Talk:243 Ida/Collaboration

Open issues

 * Should Dactyl be merged into this article?
 * I think it probably should. Reyk  YO!  00:50, 27 October 2008 (UTC)
 * No references for max/min visual magnitude.
 * Should List of geological features on 243 Ida and Dactyl be merged into this article? Perhaps everything else could be merged, leaving only the crater list, and it could be renamed.
 * Should we create a draft version of the article? If there is enough new content to be added, it could qualify for a DYK, but the improvements would have to be made to article space all at once. Wronkiew (talk) 06:53, 13 November 2008 (UTC)
 * Hi Wronkiew, sorry I've been slack about this lately. Real life has been a pain. I definitely think there's enough material for a 5-fold expansion. I suggest using the bottom of this page as a draft. Reyk  YO!  07:58, 20 November 2008 (UTC)
 * I reviewed the DYK requirements again and I don't think that this article will qualify even with a 5-fold expansion. It's just too big to start with. The requirements say that if it exists before the 5 day improvement drive, it has to be a stub or a short article. 8 KB is a bit more than a "short article". Wronkiew (talk) 17:54, 9 December 2008 (UTC)
 * I've asked at Wikipedia talk:Did you know how long is too long. If it does turn out that a DYK isn't going to happen, then we can make our changes directly to the article but I don't want to ruin any DYK chance we have by jumping in too early. BTW, I have made a small start at my snadbox (sic). Reyk  YO!  21:34, 9 December 2008 (UTC)
 * It doesn't look as though there is a maximum length. Reyk  YO!  00:58, 10 December 2008 (UTC)
 * Should we put the Exploration section nearer the start of the article than the end? Most of what is known about Ida comes from the Galileo flyby. It wouldn't make much sense to talk about the composition of Ida and the regolith, inevitably mentioning Galileo results, before talking about the flyby. Reyk  YO!  22:41, 21 November 2008 (UTC)
 * Sure. Some of the FAs combine exploration and discovery into a history section. I would prefer to keep discussion about the flyby out of the physical characteristics sections if possible, though. Wronkiew (talk) 04:48, 22 November 2008 (UTC)
 * I think we should combine the discovery and exploration section into a single section, because what little material I can find about the discovery won't allow me to write more than a few sentences. Reyk  YO!  03:04, 30 November 2008 (UTC)
 * We will need to decide soon on a citation format. My preference is for this one, but I am open to using other formats. Wronkiew (talk) 04:02, 31 December 2008 (UTC)
 * It looks as though we're going to be referring to several of the sources multiple times, so the shortened notes with wikilinks format looks quite sensible. Let's go with that one. Reyk  YO!  04:14, 31 December 2008 (UTC)
 * Still very little information regarding the discovery of Ida
 * I added two more sources with some information on the discovery. I'll see if I can find more later. Wronkiew (talk) 07:43, 2 January 2009 (UTC)
 * Hmm, I can't seem to open either of them. I get "page not found" errors. Can you double-check the URL? PS. I wish I had your genius for digging up sources. Reyk  YO!  07:46, 2 January 2009 (UTC)
 * Sorry about that, I should have checked the links. I changed the link to the book description on Google instead. Wronkiew (talk) 01:13, 3 January 2009 (UTC)

Infobox

 * Dibs Reyk  YO!  07:36, 25 January 2009 (UTC)


