Let the plants grow....

To terraform the habitat for comfortable human habitation, we need some “new ground” for plants. Of course, plants are necessary for agriculture, oxygen production, and a good quality of life.

Bulk of raw materials for the habitat's "new ground" is likely readily available from the NEO; however, I anticipate that each habitat will have to import a few tons of Terran topsoil to obtain necessary traces of life available only Earth which has spent countless millennia growing life as we Earthlings know it.

During this phase, the habitat should have achieved enough angular momentum to simulate gravity along inside of outer hull.

While angular momentum normally conserves, the habitat will probably need to adjust its spin because shape and size will likely change during subsequent construction phases.

-- Import Terran topsoil

--Trees to produce oxygen

--Import Terran flora/fauna

--Centrifugal force against inside of outer surface (hull) to create g-force (simulate Terran gravity)

JonLomberg.com Giclee Prints This painting was commissioned for the book COMET by Carl Sagan and Ann Druyan, ... These trees would use sunlight to grow to large size, and provide oxygen and shelter.....

Freeman Dyson quote

extracted from great book, Comet,Carl Sagan and Ann Druyan. Sidebar on page 353

Forested Comets

"I propose ... the Galaxy as an abode of life.Countless millions of comets are out there, amply supplied with water, cabon, and nitrogen, the basic constitutients of living cells. ... they contain all the common elements necessary to our existence. They lack only two essential requirements for human settlements: warmth and air;and now, biological engineering will come to our rescue. We shall learn to grow trees on comets. From a comet of a 10 mile diameter, trees can grow out for hundreds of miles, collecting the energy for sunlight from an area thousands as times as large as the comet, itself. ... the comet will look like a small potato sprouting an immense growth of steam and foiliage. When man comes to live on these comets, he will find himself returning to the arboreal existence of his ancestors. We shall bring to the comets not only trees but a great variety of other flora and fauna to create for outselves an environment as beautiful as ever seen ... on Earth Perhaps we shall teach our plants to make seeds which will sail out across the ocean of space to propogate life upon comets still unvisited by man."

Freeman Dyson, "The World, the Flesh, and the Devil," Third J.D. Bernal Lecture, delivered at Birbeck College, London,.

Reprinted in Communication with Extraterrestrial Intelligence (CETI),C. Sagan, Ed. MIT Press, Cambridge , Massachusetts, 1973.

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Terraformation (Following material extracted and edited from associated web page. Credit for original content go to original authors; misrakes are mine.)

Plants will need sources of water: rainfall will come from airborne water vapor and irrigation will come from habitat's water reservoirs. Water can be fortified with minerals and imported from other NEOs and even Terran topsoil to the growing patches of flora. We will also need to consider storing massive reservoirs of water in the habitat's "underground" which would be in the habitat's cylindrical outside hull. These artificial lakes will aid our seedlings. The plants will need to include a few quick growing and rapidly spreading species for the fastest possible ground cover. Certain grasses, vines or kudzu could work well. When we have a large amount of vegatation, the carbon dioxide levels will be lowered and replaced with oxygen. It is also important to introduce slow-growing trees for eventual forests.
Note-1: "Tree of life" mentioned several times in Bible.
Note-2: Trees are "lungs" of our planet, Earth.

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Paraterraforming
Also known as the "worldhouse" concept, or domes in smaller versions, paraterraforming constructs a habitable enclosure on a planet which eventually grows to encompass most of the planet's usable area. The enclosure would be a transparent roof held one or more kilometres above the surface, pressurized with a breathable atmosphere, and anchored with tension towers and cables at regular intervals. Proponents claim worldhouses can be constructed with technology known since the 1960s.
ADVANTAGES:
--Immediate payback to investors; the worldhouse starts out small in area (a domed city for example), but those areas provide habitable space from the start.
--The paraterraforming approach also allows for a modular approach that can be tailored to the needs of the planet's population, growing only as fast and only in those areas where it is required.
--Reduces the amount of atmosphere that one would need to add to planets like Mars in order to provide Earthlike atmospheric pressures. By using a solid envelope in this manner, even bodies which would otherwise be unable to retain an atmosphere at all (such as asteroids) could be given a habitable environment.
--Environment under an artificial worldhouse roof would also likely be more amenable to artificial manipulation.
DISADVANTAGES:
--Requires a great deal of construction and maintenance activity, the cost of which could be ameliorated to some degree through the use of automated manufacturing and repair mechanisms.
--More susceptible to catastrophic failure in the event of a major breach, though this risk can likely be reduced by compartmentalization and other active safety precautions. Meteor strikes are a particular concern in the absence of any external atmosphere in which they would burn up before reaching the surface.

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Finally, following links contain extensive collection of relevant readings

Planetary Engineering Bibliography

Planetary Terraforming Bibliography