Guest Blogger: Bashar Rizk – OCAMS Instrument Scientist
The OSIRIS-REx mission has the unique challenge of sending a spacecraft to the carbonaceous near-Earth asteroid Bennu, surveying the entire asteroid surface, reconnoitering potential sampling sites, then returning a sample to Earth. OSIRIS-REx must achieve all of these steps using only the observational assets he brings along.
One of our most important assets is a trio of cameras called the OSIRIS-REx Camera Suite, or OCAMS, under construction at the University of Arizona. The design of OCAMS allows us to image Bennu over 9 orders-of-magnitude in distance, from an initial distance of 1-million kilometers (over 620,000 miles) down to 2 meters (6.5 feet). The camera suite is similar to a lookout nestled in the crow’s-nest of a tall ship. This scout must observe Bennu as we approach, survey, and recon the asteroid, and tell the ship’s navigator where to go.
The OCAMS medium-resolution Mapping Camera, or MapCam, represents the sharp eyes of our scout. The OCAMS cameras share many similarities to human eyes. Only the central field of view of your eye – called the foveal region – is in focus. It is this portion of your vision that you use the most, for viewing objects both near and far, recognizing human faces, and reading. It is also the only part of your eye that senses color. The one arc-minute resolution of your foveal region is the same resolution possessed by MapCam (one arc minute is roughly the angle subtended by a stoplight seen from a mile away).
MapCam will map the entire surface of Bennu from a safe and convenient standoff distance of 5 kilometers (3 miles), watching Bennu spin through a whole asteroid day every 4.3 hours. MapCam has a moderate field of view of 4°, approximately the angular width of 3 fingers held at arm’s length and a resolution of less than 1 arc minute. It has a filter wheel that allows it to image in 4 different colors: blue, green, red and infrared, similar to your foveal region (except for the infrared). It can also rotate a glass plate into its optical train to allow it to refocus light from the relatively short range of 30 meters (100 feet) – our scout needs put on his reading glasses to consult his map from time to time.
The PolyCam is the spyglass our lookout uses to investigate faraway objects in detail – magnifying the sharpness of his natural vision by a factor of 5. The PolyCam is the largest camera of the OCAMS suite. It is a 20.3-centimeter (8-inch) wide telescope with a 63.5-centimeter (25-inch) focal length. It has a resolution of about one-sixth of an arc minute. Its field of view is just less than 1°, about the width of your pinky finger held at arm’s length. It is used for several critical tasks: to acquire the asteroid while it is still a point of light against the background stars, to identify and exclude dangerous areas on the asteroid’s surface by spotting and mapping large boulders and rocks, and to characterize a dozen prospective sample sites in detail. PolyCam has a focus mechanism that converts this telescope into a microscope – allowing our scout to scrutinize the tiniest rocks and pebbles on the asteroid to ensure that they are small enough to fit into the sample head. It is this dual nature – telescope turned microscope – that gives the PolyCam its name.
The third member of camera suite is our Sampling Camera or SamCam. SamCam represents our Scouts’ peripheral vision. Possessing a resolution of almost 4 arc minutes and a wide field of view of 21°, SamCam is designed to document sample acquisition. Just as your peripheral vision routinely captures the sudden movements that, in nature, may so often demand a fight-or-flight response, SamCam’s field of view captures the spurt of gas and regolith mobilizing out from beneath the sampler head at the moment of sample capture. Your vision relies on a steady update of the scene that is almost video-like in nature, similar to the rapid succession of images (approximately 3 every 5 seconds) that the SamCam will capture during sample acquisition. This fight-or-flight analogy is not to say that the surface of the asteroid is ready to attack us. However, if it does, SamCam’s filter wheel sports three identical filters ready to be rotated into its optical train in case one or more of them get covered by the dust mobilized during the sampling event. These filters are the safety goggles for our scout – allowing him to continue to serve the mission in the event that we need to attempt a second sample collection.
SamCam has another very important function. Using its own reading glasses, it observes the bottom of the sampler head to verify sampling success. These observations also ensure that no rocks large enough to prevent the head being inserted into the Sample Return Capsule cling to the surface.
The three OCAMS Cameras differ from each other, but belong to the same family. They use identical detector assemblies to record the images they acquire (the same retinas). They each possess mechanisms actuated by identical motors (the same muscles). They share the same unified electronics controller (the same brain). As a group, these three visual siblings greatly increase the OSIRIS-REx spacecraft’s ability to successfully navigate, explore and sample asteroid Bennu.