This past week and a half has been rather quiet as we performed two orbital maneuvers. The first maneuver placed the spacecraft at an altitude closer to its final orbit. The second one slightly adjusted the inclination of the orbit to ensure that SMAP goes over the equator at approximately 6:00 am and 6:00 pm every day, which are the ideal times to obtain our science measurements.
For those of you wondering how SMAP maneuvers around, the spacecraft contains a single pressurized propellant tank carrying 81 kilograms (179 pounds) of hydrazine. The spacecraft adjusts its orbit by firing any combination of its eight onboard thrusters, each of which provides about 4.5 Newtons (~1 pound) of thrust. The hydrazine fuel is budgeted for different maneuvers throughout the life of SMAP, with the primary allocation used for placing the spacecraft in its final orbit. Other allocations are made to maintain SMAP’s orbit, which can decay due to factors such as atmospheric drag, solar radiation pressure, and the gravity of the Earth, sun and moon. Atmospheric drag is the most significant factor and is similar to the force felt when sticking your hand out of the window of a fast moving car. At SMAP’s orbit this force is weak but with time it results in the reduction in the altitude of the satellite. Atmospheric drag can vary greatly and is difficult to predict, hence it might be a couple of weeks to months until SMAP’s orbit needs to be corrected again. Some fuel is reserved to adjust the spacecraft orbit in case there is space debris (a.k.a. space junk) to be avoided. Although such maneuvers are uncommon, space debris is a significant concern. Finally, enough propellant is allocated to move the spacecraft to a lower “disposal” orbit at the end of its useful mission. In this orbit the spacecraft is least likely to interfere with other missions. Once SMAP is in its disposal orbit, it will take approximately 15 years for it to enter the Earth’s atmosphere and disintegrate.
All systems on SMAP are currently nominal, which means they are performing as expected. Although it is a word that does not evoke feelings of excitement, it is great news for us.
This upcoming week we will continue with additional orbital maneuvers. Then things will start to get even more exciting as we begin to spin the antenna and turn on the radar and radiometer. Stay tuned.