HiRISE and the Mars Reconnaissance Orbiter
We are the only original European Co-Is on the High Resolution Imaging Science Experiment (HiRISE) – the high resolution imaging system on NASA’s Mars Reconnaissance Orbiter (MRO). The Principal Investigator is Prof. Alfred McEwen from the Lunar and Planetary Laboratory at the University of Arizona. We became involved in the project in 1999 and participated in the instrument development through to launch.
MRO was launched on August 12, 2005 from Cape Canaveral. The spacecraft completed orbit insertion in March 2006. This was followed by an aerobraking phase where MRO used the drag produced by the atmosphere of Mars to slow the spacecraft down. This brought MRO into its final, slightly elliptical orbit of 255 km x 320 km above the surface of Mars. The inclination of the orbit with respect to the Martian equator is about 87° (i.e. it is a roughly polar orbit). The orbit is Sun-synchronous so that MRO passes directly over the equator at the point on Mars where it is 15:05 local time.
There are several experiments onboard MRO.
HiRISE is the high resolution imaging system. It is complemented by the Context Imager (CTX) which is a lower resolution system but still capable of producing images at better than 10 m/px. A wide angle imager, MARCI, which is used to monitor the atmospheric conditions completes the imaging payload.
CRISM is a high spatial, high spectral resolution infrared spectrometer which is designed to identify ices and minerals on the surface of the planet. It is also used to assess the atmosphere through investigation of specific atmospheric absorption and emission lines (notably those of CO2).
SHARAD is a radio sounding experiment which is used to determine sub-surface boundaries (e.g. changes in ice content at depth).
MCS is an experiment to study the composition and structure of the atmosphere.
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).
HiRISE entered its primary science mission in November 2006. The second image acquired by HiRISE was of a target selected by a Bern PhD student, Alexandra Lefort. Lefort was interested in some unusual features called “scallops” which are seen at mid-northern and mid-southern latitudes. It is thought that over most of Mars there was once water ice relatively close to the surface. Recently, this water ice has been subliming from the sub-surface. The sublimation occurs most rapidly at the equator and it is thought that sub-surface water ice at the equator must now be rather deep (i.e. greater than several metres depth). At mid-latitudes, the sublimation has not been so rapid and is possibly still on-going. Lefort’s idea was to assess whether scallops could be evidence of this process and it does indeed appear that, even at the highest resolution, the theory still holds.
McEwen, A.S., E.M.Eliason, J.W.Bergstrom, N.T.Bridges, C.J.Hansen, W.A.Delamere, J.A.Grant, V.C.Gulick, K.E.Herkenhoff, L.Keszthelyi, R.L.Kirk, M.T.Mellon, S.W.Squyres, N.Thomas, and C.M.Weitz, (2007),Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE), Journal of Geophysical Research (Planets), 112, 5.
McEwen, A.S. et al., (2010) The High Resolution Imaging Science Experiment (HiRISE) during MRO’s Primary Science Phase (PSP) Icarus, 205, 2.