Ilmenite Ore is an ore that can be found only on Asteroids. When mined it will drop a Titanium Shard and an Iron Shard which can be smelted and then used as a crafting ingredient for things such as tools, machines, and armor.
It may also be mined using Silk Touch to give you a Ilmenite Ore block which can be smelted directly into a Titanium Ingot.
With Galacticraft 3, the Ilmenite Ore replaced the deprecated Titanium Ore, which was mined from lower levels of the map and found near diamond levels, with similar rarity. It was required to make Silicon.
In real life, Ilmenite is the main type of titanium ore used to make titanium, and it also has a high iron content. It is a common mineral on the Moon: Further Reading. We can only speculate that it may also be common on asteroids.