 * Johann Palisa discovered Ida on September 29, 1884
 * Both Ida and moon have a temperature of 200 K
 * Ida observed bulk density 2600 +/- 500 kg/m-3
 * Ida belongs to the Koronis family
 * Ida period is 4.63 hours
 * Ida adopted mean obliquity (ϵ): 156°
 * Ida ecliptic longitude (λ): 263°
 * Ida mass 0.0042 ± 0.0006 ×1019 kg (Belton et al. 1995)
 * Ida diameter 31.4 km (Belton et al. 1995)
 * Ida bulk density 2.6 ± 0.5 g/cm3 (Belton et al. 1995)
 * Orbital information
 * Epoch: 2454800.5 (2008-Nov-30.0)
 * Aphelion: 2.991 AU
 * Perihelion: 2.732 AU
 * Semi-major axis: 2.862 AU
 * Eccentricity: 0.0452
 * Orbital period: 1768.136 d (4.84 y)
 * Mean anomaly: 191.869°
 * Inclination: 1.138°
 * Longitude of ascending node: 324.175°
 * Argument of perihelion: 107.897°
 * Time of perihelion passage: 2455626.273 (2011-Mar-05.773) JED
 * Mean motion: .2036 deg/d
 * Absolute magnitude: 9.94
 * Geometric albedo: 0.2383
 * Alternate Designations: 1988 DB1 = A910 CD
 * Surface gravity 0.3–1.1cm/s2
 * Ida north pole right ascension (α0): 168.76°
 * Ida north pole declination (δ0): -2.88°

Introduction

 * Dibs Wronkiew (talk) 23:35, 21 February 2009 (UTC)

Discoverer
Ida was discovered on September 29, 1884 by Austrian astronomer Johann Palisa at the Vienna Observatory. It was Palisa's 45th asteroid discovery.


 * Johann Palisa discovered Ida on September 29, 1884
 * Vienna Observatory
 * Either the 27" or the 12" refractor
 * Ida was the 45th asteroid discovered by Palisa

Name
Ida is named after the nymph that raised Zeus in greek mythology.


 * Ida is named after the nymph that raised Zeus

Observations

 * HST observed Ida for 8 hours on 1994-04-26
 * Dactyl was less than 700 km from Ida at the time

Galileo flyby

 * Dibs Wronkiew (talk) 05:42, 25 January 2009 (UTC)


 * NASA policy to include asteroid flybys when crossing the asteroid belt
 * Choice to attempt Ida determined the arrival date of Galileo at Jupiter
 * Ida was optional, choice based on available propellant
 * Flyby cost 34 kg of propellant
 * Galileo flybys the first spacecraft encounters with asteroids
 * Galileo spacecraft launch
 * Space Shuttle Atlantis
 * 18 October 1989


 * Flyby of Ida on 28 August 1993
 * Ida flyby during second crossing through the belt
 * Spacecraft velocity compared to Ida was 12.4 km/s
 * Galileo closest approach 2400km
 * Images from SSI at ranges between 240,350 and 2,390 km
 * Ida's short rotational period allowed Galileo to image most of the surface
 * Galileo observed ~95% of Ida's surface


 * Transmission of Ida images was delayed until Galileo was closer to the Earth
 * Low bit rate when Galileo is farther away
 * Galileo finished transmitting Ida data in June 1994
 * Best Ida image had a resolution of about 108 m/pixel
 * First five images from Ida in September 1993, the rest was transmitted the next spring
 * Five frame mosaic of Ida at a resolution of 31–38 m/pixel
 * Galileo sent five high resolution (31-38m/pixel) pictures immediately, remainder in 1994
 * Failure of Galileo's main antenna complicated data transmission
 * Galileo flyby of Ida and Gaspra provided the first high resolution images of asteroid surfaces

Discoveries

 * Dibs Wronkiew (talk) 06:50, 25 January 2009 (UTC)


 * The field of "asteroid geology" started with Galileo and NEAR probes
 * More geological diversity than Gaspra
 * Possibly due to larger size
 * Dactyl the first confirmed satellite discovered around an asteroid
 * Galileo flybys provided new insights about the relationship between meteorites and asteroids


 * Association between asteroid types and meteor types is uncertain
 * S-class either ordinary chondrite or stony-iron
 * Chondrite = primitive
 * Stony-iron = differentiated
 * Significance of finding Dactyl's orbit
 * Permitted the determination of Ida's density
 * First time the mass of an S-type asteroid could be found
 * Low bulk density rules out a half-metallic composition even if it's a rubble pile


