September 29 – Discovery of Asteroid 243 Ida (1884)

243 Ida was discovered by by the Austrian astronomer Johann Palisa on September 29th, 1884, and unlike most bodies in the main asteroid belt, this particular S-type asteroid has been studied at close quarters. Ida was visited by the Galileo spacecraft in 1993. It is one of the larger members of the Koronian family of asteroids, about 6000 strong, who orbit as a group, and are thought to be the remains of a collision involving a single, larger body. Thanks to Galileo, we have pretty good photographs of Ida, and we now know it has a small companion, Dactyl.

Ida and Dactyl, courtesy of Galileo (image: NASA/JPL)

Dactyl is pretty teeny, measuring just over a kilometre across in most directions. It is the small dot on the right side of Ida in the photograph, and was the first moon discovered orbiting an asteroid. The name comes from a race of mythical beings who inhabited Mount Ida in Crete. They are credited with discovering how to use fire to work metal.

The asteroid itself is named for the nymph who, with her sister Adrasteia, was entrusted with caring for the infant god Zeus. The name was the idea of Moriz von Kuffner, an Austrian brewer, philanthropist and astronomer, who founded the Kuffner Observatory in Vienna. Kuffner eventually got his own named asteroid, but not until 2006 (number 12568).

Also as a result of the Galileo visit, I will be abandoning my usual use of the words “about” and “approximately”, and will be stating that Ida’s principal dimensions are 59.8 km by 25.4 km by 18.6 km, with a mean radius of 15.7 km (Belton et al, 1996).

September 29 – OSO-7

The Orbiting Solar Observatory 7 (OSO-7 ) was the seventh of eight similar satellites, and was launched on September 29th 1971, straight into a drama, when a problem with the second-stage guidance system left it in an unplanned orbit, and pointing in unexpected directions. Fortunately, John Thole at NASA (who we might well come across again in connection with Echo 2) managed to get the mission back on course with a mix of skill and luck, saving the day. I can’t imagine how he must have felt, slewing this huge craft around from so far away he couldn’t even see it. It’s bad enough when your spaceship is made by Nintendo and you get to start again if you crash.

OSO-& (image credit: NASA)
OSO-& (image credit: NASA)

OSO-7, like its brethren, came in two main parts, known as the “sail” and the “wheel”. The sail faced the Sun (during the satellite’s day) and measured solar x-rays and coronal activity. The wheel, as the name suggests, rotated continuously, measuring solar x-rays and cosmic x- and gamma-rays. The spinning of the wheel, combined with the axial movements that kept the sail pointing at the Sun, allowed two of the wheel-based experiments to cover the whole sky every six months.

OSO-7 re-entered the Earth’s atmosphere on July 9th 1974.

ALSO TODAY . . . .

1884  —  Discovery of asteroid 243 Ida.

1962  —  Launch of satellite Alouette 1.

1988  —  Launch of shuttle mission STS-26 (“Discovery“).