We haven't heard much about the Psyche mission to the asteroid by the same name since its launch in early October. As usual, not seeing headlines about a deep space mission follows the old axiom of "no news is good news" and the probe is steadily flying away from Earth. There won't be much to say about it because the probe isn't going to reach the asteroid itself for six years.
It's not like the crew working on the satellite goes home and into hibernation until 2029, though. Last week, on the 14th, an interesting test of a system to experiment with communication by lasers was carried out successfully. Psyche is currently around 10 million miles from Earth, or 40 times the average, 250,000 mile, Earth-moon distance. This isn't difficult radio communications for the deep space network, but without the right instruments on both ends, there's no way to test it with a laser. Laser communications tests have stopped at the moon.
The moment marked the first successful test of NASA's Deep Space Optical Communications (DSOC) system, a next-generation comms link that sends information not by radio waves but instead by laser light. It's part of a series of tests NASA is doing to speed up communications in deep space, on different missions.
"Achieving first light is a tremendous achievement. The ground systems successfully detected the deep space laser photons from DSOC," Abi Biswas, the system's project technologist at NASA's Jet Propulsion Laboratory (JPL) in Southern California, said in an agency statement.
"And we were also able to send some data, meaning we were able to exchange 'bits of light' from and to deep space," Biswas added.
The test began at JPL's Table Mountain Facility in California. There, in the hills outside Los Angeles, engineers switched on an uplink beacon, a near-infrared laser pointed at Psyche. About 50 seconds later when the light reached the probe, an optical transceiver on Psyche received the laser and replied with its own laser signal back to the Mount Palomar Observatory, near San Diego.
Just as here on the ground, we go to lasers because of they operate on higher frequencies than radios. All electromagnetic radiation, whether what we see as light or detect as radio has the property that the bandwidth they can support goes up as the frequency goes higher. Higher bandwidth generally means faster data transmission rates, which is their goal.
There's a ton of things I'd like to know about this that the source doesn't say a single word about, basically related to how it works. What frequency? How wide is optical beam width at 10 million miles? Does the laser illuminate all of the Earth? In other words, what kind of pointing accuracy is needed? Is some sort of tracking needed?
While stumbling around on NASA's Psyche website, I found this picture of the spacecraft on some sort of carrier, apparently all folded up for some unknown reason. The red highlighted instrument with the gold foil cap is the DSOC instrument. Their caption says this was in a clean room on June 26, 2023, at the Astrotech Space Operations facility near the Kennedy Space Center in Florida so this is probably long after the testing was completed. Image credit: NASA/Frank Michaux
Since everything about an optical system's resolution, image size and in this case, how far the beam diverges over millions of miles is determined by its aperture - how big the first element is - a starting point is that gold foil cap seems to be around 12" diameter. Let the calculations begin!
A small update to the Starship IFT 2 data.
A YouTube channel called Astronomy live was showing off video he took from the Florida Keys over the weekend (linked in the comments on Sunday's post). He said he was working on the video with editing software that could keep the ship centered and easier to look at. That video was finished and released Monday afternoon, here. While the video from the weekend was over an hour long, this one is just over three minutes long and it's easy to see some of things he laboriously pointed out, such as the remaining upper portion of the Ship 25 rotating around its long axis, are very noticeable in that video.
Great video. Is that sort of helical pattern the norm for a SpaceX launch?
ReplyDeleteSiG, I imagine the laser frequency is going to be shifted out of the red spectrum and probably end up as green since that's what will punch through the atmosphere easily. Around 532 nm, since it's a tradeoff between higher frequencies that require more power (blue to UV) and lower power (red) that will be absorbed easily by the atmosphere.
ReplyDeleteJust spitballing stuff here.
"Yes, but..." that matters less the closer the target is to overhead because it minimizes the path length through the atmosphere. That could be a simple planning step in the operation. Or they could only put links on mountaintops where the atmosphere is the thinnest. Which was part of this demonstration and the way all astronomical observatories are sited.
DeleteThe only references I can find just refer to it being near-infrared, which covers from just below the red, say 500nm out to 3000. 375 to 100 THz (Terahertz = 1000 Gigahertz). A 275,000 GHz spread. I'd be surprised if there weren't spans in there that are "windows", which are better than the surrounding spectrum.
980nm, 1064nm, and 1550nm are all widely used in laser communications, range finding, and target marking applications with excellent atmospheric penetration. If I had to guess, I would think 1064nm would be the most likely due to readily available high-power laser sources at that band.
DeleteBut the sky is blue - because it scatters blue light best. The dawn or dusk is red, because that's what's left after the blue gets scattered at the optical horizon. So - I think red light, or IR would be best.
DeleteThe Apollo program left one (or maybe more) optical retroreflectors on the moon. Is anyone still using them for some optical EME QSOs?
The font of all knowledge has some fairly up to date info:
ReplyDeletehttps://en.wikipedia.org/wiki/Deep_Space_Optical_Communications
Looks like 1064nm up, 1550 down.
SMH! Of all the places to not think to look for that...
DeleteWikipedia is useful for things that absolutely can't be made political, like this. Like fundamentals in lots of technical fields. Until they can figure out an argument that one wavelength of light is trans, or "just like Hitler" they're probably worth looking at.