Newly released satellite images show how Katrina built up force over the sea before lashing the Louisiana coast.
Warm waters in the Gulf of Mexico caused Katrina to strengthen into a category five hurricane.
When she hit land, she slowed down, releasing huge amounts of rain. It was this, rather than her winds, that caused much of the damage.
The US space agency and the European Space Agency are monitoring the situation to help in disaster relief.
On Sunday, Esa's Envisat satellite captured a unique view of Hurricane Katrina in the Gulf of Mexico.
An optical image shows characteristic swirling cloud patterns around the central eye, while a radar observation shows Katrina's wind fields rippling the ocean.
A hurricane needs ocean temperatures of 26.5C (80F) or warmer to strengthen.
Data obtained by Nasa's Aqua satellite shows that conditions in the Gulf of Mexico were ideal.
After crossing the southern tip of Florida on Thursday, Katrina intensified quickly. By Friday evening local time, Katrina was a Category two storm on the Saffir-Simpson scale.
And as she moved northwards in the Gulf, her status was raised to Category three, with maximum sustained winds reported at 100 knots (185km/h;115 mph).
But during the early morning hours of Sunday (local time), Katrina's central pressure continued to drop, and the storm was classed as a powerful Category five. By then, sustained winds were 152 knots (282km/h; 175mph).
Scientists predicted that when the storm hit land, the greatest danger would be from rain rather than winds.
A joint US and Japanese satellite shows how the rainfall developed.
The Tropical Rainfall Measuring Mission (TRMM) satellite is able to look underneath a storm's clouds to study rainfall.
It shows the underlying rain intensity, with at least 0.64cm (0.25 inches) of rain per hour in the blue areas.
Katrina initially made landfall south of Buras at 0610 local time on Sunday.