Astronomy & Space Technology

Experiment NOvA

Context: The study of neutrinos is an area of immense current interest among particle physicists and astrophysicists. NOvA is an experiment designed to determine the role of neutrinos in the evolution of the cosmos.

Neutrinos

Neutrinos belong to a group of fundamental particles called leptons in the Standard Model. They have no electric charge and very little mass (nearly massless).
They are the second-most abundant particles after photons (particles of light) and the most abundant among particles that make up matter.
They very rarely interact with matter and that is why they are called “ghost particles”. This means they can travel through vast distances, including entire planets, almost undetected.
There are three main types of neutrinos: Electron neutrino, Muon neutrino and Tau neutrino. These particles are produced when particles called leptons interact with matter.
- For example, when a type of lepton called a muon interacts with matter, the interaction produces a muon-neutrino.
- The same goes for electrons (electron-neutrino) and tauons (tau-neutrino).
Source of Neutrinos: Stars, Supernovae, Galaxies, Nuclear reactions.

NOvA experiment:

The NOvA (NuMI Off-axis νe Appearance) experiment is a prominent neutrino experiment designed to study neutrino oscillations and properties.
NOvA is specifically designed to observe the transformation of muon neutrinos into electron neutrinos as they travel over a long distance. To achieve this, the experiment utilises two detectors located in the US:
- Near Detector: Located at Fermilab, Illinois, this detector studies the neutrino beam before it undergoes significant oscillation.
- Far Detector: Situated in northern Minnesota, approximately 810 kilometres from the near detector, this detector observes the neutrino beam after it has travelled a long distance and potentially oscillated.

By comparing the neutrino composition at both detectors, scientists can measure the oscillation rate and gather valuable information about neutrino properties.

Timeline: The NOvA experiment began data collection in 2014 and is currently ongoing.

Quest for three important questions:

The NOvA experiment is designed to answer three fundamental questions in neutrino physics:

Can we observe the oscillation of muon neutrinos to electron neutrinos?

Neutrinos come in three varieties: muon neutrinos, electron neutrinos and tau neutrinos. Neutrinos can oscillate or change from one type to another, for example, oscillations of muon neutrinos to tau neutrinos. But scientists have not seen muon neutrinos oscillating into electron neutrinos.
So, the aim is to understand the unknown factors that govern neutrino oscillations that would significantly improve our understanding of how the universe is constituted.

What is the ordering of the neutrino masses?

Masses of neutrinos are about a million times lighter than the masses of other particles in the Standard Model of physics.
However, the masses of the different neutrino types and their mass hierarchy (which kind of neutrino is the lightest and which is the heaviest) is not yet known, as of now. Presently, it is believed that neutrinos get their masses through a different process than the other particles.
Knowledge of the mass hierarchy also will help answer the question of whether neutrinos are their own antiparticles. Particles and antiparticles have opposite charges. Because neutrinos have no electric charge, it is possible that neutrinos and antineutrinos are fundamentally the same.

What is the symmetry between matter and antimatter?

Physicists theorise that the big bang created equal amounts of matter and antimatter. When corresponding particles of matter and antimatter meet, they annihilate one another. But presently we observe a matter-dominated universe (this is called Charge-Parity violation). So, it appears that at some point, matter and antimatter behaved differently from one another.
In order to advance the theory that neutrinos tipped the balance between matter and antimatter, neutrino physicists need to observe Charge-Parity violation in action.
- If the NOvA collaboration discovers that muon antineutrinos oscillate at a different rate than muon neutrinos, they will know the symmetry between the neutrinos and antineutrinos is broken. This could be a clue to why the universe has more matter than antimatter – the reason we exist.

ISRO has many rockets but too few satellites to launch

Context: As per the Chairman of the Indian Space Research Organisation (ISRO), the space agency’s launch vehicle capability was three-times the demand. In other words, there is less demand for space launch vehicles in the domestic Indian market.

