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propulsive activity from launch to the present.
This view of MESSENGER shows the orientation at the start of orbit-correction maneuver 18 (OCM-18). The dark blue rectangles represent the front (sunlit) sides of the solar arrays. The large white feature is the spacecraft's sunshade, which points toward the Sun. Colored arrows indicate the directions of Earth, the Sun, the spacecraft's velocity with respect to Mercury, and the course-correction velocity change (delta-V or ΔV). The "spacecraft +x axis" label identifies an axis direction in the local spacecraft body-fixed coordinate system. Above the MESSENGER spacecraft is a portion of Mercury’s northern hemisphere, with latitude lines at 15° increments and longitude lines at 30° increments. The curved purple line depicts MESSENGER’s orbit about Mercury. Active thruster plumes are no longer depicted, since such a depiction would not accurately portray the appearance of helium gas leaving the spacecraft via the primary thrusters for this maneuver.
The nineteenth OCM since Mercury orbit insertion, OCM-18 raised the spacecraft’s minimum altitude above Mercury from 5.3 kilometers (3.3 miles) to 6.3 kilometers (3.9 miles), ensuring impact onto Mercury during the desired orbit. During OCM-18, a ΔV of 0.45 meters per second (1.00 miles per hour) was imparted by releasing helium gas pressurant through the four largest monopropellant thrusters. Implemented when the spacecraft was close to the farthest point in its orbit from Mercury, OCM-18 increased the spacecraft’s speed relative to Mercury. OCM-18 also increased the spacecraft’s orbit period to 8h 21.3m. During OCM-18, the sunshade protected heat-sensitive parts from direct sunlight. OCM-18 was the fourth MESSENGER course-correction maneuver to use helium gas pressurant to change the spacecraft’s orbit. The final OCM in the MESSENGER low-altitude hover campaign, also called the extension of the Second Extended Mission, OCM-18 used the four medium thrusters that point in about the same direction as the large bi-propellant thruster.
Mission controllers at The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, verified the start of the maneuver about 8.6 minutes after the start of OCM-18, at 5:28 pm Eastern Daylight Time on April 28, 2015, when the first signals indicating spacecraft thruster activity reached NASA’s Deep Space Network (DSN) tracking station in Goldstone, California. The 3.02-minute-long maneuver began at about 5:20 pm Eastern Daylight Time.
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