# How could the landing date of Perseverance Rover be fixed irrespective of 26 days launch window?

I am very much aware of this question from 2018 by @uhoh - Why would InSight's arrival date at Mars be fixed, and independent of the launch date?. I was led here by a similar question which arose in my mind after reading this article which says -

No matter what day Perseverance lifts off during its July 17-Aug. 11 launch period, it will land at Mars' Jezero Crater on Feb. 18, 2021.

I am not satisfied with the answers to the earlier question which mainly talks about a couple of reasons -

1. 'Operations planning' referring to the sequence of sending data for EDL and afterward by MRO and Mars Odyssey orbiter. I checked for the orbital period of these two spacecraft and they are 35.5 hrs and 2 hrs respectively. So, whichever configuration you start with, they will attain the same configuration in 71 hrs [LCM(35.5, 2)]. That doesn't justify the fixed landing date independent of 26 days launch window.

2. About the launch vehicle excess energy ($$C_{3}$$) which, considering the respective payload mass (InSight [ $$\approx$$700kg ] and Mars 2020 [ $$\approx$$1100 kg]), have C3 value of around 60$$Km^{2}/s^{2}$$. See my Payload Mass vs. $$C_{3}$$ plot for Atlas V in 401 and 541 configurations here made using NASA's Launch Vehicle Performance website. The answer then says that since we have far more energy than requires for Mars mission ($$C_{3} = 12 Km^{2}/s^{2}$$; from Wikipedia), hence it allows us to select the landing date with high precision. How? The discussion then leads to landing date being chosen so as to fall between Thanksgiving and Christmas but I don't think such a PR case can be made for February 2021.

I am looking for a comprehensible explanation as to why the landing date is independent of the launch date (within the window) for the Mars 2020 mission.

• I'm glad you asked this and I agree that your question is not answered by those answers, but together with answers to How was Juno's arrival set up to be on the evening of July 4th? (linked below my question) I'd felt I had a good enough idea; the choreography of the direct interplanetary-to-landing trajectory to a specific site with a narrow landing ellipse and the two MarCO cubesats that traveled together with it required the whole thing to be carefully optimized. Hopefully an answer here based on the Perseverance mission will be definitive.
– uhoh
Jun 4 '20 at 22:36
• I have no links currently, but in my vision it's part of NASA's "lessons learned" from many interplanetary missions. It's normal practice now, not only for Perseverance and Insight. Personnel management and operations scheduling/recheduling appeared much more difficult and stressful than calculation of different rocket trajectories for different dates. I know that Dragonfly probe has fixed date of arrival to Titan, too. Jun 18 '20 at 7:02
• Huge update to this. I now actually fully understand the reasoning:-) Jul 29 '20 at 12:42
• Item 1 mentions the 35.5 hr and 2 hr orbit of MRO and Mars Odyssey. You also have to consider the 24.6 hour rotational period of Mars. It does not matter if the two satellites "meet" every 71 hours if the landing site is on the other side of the planet! (Or even if just out of view of either spacecraft.) Aug 1 '20 at 0:15
• @JohnHoltz TRUE! How did I forget that? It's not just about the two spacecrafts "be on the same line" (I suppose?) for EDL coverage but also at which side of the planet do they align relative to the landing site. Thanks a lot for pointing that out! Aug 1 '20 at 4:42

It is deliberate choice. See this tweet from Tory Bruno, CEO of ULA who is launching the mission. This of course doesn't tell us the exact reasoning behind the date, however.

Why this is is actually quite a bit more complex. JPL published a paper on this very subject recently. There are a number of factors to picking a launch date. For Perseverance, as with any lander, they want to maximize coverage during the EDL period. It turns out the delta-v requirements are almost identical for a launch during the entire time window, a few days won't make a huge difference. There are a lot of factors in play, which you can see the ones below.

Basically there is a trade off of 3 different factors that determined the arrival date. The delta-v required when arriving at Mars (Red curves), the energy required at leaving from Mars (Red curve), and the coverage of satellites available during EDL at Mars, specifically MRO. To position these satellites in exactly the right spot takes a lot of time, as they don't have a lot of fuel, and they had to plan where to be actually years in advance. As early as 2018 they (MRO and MAVEN) started to adjust their orbit to be in exactly the right spot to optimally receive the landing telemetry of Perseverance. As you can see from the above plot, the window will only last a few days, and given the fact that the extra fuel requirements are negligible in that time period, why not aim for exactly the optimal time period to receive signals from MRO during EDL?

• 'Orbital design wizardry' haha!!! Maybe I need few lessons in orbital sorcery to decipher it. Thank you for asking it to him though :) Jun 17 '20 at 23:23
• @OrangeDurito we can ask as many (reasonable) questions as we like in Stack Exchange. If you'd like to ask a new question about orbital design wizardry, such as "Why was it also necessary to start changing MRO and MAVEN's orbit in 2018 to get ready for Perseverance since the landing time was already optimized for them?" I think it will generate some new and interesting answers.
– uhoh
Aug 1 '20 at 1:40
• Thank you so much for the paper! This does indeed clarify the mystery. Btw, could you please explain the second factor i.e 'the energy required at leaving from Mars (Red Curve)'? Thanks again! Aug 1 '20 at 4:33
• There are two pieces, the energy to leave from Earth to Mars, and the energy that you have to get rid of when you get to Mars. The red curve is the second of those. Aug 1 '20 at 11:23