Alpha-5 is the scheme that lets a five-character TLE field carry catalogue numbers above 99,999: the first digit becomes a letter, so A0000 means 100000. It extends the classic TLE format's ceiling to 339,999 objects while newer data formats carry full numbers natively.
Understanding Alpha-5
Why the catalogue hit five digits
The catalogue number is issued sequentially as objects are identified, so its growth curve tells the story of the space age: it took nearly five decades to reach 40,000, then fragmentation events — the 2007 Fengyun-1C anti-satellite test and the 2009 Iridium–Cosmos collision chief among them — and the constellation boom accelerated it sharply. Numbers are consumed not only by satellites but by every tracked debris fragment, spent stage and mission-related object, plus analyst objects awaiting formal cataloguing. With single launches now releasing dozens of payloads and next-generation radars such as the Space Fence detecting ever-smaller fragments, the six-digit era arrived decades sooner than the TLE's designers imagined.
The decode table
| TLE characters | Decodes to | Note |
|---|---|---|
| 00001–99999 | 1–99999 | classic numeric range, unchanged |
| A0000–A9999 | 100000–109999 | A = 10 |
| B0000–H9999 | 110000–179999 | B–H = 11–17 |
| J0000–N9999 | 180000–229999 | I skipped (looks like 1) |
| P0000–Z9999 | 230000–339999 | O skipped (looks like 0) |