After Samus gets missiles, her path is blocked by a giant centipede called Deorem.

Defeat Deorem

As with the first encounter, Deorem will block the left and right side of the room, then its head will come down from the ceiling. Hold R and press B to shoot missiles at its eye when the eye is open. When it closes its eye and clicks its pincers, it is about to come down toward you, so try to avoid it. When a side of its body shakes, shoot the spikes that come off of it to get health and missile refills.

After you hit Deorem's eye several times, it will be defeated, and it will leave behind the Charge Beam. Collect it. Now you can hold B to charge and release B to fire a charged shot. Also, while you are charging an attack, you will generate an energy field around you when you jump, which can damage enemies that you jump through.

Now you can continue on to Brinstar Part 2.