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Astronomers detect three supermassive black holes at the center of three colliding galaxies

Astronomers detect three supermassive black holes at the center of three colliding galaxies

Three supermassive black holes (SMBHs) glowing in x-ray emissions have been identified by astronomers at the center of three colliding galaxies a billion light-years away from Earth. All three black holes are active galactic nuclei(AGN), consuming material. This finding may clarify a long-standing issue in astrophysics and black hole mergers known as “final parsec problem”. The study appears in The Astrophysical Journal.

Researchers detected the three SMBHs with the data from several telescopes, Sloan Digital Sky Survey (SDSS,) the Chandra X-ray Observatory, and the Wide-field Infrared Survey Explorer (WISE)A nearly unbelievably astronomical event, the fusion of three galaxies may play a crucial role in how the most massive black holes expand over time.

Ryan Pfeifle from George Mason University in Fairfax, Virginia, the paper’s first author said that they found this incredible system through their selection technique while they were only looking for black hole pairs. He also added that this is the most powerful evidence found for such a triple system of active supermassive black holes. It is very challenging to locate triple black hole systems since they are wrapped in gas and dust. It took several telescopes functioning in different parts of the electromagnetic spectrum and also the work with researchers to detect these black holes.

Co-author Shobita Satyapal, also belonging to George Mason said that dual and triple black holes are extremely rare but such systems are actually a natural outcome of galaxy mergers, through which galaxies grow. This triple-merger was first spotted in visible light by the SDSS and only through a citizen science project named Galaxy Zoo the system of colliding galaxies was detected. The system was in a state of galaxy merger glowing in the infrared as seen by the WISE when more than one black holes were expected to be feeding.

Researchers shifted to the Chandra Observatory and the Large Binocular Telescope (LBT) for confirmation as Sloan and WISE data were fascinating clues. Chandra observations revealed bright x-ray sources in the galactic centers where SMBHs are expected to detect. Chandra and Nuclear Spectroscopic Telescope Array (NuSTAR) satellite of NASA discovered more shreds of evidence showing the presence of SMBHs and the existence of large amounts of gas and dust near one of them. It was expected in merging of black holes. Spectral evidence received by optical light data from SDSS and  LBT shows that these are characteristics of the feeding SMBHs.

Christina Manzano-King, co-author from the University of California, Riverside said that optical spectra include plenty of information about a galaxy which is frequently used to detect active accreting supermassive black holes and can tell about their influence on the inhabitant galaxies. Pfeifle said that they have found a new method of identifying triple supermassive black holes using these major observatories as each telescope gives them a distinct idea about these systems. They expect to extend their work to find more triples using the same method.

The final parsec problem is a theoretical problem that is fundamental to our understanding of binary black hole mergers that states that the enormous orbital energy of two approaching black holes stops them from merging. They can get separated by a few light-years, then the merging process stables.

The hyperbolic trajectories of two initially approaching black hole carry them right past each other. The two holes catapult the stars as they interact with them in their proximity transferring a fraction of their orbital energy to a star every time. The energy of the black holes gets reduced by the emission of gravitational waves. The two black holes finally slow down and approach each other more closely shedding enough orbital energy finally getting within just a few parsecs of each other. More matter is discharged via sling-shotting as they come closer. As a result, for the black holes, no more matter is left to interact with and shed more orbital energy. The merging process halts.

Astronomers know that strong gravitational waves are responsible for black hole mergers.LIGO (Laser Interferometry Gravitational-Wave Observatory) discovers a black hole merger almost every week. The final parsec problem is about how they merge with each other finally. Researchers think that a third black hole like seen in this system could give the push needed for the black holes to get merged. Nearly 16% of supermassive black hole pairs in colliding galaxies are expected to interact with a third supermassive black hole before they merge.

The challenge is that gravitational waves produced during merging would be too low-frequency for LIGO or the VIRGO observatory to detect. Researchers may have to depend on future observatories like LISA, ESA/NASA’s Laser Interferometer Space Antenna to detect those waves. LISA is better-equipped than LIGO or VIRGO to detect merging of giant and massive black holes as it can detect lower frequency gravitational waves.

Reference: The Astrophysical Journal.

Astronomers identify 21 more stars with erratic dimming as Tabby's Star

Astronomers identify 21 more stars with erratic dimming as Tabby’s Star

The KIC 8462852, fondly recognized as Tabby’s Star made news when spotted in 2015. Over time its brightness decreased in a manner defying all explanations. A new sky survey research has identified a group of 15 stars that show similar dimming behavior like KIC 8462852 and a further six which are more extreme. The work appears in The Astrophysical Journal Letters.

