SpaceX launched its 45th mission of the year, carrying the Euclid telescope from the European Space Agency (ESA). Liftoff was Saturday, July 1, at 11:12 AM EDT (15:12 UTC) from Space Launch Complex 40 (SLC-40) in Florida.
ESA’s Euclid will observe a large portion of the sky and deep into the past of the universe to understand the distribution and nature of dark matter and dark energy. The Euclid project is a medium-class mission as part of the agency’s Cosmic Vision science program and has a total cost of 1.4 billion euros.
Construction of the telescope has been led by Thales Alenia Space who integrated the telescope’s components from more than 80 different companies across Europe. These included major aerospace companies like Airbus Defence and Space, Beyond Gravity, and OHB. After construction and pre-launch testing, the spacecraft was transported via boat to Florida where it arrived on April 30.
For the last two months, the telescope has been at the Astrotech payload processing facility in Titusville where it has undergone final checkouts and fueling ahead of launch. Encapsulated inside Falcon 9’s payload fairing, the spacecraft was transported to the pad on the night of June 28 for integration with the rocket.
Welcome to the Space Coast Euclid! Transport from Port Canaveral is underway for final processing before launch. What a treat it was to catch it being escorted on US-1.
— Julia Bergeron (@julia_bergeron) May 1, 2023
On launch day, Falcon 9 followed its traditional 35-minute-long automated propellant load sequence, culminating in the ignition of its nine Merlin 1D engines on the first stage, which was then followed by liftoff. These engines burned until the T+2 minute and 37-second mark when they all shutdown, with both the first and second stages separating three seconds later.
The first stage for this mission, B1080, flew for its second time, having previously supported the launch of Axiom-2. After stage separation, it landed on SpaceX’s drone ship A Shortfall Of Gravitas, which is stationed 691 kilometers downrange.
Eight seconds after stage separation, the Merlin 1D Vacuum engine (MVacD) on the second stage ignited for five minutes and 15 seconds to insert Euclid onto a preliminary low-Earth parking orbit. About 42 seconds into the MVacD’s first burn, the fairing halves protecting the spacecraft separated, exposing it to space.
For this mission, the fairing halves are brand-new and are specially treated and cleaned to avoid any contamination on the telescope’s delicate instruments. They will attempt a splashdown approximately 820 kilometers downrange where they will be recovered by SpaceX’s multi-purpose recovery vessel Doug.
@esa’s Euclid telescope is on the move!
The near $1 billion telescope is currently being transported along public roads towards the Cape and it’s ultimate destination at SLC-40.
Spacecraft spotting & chill?
— Max Evans (@_mgde_) June 29, 2023
After an approximate nine-minute coast phase, the MVacD engine on the second stage ignited once more for 78 seconds to insert Euclid into a transfer orbit that will bring the telescope to the Sun-Earth Lagrange point 2 (L2). Euclid separated from the Falcon 9 second stage approximately 22.5 minutes after the end of that burn.
Euclid is expected to arrive at the L2 point about four weeks after launch and will spend several weeks checking out its systems before beginning a survey. The start of its science collection campaign is expected to occur within 3 months of the launch.
Euclid was originally planned to launch on a Soyuz rocket from French Guiana, but, as a result of the Russian invasion of Ukraine, ESA decided to switch launchers. An extensive six-month study began a few months later to explore the compatibility of Euclid with Falcon 9’s interfaces and launch environment.
The results of this study were positive and it was found that Falcon 9 was the only rocket that could meet the earliest launch date at the lowest cost without any major modification to the telescope.
Euclid Telescope: the Infrared Surveyor
Euclid is set to map 36% of the sky over the duration of its primary mission, which is currently expected to last six years. This survey will be performed using two instruments, the Near-Infrared Spectrometer and Photometer (NISP) and the visible instrument (VIS).
NISP is a 64-megapixel array of 16 near-infrared sensors. The instrument is sensitive to electromagnetic waves between 900 nanometers and 2,000 nanometers in wavelength — which correspond to the near-infrared part of the electromagnetic spectrum.
With its three different photometric filters, it can capture images in three different ranges of wavelength. The NISP-Y filter provides images at wavelengths between 920 and 1,146 nanometers, the NISP-J filter provides images at wavelengths between 1,146 and 1,372 nanometers, and the NISP-H filter provides images at wavelengths between 1,372 and 2,000 nanometers. NISP can also perform slitless spectroscopy in the 1,100 to 2,000-nanometer wavelength range.
On the other hand, VIS is a visual imaging camera of 609 megapixels and consists of 36 charge-coupled device (CCD) sensors that will be taking sharp images in the 550 to 900-nanometer wavelength range.
Both NISP and VIS are wide-field instruments, meaning that they see a large portion of the sky at each moment. The angular size of each imaging field is of about 0.5 square degrees – approximately 2.5 times larger than the size of the full Moon.
