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Planetary News: Mars (2006)Mars Reconnaissance Orbiter Reshapes its Orbit as More Images are ReleasedBy A.J.S. RaylApril 18, 2006
The Mars Reconnaissance Orbiter (MRO) has started the process of reducing and reshaping its orbit to get in closer to the planet for its science phase slated to begin in November. So far, MRO is steady as she goes and all systems on the spacecraft are performing well. MRO -- which arrived at the Red Planet and slowed into a highly-elliptical orbit there on March 10 – is aerobraking rather than using onboard thrusters to get to the desired, nearly circular, 2-hour orbit. Aerobraking is a technique in which the spacecraft uses drag within the planetary atmosphere to reduce its velocity and customize its orbit. The drag actually works for the spacecraft in much the same way it works for children who literally drag their feet through the sand to slow down and lower the height of their swing on a playground swingset. In essence, the spacecraft dips into the uppermost atmosphere at the closest approach to the planet or periapsis and the resulting drag created on the spacecraft during each pass progressively reduces the distance between the spacecraft and the planet at the farthest point in the orbit, the apoapsis. MRO first “tasted” the Martian atmosphere about two weeks ago, when each revolution around the planet took about 35 hours as the orbit swung the spacecraft about 27,000 miles (43,000 kilometers) away from the planet before swinging back in close. Then last Wednesday, the spacecraft initiated a short burn of its intermediate-sized thrusters while at the most distant point in its highly elongated orbit around the planet. That maneuver basically nudged the spacecraft to within 107 kilometers (66 miles) of Mars' surface and essentially put the spacecraft into the “corridor” in which it needs to be to continue reducing and reshaping the orbit. “Right now we are in the drag pass corridor, at roughly 108 kilometers altitude at periapsis, so we have established the corridor and are now in main phase aerobraking,” Dan Kubitschek, deputy aerobraking phase lead for MRO, told The Planetary Society yesterday. “Everything is completely nominal. Everything is operating as expected,” he added. After making hundreds of passes through the upper atmosphere, MRO will gradually reduce the apoapsis of the orbit to about 320 kilometers (198 miles), with the periapsis being 255 kilometers (about 158 miles). For comparison, space shuttles orbit the Earth at an altitude between 322 kilometers (200 miles) and 619 kilometers (385 miles), depending on mission requirements, so MRO will actually be closer to Mars in its orbit than the shuttles are to Earth. Since last weeks’ maneuver, MRO’s orbit has already been reduced. “We started out [in an orbit that takes] a little over 35 hours and in the last 5 or 6 passes have reduced that to 33 hours,” Kubitschek said. “We should be down to a 24-hour period in mid-May, an 8-hour orbit by early July, and then down to the two-hour orbit in early September.” At that point, MRO will exit the aerobraking orbit. “We will then go through a solar conjunction in October and establish the final primary science orbit in November,” he added. For the next several months, MRO will continue the process of aerobraking and the six science instruments will remain in hibernation until fall. In other MRO news, however, Michael Malin and colleagues at his Malin Space Science Systems office in San Diego have released the first test images from two of the three cameras onboard MRO, the Context Camera (CTX) and Mars Color Imager (MARCI), a descendant of a similar instrument that flew on the Mars Climate Orbiter, which was lost in September 1999 during orbit insertion. These images, along with the first images from the High Resolution Imaging Science Experiment Camera (HiRISE), were taken as part of a checkout of the orbiter's payload during its 10th close approach to Mars on March 24, before the start of aerobraking. The first images taken by HiRISE, the most powerful camera ever sent to another planet and the one that will offer the best resolution of all the cameras on MRO, were released in black and white by Alfred McEwen and colleagues at the University of Arizona in late March, and in color on April 7. The region of Mars imaged by the cameras is south of the Valles Marineris. It includes the Argyre Basin's interior plains, Argyre Planitia, and mountains forming the basin rim, Nereidum Montes to the northwest (middle of images) and Charitum Montes to the southeast (bottom of images). Although the CTX and MARCI images acquired were taken from nearly 10 times as far from the planet as MRO will be once it finishes reshaping its orbit, these test images "show that both cameras will meet or exceed their performance requirements once they're in the low-altitude science orbit," Malin, team leader for the context camera and principal investigator for the MARCI, announced late last week. In addition to giving the teams time to make sure their cameras are functioning properly, the process of collecting test images also allows team members the opportunity to fine-tune calibrations for processing the images, although all three cameras will be further calibrated before beginning science operations.