Tabetha Boyajian, astronomer at Louisiana State University and co-workers first published the uncommon behavior of KIC 8462852 in 2015. One of the main ways to detect exoplanets is with the help of dimming stars. Exoplanets passing between the star and us generally reduce the brightness of the star by less than 1 percent. Dimming behavior KIC 8462852 is peculiar although it appears to be a normal yellow-white dwarf.

The dimming of KIC 8462852 is entirely random and unpredictable and the extent of light blocked varies from 1 percent to 22 percent, unlike the planetary dips that are usually on a standard time-frame and dim the star by the same amount in every transit. These dimming durations have varying time limits that are not caused by planets. Researchers said that some wavelengths of light are blocked more than others ruling out a large, solid opaque object such as an alien megastructure. Identifying more stars exhibiting the same behavior in the same kind of environment could provide further clues.

Edward Schmidt, astronomer and physicist of the University of Nebraska-Lincoln combined the information collected between April 1999 and March 2000 by the Northern Sky Variable Survey to find such stars with irregular variability excluding the stars with explicable dimming. This resulted in 21 stars. He downloaded the light curve data from the All-Sky Automated Survey for Supernovae (ASAS-SN) for the 21 stars, then compared against KIC 8462852’s light curve. Some interesting similarities were obtained.

Schmidt said that he found no periodicity and inconsistent depths of dips in the candidate stars similar to KIC 8462852. This irregularity is a crucial differentiation since KIC 8462852 isn’t the only strangely dimming star. EPIC 204376071 was observed dimming to a depth of 80 percent like an abnormally large, ringed planet had passed in front of it. EPIC 249706694 has unevenly synchronized dips in light that have similar depth.

Schmidt divided the stars into two categories, slow and rapid dippers. 15 “slow dippers” were recognized which were similar to KIC 8462852 in terms of the timing and also six “rapid dippers”, which had similar dips, but with greater frequency. This indicates that the dimming behavior shows a series of characteristics and the event that causes it has a range into which KIC 8462852 could fall. Besides their behavior, they occupy the same area of the temperature-luminosity diagram as KIC 8462852 i.e. they are the same type of star.

However, he has not yet checked whether the dips of the 21 new stars block precise wavelengths which shows that there is still work to be done. Based on archival data, KIC 8462852 was determined to be gradually fading – between 1890 and 1989, by 0.193 magnitudes.

To decide whether these newly recognized 21 stars are Tabby-alike or not these factors have to be studied along with the cause of dimming. Schmidt thinks that it is most likely that the dips are caused by moving objects which are likely dust based on the color changes although it does not explain the long-term dimming.

Journal Reference: The Astrophysical Journal Letters.

Second-fastest, gamma rays pulsations detected

Second-fastest, gamma rays pulsations detected

An international research group led by the Hannover-based Max Planck Institute for Gravitational Physics found that the J0952-0607 radio pulsar also emits pulsed gamma radiation. J0952-0607 spins 707 times in a second.  This remains second in the list of fast-paced neutron stars. By analyzing about 8.5 years’ worth of data from NASA’s Fermi Gamma-ray Space Telescope, LOFAR radio observations from the past two years, observations from two large optical telescopes, and gravitational-wave data from the LIGO detectors, the team used approach to study the binary system of the pulsar and its lightweight companion in detail.

The gamma radiation of millisecond pulsar PSR J0952−0607 is so weak, it requires some smart, innovative exploration techniques to be detected-and these enabled original star measurements.

Pulsars are a type of dead star called neutron stars, the end result of a star that’s too massive to become a white dwarf, and not extensive enough to become a black hole. But the rotation of these pulsars is such that, as they spin, they sweep Earth with a beam of radiation, sometimes on timescales, so that are so precise they can help us measure the Universe.

Some of these pulsars are rotating so fast, and they go round on millisecond scales; fittingly, we call those millisecond pulsars, and they are usually found with a binary companion. It’s thought that their rotation speeds up as they absorb material away from that companion.

Pulsars are the compact remains of stellar explosions that have strong magnetic fields and are rapidly rotating. They emit radiation like a cosmic lighthouse and can be observable as radio pulsars and gamma-ray pulsars depending on their bearings towards Earth.

gamma rays

(Credits: NASA’s Goddard Space Flight Center/Cruz deWilde)

As per the official Fermi website in 2016, 17 percent of millisecond pulsars have been detected emitting gamma rays, compared to just 3 percent of the average pulsar population.

