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I see lots of questions how landing sites are chosen, but none so far how these landing sites are reached.

It already looks like magic how the target planet can be reached at all, but when are corrections being made to reach the site itself? is it after planet encounter is unavoidable? I reckon latitude can be adjusted quite early on and throughout most of the transfer, but how do we adjust longitude? If we brake or accelerate prograde, we might miss the planet. Do we actively play on the angle of entry to delay landing by a few 1000s of km by aerodynamic lift? Do we play on attitude for significant adjustments once there?

Also, do we have to land on the edge, or are the maneuver tricks that allow to land on the near or far side?

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    $\begingroup$ I dont believe a few 1000s of km by aerodynamic lift are possible in the very thin atmosphere of Mars. $\endgroup$ – Uwe Nov 24 '18 at 18:29
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There are typically five planned trajectory correction maneuvers on the way to Mars, referred to as TCM-1 to TCM-5. (Also there is a slot for an emergency TCM-6 a few hours before entry, but it is not expected to be used.) Also I sometimes refer to launch as TCM-0. That's the really, really big TCM.

TCM-0 provides the energy to place the aphelion of the spacecraft at Mars orbit distance, and is timed and directed to get there when Mars will be there. Almost. Due to planetary protection requirements that the final stage of the launch vehicle not impact Mars, TCM-0 is actually designed to just miss Mars.

TCM-1 is about ten days after launch. It's job is to a) take out the planetary protection bias, and b) correct for launch vehicle injection errors. That maneuver is on the order of 10 m/s. Due to execution uncertainty in that maneuver, the spacecraft may not even be on a Mars impact trajectory after TCM-1. But now it is much closer to the desired trajectory.

TCM-2 is about two months after launch. It is on the order of 1 m/s, and corrects for errors in TCM-1, putting the spacecraft close to the desired entry point and time in the Martian atmosphere. While the launch will generally target the desired arrival time to get the selected landing longitude, this can be varied by hours at TCM-2, as was done for Spirit, due to late decisions on the landing site. Note that with months to go, changing the arrival time by a few hours is still a small amount of $\Delta V$ at the time of TCM-2.

In addition to the landing latitude and longitude (the latter determined by the entry time), the flight path angle also needs to be with a fraction of a degree of the target, in order for the heat shield to experience the designed-for environment in both maximum heat rate and total heat load. I'm not sure what you mean by "land on the edge", but you may be referring to targeting the entry point to be just barely inside the impact circle. That is required to have the designed shallow entry flight path angle, e.g. –11.5° for MER. The flight path angle target is fixed. It is not altered to hit a landing site, lifting entry or not. The trajectory is altered to hit the landing site and the desired entry flight path angle.

The remaining TCMs fine-tune the trajectory, and are small or even zero, i.e. cancelled. TCM-3 is about two months before entry, TCM-4 is a week before entry, and TCM-5 is two days before entry. For Opportunity, TCM-3 and TCM-5 were cancelled.

In the end, the entry point in the atmosphere is reached within about one km of the target, and the time of entry within about one second. The entry flight path angle is then within a tenth of a degree of the target. That is where I agree with your "magic" comment. It's pretty amazing, and is due in large part to incredibly high-precision X-band radiometric tracking using Doppler, range, and very-long baseline interferometry.

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    $\begingroup$ +1 for great overall explanation, +1 for "TCM-0". Can you address OP's sub-question about aerodynamics? $\endgroup$ – Russell Borogove Nov 24 '18 at 19:05
  • $\begingroup$ I thought I did, but I'll be more explicit. $\endgroup$ – Mark Adler Nov 24 '18 at 22:49
  • $\begingroup$ Thanks. By edge of the disc I meant something else altogether. A planet is a sphere, so from far away it's a disc. The most shallow entry would be tangential to the sphere, with an intersection at the edge of the disc. Closer to the center would be steeper, farther would miss the planet altogether. In theory we could have lots of transfer orbits that could intersect tangentially anywhere along the planet's ecliptic plane, but at first sight many would have aphelia much further than the planet, and thus be much more costly. And indeed the edge of the disc is not necessarily the best place $\endgroup$ – nmajoros Nov 25 '18 at 0:13
  • $\begingroup$ Still not getting it. It sounds like you are talking about the entry flight path angle. If you just touch the entry interface, then that is a 0° EFPA. If you target the center of the disc, that is a –90° EFPA. It is very cheap in $\Delta V$ to move between those even a few weeks out. The aphelia of those orbits are changed only minutely by changing the EFPA. $\endgroup$ – Mark Adler Nov 25 '18 at 2:59
  • $\begingroup$ Yes, extremely cheap, but -°90 is probably to be avoided, only a few 10s of km to aerobrake you. So if you still want near zero EFPA AND arbitrary entry point the orbit aphelion may be much higher. Not needed for Mars because it's spinning at a convenient rate so you can TIME your entry, but imagine a slow spinning planet. The point you may want to reach might not be where Hohmann transfer with a small correction will give you shallow EFPA. $\endgroup$ – nmajoros Nov 25 '18 at 6:27
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@Mark Adler's answer addresses cruise guidance. This answer addresses guidance during entry for the Curiosity rover.

During entry, the Curiosity rover used active guidance. Like the Apollo capsules, it had an offset center of gravity (COG) and then performed banking turns during entry in order to steer the spacecraft.

During cruise, the spacecraft had a balanced COG. Before entry, it ejected two cruise balance masses in order to achieve the necessary offset COG for landing.

Guided entry started with the "range control phase," which began at 0.2 gees:

During the range control phase, the rover computer predicted the downrange distance it would fly and adjusted lift as necessary in order to shoot for the correct range... the way that the spacecraft adjusted its range was to perform a series of banking turns, rotating its center of gravity around the axis of its blunt nose. Its initial entry point was biased to the left (North) of the intended landing site, so it began with a banking turn to the right. The computer monitored the spacecraft's cross-range drift, and commanded a bank reversal when the drift passed a thresshold. - The Design and Engineering of Curiosity by Emily Lakdawallaibid, p. 79

After that, the spacecraft changed to "heading alignment" phase:

At an altitude of 14 kilometers and speed of 1.1 kilometers per second, the spacecraft transitioned into the "heading alignment" phase of guided entry. The spacecraft banked left to correct its cross-range heading, probably to compensate for a 10-meter-per-second crosswind. It flew downrange for 100 seconds at a near-constant altitude, steering lightly to arrive at the optimal location for parachute deployment. - ibid, p. 83

The offset center of gravity was achieved through the use of "entry balance masses." At the end of the heading alignment phase, the spacecraft ejected the entry balance masses in order to center-up the COG in preparation for parachute deployment.

See also:

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