The HiRise camera on the Mars Reconnaissance Orbiter snapped a beautiful photo of Perseverance falling through the Martian atmosphere hanging from its parachute.

This photo is the latest entry in this catalog of "Images with caption".

This answer explains that the HiRise camera is a push broom type; it scans a 1D-like sensor across a long swath of Mars, like older photocopy machines and fax machines. I suppose this means it couldn't shoot a sequence of shots.

HiRise Specifications: https://www.uahirise.org/specs/

The image page says:

HiRISE was approximately 435 miles (700 kilometers) from Perseverance and traveling at about 6750 mile per hour (3 kilometers per second) at the time the image was taken. The extreme distance and high speeds of the two spacecraft were challenging conditions that required precise timing and for Mars Reconnaissance Orbiter to both pitch upward and roll hard to the left so that Perseverance was viewable by HiRISE at just the right moment.

Question: Why exactly did MRO have to "both pitch upward and roll hard to the left" to point exactly at Perseverance during the entry, descent and landing phase? What does that even mean?

Since the target object is tall and skinny, did it somehow have to maneuver to "pushbroom" horizontally so that the scan was across their short direction rather than bottom-to-top?

PIA24270: HiRISE Captured Perseverance During Descent to Mars

Source: PIA24270: HiRISE Captured Perseverance During Descent to Mars

  • $\begingroup$ possibly helpful: High-Performance Pushbroom Imagers for Planetary Missions Bergstrom and Dissley 2012 (Ball Aerospace & Technologies Corp( $\endgroup$
    – uhoh
    Feb 20 '21 at 2:39
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    $\begingroup$ Are you sure it wasn't just to make sure that the telescope was pointing at the lander? $\endgroup$
    – Phiteros
    Feb 20 '21 at 5:19
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    $\begingroup$ The MRO was pretty low in the sky during the landing. They probably typically image just below the orbiter under normal circumstances, so it had to maneuver to be pointing at Perseverance. Since it was also being used as a communications relay, they probably also didn't want the antenna to be pointing away from Earth. That would be my guess. $\endgroup$
    – Phiteros
    Feb 20 '21 at 5:33
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    $\begingroup$ If you substitute "yaw" for "roll" does it make more sense? My guess is they felt like avoiding the term for whatever reason, substituting it with a more public friendly "synonym" $\endgroup$ Feb 21 '21 at 6:28
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    $\begingroup$ They probably did mean pitch and roll. Yaw doesn't do much for a nominally down-looking spacecraft. $\endgroup$ Feb 24 '21 at 4:35

Why exactly did MRO have to "both pitch upward and roll hard to the left" to point exactly at Perseverance during the entry, descent and landing phase?

Perseverance was not directly underneath MRO at the time the images were taken. Perseverence was instead forward of and to the left of MRO's ground track. The fairly standard meaning of yaw, pitch, and roll for an orbiting spacecraft are

  • Yaw is a rotation about the vertical axis,
  • Pitch is about the across-track axis, and
  • Roll is about the along-track axis.

Yaw is the last thing one wants to do with a nominally down-looking push broom camera. Think of sweeping the leaves from your sidewalk with a push broom. Yaw (in spacecraft convention) is the equivalent of turning the broom sideways. Instead of sweeping a half meter swath of the sidewalk you're sweeping a swatch that's a few centimeters wide.

Pitch also isn't normally used, but it may be needed to see a dynamic event. For a static event, the spacecraft's travel will soon take the spacecraft over the spot in question. But for a dynamic event such as seeing parachute deployment from above, the spacecraft may need to pitch up or pitch down so as to have the event in view.

The HiRISE instrument has a very narrow field of view. This is where roll becomes important. If the camera is to see some object (dynamic or static) that is not directly underneath the satellite's path, the satellite will have to roll to bring that object into the camera's field of view.


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