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Unraveling the Mysteries of Venus Based on "Occultation"

A Space Probe's Miraculous Recovery

When talking on the phone with someone you know well, you can usually guess their current state of health based on how their voice sounds. With exploration vehicles that are sent into deep space—meaning space farther from us than the Moon—it is similarly possible to monitor the vehicle's condition based on the "tone" of the signals it sends back.

I spent time at the Japan Aerospace Exploration Agency (JAXA) in Sagamihara City, Kanagawa Prefecture on the same day in December in both 2010 and 2015 to cover important events related to the Venus Climate Orbiter (VCO) Akatsuki. On those days, the path of the Akatsuki intersected with the orbit of Venus.

The vehicle had been launched in May 2010, and after spending roughly half a year traveling along an interplanetary transfer orbit, it drew near to Venus for the first time on December 7. The plan was to rapidly decelerate the vehicle and insert it into Venus' orbit, which necessitated powerful main engine firing at a precise time and with accurate directionality. It was to be a tense moment, and all-or-nothing maneuver like one might see in an action movie. One could compare it to having a passenger on a high speed Shinkansen ("bullet train") running parallel to a slow-paced Yamanote commuter loop line train jump from one car to the other while both trains are still in motion. Many members of the press and space exploration fans were present at the JAXA facility, anxiously watching the event.

Sadly, the Akatsuki's main engine used for deceleration malfunctioned and the orbital entry maneuver ended in failure; the spacecraft could not reduce its speed sufficiently, and overshot Venus entirely. Those of us back on Earth didn't have a way of knowing these details until the next day, and in the meantime, all of us journalists at JAXA sat for many hours waiting to hear news.

If the Akatsuki had, indeed, lost its main engine, then many considered the mission to be over right then and there. However, mission control had other ideas in mind. They decided to have the Akatsuki act as a satellite, orbiting the Sun for five years on a path that would bring it back close to Venus' orbit again, and then have it attempt another orbital entry. At that time, it would utilize an unusual deceleration method not included in the original mission plan: using the four attitude control thrusters, each of which has only about one-twentieth the output of the main engine, to carry out long-term burns not initially envisioned by the mission control team in order to position the vehicle. This unorthodox approach worked, and on December 7, 2015, the team successfully inserted the Akatsuki into Venus' orbit. As some of you may remember from the news, this was reported in the press as a nearly unprecedented recovery in the history of space exploration.

Monitoring Orbiter Status Based on Frequency Changes

By the way, this type of one-shot-only, vital operation is known as a "critical operation" among the orbiter mission control team. Let's take some time to think about how the team would have monitored the behavior of the vehicle from moment to moment in a situation like this.

The Akatsuki as well as other deep-space probes use a large antenna fixed to the outside for communication with Earth. When conducting data transmissions, receiving orders, and conducting other such communications, the attitude of the probe itself is changed in order to aim the antenna toward our planet. During a critical operation such as that mentioned above, however, it is seldom possible to realize the needed angle for communication due to requirements related to thrust direction, camera view and so forth—in short, the antenna is almost always pointed away from Earth.

When I talked about SLIM, the Smart Lander for Investigating Moon, in a previous article, I noted that it was possible to receive detailed information in nearly real time. This was possible thanks to the Moon's extremely close location, relatively speaking. The distance to Venus, even when it is at its closest position to Earth, is more than 100 times farther, and it takes anywhere from several to several dozen minutes (or more) for a signal to travel to or from the probe's location, meaning communications like those seen with SLIM are just not possible.

In order to attain the minimum necessary information for the critical operation, a separate, nondirectional antenna on the orbiter was used to continuously transmit beacon signals. The use of a nondirectional antenna meant that signal strength was lower, but mission control could garner the needed information by monitoring frequency changes in its signals. As you may have guessed, taking consideration of the Doppler effect enabled them to monitor the probe's radial (line-of-sight) velocity.

Therefore, it was possible for them to calculate in advance the frequency changes which would occur if the orbiter were proceeding along the intended route, and by graphing this data and then checking it against actual beacon signal data, they could tell whether the numbers matched up, and thus whether the vehicle was proceeding along the correct route. During actual operations, team members used shorthand phrases such as "check the orbiter Doppler" when talking with one another.

It's worth noting that the calculations required consideration of velocity changes at the receiving end as well. The receiving station was being moved through space due to Earth's rotation and orbital motion, so changing station latitudes/longitudes and angles relative to the orbiter had to be factored in for final radial velocity calculations. Back in 2010, while sitting for hours on end waiting for the results to come in, journalists listened with rapt attention to an explanation of these mechanisms. Personally, I found it quite fascinating.

The USO and "The Occult": Atmospheric Observation

As expressed in the Akatsuki's official naming, "Venus Climate Orbiter," the probe was designed to monitor meteorological conditions on Venus. It is equipped with five cameras used to monitor the infrared spectrum, ultraviolet waves and other information, and also with a measuring device that doesn't measure for its own sake (but helps other devices measure) known as an ultra-stable oscillator, or USO. For those of you who are familiar with radio communication technologies, you might understand more easily if I were to call this a "space-system OCXO" instead. Let's explore how the USO served the orbiter's mission.

