The Ghastly Trail Jetpack customisation was added to No Man's Sky as part of Expedition 4. It adds a green gas cloud effect to the Jetpack, and also spews out small worms and other biological matter. Here's how to get your hands on it.
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No Man's Sky: How to Get the Ghastly Jetpack Trail
Currently, the only way to get the Ghastly Trail in No Man's Sky is to complete Phase Two of Expedition 4: Emergence. To start the Expedition, select a new save slot and choose 'Expedition' from the starting options. You will be taken to a new system, starting with a handful of resources. It's a self-contained mission that spans multiple phases. Once you have completed Phase Two, you can head into the Expeditions menu to claim it. This will be next to your Options tab in the main menu.
How to Redeem the Ghastly Jetpack trail On Another Save
If you'd like to redeem the Ghastly Trail on another save file, then make sure you've completed Phase Two. Don't redeem the reward yet on your Expedition save. Load up your main save and head to the Anomaly. Speak to the Quicksilver merchant, and select Expedition Rewards. The Ghastly Jetpack can be redeemed on this screen.
How to Equip the Ghastly Trail
Once you have redeemed the Ghastly Trail, you will need to equip it. Head to an appearance modifier to do so. There's one on every space station, next to the Exosuit Upgrades Merchant. Jetpack Trails are kept under the 'Backpack' appearance options. Make sure to save your changes and you'll be able to trigger the Ghastly Trail when using your jetpack.