Over its six-year primary mission, Euclid will take multiple images of 40,000 of these fields that, combined, will cover 15,000 square degrees of the sky. This is the key differentiator of Euclid against other infrared space telescopes like the James Webb Space Telescope (JWST).
While Euclid’s mirror is 1.2 meters in diameter, much smaller than JWST’s 6.5-meter diameter primary mirror, its wide field of view will allow it to cover a much larger portion of the sky. It would take JWST hundreds of years to survey the same area of the sky as Euclid is set to observe in just six years.
These observation fields will be located in the portions of the sky where the dust from the solar system and the stars, gas, and dust from our own galaxy are less common or not present at all. This is done to gather as much clean data as possible from distant galaxies.
In order for Euclid’s near-infrared sensors to detect the light of distant galaxies, the nearly two-tonne space telescope needs to operate at the L2 point, where the sources of light from the Sun, the Earth, and the Moon are all in the same direction.
The telescope sports a sun shield that protects the mirror assembly and instruments from these infrared sources and cools them down to about 90 Kelvin. This shield also has a solar panel that provides 1.8 kilowatts of power to Euclid’s computers and instruments.
Its main structure is made out of silicon carbide which has ceramic and metal materials. Its ceramic properties allow it to be stiff and rigid across a wide range of temperatures while its metal properties allow it to conduct heat evenly and have a similar temperature all across the structure.
The spacecraft stands 4.7 meters tall and is 3.7 meters in diameter, fitting well inside the Falcon 9 payload fairing. Its mass at launch is expected to be about 1,921 kilograms, which will make it the heaviest payload the rocket has ever launched beyond Earth orbit.
Euclid’s technical capabilities would be left unused without the proper data storage and analysis. To this end, the Euclid Consortium of companies and agencies has set up the Euclid Science Ground Segment that will store, process, and analyze the data gathered by the telescope.
For each observation field, Euclid will take six visual images, 12 near-infrared images, and four spectrum exposures. This means it will take about 5 gigabytes of raw data per field. This means approximately 200 terabytes of raw data are expected to be taken over the span of Euclid’s primary mission.
Euclid’s Purpose: Exploring the Dark Universe
The primary goal of Euclid’s mission is to answer five major questions in cosmology: What is the structure and history of the cosmic web? What is the nature of dark matter? How has the expansion of the universe changed over time? What is the nature of dark energy? Is our understanding of gravity complete?
Scientists’ best model to explain the beginning of the universe and its composition is commonly known as the Lambda Cold Dark Matter model — also known as ΛCDM or Lambda-CDM.
This model combines the existence of two entities, dark matter and dark energy — along with the normal matter and radiation that we see and can directly detect — in order to best fit the data that humanity currently has about the evolution of the universe.
While current observations support this model, it does not explain the nature of dark matter and dark energy. Euclid’s mission could provide enough data to characterize these two entities.
According to Lambda-CDM, dark matter is characterized by being a type of matter that only interacts through gravity and is, therefore, unable to emit or absorb any light. For that reason, its existence and distribution must be inferred indirectly from its effects on the structure of the universe.
Some of the effects that Euclid scientists are aiming to observe are the Baryonic Acoustic Oscillations (BAO) and the weak gravitational lensing on galaxies.
Under Lambda-CDM, the universe contained a homogeneous mix of photons, electrons, neutrinos, baryonic matter, and dark matter for the first 370,000 years of its life. Baryonic matter is what makes up the great majority of the matter that we’re commonly used to seeing and interacting with.
At that stage in the universe’s life, the mix was so hot that photons and electrons interacted with each other constantly — something that would not allow the combination of electrons with nuclei to form atoms. This also made the universe opaque to light as it was not able to freely travel across space.
However, as the expansion of the universe continued, this temperature dropped down enough to allow electrons to combine with nuclei and form atoms, which set photons free. This freeing of photons left an imprint across space that we now call the cosmic microwave background radiation.
Through the study of this background radiation, scientists have been able to identify subtle changes in temperature across it. These changes in temperature correspond to the fluctuations in the density of matter in the early universe at the time of this event.
By studying these density fluctuations, it can be inferred what kind of matter makes up the universe. In regions where the density is higher, matter would be attracted to each other and create an inward pressure. However, at the same time, the interactions between photons and baryonic matter would create an outward pressure that would try to counteract it.
In such a case, dark matter would not interact with the photons and therefore not contribute to this outward pressure. It would instead help to increase the accretion of regular matter inside this higher-density region of space and account for more inward pressure.
As the local baryonic matter density increases, interactions with photons increase and the outward-going pressure attempts to cancel the effect of dark matter. This kind of back-and-forth effect created oscillations, resulting in the propagation of baryonic matter outwards from most of these original fluctuations. These are like sound waves, and it is why they are called Baryonic Acoustic Oscillations (BAOs).
Once the electrons and nuclei combined to form atoms, the photons stopped creating the outward pressure related to this oscillation, which, in turn, left the baryons behind as a “shell” around these density fluctuations. It is believed that these later were the basis for galaxies to form and group to form galaxy clusters.