 * Ida demonstrates the space weathering process
 * Space weathered material becomes redder as it ages
 * Space weathering is a superficial process
 * Space weathering affects the appearance of Ida's surface over time
 * Spectrum indicates Space weathering prominent on Ida
 * Not as prominent on Dactyl
 * Taking weathering into account resolves the differences between S-type asteroids and OCs


 * S-type asteroids are the most common in the inner part of the main asteroid belt
 * OC meteorites are the most common type found on the Earth's surface
 * Ida and Dactyl may share the mineral composition of OC meteorites
 * Extrapolating spectra of weathered regions on Ida, to less weathered regions, to Dactyl indicates ordinary chondritic (OC) composition
 * Ida and other Koronis asteroids may be the source of OC meteorites

Physical characteristics

 * Both Ida and moon have a temperature of 200 K

Mass

 * Dibs Wronkiew (talk) 06:56, 31 January 2009 (UTC)


 * Mass is 3.65–4.99 x 1016 kg
 * Inferred from the long term stability of its moon
 * Ida mass 0.0042 ± 0.0006 ×1019 kg (Belton et al. 1995)
 * Escape velocity from Ida is 20 m/s
 * Surface gravity 0.3–1.1cm/s2

Size

 * Best fit ellipsoid 59.8 x 25.4 x 18.6 km
 * Ida's volume 16,100 ± 1900 km3
 * Ida diameter 31.4 km (Belton et al. 1995)
 * Ida's volume determined by Galileo to within 12%

Shape

 * Dibs. Reyk  YO!  01:57, 26 January 2009 (UTC)

Ida is a distinctly elongated asteroid, with an irregular surface, and is somewhat "croissant-shaped". The elongation of the best-fit triaxial ellipsoid is 2.35, and the asteroid possesses a "waist" that separates it into two geologically dissimilar halves. This constricted shape is consistent with Ida being made of two large, solid components with loose debris filling the gap between them, but the constriction was imaged by Galileo in high resolution and no such loose material was seen. There are steep slopes, up to about 50°, present on Ida but few areas where the slope exceeds 35°.

Ida's irregular shape is, together with its low surface gravity and fast rotation, responsible for the asteroid's highly uneven gravitational field. The surface gravity is lowest at the ends of the asteroid, due to the fast rotation, and near the minimum radius due to less mass being present interior to that location.


 * Elongation of Ida 2.35
 * Ida "croissant-shaped"
 * "Waist" around the asteroid separating two geologically dissimilar halves
 * Ida's constricted shape is consistent with it being made of two large components with loose material filling the gap between them
 * However, the constriction was imaged in high resolution and does not display any such loose debris
 * Ida is elongated in shape
 * Irregular surface
 * Ida's irregular gravitational field due to low surface gravity, fast rotation and irregular shape
 * Surface gravity lowest at the ends and middle
 * low at ends because of rotation
 * low in middle because little mass interior to location
 * Maximum slopes about 50°, but few areas with slopes of more than 35°
 * Ida ephemeris position of the prime meridian at J2000 (W): 265.95° + 1864.6280070d

Surface features

 * Dibs Wronkiew (talk) 07:49, 20 February 2009 (UTC)

Regolith

 * Dibs Wronkiew (talk) 05:10, 26 January 2009 (UTC)


 * Ida regolith 50–100 m thick
 * Movement of debris downslope on Ida suggests the presence of regolith
 * Blocks indicate the presence of regolith
 * Upper limit for Ida regolith depth is 130 m


 * Ida's surface is mostly olivine with small amounts of orthopyroxene
 * Ida shows color variations across its surface
 * Fresh craters are less red than the rest of the surface
 * Similar colors from the ejecta of the fresh crater Azzurra
 * NOTE: "Earlier premature reports by NIMS investigators that Ida is an exceptionally olivine-rich object (Carlson, 1994) and that Dactyl is rich in clinopyroxene (Carlson et al., 1994) have been effectively withdrawn (J. Granahan, pers. comm., 1995; R. Carlson [pers. comm., 1996] agrees that the evidence for clinopyroxene is 'weak')."
 * Pyroxene and olivine are silicate minerals
 * Spectrum indicates Space weathering prominent on Ida

