While companies like Amazon and Rackspace mightily tout the power of the public cloud, some large enterprises still aren’t sold. These enterprises like the option of an on-premise private cloud that gives them a bit more control and security.

Palo Alto, Calif.-based Nebula, which has raised $25 million in new funding today, takes serious advantage of these enterprise-level concerns and gives big companies tools to enable less-expensive private clouds. As we wrote about previously, the company takes advantage of Openstack technology and creates hardware appliances that make building and managing low-cost data centers a reality.

Nebula’s team is led by CEO Chris Kemp, who was the former CTO of NASA. Many other folks in the company come from places like NASA, Amazon, Google, and Microsoft. If any team can make it easier for enterprises to improve the data-center building process and make data centers more functional, this might just be the group.

“We’re democratizing what Google and Facebook have done with their data centers,” Kemp told me. “You can use your favorite server vendor and plug those servers into our boxes. You plug in and 15 minutes later, you have Amazon running on premise.”

So what’s the company’s biggest threat? Public cloud and virtualization powerhouses that are already dominating the market. “Amazon and VMware are the two that keep me up at night,” Kemp said.

Nebula is still in beta and has a “couple dozen” customers. Kemp said he hopes to launch with general availability before mid-2013.

The new funding round was led by Comcast Ventures, with participation by Highland Capital Partners, Kleiner Perkins Caufield & Byers, Innovation Endeavors, Andy Bechtolsheim, David Cheriton, Ram Shriram, Harris Barton, William Hearst III, Scott McNealy, and Maynard Webb. Silicon Valley Bank is additionally providing Nebula with debt and credit facilities.

Nebula also previously raised a first round of funding in July 2011, but it has not revealed the specific amount. Kemp told me it was more than $8 million (after I guessed that), but less than this new $25 million round. The company has nearly 60 full-time employees, almost all of them engineers.

Star formation photo via NASA/Flickr