HiRISE and the PIG Connection
One of the novel aspects of the Giotto camera, the Halley Multicolour Camera (HMC), was its use of a special technique called Time Delay and Integration (TDI). Imagine that the subject of your image is moving quickly with respect to your camera. Normally, if you need to increase your exposure time (because the image will be too dark), then the motion of the subject will smear your image. However, if the speed and direction of that motion is known in advance (at the time when the experiment is designed), then this motion can be compensated for by “moving” the image across the detector at the same apparent speed as the motion of the subject. This trick of increasing the exposure time is known as TDI. HMC used this trick to compensate for the relative rapid rotation of the Giotto spacecraft.
In 1999, based on his experience and knowledge of TDI from his work on HMC, Nicolas Thomas was asked to support another camera system which was planning to use the same technique. The High Resolution Imaging Science Experiment (HiRISE) was going to be proposed to NASA to fly on its Mars Reconnaissance Orbiter (MRO) spacecraft for launch in August 2005. The principal investigator was Alfred McEwen from the Lunar and Planetary Laboratory of the University of Arizona in Tucson. Like HMC, HiRISE also needed to use TDI but for a slightly different reason. The scientific aim of HiRISE was to image Mars at less than 1 metre resolution from an orbiting spacecraft 250 km above the surface. The speed with which MRO would pass over the surface meant that, without TDI, images would be exposed for just 74 microsecs – nowhere near long enough to get good quality data without an enormous telescope. So TDI was implemented allowing longer exposures and thereby providing far superior data.
In the first meetings, the newly formed science team was able to discuss the design of the experiment. HiRISE was based on spy satellite technology so that several aspects (including the way to implement TDI) had already been worked out. But on one occasion, the team was prompted by Thomas to discuss whether it was possible to produce colour data. McEwen loved Thomas’s idea and, together with Alan Delamere, the project manager from Ball Aerospace, the concept was agreed. Subsequently, McEwen had to fight NASA officials and the MRO program to keep the colour when the project was under financial and technical pressure. But he held firm and the colour data from HiRISE has produced some of the most spectacular images even returned from an interplanetary mission.
MRO was launched on August 12, 2005 from Cape Canaveral. After a 7 month cruise, the spacecraft entered orbit about Mars in March 2006 and began a 6 month aerobraking phase which was designed to reduce the altitude of the spacecraft over the surface. In November 2006, MRO entered its primary science orbit (250 km x 320 km above the surface) and began imaging. The images were instantly remarkable and beautifully exposed (the latter was partially the result of a detailed computer code written for the project by Thomas and Delamere).