Rockoons are a hybrid of a rocket and a balloon that split the process of reaching orbit into two parts: First, the balloon rises into the upper atmosphere, then the rocket adds the needed momentum to launch something into orbit or to another planet.
In March of 2017, Zero2Infinity, launched a balloon to about 82,000 feet. At that point, they were above 99 percent of the atmosphere, so they ignited a small solid rocket motor. Another company, ACRA, was using a rockoon-type launch system for its entry into the Google Lunar X Prize competition.
But if using a balloon to lift a rocket seems like old technology, it's because it is... The concept originated in March of 1949 for better, high-altitude atmospheric science.
Rockoons are actually a simple and ingenious approach to spaceflight. Instead of fighting against gravity, they turn gravity into an advantage.
Encyclopedia Astronautica: Rockoon
Zero2Infinity: Bloostar - How We Do It
NASA: Development of the First Sounding Rockets