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Despite the 34 day long launch window, the day that InSight will arrive has already been chosen. See mars.nasa.gov's insight mission timeline below, a site I found linked in this answer.

I'm not asking if this is kind of variation is common or not, I'm asking why exactly InSight's arrival date is pinned down. Are there aspects of the mission that require this? Coordination with other spacecraft perhaps?

About InSight's Launch

InSight is scheduled to launch under pre-dawn skies from Vandenberg Air Force Base on the central coast of California in May 2018.

The mission's launch period is May 5 through June 8, 2018, with daily launch windows that last two hours per day. Launch opportunities are set five minutes apart during each date's launch window. The first opportunity begins at approximately 4:00 a.m. Pacific Standard Time on May 5.

InSight will launch from Launch Complex 3 and ride atop an Atlas V-401 rocket provided by United Launch Alliance, Centennial Colorado, a joint venture of Boeing Co. and Lockheed Martin Corp.

The Atlas V is one of the biggest rockets available for interplanetary flight. This is the same type of rocket that launched the Mars Reconnaissance Orbiter in 2005.

The launch is only the beginning; the trip to Mars takes about six months. The journey is about 301 million miles (485 million kilometers).

No matter at what particular time and date InSight launches during its launch windows, its date with Mars is set for Nov. 26, 2018.

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    $\begingroup$ They have got a specific place they want to land, and got a specific and have got a specific entry corridor for that. So they will aim for that point. However I could imagine them doing the same approach one sol later... $\endgroup$
    – Hans
    Mar 5, 2018 at 15:39
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    $\begingroup$ @Hans those are both good points, thanks. In this case it's not going to be exactly a solar sol or a sidereal sol, but somewhere in between perhaps? $\endgroup$
    – uhoh
    Mar 5, 2018 at 15:44
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    $\begingroup$ There is a part of me that things it is timed around US holidays. The date of arrival is the Monday after Thanksgiving, giving almost an entire month before Christmas, allowing the science team to get the spacecraft working well without having holiday conflicts. But still... $\endgroup$
    – PearsonArtPhoto ♦
    Mar 5, 2018 at 15:57
  • $\begingroup$ @PearsonArtPhoto How was Juno's arrival set up to be on the evening of July 4th? ;-) $\endgroup$
    – uhoh
    Mar 5, 2018 at 16:00

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There's a bit in the NASA press kit about this:

If launch is at the start of the launch period, May 5, the trip to Mars will take 205 days. If launch is at the end of the launch period, June 8, the trip will take 171 days. The use of a constant arrival date -- Nov. 26, 2018 -- for any launch date helped simplify operations planning.

(They must have liked that text, because it appears twice on the page) Some indications of the kinds of "operations planning" needed appear later:

NASA's Mars Reconnaissance Orbiter (MRO) is expected to be in position to receive the transmissions during InSight’s entry, descent and landing. MRO, passing over InSight's landing region on Mars, will record the data for transmitting to Earth during a later orbit.

On Earth, three radio telescopes will be listening for a very basic indicator of InSight’s status: They may be able to confirm that InSight is transmitting during descent and after landing. They are the Max Planck Institute for Radio Astronomy's facility at Effelsberg, Germany; the Institute of Radio Astronomy of Bologna’s Sardinia Radio Telescope, on the Italian island of Sardinia, and the National Science Foundation's Green Bank Observatory in Green Bank, West Virginia.

NASA’s Mars Odyssey orbiter is expected to provide information about InSight after the landing because it is scheduled to fly over InSight after the entry, descent and landing process.

So lots of sequencing to do, which they'd probably prefer to do well in advance to make sure constraints can be satisfied.

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Looking at NASA's Performance Vehicle Calculator, we can see that the C3 for an Atlas 401 is about 60 km^2/s^2. Some of that will be eaten up for the West coast launch. It's hard to say exactly, but let's say 200 m/s, which translates to an actual C3 of 56 km^2/s^s. That is far more energy then is required for a Mars mission, allowing one to select the date to a high degree of precision.

I suspect that the date has been picked to be of maximum benefit to the Science/Engineering staff, and also to generate some PR interest. November 26th is the Monday after Thanksgiving in the United States. That essentially means that the mission's critical first month of operation will be between Thanksgiving and Christmas. Assuming that NASA doesn't hate it's employees, that would be a fantastic time to have the mission take place.

Basically, there is enough extra energy from the rocket to pick the launch window, so why not pick one that is of advantage to the employees?

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  • $\begingroup$ This certainly sounds highly plausible. $\endgroup$
    – uhoh
    Mar 5, 2018 at 16:17
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    $\begingroup$ To avoid any risk for the mission it may be better to avoid the time between Christmas and New Years's Eve. $\endgroup$
    – Uwe
    Mar 5, 2018 at 16:43
  • $\begingroup$ @Uwe especially New Year's Eve! ;-) $\endgroup$
    – uhoh
    Mar 6, 2018 at 8:35
  • $\begingroup$ I have to admit I was expecting the answer to be it's going to sit in its parking orbit until the phase angle date actually arrives, but that would require an upper stage fuel other than liquid hydrogen. $\endgroup$
    – Joshua
    Apr 8, 2018 at 18:43
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    $\begingroup$ Considering the payload mass, 60 km^2/s^2 seems reasonable. I plotted the graph here. But Wikipedia says InSight mission to Mars launched with a C3 of 8.19 km^2/s^2. I think the later needs to be edited. $\endgroup$
    – OrangeDurito
    Jun 4, 2020 at 19:47

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