Structures
Dibs Wronkiew (talk) 22:01, 14 February 2009 (UTC)
 * "Waist" around the asteroid separating two geologically dissimilar halves
 * Differences between the two halves might be spurious
 * "waist" might be composed of overlapping craters rather than structural


 * Linear structure on the surface named Townsend Dorsum
 * Covers 150 degrees of arc
 * ~40 km long
 * Not visible during closest approach by Galileo
 * Ida Region 1 has a prominent ridge
 * 40 km long ridge
 * The morphology of Townsend Dorsum suggests displacement along a fault line
 * However, the projection of the ridge into higher resolution area does not show fault topography
 * Possibly an older feature like a compositional discontinuity
 * Ida Region 1 contains a large indentation Vienna Regio

Craters

 * Dibs. Reyk  YO!  06:26, 25 January 2009 (UTC)

Ida is one of the most densely cratered bodies in the Solar System, and impacts have been the primary process shaping its surface. Ida's surface is covered with craters of all sizes and stages of degradation, and range in age from fresh to very old. Some of Ida's craters may have been formed during the breakup of the Koronis family parent body. The largest crater is 12 km across. The craters are not distributed evenly over Ida's surface. Region 2 contains nearly all of the large (>6 km) craters but Region 1 has no large craters at all. Chains of craters are also apparent.

The craters on Ida are simple in structure. They are bowl-shaped with no flat bottoms and no central peaks, with the exception of Fingal, a fresh, asymmetric crater that has a sharp boundary between the crater floor and wall on one side. The ejecta excavated by impacts is, like on many asteroids, deposited differently on Ida than on planets because of its rapid rotation, low gravity and irregular shape. Ejecta blankets that settle around craters are asymmetrical and material that escapes from the asteroid is permanently lost.

The Azzurra basin seems to be the most recent major impact on Ida. The ejecta from this collision is distributed discontinuously over Ida and is responsible for the large-scale color and albedo variations across the asteroid's surface. The protrusion north of crater Choukoutien is smoother and less cratered than the rest of Ida. The 0° meridian on Ida is defined to pass through the crater Afon.


 * Ida meridian goes through Afon crater


 * Impacts are the dominant process shaping the surface of Ida
 * Deposition of ejecta is different on asteroids than on planets because of rapid spin, odd shapes, low gravity
 * Escaped ejecta from main belt asteroids is permanently lost
 * Ida Region 2 contains nearly all of the large (>6 km) craters
 * Ida Region 1 has no large craters
 * Ida has a full range of craters, from fresh to very old
 * The rapid rotation and low gravity of Ida produces asymmetrical ejecta blankets around craters
 * One of the most densely cratered objects in the Solar System
 * Chains of craters are apparent
 * Simple craters only, bowl shaped, no flat bottoms, no central peaks
 * Except for Fingal, which is fresh, asymmetric, and has a sharp boundary between the floor and wall on one side
 * Surface covered in craters of all sizes and stages of degradation
 * One of the most densely cratered bodies in the solar system
 * Ida's crater Azzurra
 * Ejecta deposited discontinuously around Ida
 * Azzurra basin appears to be the most recent major impact
 * Large-scale color and albedo variation across Ida's surface is associated with Azzurra Crater
 * Ida's largest crater is 12 km
 * Protrusion north of crater Choukoutien is smoother and less cratered than the rest of Ida
 * Some of Ida's craters may be from the breakup of the Koronis parent body

Ejecta blocks

 * Dibs Wronkiew (talk) 02:01, 11 February 2009 (UTC)


