reference map of Mars with place names; Gale Crater is almost all the way on the right of the map, next to Gusev Crater.
Michael Malin is the principal investigator for the Mars Descent Imager (MARDI) which shares a unified imaging-science team with the other two instruments from his company, Malin Space Science Systems, San Diego, California. Ken Edgett is the deputy principal investigator, also MSSS.
MARDI consists of two parts: the wide-angle camera mounted toward the front of the right hand side of Curiosity and a digital electronics assembly inside the warm electronics box of the rover’s chassis. The instrument’s electronics, including the 1,600-pixel-by-1,200-pixel charge-coupled device (CCD) in the camera, are the same design as used in the Mast Camera (MastCam) and Mars Hand Lens Imager (MAHLI).
The rectangular field of the CCD sits within a 90-degree circular field of view of the camera lens, yielding a recorded field of view of 70 degrees by 55 degrees. During the descent of Curiosity MARDI will start giving us pictures of that descent from an altitude of 2 kilometers (.2 miles). The resolution of the images will be about about 1.5 meters (5 feet) per pixel, though swinging and shaking of the spacecraft will likely blur some frames despite a fast (1.3 millisecond) exposure time.
As with the MastCam and with MAHLI, both also cameras manufactured and managed by Michael Malin’s team, the Color in the images comes from a Bayer pattern filter array, as used in many commercial digital cameras. The camera’s CCD is covered with a grid of green, red and blue filters so that each exposure samples all of those colors throughout the field of view (read for a more in dept description my blog on MastCam and MAHLI).
A piece of white material on the inside surface of the heat shield will serve as a white-balance target as the heat shield falls away at the beginning of the recorded descent video.
During the final few minutes of Curiosity’s flight to the surface of Mars, MARDI, will record a full-color video of the ground below. This will provide the Mars Science Laboratory team with information about the landing site and its surroundings, to aid interpretation of the rover’s ground-level views and planning of initial drives. Hundreds of the images taken by the camera will show features smaller than what can be discerned in images taken from orbit.
The video will also give fans worldwide an unprecedented sense of riding a spacecraft to a landing on Mars. MARDI will record the video on its own 8 gigabyte flash memory at about four frames per second and close to 1,600 by 1,200 pixels per frame. Thumbnails and a few samples of full-resolution frames will be transmitted to Earth in the first days after landing.
The nested set of images from higher altitude to ground level will enable pinpointing of Curiosity’s location. The pace of sending the rest of the frames for full-resolution video will depend on sharing priority with data from the rover’s other investigations.
The full video — available first from the thumbnails in YouTube-like resolution and later in full detail — will begin with a glimpse of the heat shield falling away from beneath the rover. The first views of the ground will cover an area several kilometers (a few miles) across.
Successive frames taken as the vehicle descends will close in and cover successively smaller areas.
The video will likely nod up and down to fairly large angles owing to parachute-induced oscillations. Its roll clockwise and counter clockwise will be smaller, as thrusters on the descent stage control that motion. When the parachute is jettisoned, the video will show large angular motions as the descent vehicle manoeuvres to avoid re-contacting the back shell and parachute. Rocket engine vibration may also be seen. A few seconds before landing, the rover will be lowered on tethers beneath the descent stage, and the video will show the relatively slow approach to the surface. The final frames, after landing, will cover a bath-towel-size patch of ground under the front-left corner of the rover.
Besides the main objective of providing geologic context for the observations and operations of the rover during the early part of mission on Mars, MARDI will provide insight about Mars’ atmosphere. Combining information from the descent images with information from the spacecraft’s motion sensors will allow for calculating wind speeds affecting the spacecraft on its way down, an important atmospheric science measurement. The descent data will later aid in designing and testing future landing systems for Mars that could add more control for hazard avoidance.
Throughout Curiosity’s mission on Mars, MARDI will offer the capability to obtain images of ground beneath the rover at resolutions down to 0.06 inch (1.5 millimeters) per pixel, for precise tracking of its movements or for geologic mapping. The science team will decide whether or not to use that capability. Each day of operations on Mars will require choices about how to budget power, data and time.
Malin Space Science Systems also provided descent imagers for NASA’s Mars Polar Lander, launched in 1999, and Phoenix Mars Lander, launched in 2007.
However, the former craft was lost during its landing and the latter did not use its descent imager due to concern about the spacecraft’s data-handling capabilities during crucial moments just before landing. The Payload and Attitude Control Interface (PACI) and the Inertial Measurement Unit (IMU),critical to controlling the decent and landing, both shared the same computer card. It was deemed too risky to use the card for images of the descent while it was also being used to control the descent of Phoenix.
The primary objectives of descent imaging are to identify where the lander has landed, and to bridge the gap between orbiter science views and those acquired by the lander. The images are not meant to give us real time data and real time capability to steer the landing, as we can not interfere anymore with the landing while it is taking place. This is due to the nature of data travelling not faster than light and Mars and Earth being more (light)minutes apart (at least 10 minutes) for even one way data transfer than that the operation of Entry Descent and Landing takes (about 6 minutes), you could ask what good will images during descent do us?
Well, there is no argument that the images taken by the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) are extremely good images. There is also no argument that images taken after landing can show where the lander is as is demonstrated by the following images: VL1, VL2, MER-A, MER-B.
However, we do not know how soon after landing HiRISE will be overhead and be able to image a lander, and a number of time-critical mission functions still would greatly benefit from having an overhead view of the landing site immediately after landing.
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