2444: Ingenuity

2023-05-16T19:52:42Z

Volatile: Added brief explanation of Rayleigh-Taylor instability, removed incomplete tag

{{comic
| number = 2444
| date = March 31, 2021
| title = Ingenuity
| image = ingenuity.png
| titletext = Plot twist: Thanks to [mumble mumble] second-order [mumble] Rayleigh-Taylor [mumble] turbulent [mumble] shear, it turns out powered flight is way EASIER on Mars!
}}
==Explanation==
{{w|Ingenuity_(helicopter)|Ingenuity}} is a drone-like helicopter deployed to the surface of Mars. It rode on the underside of the Perseverance rover and at the time of publication its protective housing had been released from the rover and it was being prepared for a flight in early April. The helicopter is supposed to take off after the rover fully releases it and clears its takeoff trajectory.

The comic projects what might happen if the mission controllers activated the helicopter early. In this case, the process is approaching the point of detaching the part-deployed Ingenuity. Ponytail and Cueball are present in mission control when Cueball trips and hits a button that clearly triggers the Ingenuity drone to take off. Perseverance, still firmly above/attached is seen to easily ride atop it. The rover exclaims "Wheee!", presumably from excitement or happiness.

In the title text, some character discovers powered flight is easier on Mars, which contradicts our current understanding [https://www.youtube.com/watch?v=GhsZUZmJvaM that powered flight is very difficult on Mars]. Mars may have less gravity, but Mars's atmosphere is 1% the density of Earth's. [https://www.youtube.com/watch?v=aYe3sIaNHVs It's so thin that you couldn't move a feather with a fan.] This is why the character mumbles his explanation of the science, because they know any explanation doesn't actually make sense.

The total mass of the two vehicles is about 556 times that of the helicopter alone, meaning the unexpected lift effect 'described' would have to be several hundred times more effective than that anticipated, depending upon the factor of overdesign already built in to avoid an expensive marginal failure. It also seems to be trivially easy to balance the extremely top-heavy loading upon the small solar-panel that tops out the counter-rotating coaxial blades, which adds yet more questions of both the dynamic and structural performance, never mind questions about the available power to accomplish this and the later possibilities to recharge.

The {{w|Rayleigh-Taylor instability|Rayleigh–Taylor instability}} is a fluid instability that occurs at the interface between two fluids of different densities when the lighter fluid is supported by the denser fluid, leading to the mixing of the fluids due to the growth of perturbations at the interface.

It is not the first time that we have seen a Mars vehicle [[1504|vastly exceed expectations]] in these pages.

It is also not the first time a character has caused an incident by [[1620|tripping and hitting a control panel]].

==Transcript==
:[Ingenuity/Perseverance is on the surface of Mars.]
:Perseverance: ''Ingenuity'' helicopter has been lowered.
:Perseverance: Preparing to release it onto the surface.

:[A cut to mission control on Earth. Cueball trips and clicks on a key on his workstation, while Ponytail stands nearby.]
:Cueball: Oops
:Control panel: Click
:Trip

:[Back on Mars, Ingenuity's rotor blades start spinning.]
:Ingenuity: Bzzzzzz

:[Perseverance is being lifted into the air atop Ingenuity.]
:Perseverance: Wheeee!
:Ingenuity: Bzzzzzz

{{comic discussion}}
[[Category:Mars rovers]]
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]

2764: Cosmological Nostalgia Content

2023-05-16T19:36:37Z

Volatile: Removed multimanteau reference and updated incomplete tag accordingly

{{comic
| number = 2764
| date = April 17, 2023
| title = Cosmological Nostalgia Content
| image = cosmological_nostalgia_content_2x.png
| imagesize = 316x386px
| noexpand = true
| titletext = Later renouncing clickbait, Einstein called his inclusion of cosmological content in general relativity the biggest blunder of his career.
}}

==Explanation==
{{incomplete|1=Created by a z=90s KID - Needs more explanation of what the click bait in the title text refers to in the context of the Einstein blunder and why that belongs in this nostalgia comic. Do NOT delete this tag too soon.}}

The title of this comic is a combination of "{{w|cosmological constant}}" (an astrophysics term related to {{w|dark energy}} and to the accelerated {{w|expansion of the universe}}) and "[https://movementstrategy.com/editorial/nostalgia-social-media/ nostalgia content]" (clickbait marketing aimed at a specific age group referencing pop culture from their youth). The [https://tvtropes.org canonical examples] of nostalgia content are "[https://knowyourmeme.com/memes/only-90s-kids--2 Only 90s Kids Remember...]" and "[https://knowyourmeme.com/memes/feel-old-yet Feel Old yet?]". Some people of relatively advanced years like to make comparisons to others in their age group of where events that they remember fit into history; e.g., "The first moon landing was closer to the end of World War I than to today."

