Spoilers for Halo: Envoy

In the new novel, it's been revealed that Gray Team had been responsible for reducing Glyke, a Sangheili colony world, to bits of debris using a NOVA bomb during Operation: SUNSPEAR

“Spartan, you don’t have authorization to—”

“No,” Adriana continued. “We do. What happened next was that an emergency order came through. Condition Endgame, it was called. It meant the Covenant had appeared in the Sol system, that the Cole Protocol had finally failed to protect us after all these years. We couldn’t reach Sanghelios itself—it was too well protected. But ONI made a breakthrough with intel in 2552. They learned the locations of a number of enemy strongholds, so we were immediately deployed to the Sangheili world of Glyke.

“We brought a NOVA bomb with us.” Now Melody took a step back. Glyke, she realized. For Rojka ‘Kasaan, it was all about Glyke. She had to suppress any emotion, any judgment, as she continued to listen. All her Diplomatic Corps training was kicking in. Glyke had been utterly destroyed in the wake of the war, only days after the UEG established a tentative peace treaty with the Arbiter. Billions of Sangheili, civilian and military, gone in a blink of an eye. Some had blamed the Jiralhanae, others had more elaborate theories of clan disputes on the planet itself—but now it appeared to have been ONI. And Gray Team was responsible for carrying it out.

“We voted,” Jai said. “We took the intel we had and we made a decision. We followed protocol. With Earth backed into a corner, we were confident humanity was about to be destroyed. So. What would you do if you knew everything you’d spent your life fighting for was about to be erased?”

This was it. This was the darkness that had settled over Gray Team and dogged them since Melody had met them, threatening to rip them apart. Again, Melody would have to draw on her Diplomatic Corps training. She needed to help them come back from the edge of whatever precipice they now found themselves on. Melody bit her lip. ”You had to follow orders. You retaliated for Earth. You hoped the slaughter would stop our extinction.”

“And now you know why your Sangheili friend was willing to risk everything and come hunt for us,” Jai said. “Those same Sangheili glassed entire human worlds!” Melody said. “Killed billions themselves! It was war, Jai. And we were all in a war for survival. Humanity was on the brink of extinction.”

“Our deep-space comms equipment was damaged,” Mike said. “We never got the orders to cease or withdraw. We didn’t know the war was over, never mind that there was peace. So we followed through with SUNSPEAR and lit the nuke.”

“We got out before the destruction,” Adriana said. “We watched from orbit in our escape shuttle. An entire planet ripped apart. Every living thing on it killed in a moment. Zero probability for survival, no matter what kind of lifeform. By the end, it was just empty debris where a planet once had been. And that’s when I actually started wondering—where does this armor stop and flesh begin?”

This is the first mention of the NOVA since First Strike (2003) and Ghosts of Onyx (2006), which was technically the same bomb anyway. This tells us two things:

• The NOVA bomb used by Vice Admiral Whitcomb was not the only one in the UNSC's possession, and they may have several more; considering it's been six years since they were first used, it's very likely the UNSC has developed more

• The NOVA has now demonstrated planet busting capabilities (previously it busted a moon and lifewiped a planet from orbit); note that Covenant held worlds, particularly Sangheili and Jiralhanae, are usually more massive than Earth (Sangheilios has 1.375G, Rahnelo slightly less than that, etc.) suggesting the Glyke had similar or greater GBE than Earth

Make of that what you will.