Elon Musk and Jeff Bezos are arguing again with their rival rocket companies. It's the latest in a 20-year feud.

• Tesla and SpaceX CEO Elon Musk and Amazon and Blue Origin founder Jeff Bezos have been rivals for 20 years.

• They've sparred over their space ambitions, traded barbs, and swapped places as the world's richest person.

• Here's how their rivalry began and everything that's happened in the years since.

They're two of the world's richest men, and their rivalry dates back to the early 2000s.

Elon Musk and Jeff Bezos have sparred for years, often publicly, ever since a reported contentious dinner they had in 2004.

They've argued over their space ambitions at SpaceX and Blue Origin, respectively, as well as their wealth as they repeatedly leapfrogged each other for the title of the world's richest person.

Here's a timeline of their relationship over the years:

Back in the early 2000s, Jeff Bezos wasn't yet the titan he is today.

Bezos launched Amazon in 1994, and the company went public in 1997. But Amazon wasn't yet the powerhouse it would become — it was years before the company would launch Prime, start its own streaming service, or create its cloud infrastructure service, Amazon Web Services.

But Bezos had always been interested in space. He told the Miami Herald in 1982, after he graduated high school as valedictorian, that he wanted to create outer space colonies for millions of people.

As a result of that long-held interest in leaving Earth, Bezos in 2000 launched Blue Origin, a startup focused on human spaceflight.

Elon Musk was already a millionaire several times over, but he hadn't become CEO of Tesla yet.

Around the time Bezos was launching Blue Origin, Musk had already sold Zip2, a startup he launched with his brother, Kimbal, to Compaq for roughly $300 million. Musk was in the process of building PayPal, while would later be sold to eBay for $1.5 billion.

Musk made about $160 million off the PayPal sale and used that money to launch SpaceX in 2002.

"In the beginning, I actually wouldn't even let my friends invest because everyone would lose their money," Musk said during an interview at South by Southwest in 2018. "I thought I'd rather lose my own money."

The Musk-Bezos rivalry appears to date back to 2004 when the two CEOs met for dinner.

By 2004, both Blue Origin and SpaceX were still in their infancy — neither company had completed any launches yet.

But that didn't stop a rivalry from heating up: When the two met to discuss their respective reusable rocket ambitions it apparently did not go well.

"I actually did my best to give good advice, which he largely ignored," Musk said after the meeting, according to Christian Davenport's book, "The Space Barons."

In 2021, Trung Phan, then a writer for business newsletter The Hustle, tweeted a photo of Musk and Bezos smiling and sitting in a restaurant. Phan said the photo was from 2004, meaning it may have been taken at that fateful dinner.

Musk responded to the photo, tweeting, "Wow, hard to believe that was 17 years ago!"

Fast forward to 2013, and things became contentious again over leasing a NASA launchpad.

In 2013, SpaceX tried to get exclusive use of a NASA launchpad. Blue Origin (along with SpaceX rival United Launch Alliance) filed a formal protest with the government to prevent SpaceX from using the pad — Bezos proposed converting it "into a commercial spaceport available to all launch companies."

Musk called the move a "phony blocking tactic" and took another swipe at Blue Origin.

"[Blue Origin] has not yet succeeded in creating a reliable suborbital spacecraft, despite spending over 10 years in development," Musk told Space News at the time. "If they do somehow show up in the next five years with a vehicle qualified to NASA's human rating standards that can dock with the Space Station, which is what Pad 39A is meant to do, we will gladly accommodate their needs."

"Frankly, I think we are more likely to discover unicorns dancing in the flame duct," he added.

SpaceX eventually won the right to take over the pad.

In 2014, the two companies got into a patent battle when Blue Origin was granted a patent for drone ships, which are used for landing rocket boosters. SpaceX petitioned to invalidate the patent.

Blue Origin's ownership of the patent would mean SpaceX would need to pay to use the technology. SpaceX argued that the science in the patent was "old hat," given that the concept of drone ships has been around for decades.

A judge sided with SpaceX, leading to Blue Origin withdrawing most of the claims in the patent.

The feud isn't just about space ambitions, however. Musk has taken issue with Blue Origin's hiring practices and has taunted Bezos in interviews.

Musk told one of his biographers, Ashlee Vance, that Blue Origin has repeatedly tried to snag talent away from SpaceX.

"Blue Origin does these surgical strikes on specialized talent offering like double their salaries," Musk said in Vance's 2015 biography. "I think it's unnecessary and a bit rude."

Musk also revealed that SpaceX set up an email filter for the words "blue" and "origin," according to Space News.

When the BBC asked Musk about Bezos in 2016, he responded, "Jeff who?"

In recent years, Musk and Bezos have been more public about their feud, taking their rivalry to Twitter.

Both execs have seized on opportunities to take shots at the other, most often sniping at each other over reusable rockets. After Blue Origin successfully landed its New Shepard rocket in 2015, Bezos tweeted a video calling it "the rarest of beasts — a used rocket."

Musk responded, saying SpaceX had performed the feat three years prior.

