Infrared

The easiest to earn is tesla, which you get by earning 10,000,000 EPs.

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2. General Khan Academy Questions
3. black hole badges

The number still remains after you achieve the badge. Just because the number is still there that does not mean you did not get those points.

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2. General Khan Academy Questions
3. What does the numbers on the sun badges mean?

I don't even know if it is available. Even if it was, I would not bother trying to get it.

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2. General Khan Academy Questions
3. black hole badges

It really does not seem fair to new users. I do though, see their perspective for retiring badges, but I still wish they could re-instate them when they get time.

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2. Feature Requests and General Site Feedback
3. Retired Badges

Also https://www.khanacademy.org/badges/herculean (I don't know if it is achievable)

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3. black hole badges

Herculean also exists. Quoting the description: 'Attained proficiency in all exercises to date' https://www.khanacademy.org/badges/herculean

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3. Black Hole Badges

Same! Really want the retired badges back.

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2. Feature Requests and General Site Feedback
3. Badges

Please bring the speed badges back!   

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2. General Khan Academy Questions
3. Recently-retired Badges (December 2020)