ISRO’s launch vehicles

India currently has four launch vehicles:
- Small Satellite Launch Vehicle (SSLV)
- Polar Satellite Launch Vehicle (PSLV)
- Geosynchronous Satellite Launch Vehicle (GSLV)
- Launch Vehicle Mark-III (LVM-3)
These rockets can launch satellites weighing up to four tonnes to the geosynchronous orbit. India relies on foreign launch vehicles, like Europe’s Ariane V and SpaceX’s Falcon 9, when a satellite weighs more than four tonnes.
ISRO also needs launch vehicles for space missions like Chandrayaan 3 and Aditya L1.
Applications: The satellites have applications in communications, remote sensing, positioning, navigation and timing (PNT), meteorology, disaster management, space-based internet, scientific missions, and experimental missions.

Supply-driven vs Demand-driven model:

The Indian space programme used to follow a supply-driven model: ISRO would build and launch satellites and then look for customers who needed the services provided by the satellites.
After the space sector reforms in 2019-2020, ISRO changed this to a demand-driven model. Here, a satellite needs to be built and launched only if there is already demand for it.

Present challenges:

Lack of demand:
- Lack of demand from consumers.
- Lack of demand is also because of the fact that:
  - Due to technological advancement, the mission life of satellites has become longer. This also reduces the frequency of launch and demand for launch vehicles for replacing the old/defunct satellites.
  - Launch vehicles are improving. In a single launch, the PSLV can deliver multiple satellites in multiple orbits.

Launch capability limitations: India’s launch vehicles are not powerful enough to undertake certain missions, like Chandrayaan 4.
China used its Long March 5 launch vehicle to launch its Chang’e 4 and Chang’e 5 missions in a single launch.
India’s LVM-3 (most powerful launch vehicle) has less than one-third of Long March 5’s capability (28%) and will need two LVM-3 launches to launch all the components of Chandrayaan 4.
Interplay between the government vs Private sector:
- Government's Vision: Indian government wants the private sector to:
  - Creating demand among customers.
  - Building and launching satellites.
  - Providing launch services to customers both in India and internationally to generate revenue.
  - Upskilling workers and providing jobs.
- Private-sector’s concern: Private companies prefer that the government does not compete with them in the launch business. Instead, they want the government to:
  - Be their customer.
  - Provide rule of law and reliable regulations.

Way Forward:

Educating customers:
- The customer of the services provided by the satellite needs to be educated about the need for the service. The customer will then create a demand for a service that will need a satellite to be launched.
  - E.g., There needs to be a demand for space-based internet in a country already filled with affordable fibre and mobile-based internet services, so a company will launch a constellation of satellites into orbit to provide that service.
- The customers include companies, government institutions, defence enterprises, and ordinary people including farmers, bankers, etc.

Upgrading existing launch vehicles:
- ISRO plans to upgrade the LVM-3 with a semi-cryogenic engine to boost its payload capacity to six tonnes to the geostationary transfer orbit (GTO).
- Further, ISRO should speed its Next Generation Launch Vehicle (NGLV) project, a.k.a. Project Soorya which aims to carry 10 tonnes to GTO.

More autonomy to Private-sector:
- Government is already moving towards commercialisation of the space-sector. So, in this spirit, ISRO should phase out of the launch vehicle business, leaving it entirely to the private sector. This is akin to the U.S. model, where government agencies contract private companies like SpaceX and Blue Origin for launches.
- This will help the private sector in revenue generation, focus on technological innovation and invest in creating demand among customers, whereas would free ISRO time and resources to invest in space research and development.

SpaceX Falcon 9 Rocket

Context: In a rare event, SpaceX’s Falcon 9 Rocket recently suffered a failure in its upper stage engine that left 20 Starlink internet satellites in a low, non-survivable orbit. The rocket’s second stage experienced a liquid oxygen leak, and failed to complete a second, short engine burn needed to place the satellites in the correct/intended orbit.

Falcon 9 Rocket has been the workhorse of SpaceX, and till date has launched successfully 364 times, carrying astronauts, payloads for SpaceX's commercial clients and thousands of Starlink satellites to orbit.
The last time a Falcon 9 experienced a serious incident was when the rocket blew up on the launchpad in September 2016.