When SpaceX landed its Falcon 9 spacecraft, Bezos took the opportunity to needle Musk on Twitter.

Musk is known for being outspoken on Twitter, and that has included jabs at Bezos.

Musk has repeatedly and publicly called Bezos a "copycat" — once after Amazon announced its plan to launch internet-beaming satellites, and again when Amazon acquired self-driving-taxi company Zoox.

Musk poked at Bezos in 2019 after the unveiling of Blue Origin's concept for a lunar-landing vehicle, called Blue Moon.

"Putting the word 'Blue' on a ball is questionable branding," he tweeted.

Musk later mocked up a screenshot of a New York Times article that changed the name from "Blue Moon" to "Blue Balls."

"Oh stop teasing, Jeff 😉," Musk wrote.

For his part, Bezos has been less overt about his distaste for Musk and SpaceX, but he's made veiled comments about his thoughts on the company's plans.

While Bezos has stopped short of calling out Musk directly, he has taken aim at Musk's biggest ambition: colonizing Mars, the main goal of SpaceX.

Bezos' focus is on getting humans to the moon, and he's described the idea of reaching Mars as "un-motivating."

"Go live on the top of Mount Everest for a year first and see if you like it, because it's a garden paradise compared to Mars," Bezos said in 2019.

During his presentation for Blue Moon, Bezos referenced SpaceX's Mars ambitions once again, titling a slide about Mars "FAR, FAR AWAY."

Things grew more contentious in the spring of 2020, when Blue Origin and SpaceX were both asked to submit designs for lunar landers to NASA for a mission to return humans to the moon by 2024, yet another avenue for the two companies to compete.

Along with a third company, Dynetics, Bezos' and Musk's companies were asked to compete for a multibillion-dollar contract with NASA.

All three companies had 10 months to work on their designs for a mission known as Artemis — the mission would be the first time a manned spacecraft has been sent to the moon since the Apollo 17 mission in 1972.

In July 2020, Musk appeared to make a dig about Bezos' age in an interview with The New York Times.

In a wide-ranging Times interview in 2020, Musk, who is seven years younger than Bezos, took the opportunity to comment on Blue Origin, appearing to imply that Jeff Bezos is too old to ever make real progress.

"The rate of progress is too slow and the amount of years he has left is not enough, but I'm still glad he's doing what he's doing with Blue Origin," Musk said.

Though the pair's main point of contention appears to be space, Musk has made other pointed remarks about Amazon, including calling the company a monopoly.

After Amazon's publishing service refused to publish a book about the coronavirus by writer Alex Berenson, Musk tweeted at Bezos that the situation was "insane" and called for Amazon to be broken up.

Musk's comments were in response to Amazon's Kindle Direct Publishing service refusing to publish Berenson's book titled "Unreported Truths about COVID-19 and Lockdowns." Berenson tweeted a screenshot of an email he says he received from Amazon and said the company "censored" his book. The screenshot appeared to show the publishing division saying the book does not comply with its guidelines.

Amazon later told Business Insider the book was removed in error and would be reinstated.

It's possible Bezos softened his stance on SpaceX a bit: After the rocket company conducted a test of its Starship spacecraft in December 2020, Bezos publicly complimented the company for its ambitious attempt.

SpaceX launched the rocket thousands of feet in the air during during a seven-minute test flight, but the rocket exploded during landing.

Still, the audacious test garnered praise from Musk's space rival, Bezos.

"Anybody who knows how hard this stuff is is impressed by today's Starship test," Bezos wrote in an Instagram post, accompanied by a low-resolution photo of the rocket. "Big congrats to the whole @SpaceX team. I'm confident they'll be back at it soon."

But any goodwill Bezos felt toward SpaceX was short-lived. In April 2021, NASA announced that SpaceX was the sole recipient of the contract for landing humans on the moon, worth $2.9 billion. The decision infuriated Blue Origin, which immediately challenged NASA's decision.

Blue Origin filed a 50-page protest with the Government Accountability Office, challenging NASA's decision as "flawed," according to The New York Times.

NASA had initially said it would award the contract to two companies, but budget concerns and a lack of Congressional funding meant it could only choose SpaceX.

Blue Origin told CNBC that NASA's decision was unfair because it had "moved the goalposts at the last minute" and had negotiated a proposed price with SpaceX, but not with Blue Origin.

Musk responded to Blue Origin's protests with a thinly veiled jab about male anatomy.

In response to The New York Times report, Musk tweeted: "Can't get it up (to orbit) lol."

He followed the tweet up with the photoshopped image of Blue Origin's lunar lander that changed the name from "Blue Moon" to "Blue Balls."

In June 2021, Bezos announced that he'd be making a major commitment to the future of Blue Origin by heading to space aboard one of his own spacecrafts.

That year, Bezos and his younger brother, Mark, took an 11-minute flight aboard Blue Origin's New Shepard spacecraft. The flight sent the crew 62 miles above the Earth's surface before landing safely back on the ground.