The image above shows a color view cropped from a Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) daily global map acquired on March 24, 2006, the same day as the MRO observations, with the two grayscale CTX views inserted into it. The MOC map, which shows the planet as if every part was imaged at some time between 1 and 3 pm -- that is, with early afternoon illumination, was obtained about 4 hours later in the day than the MRO data, which was acquired during the morning hours. The CTX first began the test imaging when it was over the southernmost portion of the chaotic terrain at the east end of the Valles Marineris. A second image was acquired several minutes later, as MRO orbited southward towards the west side of the large Argyre impact basin. The top of each of the two CTX images were obtained about 13 minutes apart. Since these are initial, test images, there is some linear striping in the images caused by incomplete removal of pixel to pixel variations in the CTX detector by the present calibration software.
The Mars Color Imager (MARCI) camera acquired a seven-band color, wide-angle view of the Red Planet on March 24, 2006. The image above shows a color composite made from the MARCI red, green, and blue bands. The view looks northward and includes the large Argyre Basin in Mars' southern hemisphere. The test imaging gave Malin and crew the opportunity to fine-tune calibrations used for processing the separate bands into "true" color -- as the color would appear to a human eye looking down from orbit.
Illustrated above are 3 views MARCI acquired compared to a color composite of two views acquired about four hours later by the wide-angle imager of Mars Orbiter Camera (MOC) on Mars Global Surveyor (MGS). The MARCI imaging occurred during the morning on Mars, while the MOC observations were made at about 2 pm local solar time. The region of Mars imaged by MARCI is south of the Valles Marineris. It includes the Argyre Basin's interior plains, Argyre Planitia, and mountains forming the basin rim, Nereidum Montes to the northwest (middle of images) and Charitum Montes to the southeast (bottom of images). The color composite from MARCI differs from the MOC wide-angle color composite because, to create a color image with MOC data, camera-team members synthesize (fake) a green channel by adding the red and blue channels together and dividing by two. The slightly greenish tint of the MARCI image shows that the approximation used for MOC images underestimates the amount of green. The test image labeled 260 nm shows how the planet appears at an ultraviolet (UV) waveband of 260 nanometers, where ozone absorbs the UV light. Relatively darker areas in this band normally will indicate the presence of ozone, and relatively lighter areas will indicate the absence of ozone. Water vapor in Mars' atmosphere is in an inverse relationship with ozone; where there is more of one, there is less of the other. So, lighter areas in images can be used to track water vapor. The term "relatively" is used here because Mars itself is very dark in the UV owing to absorption of UV light by iron-bearing minerals, and sunlight is deficient in UV relative to visible light, so in general Mars will always look dark in the UV. A second UV band on MARCI (not shown in the figure above) at a longer wavelength allows these differences to be quantified. While the MOC wide-angle image shows wispy, light water-ice clouds to the northwest of Argyre in the afternoon, researchers cannot yet correlate these clouds with the UV information from MARCI, especially because the times of day are different. When in its final mapping orbit, MRO will view the same area as MGS separated by only one hour, and such correlations, according to Michael Malin, will be much more direct. To put all the cameras in context: MARCI will view the planet's entire atmosphere and surface every day to monitor changes in clouds, wind-blown dust, polar caps and other variable features. Images from the Context Camera, meanwhile, which will will cover swaths 18.6 miles (30 kilometers) wide, will have a resolution of 20 feet (6 meters) per pixel, allowing surface features as small as a basketball court to be discerned. The Context Camera will show how smaller areas examined by HiRISE -- which will have the best resolution ever achieved from Mars orbit -- and by the mineral-identifying Compact Reconnaissance Imaging Spectrometer fit into the broader landscape. It will also allow scientists to watch for small-scale changes, such as newly cut gullies, in the broader coverage area. Once MRO gets into the proper orbit for its primary science phase, the three cameras and three other instruments onboard will begin their systematic examination of Mars. "We're looking forward to that time with great anticipation," said Malin, echoing the sentiments of McEwen and his HiRISE team. |
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