 * Ida has about 20 large ejecta blocks, up to 150 m across
 * Most 40–150 m ejecta blocks in a cluster on one side of Ida near Mammoth and Lascaux
 * Impact blocks are the largest debris particles on the surface
 * Ida's irregular gravitational field due to low surface gravity, fast rotation and irregular shape
 * May explain placement of ejecta blocks
 * Ejecta blocks rapidly broken down by other impacts
 * Those present must be fairly young
 * Azzurra is on the trailing rotational edge of the asteroid opposite the ejecta blocks
 * Azzurra contains a smaller, 3km, crater in its eastern rim
 * this smaller crater might be the source of the ejecta rather than Azzurra itself
 * Blocks indicate the presence of regolith
 * The two largest blocks on Ida are about half as tall as they are wide
 * The blocks are probably fragments of Ida ejected in an impact
 * Blocks could be leftover fragments from the parent body
 * Most blocks are located in Ida craters Lascaux and Mammoth

Grooves
Dibs Wronkiew (talk) 06:03, 19 February 2009 (UTC)
 * Ida Region 2 contains all the grooves
 * Ida's grooves are concentrated in three areas but are not obviously related to individual craters or structures.
 * One set of grooves is opposite Vienna Regio. Possibly caused by that impact and "reactivated" by later impacts such as Azzurra.
 * If grooves are the result of focussed seismic waves, Ida is probably not a rubble pile
 * Grooves up to 4 km long, 350 m depth, usually 100 m deep
 * Ida grooves near Mammoth, Lascaux, and Kartchner
 * Grooves indicate internal fractures
 * Computer simulations of impacts on Ida reproduce the grooves if the interior is homogeneous

Classification and mineral content

 * Dibs Wronkiew (talk) 06:29, 26 January 2009 (UTC)

)
 * Ida is S-class
 * Ida is S{IV} class
 * Association between asteroid types and meteor types is uncertain
 * S-class either ordinary chondrite or stony-iron
 * Chondrite = primitive
 * Stony-iron = differentiated
 * Ida's surface is mostly olivine with small amounts of orthopyroxene
 * NOTE: "Earlier premature reports by NIMS investigators that Ida is an exceptionally olivine-rich object (Carlson, 1994) and that Dactyl is rich in clinopyroxene (Carlson et al., 1994) have been effectively withdrawn (J. Granahan, pers. comm., 1995; R. Carlson [pers. comm., 1996] agrees that the evidence for clinopyroxene is 'weak')."
 * Pyroxene and olivine are silicate minerals
 * No large variations in mineral content across the surface
 * Extrapolating spectra of weathered regions on Ida, to less weathered regions, to Dactyl indicates ordinary chondritic (OC) composition
 * Low bulk density rules out a half-metallic composition even if it's a rubble pile
 * Ordinary chondrites contain varying amounts of olivine, pyroxene, iron, and feldspar ( "The chondrites fall naturally into five composition classes, of which three have very similar mineral contents, but different proportions of metal and silicates. All three contain abundant iron in three different forms (ferrous iron oxide in silicates, metallic iron, and ferrous sulfide), usually with all three abundant enough to be classified as potential ores. all three contain feldspar (an aluminosilicate of calcium, sodium, and potassium), pyroxene (silicates with one silicon atom for each atom of magnesium, iron, or calcium), olivine (silicates with two iron or magnesium atoms per silicon atom), metallic iron, and iron sulfide (the mineral triolite). These three classes, referred to collectively as the ordinary chondrites, contain quite different amounts of metal."
 * If Ida is assumed to be of homogeneous density then maximum moment of inertia coincides with spin axis. This suggests no regions of differing density.