About SpaceX Falcon 9 Rocket

The SpaceX Falcon 9 is a partially reusable two-stage rocket that can propel both satellites and astronauts into a variety of orbits.
- It can lift up to:
  - 22,800 kg to Low Earth Orbit (an altitude of 2000 km or less).
  - 8,300 kg to Geostationary Transfer Orbit (GTO).
Designed and manufactured by: SpaceX

Key Features:

The rocket hastwo stages.
- The first stage or booster stage comprises nine Merlin engines (a family of rocket engines developed by SpaceX) which use RP-1 (rocket-grade kerosene) and liquid oxygen (LOX) as propellants.
- The second stage consists of a single Merlin engine.
Reusability: The first stage of the rocket is reusable. It is capable of re-entering the Earth’s atmosphere and landing vertically on Earth after separating from the second stage. This significantly reduces the cost of access to space.

Notable Missions of Falcon 9:

Starlink: Falcon 9 is the workhorse for deploying SpaceX's Starlink satellites, aimed at providing global internet coverage.
CRS Missions: Falcon 9 has been used extensively for NASA's Commercial Resupply Services (CRS) missions to the International Space Station (ISS).
Crew Dragon: It launched the first commercial spacecraft (Cew Dragon/ Dragon 2) capable of carrying astronauts to the ISS and LEO. The Polaris Dawn mission is the planned private human spaceflight mission scheduled to be launched in July 2024, using the Falcon 9 rocket.
- Crew Dragon is a spacecraft developed and manufactured by SpaceX as part of NASA's Commercial Crew Program.
- It can carry up to seven astronauts. For NASA missions, it typically carries four astronauts and some cargo.

Why does ISRO want to venture into planetary defence?

Context: Indian Space Research Organisation (ISRO) stated that the asteroid Apophis will pass by Earth at a distance of 32,000 km in 2029. The Indian space agency might send its own spacecraft, or collaborate with other space agencies. ISRO intends to develop capabilities in planetary defence-an area it has so far not entered. A mission to study an asteroid would be the first step towards building a programme aimed at preventing celestial bodies from colliding with Earth with potentially catastrophic consequences.

An alarming asteroid

When Apophis was discovered in 2004, scientists thought there was a 2.7% chance of a collision with Earth-the highest probability of any large asteroid hitting Earth in the recent past.
Initial observations showed that if not in 2029, Apophis could hit Earth in 2036 or 2068.
A collision with Earth could cause large-scale damage.
However, subsequent observations showed these initial fears to have been unfounded.
This is close enough to be visible to the naked eye, and at a distance at which some communication satellites operate.

Apophis Asteroid

Apophis is about 340 meters in width.
The orbit of Apophis crosses the orbit of Earth. It completes an orbit around the Sun in a little less than one Earth year (about 0.9 years). This places it in the group of Earth-crossing asteroids known as ‘Atens,’ those with orbits smaller in width than the width of Earth's orbit.
Apophis is classified as an S-type, or stony-type asteroid made up of silicate (or rocky) materials and a mixture of metallic nickel and iron.
It is a remnant from the early formation of our solar system about 4.6 billion years ago.
It originated in the main asteroid belt between Mars and Jupiter.
Over millions of years, its orbit was changed primarily so that it now orbits the Sun closer to Earth. As a result, Apophis is classified as a near-Earth asteroid, as opposed to a main-belt asteroid.

Asteroid:

A minor planet, an object that is neither a true planet nor an identified comet.
They are rocky, metallic, or icy bodies with no atmosphere.
Of the roughly one million known asteroids, the greatest number are located between the orbits of Mars and Jupiter, in a region known as the main asteroid belt.
Asteroids are generally classified to be of three types: C-type, M-type, and S-type. These describe asteroids with carbonaceous, metallic, and siliceous compositions, respectively.
The first close-up observation of an asteroid was made by the Galileo spacecraft.