In cosmology, z is the symbol for {{w|Redshift|redshift}}, the effect whereby photons traveling from an object that is moving away from the observer exhibit an increase in wavelength, resulting in their color shifting towards the red end of the spectrum, and the variable z quantifies this amount of shift. Due to the expansion of the universe, objects that are further away from us appear to be moving away from us at a faster velocity, resulting in higher redshift. As light has finite velocity, it {{w|Speed of light#Spaceflight and astronomy|takes a longer time}}for light from faraway objects to arrive at the observer. So the light observed at the present must have been emitted by the faraway object further back in time. In this sense, after assuming a cosmological model, redshift and cosmic time can be put in a one-to-one relation and are often used interchangeably by astronomers. Redshift z = 10 would correspond to about 500 million years after the universe was formed, or almost 13 billion years ago[https://www.astro.ucla.edu/~wright/CosmoCalc.html]. Megan is stating that only people that were born at that time (z = 10) can remember when the first stars were still forming. At redshift z = 10 the matter filling the universe was mostly constituted of neutral hydrogen gas, referred by Megan as "cold" (it had a temperature of about 300 K) and "dark" (there were no light sources in the universe before the first stars formed. This epoch is indeed called the {{w|Timeline of the early universe#Cosmic Dark Age|cosmic dark ages}}). The ultraviolet light emitted by the first stars started ionizing the hydrogen around them in expanding hot plasma bubbles. This process (called {{w|reionization}}) had probably already begun at z = 10, but was really completed only at about z = 6, when the intergalactic hydrogen was completely ionized, as it continues to be at the present day.

Megan also has a red tint (she is drawn in the color [https://www.color-hex.com/color/462424 #462424], a very dark red; here is a comparison of #462424 and black: &emsp; &emsp; ), indicating that she is one such z = 10 kid, because her image is red-shifted, in the literal sense. The irony is that even if she were actually born 13 billion years ago (which would be absurd,{{Citation needed}}) her image would not appear red-shifted to us now. She would only appear redshifted to a far away observer looking at the young Megan wandering about the primordal cold dark gas.

White Hat, meanwhile, is referencing the fact that the universe has {{w|Scale factor (cosmology)|three eras}}: radiation dominated, matter dominated and dark energy dominated. As the universe expands, the density of radiation and matter decreases due to their dilution, causing the universe, which first started off being dominated by radiation, to then become dominated by matter, then by dark energy (which is thought to not dilute as the universe expands). The dark energy dominated era, which is when "dark energy started accelerating the universe's expansion" started around 5 billion years ago, whilst {{w|Evolution of bacteria|bacteria evolved around 3 billion years ago}}, meaning that they evolved closer to dark energy domination than to today.

The title and title text play with the similarity in sound between 'content' and 'constant', segueing between web(page) content and cosmological constant.

The title text refers to Einstein's inclusion of the {{w|Cosmological constant|cosmological constant}} to his theory of general relativity in order to attain a static model of the universe, which he later removed, reportedly referring to it as his "biggest blunder". Cosmological constant has, today, been generally accepted as a part of the current cosmological model, relating to the concept of dark energy. It is a pun on content and constant. The title text makes the claim that Einstein actually had cosmological content, which he used to clickbait, and he removed it to renounce clickbait.

==Transcript==
:[Megan (with a red tint) holds both her arms out and up while she is talking to Cueball to the left of her. To the right of Megan White Hat is holding both arms out and down while he is talking to Ponytail to the right of him.]
:Megan: Only z=10 kids remember watching the cool dark gas that suffused the universe being eaten away by expanding bubbles of plasma around the first stars!
:White Hat: Want to feel old? Bacteria evolved closer to when dark energy started accelerating the universe's expansion than to today!

:[Caption below the panel]:
:Cosmological Nostalgia Content

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Megan]]
[[Category:Comics featuring White Hat]]
[[Category:Comics featuring Ponytail]]
[[Category:Comics featuring real people]] 
[[Category:Comics to make one feel old]]
[[Category:Comics with color]] 
[[Category:Cosmology]]
[[Category:Biology]] 
[[Category:Clickbait]]

explain xkcd - User contributions [en]

2444: Ingenuity

2764: Cosmological Nostalgia Content