Porosity

 * Dibs Wronkiew (talk) 22:59, 25 January 2009 (UTC)


 * Calculated density of Ida assumes chondrite
 * Ida inferred grain density 3480–3640 kg/m3
 * Based on chondrite classification
 * Ida observed bulk density 2600 ± 500 kg/m3
 * Density of Ida directly measured by spacecraft
 * Density is 2.27–3.10 g/cm3
 * Ida bulk density 2.6 ± 0.5 g/cm3 (Belton et al. 1995)
 * If Ida is assumed to be of homogeneous density then maximum moment of inertia coincides with spin axis. This suggests no regions of differing density.
 * Porosity of Ida 11–42%


 * Debris layer at least 50 m thick, corresponding to the upper layer of megaregolith
 * Megaregolith depth ranges from hundreds of meters to a few kilometers
 * Megaregolith under Mammoth, Lascaux, and Undara may go all the way to the core

Asteroid family

 * Dibs. Reyk  YO!  00:53, 26 January 2009 (UTC)

Ida is a member of the Koronis family, a group of asteroids with similar orbits. The Koronis family was recognized as a family by Kiyotsugu Hirayama, who proposed in 1918 that the group was the remnants of a destroyed precursor body. The parent body was probably about 120 km in diameter.


 * Ida belongs to the Koronis family
 * Koronis family has similar orbits
 * Hirayama recognized the family, in 1918 proposed that it comprised remnants of a precursor body
 * Some of Ida's craters may be from the breakup of the Koronis parent body
 * Koronis parent body was about 120 km diameter

Spin
Ida's rotation period is 4.63 hours, making it one of the fastest rotating known asteroids. It is in the top 10% of measured asteroids by spin. Ida's spin axis coincides with its maximum moment of inertia, if its density is assumed to be even throughout. This suggests that there are no major variations of density within the asteroid.


 * Rotation period of Ida is 4.63 hours
 * Ida has a rotation period of 4.63 hr
 * One of the fastest rotating known asteroids
 * If Ida is assumed to be of homogeneous density then maximum moment of inertia coincides with spin axis. This suggests no regions of differing density.
 * Ida period is 4.63 hours
 * Ida in the top 10% of measured asteroids by spin
 * Ida's spin is in the top 10% of measured asteroids (Lagerkvist et al. 1989)

Coordinates of north pole

 * Ida adopted mean obliquity (ϵ): 156°
 * Ida ecliptic longitude (λ): 263°

Origin

 * Dibs Wronkiew (talk) 00:00, 1 February 2009 (UTC)


 * Analysis of cratering gives two age ranges, 50 million years, or more than 1 billion
 * Ida is billions of years old
 * Two different ages for Ida and Dactyl
 * Ida is about a billion years old
 * Dactyl should have decayed in less than 100 million years
 * Ida may be ten times older than Gaspra
 * An event formed the Koronis family 1.5 billion years ago
 * Ida "formed and evolved under the overwhelming inﬂuence of collisional processes"
 * Asteroid families are created from "catastrophic disruption events"
 * Koronis parent body was about 120 km diameter
 * Ida and Dactyl believed to have originated in the disruption of the parent body about a billion years ago
 * Koronis parent was partially differentiated
 * Ida does not contain core material from parent

Moon

 * Dibs. Wronkiew (talk) 17:09, 26 February 2009 (UTC)

Discovery

 * Dibs. Reyk  YO!  02:19, 26 January 2009 (UTC)

Ida has a small moon, Dactyl, the first satellite of an asteroid to be discovered. It was found on February 17, 1994 by Galileo mission member Ann Harch, while examining the delayed image downloads. Galileo recorded 47 images of Dactyl over an observation period of 5 1/2 hours in August 1993. The spacecraft was 10760 km from Ida and 10870 km from Dactyl when the first image was taken, 14 minutes before Galileo made its closest approach to Ida.

Dactyl's original designation was 1993 (243) 1. It was renamed by the International Astronomical Union in 1994, after the mythological dactyls who inhabited Mount Ida.


