Fortnite Alien Artefacts Week 4: How to get alien artefacts and what to spend alien artefacts on in Fortnite
Build-your-own alien!
Alien Artefacts are a new resource currency in Fortnite Season 7, which can be obtained within matches.
During each match in Season 7, you'll be able to earn alien artefacts in certain locations across the island or inside cosmic chests. Once you have enough, you'll be able to spend them into a new customizable alien character called Kymera.
It's worth keeping in mind that, while during the start of the season Alien Artefacts became available on Tuesdays, they're now appearing on Thursdays alongside Epic Quests.
This page will explain how to earn and spend alien artefacts in Fortnite Chapter 2 Season 7.
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Story Trailer For Fortnite Chapter 2 - Season 7
How to get Alien Artefacts
There are two primary ways to obtain alien artefacts in Fortnite:

If you see a floating rhombus-shaped object during your expeditions, make sure to make a quick stop since it's a sign of a cosmic chest. In order to open it, you'll have to use your pickaxe to smack all hit points. Continue doing so and it will break after a while, playing an animation before opening the chest.
Alien artefacts, much as gold bars, aren't a guaranteed drop, but it's always worth keeping an eye for cosmic chests just in case.

For cosmic chests, it's worth noting some conditions apply. In a discussion started by SuspectIsDown on reddit, it appears there could be a weekly cap of 15 Alien Fragments.
In addition, the purple Alien Artefacts you can find all give 4 artefacts each - but there seems to be a golden variant that grants 7 artefacts instead, which could appear further into the season (thanks to @iFireMonkey on twitter for the datamine!).
As well as cosmic chests, there are Alien Artefact locations which spawn out in the open - and are guaranteed ways of getting your hands on some each week.
Alien Artefact Week 1 locations in Fortnite
As for locations, you can find artefacts across the map in battle royale. During the first week the spots are as follows:

Helipad tower just west of Catty Corner, right below the chest at the top.
In the destroyed Logjam, found southwest of Weeping Woods, you'll find artefacts inside the barn on the southern end (if you're looking at the map, it's the structure in the lower left corner).
There's one right in the centre of The Aftermath, located in the middle of the map (beware of enemies when you go pick this one up!).
In the staircase of the Spire Tower located northeast from Believer Beach.
Frontyard of the farm located just north of Corny Complex.
It's worth keeping in mind that each time you pick up Alien Fragments you will receive a total of four fragments per batch.
Alien Artefact Week 2 locations in Fortnite
The start of a new week brings new Alien Artefact locations, and you can find them on the spots below:

Located southwest of Lazy Lake on ground level, under the external roof of the house found on the left island.
Next to the smaller pool northeast of Retail Row, at the side of an antenna.
There's another fragment under the bridge south of Corny Complex, located on the northern entrance.
One can be found northeast of Pleasant Park just outside the house on top of the cliffs, which overlooks the named location.
The last one is located on the big satellite found in between Coral Castle and Believer Beach.
Alien Artefact Week 3 locations in Fortnite
A new week means brand new Alien Artefact to hunt down. This time around, there are four of them instead of five, and you can find them on the spots below:

Located on Coral Castle, just on the second floor of the biggest castle in the centre. You can use your pickaxe with one of the columns inside and quickly build a stair to grab it.
Within Craggy Cliffs, look for one of the shops close to the east - it's placed exactly below the 'S' in 'Cliffs' if you're looking at the map. In the storage with the two boats hanging from the ceiling, you'll see the fragment on top of one of them.
There's another fragment on Boney Burbs, found on the second floor of the first house you see to the left if you enter from the western road. If you're looking at the map, it's right below the 'BO' of 'Boney'.
The last one is located on Slurpy Swamp. For this, if you're entering from the northern road, get inside the first big building on the east - one of the entrances has two garage doors, one of them being slightly opened, as well as some slurp barrels just outside. Once you enter, climb the stairs and you'll see the fragment floating in the opposite corner of the room. Break anything nearby to make a ladder and grab it.
And that's all until next week!
Alien Artefact Week 4 locations in Fortnite
New week, new batch of Alien Artefacts. We are back to five after last week's change, and you can find them on the spots below:

Located on Steamy Stacks, on top of a few pipes next to the building that it's right in the centre - it's signalised with the number '4', and the pipes have a massive light next to them. If you see the revival truck, then it's just further down the street.
Just outside of Dirty Docks, located on the northeastern side, you'll find another artefact waiting to be picked up. If you're lost, just head towards the location from the shore, and you'll spot it in no time.
There's another fragment on Holly Hedges, found next to the house that is right at the centre. The backgarden is filled with plants and other materials, and the artefact is below one of the shelves close to the house.
Onto Weeping Woods now. There are two houses on the eastern side of the location, and if you look at the map, you're gonna want to enter the house that's on the right, facing the opposite side of the IO Base east. The artefact is close to the rooftop as you go upstairs.
The last one is located on Lazy Row. We know this is a rather packed location, but luckily the spot is far from the action. Head to the northern section and enter the shack on the left to find the artefact.
That's all until next week!
What can you spend Alien Artefacts on in Fortnite?
The new resource can be spent on customizing Kymera, a new battle pass exclusive alien character that comes with an array or purchasable parts.
All of these cosmetic options can be obtained solely with alien artefacts, and the cost vary from 2 to around 17 for the most expensive ones. You can change both the top and the bottom of Kymera's head, eyes, skin colour and so on.

As expected you don't really gain any advantage in the game, as it's for purely cosmetic purposes. But it makes for a great opportunity to bring back your inner Sims-fan self and tweak the outfit to your liking.