[masraum]

Originally Posted by Brando

I have heard the by-product of absorbing 99.995% of all light directed at an object coated with that, is the amount of heat it retains.

What the vantablack folks say

The CNT array is patterned and spaced to allow photons to enter. Most of the light, or radiation arriving at the surface enters the space between the CNTs, and is repeatedly reflected between tubes until it is absorbed and converted to heat. This heat (largely undetectable in most applications) is conducted to the substrate and dissipated. The Vantablack array is very largly free-space; the volume of CNTs only makes up about 0.05% of the coating. Consequently, only a miniscule proportion of the incident radiation is able to hit the tip of a CNT, explaining why such a small amount is reflected back to the observer.

CNTs are hollow structures with one-or-more walls formed from atom-thick sheets of carbon. Each nanotube is around one fiftieth of one millionth (!) of a metre in diameter, making it an appropriately-sized building-block for engineering structures that exhibit low-reflectivity and high-emissivity across a wide-range of frequencies. In addition to incredible light absorption, the CNT array also has many other highly attractive properties:

The high proportion of free-space within Vantablack (>99%) makes it extremely light. The height of a Vantablack coating is typically around 20 to 30 microns. One square metre of coating weighs around 2.5g (for a typical coating - growth parameters are varied to suit the application).
The CNTs have an exceptionally high modulus of elasticity and will flex and bend, making them very robust in environments subject to extreme shock and vibration.
The strength of the CNTs' bond to the substrate is high, making it difficult to remove the forest through thermal cycling, shock or vibration, even though the coating's characteristic's can be compromised through direct abrasion.
Outgassing is virtually zero.
The structures are incredibly tolerant to thermal cycling as a result of the intrinsic properties of the CNT building blocks, and their ability to adhere to a substrate.

This is an old carbon nanotube black coating from 8 years ago developed by NASA

https://phys.org/news/2011-11-super-black-material-absorbs-multiple-wavelength.html

This close-up view (only about 0.03 inches wide) shows the internal structure of a carbon-nanotube coating that absorbs about 99 percent of the ultraviolet, visible, infrared, and far-infrared light that strikes it. A section of the coating, which was grown on smooth silicon, was purposely removed to show the tubes' vertical alignment. (Credit: Stephanie Getty, NASA Goddard)

This high-magnification image, taken with an electron microscope, shows an even closer view of the hollow carbon nanotubes. A coating made of this material is seen as black by the human eye and sensitive detectors because the tiny gaps between the tubes collect and trap light, preventing reflection. (Credit: Stephanie Getty, NASA Goddard)

(PhysOrg.com) -- NASA engineers have produced a material that absorbs on average more than 99 percent of the ultraviolet, visible, infrared, and far-infrared light that hits it -- a development that promises to open new frontiers in space technology.

The team of engineers at NASA's Goddard Space Flight Center in Greenbelt, Md., reported their findings recently at the SPIE Optics and Photonics conference, the largest interdisciplinary technical meeting in this discipline. The team has since reconfirmed the material's absorption capabilities in additional testing, said John Hagopian, who is leading the effort involving 10 Goddard technologists.

"The reflectance tests showed that our team had extended by 50 times the range of the material’s absorption capabilities. Though other researchers are reporting near-perfect absorption levels mainly in the ultraviolet and visible, our material is darn near perfect across multiple wavelength bands, from the ultraviolet to the far infrared," Hagopian said. "No one else has achieved this milestone yet."

The nanotech-based coating is a thin layer of multi-walled carbon nanotubes, tiny hollow tubes made of pure carbon about 10,000 times thinner than a strand of human hair. They are positioned vertically on various substrate materials much like a shag rug. The team has grown the nanotubes on silicon, silicon nitride, titanium, and stainless steel, materials commonly used in space-based scientific instruments. (To grow carbon nanotubes, Goddard technologist Stephanie Getty applies a catalyst layer of iron to an underlayer on silicon, titanium, and other materials. She then heats the material in an oven to about 1,382 degrees Fahrenheit. While heating, the material is bathed in carbon-containing feedstock gas.)