Euclid will be able to identify the distance at which different galaxies are located in the sky, which will help identify the way they cluster together and whether or not this corresponds to the presence of BAOs.
In order to study the distances of millions of galaxies simultaneously, Euclid will use a technique called photometric redshift. As the universe expands, the light emitted by these galaxies is stretched, and its wavelength shifts to the red portion of the electromagnetic spectrum – an effect called redshift.
This means that different galaxies will only be seen under certain wavelengths and the further away we look, the more red-shifted they will be. Therefore, there’s a correlation between how far away a galaxy is and what wavelengths it can be observed in.
The photometric redshift technique makes use of this correlation by shifting the wavelengths that an instrument can detect in order to identify at which wavelengths it stops observing a galaxy. This can then tell us the redshift of the galaxy and its distance from us.
This kind of technique does not need to identify the specific spectroscopy of each observed galaxy, so it provides a rough estimation of the redshift of millions of galaxies in a much shorter timeframe and with less needed effort.
On the other hand, Euclid scientists will also be able to identify the existence of dark matter through weak gravitational lensing. The most common way to identify dark matter in galaxies or clusters of galaxies is via the deformation of light from other distant galaxies around them due to the spacetime curvature created by their mass (gravity).
This effect is called gravitational lensing and it can be present in extreme cases where the light has been curved to form Einstein rings. But it can also be present in more subtle ways where the apparent shape of galaxies has just been slightly deformed.
To identify these deformations, Euclid will use VIS’s sharp imaging capabilities to look for them. At the same time, NISP’s different photometric bands will allow scientists to identify the distance at which these galaxies are located as to better understand their distance to us.
All of this data will allow scientists to create a 3D map of the distribution of galaxies and the dark matter surrounding them. This map will also help identify the evolution of dark energy in the universe across time.
According to the Lambda-CDM model, dark energy is a form of energy that permeates the universe and has its major effects across large scales. Its density would therefore be constant across space.
During the first fractions of a second in the life of the universe, the expansion of spacetime was driven by inflation — a rapid and exponential growth in the size of the universe in a very short period of time. After this, the acceleration of the universe gradually slowed down due to the gravitational effects of matter on spacetime itself. However, about 5 billion years ago, this expansion started accelerating again and it is now understood that this acceleration is continuously growing.
Under Lambda-CDM, dark energy would be the cause of this acceleration. Since its density is constant across space and as the universe expands and more space is created, the universe would gradually contain more and more of it, eventually surpassing the effects of gravity from matter and accelerating the expansion of the universe in the process.
Euclid will be able to observe deep into the universe, up to about 10 billion years ago. This will allow scientists to observe the changes in the expansion of the universe across time and will also allow them to infer whether this expansion is the same in all directions. The Lambda-CDM model supposes this expansion is uniform and non-dependent on the observer.
The vast collection of data from Euclid will also allow scientists to study the properties of this dark energy and whether or not it acts in the way this model predicts it to act. In particular, if it were to have fluid-like properties, an equation of the state of dark energy could, in theory, be calculated from these measurements.
Alternative models propose that dark energy could be another fundamental force with a field like the electromagnetic force. Others suggest that perhaps the current theory of general relativity, upon which the Lambda-CDM model stands, could just not be correct and needs to be modified to account for these phenomena.
By using Euclid’s data, scientists will be able to narrow down the possible values of parameters for the different models proposed to explain the composition and evolution of the universe, including the Lambda-CDM model.
This data will also be able to confirm whether or not our current understanding of dark matter is correct or not. Specifically, Euclid will confirm whether or not there may be other sources of regular matter to account for that could be interacting and thought of as dark matter.
During its primary mission, Euclid will take hundreds of thousands of images of over 12 billion galaxies that will be combined with ground data to perform the study. Scientists will use the best 1.5 billion sources from this pool of data to create the best understanding of dark energy and dark matter.
During its mission, Euclid will also perform deep fields to monitor the stability of data from the main survey. These deep fields will be 50 squared degrees in size and will cover about 10% of all observations.
The Euclid Consortium plans to perform an initial data release in 2025 with 17% of the survey, which will then be followed in 2027 with a 50% survey data release. In 2030, the final data release will see the entirety of Euclid’s survey data be released.
The telescope will build upon the work performed by ESA’s Planck Observatory. Planck observed the cosmic microwave background radiation and allowed scientists to refine the Lambda-CDM model. Furthermore, Plank helped scientists understand the amount of dark matter and dark energy in our universe.
In 2027, NASA plans to launch its Nancy Grace Roman Space Telescope, another wide-field-of-view telescope that will also observe the universe in visible and near-infrared. Coincidentally, this telescope is also planned to launch on a SpaceX rocket.
Once both are up and running, Euclid and Roman will be able to work in tandem to solve the mystery and origins of the dark side of the universe.
(Lead Image: Falcon 9 and Euclid launch from Space Launch Complex 40. Credit: Julia Bergeron for NSF).