Red Dust was an Interplanetary Strategic Alliance nuclear weapon, developed by Evelyn Batton on Vekta. When the Helghast invaded Vekta, General Metrac captured the weapon. During the retreat of the Helghast from Vekta, the weapon was taken back with them to Helghan. However, the Helghast were not able to secure the codes needed to detonate the device.
During the ISA invasion of Helghan, Batton mentioned that the capture of the weapon was important. Colonel Mael Radec attempted to extract the code from Batton following her capture in the wastes, but was unable to do so. An attack on the ISA cruisers hovering over Pyrrhus followed, where Radec was able to secure the codes during the assault on the New Sun. Shortly afterwards, the weapon was detonated in the city destroying all of the ISA and Helghast forces fighting there.
Interestingly, while the weapon does, in fact, generate radioactive fallout (as demonstrated by the existence of the irradiated Petrusite) none of the ISA troops fighting in and around the ruins of Pyrrhus seem to experience any radiation sickness or any other complications from the device's detonation. With no mentions made of any anti-radiation treatments or equipment, this is either a story error or Red Dust is a nuclear Fusion bomb which unlike a fission weapon has relatively minor radioactive trace elements and the nuclear weapons used after Red Dust are standard Fission nuclear weapons.