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According to the NASA JPL web page Mars Cube One (MarCO) Mission Overview and the YouTube video MarCO: First Interplanetary CubeSat Mission linked there, after the Centaur upper stage deploys InSight, it will deploy the two MarCO 6U* cubesats, and all three spacecraft will travel to Mars.

*actually 36.6 x 24.3 x 11.8 cm.

The MarCO spacecraft will receive data from InSight as it enters the atmosphere of Mars, and relay some data back to Earth.

If they are deployed with even say 1 meter per second relative velocity in each direction transverse to the Centaur, after six months (that's about pi/2 x 1E+07 seconds) that puts them +/- 15,000 km to either side of InSight.

Deployment may incur other tiny mechanical delta-v's and there's six months of solar pressure and outgassing and possibly other things that might induce drifting between them.

Question: How can the two MarCO spacecraft reliably stay close enough to InSight for six months in order to arrive close enough in both time and space to "witness" InSight's entry into the Martian atmosphere?

edit: Coordinating the trajectory of three spacecraft on an interplanetary deep space trajectory would be hard enough if they were all standard sized, powered, and equipped interplanetary spacecraft, but two of them are 6U cubesats and this will be the first time that a cubesat will travel to another planet, let alone doing it in a coordinated fashion with other spacecraft.

below: Both MarCO cubesats, from here. Click for full size.

MarCO cubesat

below: Screnshot from YouTube showing both MarCO cubesats, the Centaur upper stage, and in the distance at right, InSight.

enter image description here

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  • $\begingroup$ Maybe to avoid confusion about a seemingly dead mission: The launch has been delayed according to wikipedia from 2016 to May 2018, and in fact the launch schedule from spaceflightnow.com shows it scheduled to depart May 5th, this year. $\endgroup$ – AtmosphericPrisonEscape Apr 20 '18 at 0:33
  • $\begingroup$ @AtmosphericPrisonEscape as stated in Why would InSight's arrival date at Mars be fixed, and independent of the launch date? the mission's launch period is May 5 through June 8, 2018. Upcoming launches are followed by most readers here, I don't think "seemingly dead mission" is a correct characterization. $\endgroup$ – uhoh Apr 20 '18 at 1:20
  • $\begingroup$ Perhaps people here follow all of the upcoming launches, but not all of your questions. $\endgroup$ – AtmosphericPrisonEscape Apr 20 '18 at 2:02
  • $\begingroup$ @AtmosphericPrisonEscape definitely not, who could! That's why I add links. $\endgroup$ – uhoh Apr 20 '18 at 3:09
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After InSight is deployed from the Centaur upper stage in the forward direction. the two MarCO 6U cubesats will be deployed sideways, with a 180 degree rotation between the first and second, as shown in the video MarCO: First Interplanetary CubeSat Mission linked in the question. About 6.5 months later the plan for the original 2016 mission was for both MarCO cubesats to pass about 3,500 km from InSight during its own 7 minutes of terror, and to try to receive telemetry during the descent, landing, and for some time after Mars' rotation quickly hides the landing site from Earth radio reception for the next ~13 hours.

For more on that see Why would InSight's arrival date at Mars be fixed, and independent of the launch date?

This is shown in the screenshot below left, of the presentation Mars Cube One (MarCO) The First Planetary CubeSat Mission by Sami Asmar, Principal Investigator and Steve Matousek Capture Lead from 2014. As shown in the screen shot of from the 2015 presentation INSPIRE and Beyond Deep Space CubeSats at JPL by Andrew Klesh, there were as many as five TCMs or Trajectory Correction Maneuvers possible along the way. Third image is cropped from the updated 2018 InSight trajectory found in NASA's InSight Mission Launch Sequence showing as many as six TCMs for InSight itself. Fourth image is a screen shot from MarCO Introduction PPO 2015.

Click for full size:

Mars Cube One trajectory Mars Cube One TCMs

InSight trajectory and TCMs MarCO Cubes on Centaur

THRUSTERS:

For this to be possible, and to keep the arrival time fixed in order to land at the proper site, both InSight and the two MarCO cubesats will require thrusters for low delta-v maneuvers. The image below cropped from Hackaday's Interview Interview: Nacer Chahat Designs Antennas for Mars Cubesats shows the eight cold gas thrusters on the 1x2U Earth-facing end.

The inner four point aft and are for axial thrust, and the upper and lower outer pairs are angled somewhat downwards and upwards. Using the upper/lower pair would result in pitch down/up, a diagonal pair would produce roll, and a left pair or right pair would produce yaw. See also this answer about Voyager's thruster patterns, thought they may not be angled in the same way. (So I've just asked How are Voyager's thrusters pointed?)

above: Cropped from Hackaday. below x2: Cold gas Thruster system, from Vacco's JPL MarCO Micro CubeSat Propulsion System.

