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The idea of surface rendezvous for supply missions to a Mars expedition (either before or after arrival of a manned expedition) seems to come up every now and then. This would allow a manned spacecraft to either land near a supply craft, or would allow a supply craft to land near a manned base.

However, that obviously requires close to pinpoint accuracy in landing position. Anything worse than that, and the two spacecraft might be too far away from each other for any supplies to be of significant use.

For our Mars missions of recent past, what has been the landing position error compared to the landing position intended during the final mission planning stage?

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Curiosity was targeted at a small area later named Bradbury Landing and came down 2.4 km from the centre.

See also: landing ellipse comparison (earlier probes were of much lower accuracy.)

Of course, for projected future landings, carrying a better fuel reserve will allow much better landings. (Edit: I've removed my suggestion that landing from a parking orbit would improve accuracy; Mark Adler has convinced me in comments that this doesn't make any difference, aero manoeuvres during entry can accommodate reasonable errors in the entry window.)

(I'd add that Apollo 12 landed within walking distance of an old Surveyor probe - obviously that was the Moon, not Mars, but take that as an indication that pinpoint landings are possible if really needed..)

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    $\begingroup$ Even with an Apollo 12/Surveyor style powered landing, you still need to be rather close on the incoming landing trajectory before you do the precision navigation. Still, point well taken. $\endgroup$
    – user
    Feb 8 '16 at 15:27
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    $\begingroup$ Entering from orbit wouldn't help. Yes, you'd need more fuel and a sensor to determine your location relative to the target. Such a terrain-relative navigation sensor is being developed for the Mars 2020 mission. Given those two, landing within 100 meters of the target or less would not be a problem. $\endgroup$
    – Mark Adler
    Feb 8 '16 at 16:50
  • $\begingroup$ It would probably be much easier if you had some sort of beacon on the first lander. Then if followup landers had a bit of maneuvering capability, you should be able to land within a few meters. After all, the current 20 km landing ellipse is achieved purely by ballistics, isn't it? $\endgroup$
    – jamesqf
    Feb 8 '16 at 18:29
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    $\begingroup$ @Andy: our atmospheric entries from hyperbolic approach are already accurate enough that the rest of the error can be flown out hypersonically. $\endgroup$
    – Mark Adler
    Feb 8 '16 at 21:28
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    $\begingroup$ @jamesqf: No, the 20 km landing ellipse is not ballistic. It requires a guided entry using a lifting body and inertial knowledge. A ballistic entry is more like 80 to 100 km. $\endgroup$
    – Mark Adler
    Feb 8 '16 at 21:30
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For Curiosity, the target area was a 20-km ellipse:

Where the Mars Exploration Rovers could have landed anywhere within their respective 150 by 20 kilometers (about 93 miles by 12 miles) landing ellipses, Mars Science Laboratory landed within a 20-kilometer (12-mile) ellipse.

Curiosity landing

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  • $\begingroup$ The source is incorrect about the MER landing ellipses. They were about 80 km long, not 150 km. $\endgroup$
    – Mark Adler
    Feb 8 '16 at 21:32
  • $\begingroup$ @MarkAdler the source is nasa, I would assume they know about their rovers? do you have a contradicting source? $\endgroup$
    – njzk2
    Feb 8 '16 at 22:45
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    $\begingroup$ Yes, one would assume that. I'll see if I can find whose web page that is and have them fix it. $\endgroup$
    – Mark Adler
    Feb 8 '16 at 23:52
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    $\begingroup$ @DavidHammen: that's true about over-delivering, but the ellipses at Gusev and Meridiani were never as big as the linked reference. They started off at around 100 km x 20 km. Five months before launch they were 81.5 x 11.5 and 81 x 12 respectively. (The Opportunity ellipse actually got a little longer by the time of landing, as I recall due to launching late in the launch period.) $\endgroup$
    – Mark Adler
    Feb 9 '16 at 0:08
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    $\begingroup$ @MarkAdler -- We're losing track of the big picture, which is that a 20km×6km ellipse is at least 2 orders of magnitude too big compared to people who think we can do this, and without any effort. $\endgroup$ Feb 9 '16 at 0:34
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This paper on the Insight Lander: 2018 MARS INSIGHT TRAJECTORY RECONSTRUCTION AND PERFORMANCE FROM LAUNCH THROUGH LANDING has a lot of information on landing accuracy and entry delivery accuracy. It even shows some time-progression error values that show how different trajectory correction maneuvers reduced those errors.

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