**List of unnumbered minor planets: 1999 S–T**
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**Definition**
The „List of unnumbered minor planets: 1999 S–T” is a catalog of minor planets discovered in the second half of September 1999, specifically those with provisional designations beginning with the letters S and T, which have not yet been assigned permanent numbers by the Minor Planet Center.
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## List of unnumbered minor planets: 1999 S–T
### Introduction
Minor planets, commonly known as asteroids, are small celestial bodies orbiting the Sun primarily within the asteroid belt between Mars and Jupiter, though many also inhabit other regions of the Solar System. When newly discovered, these objects receive provisional designations based on the date of discovery. Once their orbits are sufficiently well determined, they are assigned permanent numbers by the Minor Planet Center (MPC). However, many minor planets remain unnumbered for extended periods due to insufficient observational data or orbital uncertainties.
This article presents an overview of the unnumbered minor planets discovered in the latter half of September 1999, specifically those with provisional designations starting with the letters S and T. These designations correspond to discoveries made between September 16 and September 30, 1999. The list includes information on their discovery circumstances, orbital classifications, physical characteristics, and observational status.
### Provisional Designation System
The provisional designation system for minor planets is a standardized method used by the MPC to identify newly discovered objects before they receive permanent numbers. The system encodes the year of discovery, a half-month code, and an alphanumeric sequence indicating the order of discovery within that half-month.
– The letter „S” corresponds to the half-month period from September 16 to 30.
– The letter „T” follows „S” and continues the sequence for the same half-month.
For example, a minor planet designated as „1999 SA” was discovered in the first half of September 1999, while „1999 SS” or „1999 ST” would be discoveries from the second half of September 1999.
### Overview of Discoveries in 1999 S–T
The second half of September 1999 was a productive period for asteroid discovery, with numerous surveys and observatories contributing to the identification of new minor planets. Many of these objects were detected by large-scale sky surveys such as the Lincoln Near-Earth Asteroid Research (LINEAR) program, the Spacewatch project, and the Lowell Observatory Near-Earth Object Search (LONEOS).
The minor planets in this list vary widely in size, orbital characteristics, and classification. Some are main-belt asteroids with relatively stable orbits between Mars and Jupiter, while others may be near-Earth objects (NEOs) or belong to other dynamical groups such as Jupiter Trojans or Mars-crossers.
### Classification of Minor Planets in the List
Minor planets are classified based on their orbital parameters, spectral properties, and dynamical behavior. The primary orbital classes represented in the 1999 S–T list include:
– **Main-belt asteroids (MBA):** Objects orbiting primarily between 2.1 and 3.3 astronomical units (AU) from the Sun.
– **Near-Earth objects (NEO):** Asteroids with orbits that bring them close to Earth’s orbit, including subgroups such as Atiras, Atens, Apollos, and Amors.
– **Mars-crossers (MC):** Asteroids with orbits that cross that of Mars.
– **Jupiter Trojans:** Objects sharing Jupiter’s orbit around the Sun, clustered near the L4 and L5 Lagrangian points.
– **Outer Solar System objects:** Including Centaurs and trans-Neptunian objects (TNOs), though these are less common in this particular list.
### Observational Status and Numbering
The process of numbering minor planets requires precise orbital determination, which depends on multiple observations over time. Many objects in the 1999 S–T list remain unnumbered due to:
– Limited observational data or short observation arcs.
– Orbital uncertainties that prevent reliable long-term predictions.
– Loss of the object due to insufficient follow-up.
As observational techniques and survey coverage improve, some of these objects may eventually be numbered and named.
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## Detailed Information on Selected Minor Planets from 1999 S–T
Due to the extensive number of unnumbered minor planets discovered during this period, this section highlights representative examples illustrating the diversity of objects within the 1999 S–T designation range.
### Example 1: 1999 SS1
– **Discovery:** Detected by LINEAR on September 18, 1999.
– **Orbital classification:** Main-belt asteroid.
– **Orbital parameters:** Semi-major axis approximately 2.5 AU, eccentricity 0.12, inclination 5°.
– **Physical characteristics:** Estimated diameter ~1.2 km based on absolute magnitude.
– **Observational status:** Multiple oppositions observed, but orbit not yet precise enough for numbering.
### Example 2: 1999 ST12
– **Discovery:** Discovered by Spacewatch on September 22, 1999.
– **Orbital classification:** Mars-crosser.
– **Orbital parameters:** Semi-major axis 2.3 AU, eccentricity 0.3, inclination 15°.
– **Physical characteristics:** Diameter estimated at 0.8 km.
– **Observational status:** Limited follow-up observations; orbit determination remains uncertain.
### Example 3: 1999 SU45
– **Discovery:** Found by LONEOS on September 25, 1999.
– **Orbital classification:** Near-Earth Apollo asteroid.
– **Orbital parameters:** Semi-major axis 1.1 AU, eccentricity 0.4, inclination 7°.
– **Physical characteristics:** Approximately 300 meters in diameter.
– **Observational status:** Observed during discovery apparition; no subsequent observations recorded.
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## Importance of Cataloging Unnumbered Minor Planets
Maintaining comprehensive lists of unnumbered minor planets is crucial for several reasons:
– **Tracking and recovery:** Enables astronomers to locate and observe these objects in future apparitions.
– **Orbit refinement:** Additional observations improve orbital models, facilitating numbering and naming.
– **Risk assessment:** Identifying potentially hazardous asteroids (PHAs) among unnumbered objects is vital for planetary defense.
– **Scientific research:** Understanding the population distribution, composition, and dynamics of minor planets informs models of Solar System formation and evolution.
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## Challenges in Numbering Minor Planets
Several factors contribute to the delay or difficulty in numbering minor planets:
– **Short observation arcs:** Initial discovery observations may span only a few days, insufficient for precise orbit determination.
– **Faintness and size:** Smaller and darker objects are harder to detect and track.
– **Orbital complexity:** Objects with highly eccentric or inclined orbits require more data to model accurately.
– **Observational constraints:** Weather, telescope time, and survey priorities affect follow-up opportunities.
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## Summary
The list of unnumbered minor planets with provisional designations from 1999 S–T represents a significant subset of small Solar System bodies discovered in late September 1999. These objects, ranging from main-belt asteroids to near-Earth objects, remain unnumbered due to various observational and orbital challenges. Continued efforts in observation and orbit refinement are essential to fully catalog and understand these minor planets.
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**Meta Description:**
This article provides a comprehensive overview of unnumbered minor planets discovered in the second half of September 1999, designated 1999 S–T, detailing their discovery, classification, and observational status.