Nysa–Polana complex

Proper orbital elements (inclination $i_{p}$ vs. eccentricity $e_{p}$) of asteroids and their families (highlighted in red) in the inner main belt. The Nysa–Polana complex includes Nysa, Polana, Hertha, Eulalia, and their respective families.

Same as left, but inclination $i_{p}$ vs. semi-major axis $a_{p}$. The vertical stripes correspond to mean motion resonances with the major planets, especially the 2:1 resonance with Mars (occupied by Polana) and the 1:3 resonance with Jupiter (right of Eulalia).

The Nysa–Polana complex is a collection of overlapping asteroid families in the inner region of the main asteroid belt. Asteroids in this complex orbit the Sun at proper semi-major axes between 2.3 and 2.5 AU, eccentricities between 0.12 and 0.21, and inclinations between 1°–10°. The Nysa–Polana complex consists of asteroids with a variety of albedos and spectral types, which include dark carbonaceous B-, C-, and F-types, bright silicaceous S-types, and intermediate X-types. The largest asteroids in the Nysa–Polana complex include the namesakes 44 Nysa (E-type) and 142 Polana (C-type), as well as 135 Hertha (X-type) and 495 Eulalia (C-type). The last three of these are known to host collisional families—asteroid families formed by disruptive collisions long ago.

The Nysa–Polana complex received its name under the historical presumption that Nysa and Polana were the parent bodies of this group. However, whether these two asteroids actually share a common origin is debated among astronomers. Nysa is often considered an interloper due to its enstatite-rich (E-type) composition, which is rarely seen in the complex. The Nysa–Polana complex's low-albedo asteroids are believed to be members of the Eulalia and New Polana families—collisional fragments ejected from Eulalia 0.9–1.5 billion years ago and from Polana over 2 billion years ago, respectively. The origin of the complex's S-type asteroids is less clear, although Hertha is suspected to be a possible source.

Subdivision

Proper orbital elements (inclination $i_{p}$ vs. eccentricity $e_{p}$) of the Nysa–Polana complex, with each asteroid represented as a circle scaled by diameter and color-coded by albedo. Low-albedo (C-type) asteroids are clustered near Polana, while moderate-albedo (S-type) asteroids are clustered near Hertha. Nysa does not have a cluster of high-albedo asteroids.

Asteroids in this complex are typically divided into the stony Nysa and carbonaceous Polana subgroups, two mineralogically different families:

• The much brighter S-type subgroup. It has been referred to as the Hertha family, and historically the Nysa family. The X-type asteroid Hertha is believed to be the parent body of the S-types.
• In the low-albedo subgroup of the complex lies the Polana family (adj Polanian), a family of dark F-type asteroids named after 142 Polana, the largest asteroid in this section. New JWST spectroscopy of (142) Polana suggests that the near-Earth asteroids (101955) Bennu, which was the target of NASA’s OSIRIS-REx mission, and (162173) Ryugu, which was the target of JAXA’s Hayabusa-2 mission, are also members of this family. An additional family, the Eulalia family has also been identified inside this subgroup.

Members

Name	Semimajor axis (a)	orbital eccentricity (e)	inclination (i)
44 Nysa	2.423	0.149	3.703°
135 Hertha	2.428	0.206	2.306°
142 Polana	2.418	0.136	2.238°
750 Oskar	2.444	0.130	3.952°
2984 Chaucer	2.470	0.135	3.054°
2391 Tomita
2509 Chukotka
2710 Veverka
3048 Guangzhou
3069 Heyrovsky
3172 Hirst
3467 Bernheim	2.409	0.149	4.112°
3952 Russellmark
4797 Ako
5075 Goryachev
5394 Jurgens
7629 Foros
7655 Adamries
7866 Sicoli	2.428	0.210	3.480°
9922 Catcheller	2.402	0.190	2.492°

History of studies

Members of the Nysa–Polana complex were first identified as a family in 1951 by Dirk Brouwer, who noted that Nysa, Hertha, Polana, and six other asteroids shared similar proper orbital elements. Brouwer assigned the identification number "24" to this family, while a 1969 study by James R. Arnold assigned the label "A-74". During this time, the Palomar–Leiden Survey discovered numerous small asteroids within the vicinity of Brouwer's purported family, revealing it to be an aggregation of several neighboring unrelated families. Thus in 1971, Bertil Lindblad and R. B. Southworth proposed that the family should be split into the Nysa and Hertha families. However, few asteroids in these proposed families had known colors and compositions, which raised the question of whether the Nysa and Hertha families truly originated from their namesakes, and whether the two families shared the same origin.

In 1982, E. F. Tedesco and colleagues analyzed the colors and albedos of asteroids in Lindblad and Southworth's proposed Nysa family and found that they were mostly F-type asteroids, unlike Nysa. This led Jeffrey F. Bell to argue in 1989 that these asteroids belonged to Polana instead of Nysa, which he labeled an interloper. In 1995, Vincenzo Zappalà and colleagues found that this family was actually two overlapping families (which does not include Hertha), which he dubbed a "complex clan" associated with Nysa. In response to this, Alberto Cellino and colleagues measured the spectra of multiple asteroids in this proposed clan and found that none matched Nysa's spectral type. They proposed in a 2001 study that the Nysa–Polana clan contains an S-type Mildred family (renamed from the Hertha family) and a F-type Polana family, neither of which share a common origin.

See also

• 101955 Bennu, probably part of the Polana family, visited by the OSIRIS-REx spacecraft in 2018