Threats from space

Thousands of Asteroids enter the Earth’s atmosphere every day. Most are very small and burn up in the atmosphere due to friction and some of the larger ones burn and show up as fireballs in the sky.
In some cases, unburnt fragments make it to surface, although they are not large enough to cause much damage.
Once in a while, however, asteroids do cause damage.
- In 2013, a 20-metre-wide asteroid entered the atmosphere and exploded about 30 km above a Russian town, releasing energy.
- While most of this energy was absorbed by the atmosphere, shock waves travelled to the ground, flattened trees, damaged buildings, and injured 1,491 people.
- Worryingly, the asteroid was detected only after it entered the atmosphere.
- This was in part because it came from the direction of the Sun, and was hidden by its glare.
A planetary defence programme seeks to track and neutralise these threats.

Planetary defence programme

NASA launched a spacecraft that crashed into an asteroid named Dimorphos, and changed both its shape and its trajectory.
- Dimorphos did not pose a threat to Earth, and was circling the Sun some 11 million km away from our planet.
- The Double Asteroid Redirection Test spacecraft, or DART, was launched in 2021 and intentionally impacted Dimorphos in September 2022, successfully altering its orbit by crashing into it.
Asteroids are yet to be studied in detail, and very few missions have been dedicated to them.
This is why the approach of Apophis has generated huge interest among space agencies around the world.
- NASA has already redirected one of its Space-craft, one that previously studied the asteroid Bennu, to track Apophis.
- After successfully completing its mission to gather a sample of asteroid Bennu in September 2023, OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security – Regolith Explorer) was renamed OSIRIS-APophis EXplorer (OSIRIS-APEX).
- The spacecraft was sent to study Apophis during the asteroid's 2029 Earth flyby.

Asteroid Bennu:

It is a near-Earth asteroid that was discovered in 1999.
It is about 500 meters.
Bennu is classified as a carbonaceous asteroid, which means that it is rich in carbon and other organic molecules.
In 2016, NASA launched the OSIRIS-REx spacecraft to study Bennu and collect a sample of its surface material.
The OSIRIS-REx spacecraft arrived at Bennu in 2018 and spent two years studying the asteroid.

Why is the success of China’s sample return lunar mission significant?

Context: China’s Chang’e-6 became the first spacecraft to bring back samples from the far side of the Moon to the Earth.

Timeline of Chang’e-6:

The Chang'e-6 probe was successfully launched from China on a Long March-5 rocket on May 3, 2024.
The lander descended on the Moon’s surface on June 1 and spent two days collecting rocks and soil from one of the oldest and largest of lunar craters — the 2,500 km-wide South Pole-Aitken (SPA) basin — using a robotic arm and drill.
- The lander then launched an ascent module that transferred the samples to the Chang’e-6 orbiter that was orbiting the Moon.
On June 21, the orbiter released a service module that brought back the samples to Earth. The Chang'e-6 probe returned to Earth on June 25, bringing back the first-ever samples from the far side of the moon.
Aim: The sample return mission aims to collect and return samples from the far side of the Moon to Earth for analysis. The sample can be rocks or soil or even some molecules.

Is this the first time a spacecraft has brought lunar samples to Earth?

Back in July 1969, the US Apollo 11 mission brought 22 kg of lunar surface material, including 50 rocks, to Earth.
In September 1970, the Soviet Luna 16 mission — the first robotic sample return mission — too, brought pieces of the Moon to Earth.
In December 2020, China’s Chang’e-5 brought back 2 kg of lunar soil.

All these samples came from the near side of the Moon.

Note:
- China is the only country to achieve a soft-landing on the far side of the Moon. In 2019, its Chang’e-4 mission landed on the far side and explored the Moon’s Von Karman crater with the help of a rover.
- Difficult terrain, giant craters, and the difficulty in communicating with ground control made it technically challenging to land a spacecraft on the far side that never faces the Earth.

Facts about the Moon:

The Moon is tidally locked to Earth, meaning that the same side of the Moon always faces Earth. This side is known as the near side, while the opposite side is called the far side or the "dark" side (although it does receive sunlight).
- The Moon takes roughly the same amount of time to complete one full orbit around the Earth as the Moon takes to complete one full rotation on its axis.
- As a result, one side of the Moon always faces the Earth, while the other side (far side) faces away from Earth. Thus, we can see only one side of the Moon.

