Foresight: Designing a Radio Transponder Mission to Near Earth Asteroid Apophis
Charania, A.C.¹; Olds, J.¹; Koenig, J.^2
1SpaceWorks Engineering, Inc. (SEI); ²SpaceDev, Inc.

The Foresight spacecraft is a concept design for a radio tagging mission to Near Earth Object (NEO) Apophis. The spacecraft is designed to be a low-cost, low-risk, minimal science mission in order to conduct precise in-situ measurements of the position of Apophis in order to enable a very accurate determination of the NEO's heliocentric orbit. The specific goal is to obtain accurate tracking information for Apophis by 2017 such that the long dimension of the plus/minus 3-sigma error ellipse is reduced to less than 14 km for the 2029 passage of Apophis through a keyhole near Earth. Previous papers have predicted the 2029 Apophis-Earth encounter distance to be approximately 5.89 Earth radii based on current knowledge. This equates to an approximate largest dimension of the 3-sigma error ellipse of 4500 km during the 2029 encounter. From a trailing heliocentric orbit, a series of range measurements from the asteroid to the spacecraft are used in conjunction with the Deep Space Network (DSN) to predict the range from the Earth to the Foresight spacecraft.

Our analysis indicates that at a given epoch on the 2017 timeframe, the position and velocity of Foresight (and therefore Apophis' center of gravity) must be known to within tens of meters and thousandths of a millimeter per second in velocity in order to reduce the error ellipse in 2029 to the required accuracy. A series of range measurements can be used as the basis of a batch filter algorithm to accurately predict the orbital state at the beginning of the measurement arc. We propose to baseline a weekly measurement frequency for a period of 300 days (approximately 43 DSN range measurements). This solution represents a reasonable compromise between orbit determination accuracy, mission cost, and operational complexity. With this observation schedule, the error ellipse of Apophis' trajectory ("keyhole" or b-place encounter) in 2029 is reduced to less than 6 km, (± 3-sigma) by 2017. Adding a full kilometer for Yarkovsky effect and solar pressure acceleration uncertainties, the error ellipse is still well within the desired 14 km target. Five launch windows have been identified for the mission spanning the years 2012 to 2014 for the Minotaur IV launch vehicle. The mission requires a chemical propulsive transfer vehicle to perform the outbound burn to Apophis (3,600 m/s) with the Foresight encounter spacecraft performing a portion of the Earth departure, and the Apophis capture burn (total less than 2,400 m/s). The mass of the Foresight spacecraft is 220 kg while the propulsive transfer vehicle is 1,387 kg. The solar powered spacecraft has two main instruments: a multi-spectral imager and laser altimeter. The spacecraft leverages off the shelf technologies where possible, incorporating leaner approaches to spacecraft design. The total life cycle cost for this mission (including operations and launch) is estimated to be $131 M. Overall system reliability is estimated to be 90.2%.