"Ever since I was 5 years old, I've dreamed of traveling to space," Bezos said in a video announcing the trip posted on Instagram. "I want to go on this flight because it's a thing I wanted to do all my life. It's an adventure — it's a big deal for me."

The short trip was Blue Origin's first human flight — SpaceX launched its first human passengers into orbit in May 2020. While it's likely Musk could have gone to space himself by now, the trip would carry more risk for his business dealings, given that he's also the CEO of a public company. Bezos, on the other hand, stepped down as CEO of Amazon two weeks before his visit to space.

It seems that in Musk's view, Bezos' flight wasn't all that impressive: he poked fun at the fact that the voyage was sub-orbital on Twitter.

Days before Bezos' trip to space, a Twitter user created a meme that shows Bezos and Musk talking about the flight using a popular meme format where their faces superimposed onto Anakin Skywalker and Padme Amidala from "Star Wars: Episode II — Attack Of The Clones."

The meme was making fun of the fact that Bezos' flight was to the edge of space rather than blasting him into orbit.

In response, Musk tweeted: "haha."

Musk eventually congratulated Bezos on a successful flight and has praised Blue Origin's subsequent human spaceflights.

And after Blue Origin criticized NASA once again for awarding the lucrative contract to SpaceX, Musk mocked Blue Origin's lunar lander.

Musk tweeted a photo of Blue Origin's lunar lander concept, which showed the middle portion looking deflated.

"Somehow, this wasn't convincing…" he wrote.

In August 2021, Blue Origin filed a lawsuit against NASA over the $2.9 billion contract, which resulted in NASA agreeing to temporarily pause work on the contract.

The suit, which was filed in the US Court of Federal Claims, challenges "NASA's unlawful and improper evaluation of proposals" that were submitted for its Human Landing System Program.

NASA agreed to press pause on the contract until November 1, but by early November, the court ruled against Blue Origin.

Also in 2021, an Amazon subsidiary filed a protest with the Federal Communications Commission over SpaceX's plans for more Starlink internet satellites, prompting a fiery response from Musk.

Amazon's own satellite company, Project Kuiper, filed the protest letter, telling the FCC that SpaceX broke its rules. The letter is not a lawsuit.

But it sparked a response from Musk, who made a jab at Bezos on Twitter.

"Turns out Besos retired in order to pursue a full-time job filing lawsuits against SpaceX …" Musk tweeted, misspelling Bezos' name.

In recent years, there's been a new component to the Musk-Bezos rivalry: they've been trading places as the world's richest man.

Bezos had been the richest person in the world since about 2017, but in January 2021, Musk overtook him to claim the top spot after Tesla stock hit all-time highs, nudging Musk's wealth skyward — first, past Bill Gates and Mark Zuckerberg, and then, past Bezos.

The two moguls swapped spots a few times over the course of the year and for a while, Musk was firmly No. 1.

He used that as another opportunity to poke fun at Bezos, tweeting silver medal emojis at Bezos and taunting the Amazon founder by telling Forbes he planned to send "a giant statue of the digit '2' to Jeffrey B., along with a silver medal."

They've since swapped spots a few more times, but today Musk is on top again, with $241 billion to Bezos' $204 billion, according to the Bloomberg Billionaires Index.

Musk said in 2021 that Bezos "does take himself a bit too seriously" and that he's intentionally trying to provoke the former Amazon CEO.

"In some ways, I'm trying to goad him into spending more time at Blue Origin so they make more progress," Musk said in an interview with the Financial Times.

Musk added that while Bezos has a "reasonably good engineering aptitude," he doesn't seem to be spending a lot of "mental energy" getting into the details.

"As a friend of mine says, he should spend more time at Blue Origin and less time in the hot tub," Musk said.

Bezos has poked at Musk's business ties to China, specifically in the context of his $44 billion deal to buy Twitter.

"Did the Chinese government just gain a bit of leverage over the town square?" Bezos tweeted in April 2022, the month Musk announced his plans to buy the social media site.

Bezos went on to say: "My own answer to this question is probably not. The more likely outcome in this regard is complexity in China for Tesla, rather than censorship at Twitter."

"But we'll see. Musk is extremely good at navigating this kind of complexity," he concluded.

Bezos has said he thinks he and Musk are "very like-minded."

"Well, I don't really know Elon very well," Bezos told podcaster Lex Fridman in 2023. "I know his public persona but I also know you can't know anyone by their public persona — it's impossible. You may think you do, but I guarantee you don't."

He went on to say Musk "must be a very capable leader."

"I agree with you and I think with a lot of these endeavors we're very like-minded," he added. "So I think, I'm not saying we're identical, but we're very like-minded so I love that idea." 

In 2024, their rocket rivalry got new fuel.

Blue Origin filed concerns to the Federal Aviation Administration about SpaceX's Starship rocket launches, asking that Starship's launch operations be potentially limited due to their having a "greater environmental impact than any other launch system" at Kennedy Space Center.

Musk responded to the complaint on Twitter, writing, "Sue Origin."

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