 * 5 1/2 hours of observations of Dactyl
 * 47 images of Dactyl
 * Ann Harch of the Galileo camera team first discovered Ida's moon
 * Images processed on February 17, 1994
 * Dactyl the first confirmed satellite discovered around an asteroid
 * First image of Dactyl taken at a distance of 10,870km from Dactyl, 14 minutes before Galileo's closest approach to Ida
 * IAU named Dactyl in 1994
 * Previous designation was 1993 (243) 1
 * Dactyl found in 1994-02 from data recorded in 1993-08
 * Spacecraft was 10,760 km from Ida when first pictures were taken
 * Initial designation 1993 (243) 1
 * "These things then are as I have described them. As for the Olympic games, the most learned antiquaries of Elis say that Cronus was the first king of heaven, and that in his honor a temple was built in Olympia by the men of that age, who were named the Golden Race. When Zeus was born, Rhea entrusted the guardianship of her son to the Dactyls of Ida, who are the same as those called Curetes. They came from Cretan Ida – Heracles, Paeonaeus, Epimedes, Iasius and Idas."

Physical characteristics

 * Dibs. Reyk  YO!  23:10, 6 February 2009 (UTC)

Dactyl is an "egg-shaped" object measuring 1.6 &times; 1.4 &times; 1.2 km. It is oriented with its longest axis pointing towards Ida. Like Ida, Dactyl exhibits saturation cratering. There are more than a dozen craters with a diameter greater than 80 m on Dactyl's surface, indicating that the moon has suffered many collisions during its history. About six of the easily identifiable craters on Dactyl are in a linear chain, suggesting a local origin such as material ejected from Ida. Unlike the craters on Ida, there are indications that Dactyl's craters possess central peaks. These central peaks, and Dactyl's spheroidal shape, imply that, despite its low gravity, Dactyl is gravitationally controlled.

Dactyl has an escape velocity of about 0.5 m/s and, like Ida, a temperature of 200K.


 * Dactyl about 1.5 km long
 * Both Ida and moon have a temperature of 200 K
 * 1.6 x 1.4 x 1.2 km
 * Egg shaped
 * Oriented with its longest axis pointing towards Ida
 * Saturation cratering
 * About six craters form a linear chain
 * Dactyl has more than a dozen craters greater than 80m in diameter
 * Indicates many collisions during its history
 * Dactyl appears to have craters with central peaks
 * Despite its low gravity, Dactyl seems to be gravitationally controlled
 * Escape velocity is half a meter per second

Composition
Dactyl's surface was initially thought to be composed of equal parts clinopyroxene and the silicate minerals olivine and orthopyroxene and clinopyroxene but later studies indicate that the evidence for clinopyroxene is "weak".


 * Dactyl's surface is equal parts olivine, orthopyroxene, and clinopyroxene
 * NOTE: "Earlier premature reports by NIMS investigators that Ida is an exceptionally olivine-rich object (Carlson, 1994) and that Dactyl is rich in clinopyroxene (Carlson et al., 1994) have been effectively withdrawn (J. Granahan, pers. comm., 1995; R. Carlson [pers. comm., 1996] agrees that the evidence for clinopyroxene is 'weak')."
 * Pyroxene and olivine are silicate minerals

Mass

 * "An astronaut would need to exercise care in order to walk on the surface of Dactyl, as an overly energetic step would lead to permanent departure"

Comparison with Ida

 * Dactyl's albedo and spectrum not very different from Ida's
 * Ida and Dactyl have similar compositions
 * Spectrum indicates Space weathering prominent on Ida
 * Not as prominent on Dactyl
 * Dactyl thought to have less regolith than Ida
 * Dactyl 10-20 times smaller than Ida

Orbit and rotation

 * Dibs. Reyk  YO!  21:02, 15 February 2009 (UTC)

Dactyl's orbit around Ida is not known with great precision. Galileo was in the plane of Dactyl's orbit when most of the images of the moon were taken, which made determining its exact orbit difficult. It is known that Dactyl orbits in the prograde direction and is inclined about 8° to Ida's equator. Based on computer simulations Dactyl's pericenter must be more than about 65 km from Ida if it is to remain in a stable orbit. The range of orbits generated by the computer simulations was narrowed down by the necessity of having the simulated orbits pass through points at which Galileo observed Dactyl to be, particularly a reference point approximately 90 km from Ida at longitude 85° taken at 16:52:05 UT on 1993-08-28. On 1994-04-26, the Hubble Space Telescope observed Ida for eight hours and was unable to spot Dactyl. It would have been able to spot the moon if it was more than about 700 km from Ida.

