Deep Space Optical Communications (DSOC) mission and its impact on our future
Introduction
Psyche – this is a name of the Greek goddess of the soul. But in this blog post, will observe this name not from the Greek mythology perspective but from an astrology perspective because in the year 1852, an Italian astronomer, Annibale de Gasparis, peeked through his telescope and discovered an asteroid which he named Psyche.
During that time, the asteroid was just a dot that passed through his telescope`s field of view, but years later, with modern telescopes and sophisticated imaging, it was estimated that the Psyche asteroid`s diameter is approximately 223 km and takes an ellipsoid form (sphere with scale deformation in one axis direction).
The Psyche asteroid is located in the Asteroid Belt, which is located between the orbits of Mars and Jupiter. Nowadays, it has captured scientists attention due to the fact that investigations have proven that it consist of an exposed metallic-core-like body, which is the basic building block of planets. It contains mainly nickel and iron, with possible traces of cobalt, platinum, and gold. If compared to other asteroids, they consist mainly of dust and ice.
At the start of year 2017, NASA approved a spacecraft mission to the asteroid Psyche. The spacecraft, equipped with various scientific instruments, will study the asteroid`s origins and composition. The spacecraft was named after the asteroid – Psyche.
The Psyche spacecraft was successfully launched by a SpaceX Falcon Heavy rocket on October 13, 2023, with an expected arrival at the asteroid Psyche in 2029.
Pic.1 – Illustrative representation of asteroid Psyche, and spacecraft of the same name
Spacecraft Missions
Usually, space missions maximize the potential to achieve as much scientific data as possible. So does the Psyche spacecraft mission. Its primary objective is to study the Psyche asteroid, but the spacecraft also has several additional side-objectives.
To achieve its primary goal, the Psyche spacecraft includes the following tools:
- Multispectral Imager – Two identical cameras to map the Psyche asteroid`s surface in various wavelengths. Providing data on its composition, texture, and other geological features;
- Gamma-Ray and Neutron Spectrometer – Measures the asteroids elemental composition;
- Magnetometer – Can detect any magnetic field around Psyche. Which could suggest if asteroid truly is remnant core of protoplanet (non-fully developed planet);
- Gravity measurements – tracks spacecrafts exact orbit around asteroid and can detect asteroids mass distribution, providing clues about its internal structure. [1]
Additional side missions:
- Solar Electric Propulsion – The Psyche spacecraft is one of the first missions to use Hall-effect thrusters for deep space travel. Providing testing ground for this propulsion technology in interplanetary missions;
- Long duration Spacecraft Operations – Psyche mission will test Psyche spacecraft power management, thermal control and navigation systems for longevity over multi-year travel across the space. [1]
And the most important side mission – Deep Space Optical Communications technology experiments and demonstration.
Deep Space Optical Communications (DSOC) technology demonstration
Current deep space radio frequency communication systems are becoming obsolete. Their bandwidth has been fully utilized, and they cannot no longer satisfy future data transmission requirements for space missions. Future missions are expected to transmit large volumes of scientific data and high-definition video. Obviously, a technological upgrade is required for these communication systems. Similarly, as networks on Earth are undergoing upgrade to more faster data rates by utilizing optical fibers, scientist are highly confident that future space exploration can rely on laser or optical communications. [1]
Deep Space Optical Communications (DSOC) technology experiment will be the first of its kind optical communication demonstration beyond the Moon orbit. DSOC basically consists of laser transceiver on board of Psyche spacecraft and laser transmitter, receiver ground stations on Earth.
Planned mission milestones:
- Flight laser transceiver and ground system lock on to each other laser signals.
- Demonstrate downlink data transmission as Psyche spacecraft travels further away from Earth.
- Demonstrate data uplink up to a distance of 150 million kilometres (average distance between Sun and Earth).