When the Akatsuki goes behind Venus relative to the Earth, and also when it re-emerges on the other side, its transmitted signals experience refraction as they pass through Venus' atmosphere. This is due to differing temperature distributions throughout Venus' atmosphere, and occurs based on the same principle we experience with mirages which enable us to see faraway sights which should not be within our visible range, or so-called "inferior mirages" we see when driving which appear to us as water on the road ahead.

Just as the optical path length is extended via significant refraction in these mirage examples, the Akatsuki appears to be moving away from Earth when going behind Venus and to be moving toward Earth when it comes back out on the other side. This is due to frequency changes equivalent to those resulting from Doppler shifts. This approach of detecting and analyzing extremely minor frequency fluctuations to analyze the changes in Venus' atmosphere based on altitude—its atmospheric structure, in other words—is known as radio occultation (RO). The word "occultation" has the same roots as the word "occult," as in occult practices or occult arts—something abstruse, mysterious or concealed.

Although RO sensing may seem like a difficult and highly unusual technique to the unaccustomed, it is not by any means a new approach. During the 1960s, this same technique was used for the first time by NASA via the probes of the Mariner program (Mariner 1 through Mariner 10) as well as Voyager 1 and Voyager 2 to research atmospheric structures for and (in applicable planets) rings in Venus, Mars, Jupiter and Saturn.

Since the advent of GPS technology, the same technique has also been used to observe Earth's atmosphere. In a process known as GPS radio occultation (GPS-RO), receiving stations are set up on mountaintops, on aircraft, on low-orbit satellites and in various other locations to read GPS signals refracted by the atmosphere in order to determine its structural properties. This, in turn, led to the birth of the field of study known as GPS meteorology.

One major advantage of GPS-RO is its use of "high-quality" radio waves from atomic clocks as its standard oscillators. GPS meteorology offers far more opportunities for observation activities than other-planet observation, and it is useful in confirming information accuracy via weather balloon measurements and the like. Based on these advances, a method known as radio holography was developed for more in-depth monitoring and measurements. This is described as a method of analysis which takes consideration of multipath propagation waves as well as direct signal waves, and in the Akatsuki, a specific type of radio holography called full spectral inversion (FSI) was used.

Takeshi Imamura, who worked at JAXA as an Akatsuki project scientist and is now a professor at the University of Tokyo, explains as follows: "With radio holography, we work with sets of phase changes, each having a duration of 10 minutes or more. In other words, these are not moment-by-moment snapshots of individual phase changes, but a look at larger patterns, and this type of observational data provides us with a wealth of valuable information. Then, by reverse-calculating based on this data, we can garner information about atmospheric structures with much greater resolving power than is possible using traditional methods."

This approach has enabled analyses of Venus' atmosphere with vertical resolution down to a staggering 100-meter precision and temperatures down to 0.1 kelvin. "Astonishing" is the only way to describe a method like this, which measures things with height accuracy down to 100 meters and temperature down to 0.1 degrees kelvin from a distance of tens of millions of kilometers away—a distance so great it takes radio signals minutes to arrive!

The outstanding performance of the USO, the "measuring device that doesn't measure," installed on the Akatsuki has a major part in this. It is, after all, an ultra-stable oscillator with short-term frequency inconsistencies of just 10–12 (0.000000000001), meaning stability approaching that of an atomic clock. On the receiving end, a large antenna for very-long-baseline interferometry (VLBI) utilizing a hydrogen maser frequency standard device was used—specifically, the 64-meter antenna installed at the JAXA Usuda Deep Space Center. By combining the long-known Doppler effect principle, a highly accurate clock (frequency standard device), and analytical techniques refined through GPS operations, it has become possible to analyze and clearly explain the atmospheric structure of a distant planet and realize major contributions to understanding the super-rotation* of Venus which baffled scientist for so long.
* Planet-wide winds that rotate at 60 times the speed of the planet's rotation in an opposite direction. Speeds reaches approximately 100 meters per second at the uppermost parts of Venus' atmosphere.

In the spring of 2024 (based on what we know as of November 2024), scientists have lost contact with the VCO Akatsuki, whose observations have contributed to a great number of scientific discoveries and advances. It had, of course, already far surpassed its design life, but keep in mind that this was the lucky space probe which made a truly miraculous recovery and returned to its Venusian orbit. Says Professor Imamura, "We are waiting for any opportunity to locate and continue making use of the Akatsuki." The next time you look at the western sky following sunset or the eastern sky just before dawn and spot the planet we call Venus, take a moment to think of the Akatsuki all alone up there, continuing to make its orbits.

Reference material: Semare, Akatsuki no Hoshi! (4) Kinsei Tansaki Akatsuki no Denpa Enpei wa Nani o Shitekitanoka? ("Go Forth, Akatsuki! Part Four: What the Venus Climate Orbiter's Radio Occultation Has Taught Us").
Original: https://www.isas.jaxa.jp/feature/forefront/230926.html