Dactyl's orbital period is approximately 20 hours, assuming the moon is in a circular orbit around Ida. Its orbital speed is about 10 m/s, "about the speed of a fast run or a slowly thrown baseball". The moon's longest axis points toward Ida.


 * Dactyl's orbit is prograde
 * At 16:52:05 UT on 1993-08-28, Dactyl was 90 km from Ida at longitude 85 deg
 * Dactyl pericenter greater than 65 km
 * Search for Dactyl by Hubble turned up nothing (Belton et al. 1995)
 * Galileo was in the plane of Dactyl's orbit for most of the photos of Dactyl
 * This made determining the orbit difficult
 * Dactyl's orbital speed is about 10 m/s "about the speed of a fast run or a slowly thrown baseball"
 * HST observed Ida for 8 hours on 1994-04-26
 * Dactyl was less than 700 km from Ida at the time
 * Longest axis pointing towards Ida
 * Period of Dactyl's orbit is approx. 20 hours, assuming circular orbit
 * Dactyl's orbit is inclined about 8° to Ida's equator (

Age & Origin
Dactyl is believed to have originated from the same parent body as Ida, from the disruption of the parent of the Koronis asteroid family, but it is possible that it formed formed more recently, perhaps as ejecta from a large impact on the asteroid. It is extremely unlikely that it was captured by Ida. There is evidence that it suffered a major impact around 100 million years ago, which reduced its size.


 * Dactyl probably created at the same time as Ida (Durda 1996)
 * Possible Dactyl was formed more recently (Durda & Geissler 1996)
 * Dactyl may have been impacted around 100 million years ago
 * Size reduction
 * Ida and Dactyl from the same parent body
 * Differences in composition between Ida and Dactyl indicate differentiation in the parent body
 * Dactyl has more than a dozen craters greater than 80m in diamter
 * Indicates many collisions during its history
 * Virtually impossible for Dactyl to have been captured by Ida
 * Ida and Dactyl believed to have originated in the disruption of the parent body about a billion years ago

Model articles

 * Ceres
 * Article about a similar object
 * Pluto
 * Mostly airless rock with moons
 * Mercury
 * Airless rock visited by spacecraft
 * Enceladus
 * Airless rock visited by spacecraft, lots of good diagrams
 * Craters of the Moon National Monument and Preserve
 * Less similar, but an interesting comparison
 * Antarctica
 * For comparison
 * Callisto (moon)
 * Another object that features saturation cratering

Unused source citations
List of citations for potentially useful source material:
 * Replaced by Petit 1997
 * Preprint (PS).
 * The preprint is probably not a reliable source.
 * Also a preprint
 * No new information.
 * No new information.
 * The Asteroid Orbital Elements Database
 * No new information.
 * No new information.
 * The Asteroid Orbital Elements Database
 * No new information.
 * The Asteroid Orbital Elements Database
 * The Asteroid Orbital Elements Database

Plan
This is a preliminary list of steps that need to be taken to finish the project


 * 1) Write the article
 * 2) *Reorganize the old article into the new structure
 * 3) *Remove or reference any unreferenced information
 * 4) Illustrate the article
 * 5) *Create a gallery on this page of all the images available
 * 6) *Verify source information and complete descriptions for each image
 * 7) *Find additional free images
 * 8) *Write captions and add to the article
 * 9) GAN (optional?)
 * 10) Peer Review (simultaneous with GAN?)
 * FAC
 * FAC

DYK ideas

 * ...that images of 243 Ida returned from the space probe Galileo, and processed on 17 February 1994, provided the first confirmation of a moon orbiting an asteroid?
 * ...that asteroid 243 Ida is one of the most densely cratered objects in the Solar System?