According to the system's webpage and Datasheet:

  • Smart, Self-contained MiPS
  • 755 N-Sec total impulse
  • Inherently safe non-toxic R236fa propellant.
  • 3490 gram wet mass
  • 4 axial and 4 RCS 25mN thrusters
  • Two interrupts against leakage
  • Microcontroller driven

ADCS:

Thrust is useless unless you know how to point it, so MarCO also contains a sophisticated ADCS system. Images are from the Spaceflight 101 page MarCO – Mars Cube One:

From Spaceflight 101:

The MarCO satellites are three-axis stabilized through a typical CubeSat Attitude Determination and Control System based on several attitude determination inputs and reaction wheels for actuation. Since MarCO will not have the Earth’s magnetic field to work with, magnetic torquers are no option for momentum unloading from the wheels – requiring a propulsive attitude control system to be implemented in addition to a main propulsion system for trajectory control.

Chosen for the MarCO mission was Blue Canyon’s XACT (fleXible Attitude Control Technology) unit which includes a star tracker, inertial measurement sensors, coarse sun sensors and a three-axis reaction wheel assembly for actuation. The XACT unit takes up 0.5 Units of volume (10 x 10 x 5 centimeters) and weighs 0.91 Kilograms when flying in its baseline configuration, capable of a pointing accuracy of ±0.003 degrees on two axes and ±0.007 on the third. It uses a 12V power supply, supports slew rates of over 10° per second and has been rated for a five-year lifetime on Low Earth Orbit missions.

Several modifications on the XACT baseline design were needed for the MarCO mission and include the addition of a coarse sun sensor and provisions for controlling the thruster system for momentum control, facilitated on a functionally separate unit from a different manufacturer. Changes were also needed in the flight software to account for deep space trajectories.

Through the cruise phase, the Star Tracker is the primary attitude determination device, collecting imagery of the star-filled sky that is compared to an onboard catalog of over 23,000 stars to identify known constellation and from that calculate the craft’s precise three-axis orientation.

The XACT Star Tracker typically processes up to 64 guide stars down to 7.5 mag and delivers attitude quarternions at a refresh rate of five per second. The inertial measurement system is used to propagate the state of the craft in between ST updates and it also provides guidance for the initial de-tumble of the spacecraft after separation from the launch vehicle. One interesting function of the XACT Star Tracker is its imaging mode, capable of collecting black and white images of 1024 x 1280 pixels.


DSN Command and Rate/Range:

As any other interplanetary spacecraft to date, including InSight, MarCO has no ability to self navigate. Each MarCO spacecraft will contain an IRIS Cubesat Radio Transceiver. According to the IRIS radio Datasheet of the Iris V2.1 CubeSat Deep Space Transponder, it contains three RF paths and supports several standard space communications bands.

A component critical to navigation is the Configurable Software Defined Coherent Transponder which will receive Rate/Range signals from the Deep Space network X-band at 7.145 – 7.190 GHz, and simultaneously rebroadcast them coherently at 8.400 – 8.450 GHz, with a coherent turn-around frequency ratio of 880/749.

This way NASA will be able to make precise distance and relative velocity measurements of each spacecraft throughout the journey, and to calculate the propulsive maneuver commands to then send back to them in order to keep them on course.

During InSight's entry into the Martian atmosphere and afterward, the MarCO spacecraft will receive signals from InSight via the UHF antenna on the bottom, and rebroadcast them as a "bent pipe" transponder to Earth in X-band via the high gain antenna pointed at Earth.

above: MarCO UHF loop antenna used to communicate with InSight, shown in deployed and stowed positions. below: MarCO High Gain Antenna used to communicate with Earth's DSN, shown in stowed and deployed positions. Both images from Hackaday.

below: Image from the NASA JPL IRIS radio page.

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    $\begingroup$ This is practically an article on a specific satellite's design, with a question as an excuse to post it. :/ $\endgroup$ – Erin Anne Apr 20 '18 at 4:17
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    $\begingroup$ There seems to be a missing link target that presumably should point at an answer on SpEx…. $\endgroup$ – Nathan Tuggy Nov 23 '18 at 10:24
  • $\begingroup$ @NathanTuggy Thank you for the heads-up! I'll have a look now. I'm still cleaning up after discovering Does the post editor secretly delete other links when posting an image? If so, why? I personally think that an editor that actively and quietly deletes things you've typed is "evil" but once something is suspected of being "by design" I guess it's better to learn to live with it in the short run at least. $\endgroup$ – uhoh Nov 23 '18 at 10:36
  • $\begingroup$ @NathanTuggy however, upon further investigation, this one looks like I simply forgot (blush). $\endgroup$ – uhoh Nov 23 '18 at 10:38
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They have an onboard propulsion system.

http://www.cubesat-propulsion.com/jpl-marco-micro-propulsion-system/

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    $\begingroup$ It takes more than having a propulsion system for this to work. How will the tracking and navigation be implemented? $\endgroup$ – uhoh Apr 18 '18 at 17:12

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