Editing Mars Science Laboratory (section)

==== Guided entry ====
[[File:593419main pia14834-full full Mars Science Laboratory Guided Entry at Mars.jpg|thumb|left|The guided entry is the phase that allowed the spacecraft to steer with accuracy to its planned landing site.]]
Precision guided entry made use of onboard computing ability to steer itself toward the pre-determined landing site, improving landing accuracy from a range of hundreds of kilometers to {{convert|20|km|mi|sp=us}}. This capability helped remove some of the uncertainties of landing hazards that might be present in larger landing ellipses.<ref>{{cite web |url=http://mars.jpl.nasa.gov/msl/mission/technology/insituexploration/edl/guidedentry/ |title=MSL – Guided Entry |access-date=August 8, 2012 |year=2011 |work=[[JPL]] |publisher=NASA}}</ref> Steering was achieved by the combined use of thrusters and ejectable balance masses.<ref name='Guided entry'>{{cite journal |title=The RCS Attitude Controller for the Exo-Atmospheric And Guided Entry Phases of the Mars Science Laboratory |journal=Planetary Probe |first1=Paul B. |last1=Brugarolas |first2=A. Miguel |last2=San Martin |first3=Edward C. |last3=Wong |url=http://www.planetaryprobe.eu/IPPW7/proceedings/IPPW7%20Proceedings/Papers/Session5/p453.pdf |access-date=August 8, 2012}}</ref> The ejectable balance masses shift the capsule center of mass enabling generation of a [[Lift (force)|lift vector]] during the atmospheric phase. A navigation computer integrated the measurements to estimate the position and [[Attitude control (spacecraft)|attitude]] of the capsule that generated automated torque commands. This was the first planetary mission to use precision landing techniques.

The rover was folded up within an [[aeroshell]] that protected it during the travel through space and during the [[atmospheric entry]] at Mars. Ten minutes before atmospheric entry the aeroshell separated from the cruise stage that provided power, communications and propulsion during the long flight to Mars. One minute after separation from the cruise stage thrusters on the aeroshell fired to cancel out the spacecraft's 2-rpm rotation and achieved an orientation with the heat shield facing Mars in preparation for [[Atmospheric entry]].<ref name="spaceflightnow.com_1"/> The heat shield is made of [[phenolic impregnated carbon ablator]] (PICA). The {{convert|4.5|m|abbr=on}} diameter heat shield, which is the largest heat shield ever flown in space,<ref name="nasa8"/> reduced the velocity of the spacecraft by [[Ablative heat shield|ablation against the Martian atmosphere]], from the atmospheric interface velocity of approximately {{convert|5.8|km/s|abbr=on}} down to approximately {{convert|470|m/s|abbr=on}}, where parachute deployment was possible about four minutes later. One minute and 15 seconds after entry the heat shield experienced peak temperatures of up to {{convert|2090|C|F|abbr=on}} as atmospheric pressure converted kinetic energy into heat. Ten seconds after peak heating, that deceleration peaked out at 15 [[g-force|g]].<ref name="spaceflightnow.com_1"/>

Much of the reduction of the landing precision error was accomplished by an entry guidance algorithm, derived from the algorithm used for guidance of the [[Apollo Command Module]]s returning to Earth in the [[Apollo program]].<ref name="spaceflightnow.com_1"/> This guidance uses the lifting force experienced by the aeroshell to "fly out" any detected error in range and thereby arrive at the targeted landing site. In order for the aeroshell to have lift, its center of mass is offset from the axial centerline that results in an off-center trim angle in atmospheric flight. This was accomplished by ejecting ballast masses consisting of two {{convert|75|kg|lbs|abbr=on}} [[tungsten]] weights minutes before atmospheric entry.<ref name="spaceflightnow.com_1"/> The lift vector was controlled by four sets of two [[reaction control system]] (RCS) thrusters that produced approximately {{convert|500|N|lbf|abbr=on}} of thrust per pair. This ability to change the pointing of the direction of lift allowed the spacecraft to react to the ambient environment, and steer toward the landing zone. Prior to parachute deployment the entry vehicle ejected more ballast mass consisting of six {{convert|25|kg|lbs|abbr=on}} tungsten weights such that the [[center of gravity]] offset was removed.<ref name="spaceflightnow.com_1"/>