Let us speak of HiRISE image names…

Every HiRISE image (or “observation”) is identified by a unique ID. Think of this observation ID as the name of the image. The basic form of the ID has three parts: a mission phase, an orbit number, and a target code. Our first transition phase image, for example, is TRA_000823_1720.

The mission phase is a three-letter abbreviation. “TRA” is the transition phase. Starting in November, you’ll be seeing “PSP” for “primary science phase,” which lasts for two Earth years.

The orbit number is a six-digit, zero-padded number. “000823″ is the eight hundred twenty-third orbit of MRO around Mars. MRO is in a roughly polar orbit around Mars, meaning the orbit is nearly perpendicular to the planet’s equator; this sort of orbit is typical of missions designed for mapping, because it provides coverage of the entire planet. The orbit number increments by one whenever we cross the planet’s equator on the nighttime side.

The target code indicates the target of the observation. If the number is between 0000 and 3595, it is the angle between the nighttime equator and the latitude of the center of the observation, multiplied by 10. It is measured to the nearest half-degree. The nighttime equator is 0000, the south pole is 0900, the daytime equator is 1800, and the north pole is 2700. 1720 is at latitude 8° S.

If the target code is between 9000 and 9303, it indicates an off-planet target, such as Deimos, Phobos, or a star.

And that’s how you identify a HiRISE image. In practice, the HiRISE team talks about them by dropping the mission phase code and the orbit number padding. Our first image is simply “823 1720″ amongst the team. Formally, however, it will always be TRA_000823_1720.