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So as part of the Insight mission, two Cubesats were also launched to relay data during landing (so data can be obtained if something goes wrong)

Cubesats

What becomes of these satellites once their primary mission is done?

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MarCO's primary mission isn't actually to transmit data from InSight during its entry period. That's a non-essentially function that can be done by other spacecraft, if needed. Instead, the main part of the MarCO mission is determining the functionality of cubesats during deep space missions. We've never before sent such small spacecraft this far from Earth, and the satellites are really mostly a testbed for future cubesat missions.

In terms of their orbital trajectory - well, Emily Lakdawalla wrote about this back in March:

With no ability to enter orbit at Mars, the MarCO spacecraft will shoot on past, remaining in solar orbit. I assume their Earth controllers will remain in contact for as long as possible.

As I understand it, "as long as possible" is hopefully in the area of a few weeks.

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Not an answer, but comments can't have pictures.

After the primary mission of supporting the InSight landing, one of the MarCO sats did snap a nice picture of Mars while departing:

enter image description here

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Both MarCO cubesats are flying by Mars -- not in orbit. Furthermore, during their radio coverage of InSight's entry/descent/landing, their solar arrays are pointed away from the sun, towards Mars. They are on battery power, and will eventually stop transmitting when the batteries die. Then they will continue passively around the sun.

MarCO paths

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    $\begingroup$ Why don't the MarCO sats re-orient the arrays towards the sun after EDL? Do they lack the attitude gas to do that? $\endgroup$ – Wayne Conrad Nov 26 '18 at 21:25
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    $\begingroup$ @WayneConrad "Each MarCO's attitude-control system combines a star tracker, Sun sensors, gyroscopes and three-axis reaction wheels for monitoring and adjusting orientation. Accelerating a reaction wheel rotates the spacecraft in the opposite direction from the direction the wheel is spinning." No mention of any attitude gas at all. But I don't know why they couldn't reorient with just reaction wheels. $\endgroup$ – Ross Presser Nov 26 '18 at 22:16
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    $\begingroup$ There are thrusters, fed by a single tank. They were used for course corrections, and to de-saturate the reaction wheels. MarCO A (nicknamed WALL-E) had a leak and is believed to be out of gas. MarCO B might be able to re-orient to the sun, but being aligned to send data to Earth is a higher priority. $\endgroup$ – Dr Sheldon Nov 26 '18 at 23:03
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    $\begingroup$ Clearly, orienting for data transmission is important right now. That doesn't explain why they wouldn't reorient later. Many interplanetary craft aim solar and antenna by reorienting the craft repeatedly, after all. $\endgroup$ – Saiboogu Nov 26 '18 at 23:09
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    $\begingroup$ According to space.com/42541-mars-insight-lander-success.html, "It's possible that MarCO-A and MarCO-B could observe an asteroid or other celestial body if their paths bring them close enough, and if funding for an extended mission is granted, John Baker, NASA's program office manager for the MarCO mission, told Space.com." This contradicts the assertion that they are going to run out of power in a few weeks and die. $\endgroup$ – Wayne Conrad Nov 27 '18 at 0:58

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