- Proof the capability to operate for nearly two years after Psyche mission launch, making one contact per week. [1]
The DSOC technology experiments will begin shortly after the spacecraft launch, and continue in two phases. First phase was executed in June 2024, and second phase has planned to last from January to October of 2025. [1]
Laser transceiver
Due to increasing distance between the laser transmitter and receiver stations, the laser signal becomes weaker, requiring highly sensitive sensors to detect and record the highly diminished laser light. Transmitted laser light becomes more suspectable to background noise – stray sunlight and Earth atmosphere light scattering phenomenon.[4]
To counter these issues laser transceiver on board of Psyche spacecraft consist of delicate vibration isolation platform which is located beneath the main transceiver enclosure. It isolate the vibrations generated by host spacecraft and as well helps to act as precision laser pointing device which is needed to successfully transmit sample data back to Earth.[2]
Transceiver itself consist of near-infrared laser transmitter and a sensitive photon-counting camera. The transceiver has 22 centimetres aperture telescope mirror which is located above the enclosure – it provides ability to receive and transmit laser light in precisely timed photon bursts.[3]
The laser onboard the Psyche spacecraft is based on master-oscillator power amplifier that uses optical fibers (MOFA).[3]
The MOFA laser assembly consists of laser oscillator (seed laser) and a power amplifier which acts as a fiber-based amplifier – often can be based on optical fiber doped with rare-earth elements.
Spacecraft transceiver transmission power is about 4 watts and wavelength for downlink to Earth is 1550nm which has low attenuation in optical fibers and as well in Earth atmosphere.
Ground-based laser transmission power is rated to 5 kilowatts and spacecraft receiver wavelength is 1064nm which ensures compatibility with existing ground-based laser systems and also ensures low atmospheric scattering.
Pic.2 – DSOC`s Flight laser Transceiver mounted on an assembly of struts and actuators that stabilizes the optics from spacecraft vibrations. Credit: NASA/JPL-Caltech
Receiver ground stations
The DSOC’s beaming of data from space will be received at a large aperture 5.1-meters Hale Telescope at Palomar Mountain Observatory in California. When the faint signal is received, it will be guided to a superconducting nanowire photon-counting detector, that can precisely measure and process the time of arrival of the photons. Through this back-end signal processing, the data that is modulated and encoded into the laser beam in deep space can be decoded and converter to information on the ground.[4]
The Deep Space Optical Communications (DSOC) mission`s data transmission principle can be related to free-space optical communication technology here on Earth. The principle remains the same: communication is achieved with optical laser sources and receivers using sensitive pointing and alignment mechanisms to ensure that laser beams are accurately directed from transmitter to receiver. Additionally, sophisticated, telescope-like mirrors and lenses are used to ensure the precision of the laser light.
Pic.3 – DSOC`s Psyche spacecraft with flight laser transceiver installed (large tube). Credit: NASA/JPL-Caltech
DSOC experiments
Planned DSOC experiment timeline is as follows:
- After 20 days of launch, DSOC calibration and commissioning phase is expected to begin, preparing the technology for operation.
- Roughly 50 days after launch: First expected contact opportunity between DSOC ground systems and the flight transceiver aboard Psyche.
- June 2024: First phase of this technology demonstration ends.
- January 2025: Second phase of the tech demo begins.
- October 2025: DSOC tech demo ends.
First video stream
On December 11, 2023, the first ever ultra-high-definition video was streamed from deep space to Earth. The Psyche spacecraft was at a distance of 31 million kilometers from Earth, or eight times the distance from Earth to Moon, when this streaming occurred. It took 101 seconds for the video signal to reach Earth. The data rate achieved during the stream was 267 megabits per second (Mbps). Video was downlinked from the spacecraft to Palomar Observatory, with each frame then sent live to NASA`s Jet Propulsion laboratory, where it was played live.
The test video was uploaded to the spacecraft before the mission began because the spacecraft itself has no equipment to generate a video. The video features – well, you guessed it! Of course, a cat! It is an orange cat named Taters, chasing a laser pointer, with overlay graphics of mission facts and data.[5]
This experiment paves the way for future exploration of distant worlds, as it allows us to communicate with more data at faster rates.
