A bit of a Twitter fight is happening between SpaceX founder Elon Musk and Blue Origin founder Jeff Bezos, two billionaires whose private spaceflight companies are leading the future of reusable-rocket technology.

On Tuesday, Blue Origin hit a major milestone by successfully launching and landing its New Shepard vehicle for the first time. Bezos was very proud, tweeting: 


Elon Musk quickly retaliated with the following tweet:

Musk's tweet is a bit of a low blow, but it gets at the core of the main differences between SpaceX and Blue Origin, and here's why:

Blue Origin's first successful landing came after the company had launched its unmanned vehicle 62 miles above Earth's surface.

The flight's journey is described as a suborbital spaceflight, meaning the spacecraft did not travel high or fast enough to achieve an orbit around the Earth. And that's what Blue Origin is after: suborbital spaceflight that will ferry paying customers to space and back so they can experience a brief 10 minutes of weightlessness.

If Blue Origin is going to imply that it is the first to achieve suborbital spaceflight with a reusable rocket, however, then Musk will have something to say about it. SpaceX beat Blue Origin by a few years, which explains Musk's follow-up tweet:

In his two tweets, Musk refers to one of SpaceX's major accomplishments, the Grasshopper rocket booster, which was the company's first successful reusable rocket, tested throughout 2012 and most of 2013.

The Grasshopper made eight flights and landings before it was retired in late 2013. You can watch all of the test flights and landings on YouTube.

The highest Grasshopper ever flew, however, was about half a mile on October 7, 2013, its last flight. Not very high compared with the 62 miles Blue Origin just achieved.

Musk's tweet, however, makes an important point: Suborbital flights are insignificant for his ultimate goal, which is to send and return astronauts to Mars. To do that, you must have a rocket that can achieve orbital spaceflight.

And Blue Origin's New Shepard vehicle is not powerful enough to make it -; yet. Bezos announced last September that his company was in the preliminary stages of designing a rocket that could achieve orbital spaceflight.

To Bezos' credit, New Shepard now holds the record as the suborbital reusable rocket that has traveled the highest and vertically landed in one piece. It's a significant achievement for reusable-rocket technology.

Disclosure: Jeff Bezos is an investor in Business Insider through his personal investment company Bezos Expeditions.

This story first appeared on Business Insider