C/1969 T1 (Tago–Sato–Kosaka)

Lore & Background

The comet was first discovered by Akihiko Tago on 10 October 1969 as a diffuse magnitude 10 object using a 15 cm reflector telescope. He reported his discovery to the Tokyo Astronomical Observatory about four days later. Two other independent discoveries were made by 19-year-old Yasuo Sato and 17-year-old Kozo Kosaka. Tago had previously co-discovered comet C/1968 H1, and he and Sato had spent 344 and 182 hours respectively searching for new comets.

In the following days, the comet was observed and photographed across Australia, Japan, United Kingdom, United States, and New Zealand, but its brightness remained constant throughout October. It had only two observations in November due to its proximity to the Sun. Throughout December, it gradually brightened to magnitude 4 and developed a tail reaching about 5 degrees by year's end. On 2 January 1970, Z. M. Pereyra spotted it with the naked eye and estimated its tail at 10 degrees; that day it reached maximum southern declination at –54° and peak brightness of 3.6. After early January it faded, with its closest approach to Earth on 20 January 1970. The last naked eye observation was on 2 February at magnitude 5.5. A minor outburst occurred on 6 February, observed in infrared. The comet was last observed on 4 May 1970.

C/1969 T1 was the first comet ever observed by Alan Hale (later co-discoverer of Comet Hale-Bopp), when it faded as a 5th-magnitude object on 2 February 1970.

Reader's Guide

The comet's significance lies in being the first comet observed by an artificial satellite, OAO-2, marking a milestone in space-based astronomy. Its discovery involved three independent amateur astronomers, including Akihiko Tago who had co-discovered another comet the previous year. The comet's orbital elements were calculated in 1978 by Brian Marsden, Zdenek Sekanina, and Edgar Everhart based on 305 observations over 175 days. Gravitational perturbations by giant planets increased its semimajor axis from 1,970 AU to 2,320 AU after its perihelion on 21 December 1969. Its closest approach to Earth was on 20 January 1970 at 0.38 AU. The comet's very small minimum orbit intersection distance with Earth led Zdenek Sekanina and Ichiro Hasegawa to independently consider a potential meteor shower, but no significant activity was observed. The comet also provided an early observing experience for Alan Hale, who would later co-discover Comet Hale-Bopp.

Did You Know?

• The comet was independently discovered by three Japanese amateurs, including a 17-year-old and a 19-year-old.
• Alan Hale, later co-discoverer of Comet Hale-Bopp, observed this comet as his first comet on 2 February 1970.
• Its orbital semimajor axis increased from 1,970 AU to 2,320 AU after its 1969 perihelion due to planetary perturbations.