Research highlight: Optical observations of the BepiColombo spacecraft as a proxy for a potential threatening asteroid
BepiColombo is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) designed to study the planet Mercury. Launched in late 2018, its complex trajectory involved a fly-by past Earth on April 10, 2020. We took advantage of the event to organise a coordinated observing campaign. The main goal was to compute and compare the observed fly-by orbit properties with the values available from the Mission Control. The method we designed could then be improved for future observation campaigns targeting natural objects that may collide our planet.
The incoming trajectory of the probe limited the ground-based observability to only a few hours, around the time when it was closest to Earth. The network of telescopes we used has been developed by ESA’s NEO Coordination Centre (NEOCC) with capabilities to quickly observe imminent impactors, thus presenting similar orbits. Our team successfully acquired the target with various instruments such as the 6ROADS Chilean telescope, the 1.0 m Zadko telescope in Australia, the ISON network of telescopes, and the 1.2 m Kryoneri telescope in Corinthia, Greece.
The observations were difficult due to the object’s extremely fast angular motion in the sky. At one point, the telescopes saw the probe covering twice the size of the moon in the sky each minute. This challenged the tracking capabilities and timing accuracy of the telescopes. Each telescope was moving at the predicted instantaneous speed of the target while taking images, "tracking" the spacecraft. Field stars appeared as trails, while BepiColombo itself was a point source, but only if the observation started exactly at the right moment. Because the probe was moving so fast, any date errors of the telescope images translate into position errors of the probe. To reach a precise measurement of 0.1 metres, the date of the images needed to have a precision of 100 milliseconds.
The final results were condensed into two measurable quantities that could be directly compared with the Mission Control ones, the perigee distance, and the time of the probe’s closest approach to Earth. Both numbers were perfectly matched, proving our method a success: it calculated a more accurate prediction of BepiColombo’s orbit; it also provided valuable insights for future observations of objects colliding with Earth:
• A purely optical observing campaign can provide trajectory information during a fly-by at sub-kilometre and sub-second levels of precision.
• A similar campaign would lead to a sub-kilometre and sub-second precision for the time and location of the atmospheric entry of any colliding object.
• Timing accuracy below 100 milliseconds is crucial for the closest observations.
• It’s possible to organise astrometric campaigns with coverage from nearly every continent.
Link to research paper: https://doi.org/10.1016/j.actaastro.2021.04.022
Written by OzGrav researcher Dr Bruce Gendre, University of Western Australia.
From gravitational wave science to global technology company: Liquid Instruments is a Canberra start-up bringing NASA technology to the world.
Liquid Instruments (LI) Pty Ltd, a spin-off company from the Australian National University (ANU), is revolutionising the $17b test and measurement market. Test and Measurement devices are used by scientists and engineers to measure, generate and process the electronic signals that are fundamental to the photonics, semiconductor, aerospace and automotive industries. The LI team has raised more than $25M USD in Venture Capital investment, and now has more than 1000 users in 30 countries.
LI was founded by researchers from the gravitational wave group at ANU to commercialise advanced instrumentation technology derived from both ground and space-based gravity detectors. OzGrav Chief Investigator Daniel Shaddock (ANU), CEO of Liquid Instruments, began as an engineer at NASA’s Jet Propulsion Laboratory in 2002, working on the Laser Interferometer Space Antenna (LISA), a joint project between NASA and the European Space Agency. The work on LISA’s phasemeter was the genesis for forming Liquid Instruments.
LI’s software-enabled hardware employs advanced digital signal processing to replace multiple pieces of conventional equipment at a fraction of the cost and with a drastically improved user experience. Their first product Moku:Lab provides the functionality of 12 instruments in one simple integrated unit.
On 23 June, the company launched two new hardware devices, the Moku:Go—an engineering lab in a backpack for education, and the Moku:Pro – a multi-GHz device for professional scientists and engineers. Lik the Moku:Lab, this revolutionary new hardware includes a suite of instruments with robust hardware features giving a breakthrough combination of performance and versatility.
Daniel Shaddock says: “Moku:Pro takes software defined instrumentation to the next level with more than 10x improvements in many dimensions. Moku:Pro is a new weapon for scientists. Moku:Go takes all the great features of Moku:Lab but reduces the cost by 10x to make it more accessible than ever before. We hope it will help train the next generation of scientists and engineers in universities around the world.
‘Type Ia’ supernovae involve an exploding white dwarf close to its Chandrasekhar mass. For this reason, type Ia supernova explosions have almost universal properties and are an excellent tool to estimate the distance to the explosion, like a cosmic distance ladder. Collapsing massive stars will form a different kind of supernova (type II) with more variable properties, but with comparable peak luminosities.
To date, the most luminous events occur in core-collapse supernovae in a gaseous environment, when the circumstellar medium near the explosion transforms the kinetic energy into radiation and thus increases the luminosity. The origin of the circumstellar material is usually the stellar wind from the massive star’s outer layers as they’re expelled prior to the explosion.
