The moment you finish Star Chart (do every mission location once) you will unlock two things - Steel Path and Arbitrations.
Steel Path is optional elite variant of Star Chart with 5 daily missions called Incursions. Steel Path Earth may be really extreme experience when you go there for first time. Some weapons do need specific builds and specific mods (like Galvanized) to be able to go through them. I suggest that you should try to do daily missions (Incursions) and SP Void Fissures at first, instead of doing full clearing of SP from Earth, as the Grineer missions are pretty hard missions at first. Daily SP Incursions will give you 5 Steel Essence per Incursion so it should help you progress (plus they also unlock you those nodes). You will be able to change Star Chart between Steel Path and Standard on the right side of the Star Chart (there will be a button for that).
Arbitrations are easier than SP, they are one infinite mission at the time (Defence, Survival etc), that have slightly higher level (70-100+) and unique drops and resource. Vitus Essence is somewhat similar to Steel Essence, as it is a credits for reward shop in Arbiters of Hexis, from where you can get very powerful mods, cosmetics and Grendel missions keys. I would suggest trying to focus on Arbitrations at first (while also trying to finish SP Incursions) and try to effectively strengthen yourself.
