This Mars landing is pretty bonkers stuff.

<YOUTUBE id="4czjS9h4Fpg">[media]https://youtu.be/4czjS9h4Fpg</YOUTUBE>

The voice-over mentions the "constant velocity accordion"

It seems to be called that because, unlike other portions of the descent that are of precise duration, this one is variable in length, to negate up to 100m of slop in all that came before it.

After traveling 300 million km, they have room for 100m of uncertainty

NASA:

Constant Velocity Accordion

When the altitude is computed for BSS [Back Shell Separation], the spacecraft is

still traveling horizontally and the [terminal descent sensor] may not be

illuminating the exact point on the surface where landing

will occur. This, as well as inherent system errors, will

contribute to an error of up to 50 m in knowledge of altitude

at BSS. To accommodate this, a period of constant vertical

velocity is used to fly out the altitude error. This is termed

the Constant Velocity Accordion.

Since the next sub-segment (Constant Deceleration) begins

at an altitude of 50 m, the target altitude for the beginning

of the Constant Velocity sub-segment is set to 100 m. This

will allow for the case where the surface is 50 m closer than

initially calculated. In this case, the length of the Constant

Velocity Accordion is zero. In addition, enough fuel must

be allocated for the Constant Velocity phase for the case

where the surface is 50 m further away than initially

calculated, in which case 100 m of altitude will need to be

traversed.

The Constant Velocity sub-segment ends when the 50 m

Constant Deceleration altitude is achieved.

I do this in kerbal space program all the time. NASA thinks they are so smart, but I won't be impressed until they are able to make the ground traverse the final 50m and land on the ship.

Now, I will say, I have NEVER seen video from mars. How did they do that? Also, the staff are watching like a 2.5 minute delayed feed, at least, due to the limitation of the speed of light. I don't think we have a strong enough commo satellite and relay system around mars to get video like this, even with the two minute delay. So... What are we watching? Video that took 9 hours to transmit, pasted over the NASA crew analyzing the 20kbps signal from the probe?

[quote="harrisonreed"]Now, I will say, I have NEVER seen video from mars. How did they do that? Also, the staff are watching like a 2.5 minute delayed feed, at least, due to the limitation of the speed of light. I don't think we have a strong enough commo satellite and relay system around mars to get video like this, even with the two minute delay. So... What are we watching? Video that took 9 hours to transmit, pasted over the NASA crew analyzing the 20kbps signal from the probe?[/quote]

We're watching the video sent long after the landing overlaid with the control rooms reaction to the small amount of data delayed by quite a bit. At no point do they really have any control over what happens. When earth was informed that the parachute was open the lander was probably already on the ground.

Light speed, baby.

The delay is 11-12 minutes.

I thought a constant velocity accordion was when the accordion reaches terminal velocity when thrown from a 12 story building.

[quote="hyperbolica"]The delay is 11-12 minutes.

I thought a constant velocity accordion was when the accordion reaches terminal velocity when thrown from a 12 story building.[/quote]

No, terminal velocity is when you throw a DEC VT-100 off the roof of the Comp Sci building.

Do not ask how I know this.

Ah, yes Mars must be on the other side of the sun. I think the delay can be as low as 2-3 minutes if we are both on the same side of the sun.

It is pretty incredible to see real video from mars, and ridiculously better quality than from the moon. I think I did see some quasi "video" of dust devils on Mars, but nothing like that.

Light is so slow. Some people took a video of how slow it is:

<YOUTUBE id="EtsXgODHMWk">[media]https://youtu.be/EtsXgODHMWk</YOUTUBE>

So... What are we watching?

Part of the magic of this is that it is a completely autonomous landing, because it is impossible to manage in real-time from Earth.

The first Mars landers, the Vikings in the 70s, had to be dropped into the safest, flattest areas possible and a good result merely hoped for. Since then the ability to program a computer to interpret images and radar in real time have made for landings probably as good as any on-board human could do.

The current craft is programmed and equipped to guide itself to a very specific target on Mars and even at the 100 meter level identify a safe landing spot amongst obstacles too small to have been seen from orbit.

What you are seeing is video recorded by the rover during that descent, later slowly uploaded to Earth, and now synced up with the voice of mission controllers reading off status reports received from the rover as they arrived on Earth... 10 or so minutes after they were sent from Mars.

When the sky crane takes off, that is unbelievable.

NASA:

Implementation of the Sky Crane architecture presents

many advantages over historical touchdown methods,

namely airbags and legged landers. The two body

architecture keeps the engines and thrusters away from the

surface, mitigating surface interactions like dust excavation

and trenching, while enabling closed looped control

throughout the touchdown event. The bridle decouples the

touchdown event and associated disturbances from the DS

controller. Additionally, rather than using a traditional

touchdown sensor, touchdown is detected though a

persistence of reduced throttle commands necessary to

maintain the constant descent rate.

Due to the persistence of tethering during touchdown and

low touchdown velocities, the system has greater

touchdown stability and experiences lower impact loads

than other landing systems. High stability and low loading,

on par with rover driving loads, means that a separate

touchdown system is not required and the egress phase can

be eliminated. Rather, the rover’s rocker-bogey suspension,

which is specifically designed for surface interaction, is the

touchdown system and it is properly positioned to begin

operations immediately after touchdown.

My son was on a First Robotics team in high school. Following what it took for his team to build and code a robot to toss a ball through a hole in the wall makes me absolutely in awe of the math and engineering involved in something like this.

It was amazing to watch.

[url=https://apod.nasa.gov/apod/ap210227.html]Astronomy Picture of the Day

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Rover, between the two larger scuff marks...

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