A natural question is how will type Ia supernovae look like in a dense gaseous environment? And what is the origin of the circumstellar medium in this case? Will they also be more luminous than their standard siblings? To address this question, OzGrav researchers Evgeni Grishin, Ryosuke Hirai, and Ilya Mandel, together with an international team of scientists, studied explosions in dense accretion discs around the central regions of active galactic nuclei. They constructed an analytical model which yields the peak luminosity and lightcurve for various initial conditions, such as the accretion disc properties, the mass of the supermassive black hole, and the location and internal properties of the explosion (e.g. initial energy, ejecta mass). The model also used suites of state-of-the-art radiation hydrodynamical simulations.
The explosion generates a shock wave within the circumstellar medium, which gradually propagates outward. Eventually, the shock wave reaches a shell that is optically thin enough, such that the photons can ‘breakout’. The location of this breakout shell and the duration of the photon diffusion determine the lightcurve properties.
If the amount of the circumstellar medium is much smaller than the ejecta mass, the lightcurves look very similar to type Ia supernovae. Conversely, a very massive circumstellar mass can choke the explosion and it will not be seen. The sweet spot lies somewhere in between, where the ejecta mass is roughly comparable to the amount of circumstellar material. In the latter case, the peak luminosity 100 times bigger than the standard type Ia Supernovae, which makes it one of the brightest supernova events to date.
The research paper describing this work (Grishin et al., “Supernova explosions in active galactic nuclear discs”) was recently published in Monthly Notices of the Royal Astronomical Society. The luminous explosions may be observed in accretion discs of accretion rate, or in galaxies with smaller supermassive black hole masses where background active galactic nucleus activity will not hinder observations with advanced instruments.
The underlying physical processes of photon diffusion and shock breakout can be creatively explained with poetry:
All of a sudden, the heat is intense.
We must cool down, but the path is opaque.
Every direction around is so dense,
Which one should the photons take?
They have to break out, for God's sake...
At first, they are stuck, no matter the way,
They sway side to side, they randomly walk.
The leader in front leads them astray,
How hogtied is this radiant flock...
But wait, do you also gaze at the shock?
The ominous furnace is starting to snap,
Its violent grip getting frail.
The path is now clear, the direction is "up!"
We're sitting on the shock front's tail.
We're seizing the shock, we'll prevail!
The shock front behind us, but we're still out of place,
We propel with incredible might.
We keep on ascending, increasing the pace,
Any particle is now out of sight,
In this vacuum, we're free from inside,
And can travel as fast
as the light.
Written by OzGrav researcher Evgeni Grishin, Monash University
RESEARCH HIGHLIGHT: Extraction of binary black hole gravitational wave signals from detector data using deep learning
One of the major challenges involved in gravitational wave data analysis is accurately predicting properties of the progenitor black hole and neutron star systems from data recorded by LIGO and Virgo. The faint gravitational wave signals are obscured against the instrumental and terrestrial noise.
LIGO and Virgo use data analysis techniques that aim to minimise this noise with software that can ‘gate’ the data – removing parts of the data which are corrupted by sharp noise features, called ‘glitches’. They also use methods that extract the pure gravitational-wave signal from noise altogether. However, these techniques are usually slow and computationally intensive; they’re also potentially detrimental to multi-messenger astronomy efforts, since observation of electromagnetic counterparts of binary neutron star mergers—like short-gamma ray bursts—relies heavily on fast and accurate predictions of the sky direction and masses of the sources.
In our recent study, we’ve developed a deep learning model that can extract pure gravitational wave signals from detector data at faster speeds, with similar accuracy to the best conventional techniques. As opposed to traditional programming, which uses a set of instructions (or code) to perform, deep learning algorithms generate predictions by identifying patterns in data. These algorithms are realised by ‘neural networks’ – models inspired by the neurons in our brain and are ‘trained’ to generate almost accurate predictions on data almost instantly.
The deep learning architecture we designed, called a ‘denoising autoencoder’, consists of two separate neural networks: the Encoder and the Decoder. The Encoder reduces the size of the noisy input signals and generates a compressed representation, encapsulating essential features of the pure signal. The Decoder ‘learns’ to reconstruct the pure signal from the compressed feature representation. A schematic diagram of a denoising autoencoder model is shown in Figure 1.
For the Encoder network, we’ve included a Convolutional Neural Network (CNN) which is widely used for image classification and computer vision tasks, so it’s efficient at extracting distinctive features from data. For the Decoder network, we used a Long Short-Term Memory (LSTM) network—it learns to make future predictions from past time-series data.
