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This thorough answer to Did the Curiosity and Perseverance rover brains do the driving from Earth to Mars? explains that during deep space flight the rovers' computers were doing most of the thinking even though they were clamshelled inside several outer structures. They communicated with all the sensor and thruster systems via cables that was cut when the rovers were dangling from the skycrane just above the surface of Mars.

During the deep space flight between Earth and Mars the spacecrafts communicated regularly with Earth for tracking, spacecraft health and potential trajectory correction instructions.

If the antennas were medium and especially high gain they would have to be pointed somehow towards Earth.

At the same time there were solar panels (not shown in the image below for some reason) that would have to be oriented roughly sunward.

For the three spacecraft now on their way to Mars plus ExoMars, how many have any solar power capability during transit?

Also, I don't know how often (if ever) were put into a roll for spin stabilization.

Question: When the Mars 2020 and Mars Science Laboratory missions were in deep space, what antennas did they have available on the outside of the spacecraft to communicate with Earth? Were they high gain, directional antennas requiring the spacecraft to maneuver and point towards Earth?


From this answer to Where does MSL end and Curiosity begin?:

Curiosity rover and MSL

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    $\begingroup$ Just to clarify, you mean antennas on the spacecraft? Because the antennas on the ground were those of the Deep Space Network. $\endgroup$
    – DrSheldon
    Mar 18 at 5:51
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    $\begingroup$ @DrSheldon I thought about that. The footprint of the beam coming back from a spacecraft is far larger than the Earth so I didn't think of the spacecraft as "using" a particular DSN antenna, but I'll add some clarifying language in the body of the question and adjust the title slightly. $\endgroup$
    – uhoh
    Mar 18 at 6:01
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From the DESCANSO MSL Article, cruise communications were done with a medium-gain antenna (MGA) and a low-gain antenna (PLGA).

The spacecraft was spin stabilized:

the interplanetary trajectory attitude control plan for MSL has the cruise stage spinning at 2 revolutions per minute (rpm) until shortly before entry into the Martian atmosphere. The cruise antennas (a medium gain antenna and a low gain antenna) have their boresights co-aligned with the spacecraft –Z axis. The –Z axis is closely aligned with the spin axis of the spacecraft.

And was always spin stabilized (see launch vehicle separation video).

The cruise stage's solar array is also mounted with the spacecraft –Z axis which presents an interesting tug-of-war between the power, thermal, and telecom subsystems:

the cruise-stage orientation to the Sun is driven by power and thermal subsystem constraints. The solar array normal must be pointed near the Sun line, at a Sun-view angle that optimizes the solar cell power output, which can be affected by being heated too much. On the other hand, the telecom link would be optimal if the antenna bore sight (and, hence, the spacecraft –Z axis) were pointed towards Earth.

The trajectory's Sun-Probe-Earth (SPE) angle is a key geometry in balancing these constraints (from the DESCANSO MSL Article):

SPE for MSL

The flown trajectory is (not quite , but close enough) the red dashed line.

The spacecraft therefore had to trim its attitude (while spinning) multiple times to maintain, primarily, its thermal and power state and, secondarily, telecom antenna pointing.

These attitude turns can be seen in this slideshow (2011 Mars Science Laboratory Trajectory Reconstruction and Performance from Launch Through Landing, Fernando Abilleira, 2013) on slide 9 as the ACS turns, there were 22 in total.

The telecom system's only constraint was a "80-deg offpoint from PLGA boresight" maximum. The 80° off bore sight constraint comes from the radiation pattern of the PLGA:

PLGA radiation pattern

which in theory puts the allowable Sun-view angles at between 0° and 145° (~65° SPE + 80° off bore sight PLGA) to maintain communication with Earth (I hope they didn't need >90°!).

Later in cruise the MGA was used when lower SPE angles presumably allowed for the MGA to produce better gain than the PLGA:

MGA radiation pattern

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    $\begingroup$ This is a beautiful answer, and for several reasons including 1) it's thorough and complete, 2) it's instructive, explaining several challenges to the mission design, and 3) serves as a "launch site" for several new mission-design and antenna questions! :-) $\endgroup$
    – uhoh
    Aug 19 at 0:38

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