Second data downstream
On April 8, 2024 Psyche spacecraft was 226 million kilometre (140 million miles) away from Earth. Spacecraft transmitted duplicated spacecraft data at 25 Mbps rate. If compared to previous experiment now the spacecraft is located more than seven times farther away, and the rate at which it can send and receive data is reduced. This experiment highly surpasses the initial expectations and is a huge success. Notably, during the experiment, data can even be sent to the spacecraft transceiver and then downlinked back to the Earth on the same night, as the project proved in the turnaround experiment.[6]
Pic.4 – Psyche spacecraft location at April 8, 2024.
Third data downstream
On June 24, 2024 when Psyche was 390 million kilometres from Earth, downstream signal achieved data rate of 6.25 Mbps, with peaks of 8.3 Mbps. Data payload was UHD video signal of scenes from Earth and space. This data rate is far higher than conventional radio frequency communications systems (using comparable power) can achieve over that distance.
On July 29, 2024 the last downstream signal was received which marked the end of first phase of DSOC technology demonstration. It was sent from Psyche spacecraft at about 460 million kilometres away. That is the same distance between Earth and Mars when the two planets are farthest apart. This totals of 11 terabits of data which has been downlinked during the first phase of the demo.
The DSOC technology demonstration’s second phase will start in January 2025 and end in October 2025. After which, the spacecraft will fire up its futuristic-looking electric thrusters, which emit a blue glow, and start its cruise to Mars for its gravity assist, which will greatly help it continue its way to the Psyche asteroid.
Pic.5 – Psyche spacecraft location at July 29, 2024.
Pic.6 – Psyche spacecraft as it travels to asteroid Psyche. Credit: NASA/JPL-Caltech
Pic.7 – This photo captures an operating electric thruster identical to those being used to propel NASA’s Psyche spacecraft. The blue glow comes from the charged atoms, or ions, of xenon. Credit: NASA/JPL-Caltech
Conclusion, what do we on Earth get from this space mission?
The success of DSOC could lead to adaptation of optical communications for following space missions, improving our ability to communicate with spacecraft across our solar system and deep space.
Techniques developed for pointing and acquiring laser signals over great distances could benefit ground-based systems by improving communication with satellites and even terrestrial free-space optical links.
On Earth, we have terrestrial communication system technology – free space optics (FSO). This communication technology allows us to send and receive data via a terrestrial laser link without the need for an optical fiber medium. This kind of communication system usually uses the same signal wavelength, 1550nm, as the Psyche spacecraft for downstream signal.
There have been, and still are, ongoing a lot of experiments and tests of free space optical communications systems. As an example, an experiment which was made in Australia, above the University of Wester Australia Crawley campus [7], where an FSO ground station was built to communicate with an airborne drone. The airborne drone had a Corner Cube Retroreflector (CCR) mounted on it, which allows it to mirror and precisely return the incoming laser signal to the sender. The ground station laser was generated with a commercially-off-the-shelf CFP Digital Coherent Optics transceiver. Similarly, as Psyche spacecraft had an optical amplifier based on doped optical fiber, the FSO systems on Earth also use a similar optical amplifier system – EDFA. As a result, the drone achieved a distance of about 600 meters from the ground station, and a successful link was established.
Commercial optical transceivers are used for terrestrial free space optics systems here on Earth. Not only CFP DCO transceivers, but also SFP+ and QSFP28 PAM4 modules have been used for free space communication technology. Together with EDFA amplification, they can achieve great distances and transmit vast amounts of data.
Ground stations use highly similar systems to those on the Psyche spacecraft for stabilization and signal acquisition – struts and an actuator platform, which allow for pinpoint the receiver with very high precision. Space technology will greatly advance the technology and improve our ground based terrestrial FSO systems.
