This guide has spoilers. None of which will be hidden. You have been warned.

Blowing up too many cities will give you a few hints, until you are threatened of having the health bar reduced to a pixel. The threats are not empty...

To do this, blow up about 10 or more buildings.

You're health will be greatly reduced to approximatly 5%, but after either quitting the game, or coming back later (exact time unknown) the health bar will go back up to 5. Destroying more cities will put it back down once more.

To complete this achievement you must kill your character, an immortal being. The only way to do so is to fly into a black hole. You are likely to find a black hole while going through space and finding the signs.

1. Acquire Gigaton Blast
   
2. Find a city
   
3. Hold Gigaton Blast for Approximately 1 minute or more.
   
4. Release: fire it downwards at the city

You know it worked if you are pushed into orbit and see the earth falling apart.

引用自 ๖Vɪɴʏʟ™ ✓：

you only need to hold the blast for 30 seconds!

You'll be doing flying for about 10 to 20 minutes until you get the achievement. You'll know its working if you keep flying for approximately 5 minutes, and by then you get some hints about returning. Eventually it will say that you will not get a trophy for it(but you do)

引用自 Dasorine：

for the explorer acheivement, once unlocked just reload your game rather than fly back

引用自 Will0thewhisp ：

for the explorer achievment if you dash out from a planet and hold it it'll blast you out at warp speeds. I got this acheivment after like 30 seconds easy

Once you get the achievement, you should fly back. We tested going further for about 30 minutes, then spent the next 2 hours trying to find a black hole so we could respawn. Head back as soon as you get the achievement for your own time sake.