David C. Jefferson

An Investigation of the Orbital Status of Viking-1

The dual orbiter-lander spacecraft of the Viking mission to Mars were launched in 1975 with an objective of analyzing the planet’s surface and atmospheric characteristics. The Viking-1 orbiter’s mission ended in 1980, after four years in orbit and a depleted supply of attitude control fuel, with the stipulation to the Viking navigation team that the spacecraft would not impact (i.e., contaminate) the planet before the year 2019. To date, it is presumed that no such impact has occurred. However, some preliminary analyses conducted in recent years at the Jet Propulsion Laboratory (JPL) suggest that the Viking-1 orbit could have undergone significant changes that might have led to a Mars impact as early as two decades before the original requirement. These implications prompted further investigation of the spacecraft’s orbital evolution to determine the conditions under which an impact might be possible. Trajectory propagations using varied spacecraft states and models (planetary ephemeris, gravity field, solar radiation pressure, attitude, etc.) are examined. Findings point to the solar pressure force direction having a large influence on the spacecraft’s periapsis altitude; then, depending on the altitude, atmospheric drag forces may be such that a rapid descent to the planet’s surface (in effect, an “impact”) is achieved in these simulations. An attempt to infer the current status of the Viking-1 orbiter is made, but is difficult in the absence of accurate and critical end-of-mission spacecraft data.