Anthonares

While working on my new condominium (well not exactly while I was working, in the evenings more like), I enjoyed some great reading this last week, and some really terrific news from Enceladus. I’ll be writing a published research synopsis on the Science paper later today (I’m sure you’ve all been waiting to hear about how the water geysers were detected and confirmed). Also, there was some exciting news in the alt.space community with the announcement of the Dragon manned capsule by SpaceX.

As on most Mondays, today’s fantastic skeptical examination of the Aviation Week article last week about the supposed Blackstar hypersonic spy aircraft/orbital spaceplane. Dwayne Day methodically exposes the uninformed speculation that sent so much of the apparently overly-credulous space blog community into a tizzy last week.

In that The Space Review article, Day mentions that the optical adaptive optics/laser guide star system rumored to have been installed on the Blackstar would not work from above. Let me go over just why it would not work, so you can better see the difference between science and psuedoscience in this case.

Proposed System on the Blackstar

From the Aviation Week article “Spaceplane Shelved?” by William Scott:

the spaceplane is capable of carrying an advanced imaging suite that features 1-meter-aperture adaptive optics with an integral sodium-ion-sensing laser. By compensating in real-time for atmospheric turbulence-caused aberrations sensed by the laser, the system is capable of acquiring very detailed images of ground targets or in-space objects, according to industry officials familiar with the package.

Reality

Ground-based telescopes do have adaptive optics systems with “integral sodium-ion” laser systems. Some of the premier telescopes in the world are equipped with adaptive optics, including the Very Large Telescopes in Chile, or the Keck Telescopes in Hawaii. Adaptive optics (AO) means that the shape of the telescope’s mirror is deformed very slightly in order to cancel out distortion due to atmospheric turbulence. The difficulty with this process is that turbulence is inherently chaotic and can only be canceled out if it can somehow be sensed directly.

AO systems sense atmospheric turbulence using so-called guide stars. Guide stars are typically very bright stars, or laser-induced flourescence in the upper atmosphere. By measuring the distortion of the point of light from the guide star very accurately, the mirror of the telescope is then adjusted by hundreds of heating elements that expand portions of the mirror in order to cancel out atmospheric turbulence.

The difficulties inherent in AO are that the atmospheric distortion must be continually corrected, so the time delay between sensing the distortion and correcting is typically on the order of hundreths of a second. Also, the distortion is wavelength-dependent. Infrared wavelengths, those that are longer than optical, are distorted less than optical wavelengths, thus infrared sensors are much more commonly corrected with AO than optical systems.

So, the Blackstar system suffers from two very serious engineering difficulties. If it travels near to Mach 5, say (well below orbital speeds), snapping its AO-corrected photographs, this means that in the space of 1/100 of a second, the plane will have traveled over 1.3 km. Given that significant atmospheric turbulence occurs on the scale of tens of meters or less, this would require an improvement of several orders of magnitude over current state-of-the-art. Also, optical AO is extremely tricky. The electrical systems must be very fast, and the requisite adjustment of the, in the Blackstar case, 1-meter mirror require many more precise heating elements than the infrared systems. Again, on a moving vehicle, in this case one that is vibrating rather significantly, the difficulty of implementing an AO system is even greater. Now, on orbit this would not be the case, but even the highly-credulous Mr. Scott is careful about overstating any supposed orbital capabilities of the fictitious Blackstar.

There is an even more critical problem that any orbital AO system faces: there are no guide-stars on the ground. Guide stars work because they are point-illumination sources. There are no comparable sources on the ground. The Aviation Week article mentions a sodium-laser system, but as mentioned by Mr. Day, the laser guide-star concept cannot work when directed at the Earth. The laser guide-star relies on a specific wavelength laser, 589 nm (an orangish color), to excite sodium ions in the mesosphere and thermosphere. These excited ions then fall back to their quantum ground states and emit a characteristic glow. Since the laser is positioned directly next to the telescope on the ground, the column of excited sodium ions appears as a point when viewed from below. Obviously such a system will not work from orbit. Indeed, no AO system as it is currently conceived could work from orbit, when directed at ground-based targets. (image: Lick Observatory laser guide star, constructed by the Lawrence Livermore National Laboratories,)

So, kudos to Mr. Day for an excellent bit of skepticism. Unfortunately not everyone that read Mr. Scott’s work was quite so careful in witholding their enthusiasm about such a gee-whiz airplane idea.