Our CNN-LSTM model architecture successfully extracts pure gravitational wave signals from detector data for all ten binary-black hole gravitational wave signals detected by LIGO-Virgo during the first and second observation runs. It’s the first deep learning-based model to obtain > 97% match between extracted signals and ‘ground truth’ signal ‘templates’ for all these detected events. Proven to be much faster than current techniques, our model can accurately extract a single gravitational wave signal from noise in less than a milli-second (compared to a few seconds by other methods).
The data analysis group of OzGrav-UWA is now using our CNN-LSTM model with other deep learning models to predict important gravitational wave source parameters, like the sky direction and ‘chirp mass’. We’re also working on generalising the model to accurately extract single signals from low-mass black hole binaries and neutron star binaries.
Paper status: Accepted.
Written by OzGrav researcher Chayan Chatterjee, UWA
AUSTRALIAN SCIENTISTS AT HELM OF THE EXTRAORDINARY DISCOVERY OF TWO NEUTRON STAR-BLACK HOLE COLLISIONS WITNESSED FOR THE FIRST TIME
A newly discovered astronomical phenomenon was revealed in a globally coordinated announcement of not one, but two events witnessed last year: the death spiral and merger of two of the densest objects in the Universe—a neutron star and a black hole.
The discovery of these remarkable events, which occurred before the time of the dinosaurs but only just reached Earth, will now allow researchers to further understand the nature of the space-time continuum and the building blocks of matter.
The discoveries were made by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US, and the Virgo gravitational-wave observatory in Italy, with significant involvement from Australian researchers.
According to Dr Rory Smith—from the ARC Centre of Excellence for Gravitational-Wave Discovery (OzGrav) and Monash University, and the international co-lead on the paper published in the journal Astrophysical Journal Letters—the discoveries are a milestone for gravitational-wave astronomy. “Witnessing these events opens up new possibilities to study the fundamental nature of space-time, and matter at its most extreme,” Dr Smith said.
The first observation of a neutron star-black hole system was made on Jan 5th, 2020 when gravitational waves—tiny ripples in the fabric of space and time—were detected from the merger of the neutron star with the black hole by LIGO and Virgo.
Detailed analysis of the gravitational waves reveal that the neutron star was around twice as massive as the Sun, while the black hole was around nine times as massive as the Sun. The merger itself happened around a billion years ago, before the first dinosaurs appeared on Earth.
Remarkably, on January 15th 2020, another merger of a neutron star with a black hole was observed by LIGO and Virgo using gravitational waves. This merger also took place around a billion years ago, but the system was slightly less massive: the neutron star was around one and a half times as massive as the Sun, while the black hole was around five and a half times as massive.
Dr. Smith explains the significance: “Astronomers have been searching for neutron stars paired with black holes for decades because they’re such a great laboratory to test fundamental physics. Mergers of neutron stars with black holes dramatically warp space-time—the fabric of the Universe—outputting more power than all the stars in the observable Universe put together. The new discoveries give us a glimpse of the Universe at its most brilliant and extreme. We will learn a great deal about the fundamental nature of space-time and black holes, how matter behaves at the highest possible pressures and densities, how stars are born, live, and die, and how the Universe has evolved throughout cosmic time”.
The discovery involved an international team of thousands of scientists, with Australia playing a leading role. “From the design and operation of the detectors to the analysis of the data, Australian scientists are working at the frontiers of astronomy,” Dr Smith added.
Black holes and neutron stars are two of the most extreme objects ever observed in the Universe—they are born from exploding massive stars at the end of their lives. Typical neutron stars have a mass of one and a half times the mass of the Sun, but all of that mass is contained in an extremely dense star, about the size of a city. The star is so dense that atoms cannot sustain their structure as we normally perceive them on Earth.
Black holes are even more dense objects than neutron stars: they have a lot of mass, normally at least three times the mass of our Sun, in a tiny amount of space. Black holes contain an “event horizon” at their surface: a point of no return that not even light can escape.
“Black holes are a kind of cosmic enigma,” explained Dr Smith. “The laws of physics as we understand them break down when we try to understand what is at the heart of a black hole. We hope that by observing gravitational waves from black holes merging with neutron stars, or other black holes, we will begin to unravel the mystery of these objects.”
When LIGO and Virgo observe neutron stars merging with black holes, they are orbiting each other at around half the speed of light before they collide. “This puts the neutron star under extraordinary strain, causing it to stretch and deform as it nears the black hole. The amount of stretching that the neutron star can undergo depends on the unknown form of matter that they’re made of. Remarkably, we can measure how much the star stretches before it disappears into the black hole, which gives us a totally unique way to learn about the building blocks of protons and neutrons,” Dr Smith said.
"Using one of the most powerful Australian supercomputers and the most accurate solutions to Einstein's famous field equations known to date, we were able to measure the properties of these collisions, such as how heavy the neutron stars and black holes are, and how far away these events were," he said.
Publication in Astrophysical Journal (ApJL)
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