Nereus NEAR us

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Nereus imaged by the Goldstone radar in November 2021

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<<4660 Nereus, provisional designation 1982 DB, is a small (about 0.33 kilometres asteroid. It was discovered by Eleanor F. Helin on 28 February 1982, approximately a month after it passed 4.1 million km from Earth. Nereus is potentially an important asteroid with a high albedo. It is an Apollo and Mars-crosser, with an orbit that frequently comes close to Earth, and because of this it is exceptionally accessible to spacecraft. Indeed, because of its small size and close orbit, its delta-V for rendezvous of ~5 km/s is smaller than the Moon's, which is about 6.3 km/s.

Nereus is classified as a potentially hazardous asteroid (PHA), due to both its absolute magnitude (H ≤ 22) and its minimum orbit intersection distance (MOID ≤ 0.05 AU). Nereus makes seven approaches to Earth of less than 5 million km between 1900 and 2100. The closest will be on 14 February 2060, at 1.2 million km. The next close approach is on 11 December 2021, when it will be 3.9 million km away. During the 2021 approach it will peak around apparent magnitude 12.6. Its orbital period of 1.82 yr also puts it somewhat near a 2:1 orbital resonance with Earth, which means that an approximately 4-year mission could depart for and return from the asteroid on relatively near passes to the Earth. The asteroid is classified as E-type, so it could be potentially associated with aubrite meteorites (enstatite achondrites).

Nereus was proposed for visitation by both the private Near Earth Asteroid Prospector (NEAP) probe, and the Japanese sample return mission Hayabusa. However, the NEAP probe was not realized, and the Hayabusa's launch was delayed by 10 months and the probe had to be redirected to 25143 Itokawa. 4660 Nereus was considered as a flyby target of the NEAR robotic spacecraft mission. NEAR was eventually launched, but visited 253 Mathilde and 433 Eros.

Nereus has a generally ellipsoidal shape with dimensions of 510 m × 330 m × 241 m. On the ends of its longest axis, one end appears narrower and rounder than the other, larger end, making it more of an egg shape. The larger end also appears to have a flatter region on one side of it. Nereus rotates about an axis roughly perpendicular to its longest axis much like a silver spoon spinning on a table.

Although the discoverer is given the opportunity to name the asteroid, Helin donated naming rights to the Planetary Society which organized a naming contest. The winner, Robert M. Cutler, then an employee of NASA contractor The MITRE Corporation, named the asteroid after the ancient Greek proto-god Nereus who had characteristics later attributed to Apollo (prophecy) and Poseidon (a sea god similar to Nereus but with legs rather than a fish tail).>>

[Fred the Cat]

Nereus imaged by the Goldstone radar in November 2021

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<<4660 Nereus, provisional designation 1982 DB, is a small (about 0.33 kilometres asteroid. It was discovered by Eleanor F. Helin on 28 February 1982, approximately a month after it passed 4.1 million km from Earth. Nereus is potentially an important asteroid with a high albedo. It is an Apollo and Mars-crosser, with an orbit that frequently comes close to Earth, and because of this it is exceptionally accessible to spacecraft. Indeed, because of its small size and close orbit, its delta-V for rendezvous of ~5 km/s is smaller than the Moon's, which is about 6.3 km/s.

Nereus is classified as a potentially hazardous asteroid (PHA), due to both its absolute magnitude (H ≤ 22) and its minimum orbit intersection distance (MOID ≤ 0.05 AU). Nereus makes seven approaches to Earth of less than 5 million km between 1900 and 2100. The closest will be on 14 February 2060, at 1.2 million km. The next close approach is on 11 December 2021, when it will be 3.9 million km away. During the 2021 approach it will peak around apparent magnitude 12.6. Its orbital period of 1.82 yr also puts it somewhat near a 2:1 orbital resonance with Earth, which means that an approximately 4-year mission could depart for and return from the asteroid on relatively near passes to the Earth. The asteroid is classified as E-type, so it could be potentially associated with aubrite meteorites (enstatite achondrites).

Nereus was proposed for visitation by both the private Near Earth Asteroid Prospector (NEAP) probe, and the Japanese sample return mission Hayabusa. However, the NEAP probe was not realized, and the Hayabusa's launch was delayed by 10 months and the probe had to be redirected to 25143 Itokawa. 4660 Nereus was considered as a flyby target of the NEAR robotic spacecraft mission. NEAR was eventually launched, but visited 253 Mathilde and 433 Eros.

Nereus has a generally ellipsoidal shape with dimensions of 510 m × 330 m × 241 m. On the ends of its longest axis, one end appears narrower and rounder than the other, larger end, making it more of an egg shape. The larger end also appears to have a flatter region on one side of it. Nereus rotates about an axis roughly perpendicular to its longest axis much like a silver spoon spinning on a table.