Near side and Far side of the Moon have strikingly different appearances:

Near side:

The near side is characterised by large, dark basaltic plains called maria (dark spots), which are believed to have formed from ancient volcanic eruptions. These maria cover about 31% of the near side and are less common on the far side.
The near side also has fewer impact craters compared to the far side.

Far side:

The far side is more heavily cratered and lacks the extensive maria (dark spots) found on the near side.
The far side has a thicker crust and is more mountainous, with the highest elevations on the Moon. It has a thicker crust by almost 20 km.
One of the most prominent features on the far side is the South Pole-Aitken basin, which is the largest known impact crater in the Solar System.

Why are sample return missions significant?

Lunar missions are exploring ways to stay for long on the Moon and to use its resources in situ.

Detailed analysis of samples:
- In situ robotic explorations — in which landers, orbiters, and rovers carry out experiments in space or on heavenly bodies — can carry only miniature instruments that are not very sophisticated or accurate and have limitations. For example, they cannot determine the origin or age of a rock.
- If the samples can be brought to Earth, the scientists can examine them using extremely sensitive laboratory instruments. They can study the chemical, isotopic, mineralogical, structural, and physical properties of extraterrestrial samples from the macroscopic level down to the atomic scale.
- The returned samples can be preserved for decades and can be examined by future generations using ever more advanced technology.
Insights into Moon:
- Samples collected from the SPA basin can reveal the timeframe for lunar cratering. The collision that created the basin may have excavated enough material from the Moon’s lower crust and upper mantle, which could give insights into the Moon’s history and its origin.
- An examination of the Chang’e-6 samples could throw up some answers on why the lunar far side is geologically different from the near side.
Lunar Resources:
- Far side might hold resources like water ice trapped in permanently shadowed craters. Ice can be harvested for water, oxygen and hydrogen — and the latter two can be used in a rocket propellant. These resources could be vital for future lunar settlements or exploration efforts.
- The samples can suggest ways to use lunar resources for future lunar and space exploration. For instance, lunar soil could be used to produce bricks to build future lunar research bases through 3D printing.
- Moon can be used in the near future as a launch pad to travel deeper into space and to other extraterrestrial bodies.

Note:

India’s Chandrayaan-4 mission, which is currently under development by the Indian Space Research Organisation (ISRO), will also be a sample return mission.
Chandrayaan-3 landed about 600 km from the South Pole of the Moon in August 2023.

Hydrogen Line

What is the Hydrogen Line?

The hydrogen line, also known as the 21-centimetre line, is a specific electromagnetic emission from neutral hydrogen atoms.
Wavelength and Frequency: The hydrogen line is observed at a wavelength of 21 centimetres (or 1420.4 MHz frequency) in the radio spectrum.
Origin: Hyperfine transition in the ground state of neutral hydrogen atoms. This transition involves a change in the spin state of the electron relative to the proton.

Hyperfine Transition:

Each hydrogen atom is made of one proton and one electron. Both these particles have a property called spin.
Energy States: In a neutral hydrogen atom, the spin of the electron and proton in a hydrogen atom can be either aligned (higher energy state) or anti-aligned (lower energy state).
- When the spins of both particles are pointing up (or down), they are said to be aligned. When they are pointing in opposite directions, they are anti-aligned.
Emission Process: When the spins flip from aligned to anti-aligned, the atom will shed this ‘excess’ energy, emitting electromagnetic radiation of wavelength 21 cm.

Significance in Astronomy:

Mapping the Milky Way: Hydrogen line is critical for mapping the structure of our galaxy. Neutral hydrogen emits this radiation, allowing astronomers to study the distribution and motion of clouds of cold, neutral hydrogen atomic gas in interstellar space.
Star Formation: By observing the hydrogen line, astronomers can identify regions where stars are forming, as these regions often have abundant neutral hydrogen.
SETI (Search for Extraterrestrial Intelligence): The hydrogen line is often considered a potential frequency for interstellar communication due to its significance in astronomy and the likelihood that advanced civilizations might recognize its importance.

Agnibaan Rocket

Context: Agnikul Cosmos, a private space company, carried out the first successful launch of its indigenously built rocket, Agnibaan SOrTED (Suborbital Tech Demonstrator) from India’s first and only private launchpad within Satish Dhawan Space Centre (SDSC) SHAR in Sriharikota, Andhra Pradesh. It is also India’s first flight with a Semi-Cryo engine.

