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The control problem of time-optimal, rest-to-rest slewing of a flexible spacecraft through a large angle is considered. The flexible spacecraft is modeled as a linear, elastic, undamped, nongyroscopic system suitable for analysis of planar rotational maneuvers. Minimum-time open-loop planar maneuvers are studied. The control histories are found to be bang-bang with multiple switches in each control variable. The optimal control history is shown to have an important time symmetry property. The switching times, final time, and costates at midmaneuver satisfy a system of nonlinear algebraic equations that can be solved using a homotopy method. An upper bound on attitude error due to control spillover is obtained. This helps to determine, a priori, the number of vibrational modes that need to be actively suppressed at the final time such that a prespecified pointing accuracy is guaranteed after the maneuver has been completed. A time-optimal slewing example is discussed to demonstrate the applicability of the results.

Landing (EDL) system: the large reduction in the size of the Landing Ellipse and the large increase in the Landed Mass as compared to previous missions [1,2].The reduction in the size of the Landing Ellipse was dictated by the desire to give scientists more choices when selecting sites of great scientific value and still safe for landing the Curiosity rover. Scientists made maximum use of this new capability when they selected Gale Crater as the landing site, boldly placing the ellipse at the crater's floor between the dangers of its rim at one side and Mt Sharp (a 5.5 km high mountain in its center) at the other (Fig. 1). Figure 2 illustrates the reduction of the landing ellipses since Viking against a picture of Gale Crater. Only MSL landing ellipse can fit in the tight space within the crater.The increase in the landed mass capability of MSL EDL was dictated by the mass of the Curiosity rover, which itself was driven by the size and complexity of its scientific instruments and the need for improved mobility. Figure 3 compares the size of Curiosity against the previous Mars rovers.To meet the first challenge, Landing Ellipse reduction, MSL employed the first use at Mars of Entry Guidance. This technique, which was used successfully by Gemini and Apollo to guide the capsules precisely and safely through the Earth's atmosphere to land closely to recovery assets, requires a lifting capsule (Fig. 4), on-board guidance, navigation and control software, and an RCS propulsion system to stabilize and control the roll of the capsule to point its Lift vector as commanded by the on-board guidance function.To meet the challenge of placing, a 1 ton class rover with the size of a small car on the surface of Mars, MSL had to invent a totally new landing architecture: the SkyCrane (Fig. 5). This architecture solved the rover-egress problem (i.e., how to lower the rover from the top deck of a Viking style legged lander) by placing the rover on the surface of Abstract The Mars Science Laboratory (MSL) project successfully landed the rover Curiosity in Gale crater in August 5, 2012, thus demonstrating and validating a series of technical innovations and advances which resulted in a quantum leap in Entry, Descent, and Landing (EDL) performance relative to previous missions. These included the first use at Mars of Entry Guidance to reduce the size of the landing ellipse and the first use of the SkyCrane landing architecture to enable the placement of a 1 ton class rover on the surface of the red planet. Both of these advances required innovations in the design, analysis and testing of the Guidance, Navigation, and Control system. This paper will start with a high-level description of the MSL EDL/GN&C system design and performance requirements, followed by a brief discussion of the risks and uncertainties as they were understood prior to landing, and the actual in-flight GN&C performance as reconstructed from telemetry. Finally, this paper will address areas of improvements for future Mars EDL missions.

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