. It was hypothesized that both may have been produced by the breakup of a [[Koronis family|Koronis]] parent body.{{sfn|Belton et al|1996|pp=2–3}}{{sfn|Chapman|Veverka|Thomas|Klaasen|1995|pp=783–785}}