The Challenge of Navigating Toward and Around a Small, Irregular NEO
Gil-Fernandez, Jesus; Prieto-Llanos, T.; Cadenas, R.; Corral, C.; Graziano, M.
GMV

The terminal phase of missions to NEO, either for characterization or mitigation, have very specific conditions that pose many stringent constraints on the on-board navigation system. In the case of a small, irregular body like Itokawa (the target of Japans Hayabusa mission), the uncertain environmental conditions make the objectives of the navigation system extremely challenging. This paper shows some solutions to the different sub-phases of the terminal navigation problem for impact or characterization missions to several interesting NEO such as Apophis, starting by the detection and identification and finishing with an impact or a precise, soft landing.
For the terminal phase of any mission, the trajectory must be determined relative to the NEO, i.e. the position and velocity of the SC must be referred to the center of mass of the target body. The different design parameters and environment uncertainties affecting the mission performances (e.g. final impact accuracy) must be assessed in high-fidelity simulators to achieve a robust GNC system. The most generic methods to obtain the required relative measurements to improve the knowledge of the SC state are: (1) to take images with a camera, and/or (2) to obtain range measurements with an altimeter. Typically, the camera measurements can be taken at a further distance than the range measurements. The performances of the navigation depend on the type of measurements and their quality.
The first objective of the navigation system is to detect and identify the small NEO against the starry background. Small objects mean that their visibility is difficult and special techniques are required for correct identification of the target body. This phase is particularly important in the case of a kinematic impactor, where the total time of the terminal phase is very short.
When the NEO can be tracked with the camera, the trajectory can be estimated and corrected, in order to achieve either an impact or a desired orbit in the proximity of the small body. In the case of an impact, the fast dynamics imposes an autonomous GNC for the terminal phase. There are several techniques to achieve an impact in a small, irregular body without full relative state estimation. The most demanding scenario is the absence of an orbiting SC providing additional measurements and accurate ephemeris of the target.
In the case of a rendezvous, there are specific techniques to achieve a good knowledge of the entire relative state vector from camera images. The accuracy of the trajectory determination drives the propellant expenditure, the required approach time, and the capability of delivering the SC into safe orbits close to the target body (either around or at a certain distance).
The next phase is the proximity operations where the SC remains in the vicinity of the NEO and may include descent and landing (D&L). The navigation algorithm must consider the uncertainties in the dynamics and kinematics. The image processing techniques are usually more sophisticated than in the approach phase and may include image matching, unknown feature tracking or known landmark identification. The short distance allows the use of altimeter (or analogous sensors) providing additional measurements that improve the knowledge of the SC relative state. In the sensors data processing, the shape and other characteristics of the target are necessary for attaining high accuracy. During D&L the communications delay again forces an autonomous system and the objective of achieving a soft landing demands specific data processing and sometimes-additional sensors.