Although the discoverer is given the opportunity to name the asteroid, Helin donated naming rights to the Planetary Society which organized a naming contest. The winner, Robert M. Cutler, then an employee of NASA contractor The MITRE Corporation, named the asteroid after the ancient Greek proto-god Nereus who had characteristics later attributed to Apollo (prophecy) and Poseidon (a sea god similar to Nereus but with legs rather than a fish tail).>>

But how many " Nereids" are near us?

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But how many " Nereids" are near us?

E-Type asteroid 2867 Šteins

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The E-belt asteroids

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Asteroid groups out to the orbit of Jupiter, showing eccentricity versus semi-major axis. Hungaria asteroids are the leftmost dense grouping in blue. The core region of the asteroid belt is shown in red.

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<<E-type asteroids are asteroids thought to have enstatite (MgSiO₃) achondrite surfaces. They form a large proportion of asteroids inward of the asteroid belt known as Hungaria asteroids,but rapidly become very rare as the asteroid belt proper is entered. They are thought to have originated from the highly reduced mantle of a differentiated asteroid.

E-type asteroids have a high albedo (0.3 or higher), which distinguishes them from the more common M-type asteroids. Their spectrum is featureless flat to reddish. Probably because they originated from the edge of a larger parent body rather than a core, E-types are all small, with only three (44 Nysa, 55 Pandora, 64 Angelina) having diameters above 50 kilometres and no others above 25 kilometers (the biggest three also orbit atypically far, c.3 AU, from the Sun). Aubrites (enstatite achondrite meteorites) are believed to come from E-type asteroids, because Aubrites could be linked to the E-type asteroid 3103 Eger.

The E-belt asteroids were the population of a hypothetical extension of the primordial asteroid belt proposed as the source of most of the basin-forming lunar impacts during the Late Heavy Bombardment. It describes the dynamics of an inner band of the early asteroid belt within the framework of the Nice model. The extended-belt asteroids were located between the current inner boundary of the asteroid belt and the orbit of Mars with semi-major axis ranging from 1.7 to 2.1 astronomical units (AU). In the current Solar System most orbits in this region are unstable due to the presence of the ν6 secular resonance. However, prior to the giant planet migration described in the Nice model the outer planets would have been in a more compact configuration with nearly circular orbits. With the planets in this configuration the ν6 secular resonance would be located outside the asteroid belt. Stable orbits would have existed inside 2.1 AU and the inner edge of the primordial asteroid belt would have been defined by Mars-crossing orbits.

During the migration of the giant planets the ν6 secular resonance would have moved inward as Saturn moved outward. Upon reaching its current location near 2.1 AU the ν6 secular resonance and other related resonances would destabilize the orbits of the E-belt asteroids. Most would be driven onto planet-crossing orbits as their eccentricities and inclinations increased. Over a period of 400 million years impacts of the E-belt asteroids yield an estimated 9-10 of the 12 basin-forming lunar impacts attributed to the Late Heavy Bombardment.

As their orbits evolved many of the E-belt asteroids would have acquired orbits similar to those of the Hungaria asteroids with high inclinations and semimajor axis between 1.8 and 2.0 AU. Because orbits in this region are dynamically sticky these objects would form a quasi-stable reservoir. As this population of the E-belt asteroids leaked from this reservoir they would produce a long-lived tail of impacts after the traditional end of the late heavy bombardment at 3.7 billion years ago. A remnant representing roughly 0.1–0.4% of the original E-belt asteroids would remain as the current Hungaria asteroids.

Evidence for the Moon does not support comets from the outer planetesimal belt as the source of the basin-forming lunar impacts. The size frequency distribution (SFD) of ancient lunar craters is a similar to the SFD of main belt asteroids instead of that of comets. Samples recovered from the Moon containing impact melts have a range of ages rather than the sharp spike expected if comets produced the LHB. Analysis of highly siderophile elements in these samples shows a better match for impactors from the inner Solar System than for comets. Studies of the dynamics of the main asteroid belt during giant planet migration have significantly limited the number of impactors originating from this region. A rapid alteration of Jupiter's and Saturn's orbits is necessary to reproduce the current orbital distribution. This scenario removes only 50% of the asteroids from the main belt producing 2–3 basins on the Moon.

Examination of samples recovered from the Moon indicates that the impactors were thermally evolved objects. E-type asteroids, an example of this type, are uncommon in the main belt but become more common toward the inner belt and would be expected to be most common in the E-belt. The Hungaria asteroids, which are a remnant of the E-belt in this model, contain a sizable fraction of E-type asteroids. The decay of the population of E-belt asteroids captured onto Hungaria like orbits produces a long-lived tail of impacts which continues past the LHB. The continuation of the bombardment is predicted to generate basin-forming impacts on the Earth and Chicxulub-sized craters on the Earth and Moon. Impact craters on the Moon and impact spherule beds found on the Earth dated to this period are consistent with these predictions.>>