About Agnibaan SOrTED:

SOrTeD uses the world’s first single-piece 3D-printed engine, designed and built indigenously.
It is powered by liquid fuel in the core. All the ISRO rockets have solid fuel in the core, though the strap-on rockets (small rockets that cling to the sides of the main rocket at the bottom) were liquid-fuel fired. Agnibaan is also designed to be fitted with strap-ons.
It was for the first time in India that a semi-cryogenic engine was used: ATF (Aviation turbine fuel) at ambient temperature and liquid oxygen (oxidiser) in cryogenic condition. Fuel loading had to begin only 3 hours before the lift-off.
It was also the first private rocket that was controlled during the entire flight. Its velocity, attitude and position were fully telemetered, and it could be destroyed on the ground if something went wrong.
The Agnibaan rocket is a customisable, two-stage launch vehicle that can carry up to 300 kilo-gram of payload to orbits nearly 700 kilometers in altitude.
Agnikul Cosmos:
- It is an Indian aerospace manufacturer based in National Centre for Combustion Research and Development (NCCRD) of IIT Madras, Chennai, Tamil Nadu.
- In 2017, it was formally incorporated as a company.
- AgniKul signed a framework agreement with the Department of Space in 2021 for access to ISRO facilities and technical expertise for the development of its two-stage small-satellite Agnibaan launch vehicle.

James Webb Space Telescope spots earliest-known galaxy

Context: NASA's James Webb Space Telescope (JWST) has discovered the earliest-known galaxy, JADES-GS-z14-0. This galaxy, remarkably large and bright, formed when the universe was just 2% of its current age. Until now, the earliest-known galaxy dated to about 320 million years after the Big Bang, as announced by the JADES team last year.

About JADES-GS-z14-0 galaxy:

The galaxy existed about 290 million years after the Big Bang event (that initiated the universe roughly 13.8 billion years ago). This period spanning the universe’s first few hundred million years is called cosmic dawn.
- The period in the first few hundred million years after the big bang where the first galaxies were born.
- These galaxies provide vital insight into the ways in which the gas, stars, and black holes were changing when the universe was very young.
Early galaxies were formed in an environment that was denser and gas-rich than today. In addition, the chemical composition of the gas was very different, much closer to the pristine composition inherited from the Big Bang (hydrogen, helium and traces of lithium).
The international team of astronomers used JWST to observe galaxies as part of the JWST Advanced Deep Extragalactic Survey (JADES) program.
JADES-GS-z14-0 galaxy measures about 1,700-light years across.
A light year is the distance light travels in a year, which is 9.5 trillion km.
- The galaxy has a mass equivalent to 500 million stars the size of our Sun and is rapidly forming new stars, about 20 every year. (It is dwarfed by some present-day galaxies such as the Milky Way is about 100,000 light years across, with the mass equivalent to about 10 billion sun-sized stars).
The team also disclosed the discovery of the second oldest-known galaxy, from about 303 million years post-Big Bang.
This galaxy, JADES-GS-z14-1, is smaller, with a mass equal to about 100 million sun-sized stars, measuring roughly 1,000 light years across and forming about two new stars per year.

Read about James Webb Telescope:

James Webb Telescope

China lands on the Far side of the Moon

Context: China’s Chang’e-6 lunar lander successfully landed in the South Pole-Aitken Basin (the far side of the moon), where it will begin to collect samples from the lunar surface.

About Chang’e-6 Mission:

The Chang’e-6 is a 53-day-long mission. After reaching the Moon’s orbit, the mission’s orbiter will circle the natural satellite while its lander will descend into the 2,500-kilometre-wide South Pole-Aitken basin on the lunar surface.
After collecting samples through scooping and drilling, the lander will launch an ascent vehicle, which will transfer the samples to the orbiter’s service module. This module will then return to the Earth.
Note: China is the only country to achieve a soft-landing on the far side of the Moon. In 2019, its Chang’e-4 mission landed on the region and explored the Moon’s Von Karman crater with the help of a rover.

Facts about the Moon:

The Moon is tidally locked to Earth, meaning that the same side of the Moon always faces Earth. This side is known as the near side, while the opposite side is called the far side or the "dark" side (although it does receive sunlight).
- The Moon takes roughly the same amount of time to complete one full orbit around the Earth as the Earth takes to complete one full rotation on its axis.
- As a result, one side of the Moon always faces the Earth, while the other side (far side) faces away from Earth. Thus, we can see only one side of the Moon.

Near side and Far side of the Moon have strikingly different appearances:

Near side:

The near side is characterised by large, dark basaltic plains called maria (dark spots), which are believed to have formed from ancient volcanic eruptions. These maria cover about 31% of the near side and are less common on the far side.
The near side also has fewer impact craters compared to the far side.

Far side:

The far side is more heavily cratered and lacks the extensive maria (dark spots) found on the near side.
The far side has a thicker crust and is more mountainous, with the highest elevations on the Moon. It has a thicker crust by almost 20 km.
One of the most prominent features on the far side is the South Pole-Aitken basin, which is the largest known impact crater in the Solar System.

Why is the Far side of the Moon important for us?

South Pole-Aitken Basin is the oldest known impact crater in the solar system. The impact that created the basin is thought to have dug up material from the lunar mantle. If that material can be retrieved, scientists can learn more about the history of the interior of the Moon. Chang’e-6’s sample return could also shed more light on the early evolution of the moon and the inner solar system.
The far side is completely free from radio interference from Earth. This makes it an ideal location for setting up giant radio telescopes that could detect ultra-low radio waves that emanate from the early universe and which would provide crucial information about the formation of the first galaxies.
The far side might hold resources like water ice trapped in permanently shadowed craters. These resources could be vital for future lunar settlements or exploration efforts.

Astronomical Transients

About Astronomical transients:

Astronomical transients are brief and often dramatic astronomical events that can vary in brightness over relatively short timescales, ranging from fractions of a second to several months or even years.
Understanding these violent and energetic phenomena helps us learn more about the universe, including the birth and death of stars, the formation of black holes, and the nature of dark matter and dark energy.

Types of Astronomical Transients:

1. Supernovae: When the outer layers of large stars blow up while their cores implode (collapse) because the stars have run out of elements to fuse. Many supernovae have been known to become so bright that they emit light more intensely than the stars in the rest of its host galaxy combined.

Type Ia Supernovae: These occur in binary systems where a white dwarf accretes material from a companion star until it reaches a critical mass and undergoes a thermonuclear explosion.
Core-Collapse Supernovae: These result from the collapse of massive stars (greater than 8 solar masses) at the end of their life cycles.

2. Active galactic nucleus (AGN): When the supermassive black hole at the heart of a galaxy is actively accreting material, it is called an Active Galactic Nucleus. As matter spirals inward due to the black hole's gravity, it heats up and releases tremendous energy through friction. This energy is what makes AGNs so luminous and glows with a changing brightness.

3. Fast Radio Bursts (FRBs): Millisecond-duration bursts of radio waves from distant galaxies. Their origin is still a mystery, but they are thought to be associated with highly energetic processes like neutron star mergers or magnetars. They can emit more than 10-times as much energy as the Sun in a few milliseconds.

4. Gamma-Ray Bursts (GRBs):

Short GRBs: Typically lasting less than 2 seconds, often associated with the merger of compact objects like neutron stars.
Long GRBs: Lasting more than 2 seconds, usually linked to the collapse of massive stars and associated with supernovae.

PREFIRE (Polar Radiant Energy in the Far-InfraRed Experiment) Mission

Context: On May 25, the National Aeronautics and Space Administration (NASA) launched one of the two climate satellites to study heat emissions at Earth’s poles from New Zealand. The second satellite will be launched in the following days.

About PREFIRE (Polar Radiant Energy in the Far-InfraRed Experiment) Mission:

NASA aims to launch two cube satellites, or CubeSats to measure far-infrared radiation from the Earth’s poles.
The mission has been named PREFIRE (Polar Radiant Energy in the Far-InfraRed Experiment) and was jointly developed by NASA and the University of Wisconsin-Madison (US).
Each of the PREFIRE satellites is a 6U CubeSat. They measure around 90 cm in height and nearly 120 cm in width when the solar panels (which will power the satellite) are deployed. The two satellites will be placed in a near-polar orbit (a type of low Earth orbit) at an altitude of about 525 kilometres.
Each of the PREFIRE CubeSat is equipped with a thermal infrared spectrometer — known as Thermal Infrared Spectrometer (TIRS) — to measure the amount of infrared and far-infrared radiation from the Arctic and Antarctica.

Need and significance of the mission:

The Earth’s energy budget is the balance between the amount of heat incoming to Earth from the Sun and the amount of heat outgoing from Earth into space. The difference between the two determines the planet’s temperature and climate.
A large amount of the heat radiated from the Arctic and Antarctica is emitted as far-infrared radiation — wavelengths of 3 μm to 1,000 μm within the infrared range of electromagnetic radiation. However, there is currently no way to measure this type of energy. As a result, there is a gap in knowledge about the planet’s energy budget.
The climate satellites will measure how much heat the Arctic and Antarctica — two of the coldest regions on the Earth — radiate into space and how this influences the planet’s climate. The data collected by the satellites would help scientists better understand the energy budget of the planet.
The CubeSats will also measure the amount of far-infrared radiation trapped by atmospheric water vapour and clouds at the poles and how this influences the greenhouse effect in the region.

What are SmallSats?

Small spacecraft (SmallSats) focus on spacecraft with a mass less than 180 kilograms.

Small spacecraft can be differentiated into a large variety of size and mass.

Minisatellite, 100-180 kilograms
Microsatellite, 10-100 kilograms
Nanosatellite, 1-10 kilograms
Picosatellite, 0.01-1 kilograms
Femtosatellite, 0.001-0.01 kilograms

What are CubeSats?

CubeSats are are a class of nanosatellites whose basic design is a 10 cm x 10 cm x 10 cm (which makes up for “one unit” or “1U”) cube — just a little bigger than a Rubik’s cube — and weight not more than 1.33 kg.
Depending on the CubeSat’s mission, the number of units can be 1.5, 2, 3, 6, and 12U.
Owing to their low cost and less mass in comparison to traditional satellites, they are placed in orbits for technology demonstrations, scientific research, and commercial purposes.

Space start-up sends SAR on High-Altitude Platform

Context: Bengaluru-based space start-up, Galaxeye Space has flown a synthetic aperture radar (SAR) developed by it on a High-Altitude Platform station (HAPS) of the National Aerospace Laboratories, based in Pune.

About Synthetic Aperture Radar

SAR is a powerful remote sensing tool that can see through cloud and vegetation cover to picture the ground below.
HAPS are like big drones, they can be moved, or made to hover over a certain country or a region.

High-altitude platform stations:

High-altitude platform stations (HAPS) are aircrafts that can fly at an altitude of 18-20 km from the ground (stratosphere), almost double the heights attained by commercial aeroplanes.
HAPs are of many types: Balloons, airships and unmanned aerial vehicles.
They are typically solar-powered so can remain in the air for months and even years, giving the advantage of a satellite.
They can carry payloads of up to 40 kg.

Differences between HAPS and Satellites:

S.No.	Category	HAPS	Satellites
	Altitude	Operates in the stratosphere, 18-20 km above the Earth.	Range varies from Low Earth Orbit (LEO) at 200-2000 km to Geostationary Earth Orbit (GEO) at 36,000 km.
	Coverage	Covers a specific area, typically up to 400 km in diameter.	Can offer spot coverage (LEO) as well as global coverage (GEO).
	Latency	Lower latency due to their closer proximity to Earth, ideal for real-time applications.	Higher latency, especially for GEO satellites, which can cause signal delays.
	Cost	Generally cheaper than satellites due to no need for a rocket launch.	Launching and maintaining satellites in space is expensive.
	Applications	Providing remote internet connectivity, remote sensing, disaster response communication, border surveillance and defence applications.	Global communication networks, navigation (GPS), earth observation.