But PSR J0952−0607 is one of the most extreme yet, second only to PSR J1748-2446ad, discovered in 2006 to be rocketing around at 716 rotations per second.

It’s also what we term a “black widow”. The pulsar is 1.4 times the mass of the Sun, squished down into that teeny tiny diameter, with a binary companion around 0.02 times the mass of the Sun. What makes it a black widow is that insanely low binary companion mass: the pulsar has slurped up most of its companion. When the pulsar was discovered in 2017 earlier, there were no gamma rays detected emanating from the pulsars. The pulsar in itself was found using the Low-Frequency Array (LOFAR) radio telescope. This search for gamma-ray emissions was conducted using the robust Atlas Computing Cluster. And it found the signal – but something, Nieder said, was awry.

No gamma-ray pulsations before July 2011:

The news then came with another surprise that it was impossible to detect gamma-ray pulsations from the pulsar in the data from before July 2011. The reason for why the pulsar only seems to show pulsations after that date remains unknown. Variations in how much gamma rays it emitted might be one reason, but the pulsar is so faint that it was not possible to test this hypothesis with sufficient accuracy. Changes in the pulsar orbit seen in similar systems might also explain, but there was not even a hint in the data that this was happening.

Journal Reference: The Astrophysical Journal.

The all-new Event Horizon Telescope is being designed

The all-new Event Horizon Telescope is being designed

Radio telescopes around the globe were used in April 2017 to conduct a fantastic feat: two supermassive black holes were observed all together. This was the Horizon Telescope event and the first outcomes, released back in April, gave us the very first image of one of these new artifacts.

The National Science Foundation has granted a $12.7 million grant to the cooperation to schedule the next-generation Horizon Telescope (ngEHT) event. It will use the funds to improve the current design. The telescope is currently being produced by linking current installations worldwide. This technique, which is known as interferometry, allows astronomers to have a radio telescope that is effectively the size of the Earth.

Led by Principal Investigator Shep Doeleman at the Center for Astrophysics at Harvard, and Smithsonian (CfA), the new ngEHT award will fund design and prototyping efforts by researchers at several US institutes. These include Dr. Gopal Narayanan at the University of Massachusetts, Amherst, Dr. Vincent Fish at the MIT Haystack Observatory, and Drs. Katherine L. (Katie) Bouman and Gregg Hallinan at Caltech. At the CfA, Drs. Michael Johnson, Jonathan Weintroub, and Lindy Blackburn are co-Principal Investigators of the ngEHT program.

On April 10th, 2019, the International Event Horizon Telescope Collaboration released the first image of a supermassive black hole. A bright ring of emission at the heart of the Virgo A galaxy revealed a black hole, known as M87, that has a mass of 6.5 billion Suns. Einstein’s theory of gravity spectacularly passed this new test in this extreme cosmic laboratory. For this work, the EHT Collaboration will receive the Breakthrough Prize in Fundamental Physics this November.

Black holes are now accessible for direct imaging, objects with gravity so sharp that light can not escape. More precise gravity tests can now be considered, and it is possible to study in detail the processes by which supermassive black holes energize the brightness and dynamics of most galaxy cores. The next-generation EHT (ngEHT) will sharpen the focus on black holes and allow researchers to move on to the event horizon from still-imagery to real-time space-time videos.

Announcement of the Next Generation Event Horizon Telescope

(Credits: Harvard CFA)

New telescopes and technology will allow scientists to study the black holes in more frequencies, significantly improve our understanding. Each observatory will have to be equipped with ways to record and transfer an incredible amount of data.

The team observed the supermassive black hole in the Milky Way two years ago, but its mercurial character has made the analysis quite tricky. Doeleman confirms that they are working hard on it, so we might soon see an equally breathtaking image. As he said, the best is truly yet to come.

 

Researchers report that Venus might have been habitable for millions of years

Researchers report that Venus might have been habitable for years

Venus is poisonous and a super-heated for a planet that is named after a Roman goddess of love and beauty. All early missions to Venus have provided researchers with a valuable lesson: Venus is like a giant pressure cooker. Although the planet is the second closest planet to the sun, it’s considerably the hottest of the eight planets in our solar system. Its thick atmosphere is mostly made of carbon dioxide with clouds of sulfuric acid, which traps the sun’s heat and creates a runaway greenhouse effect.

But as it turns out; it hasn’t always been this way.

In a recent research study, submitted at the 2019 Joint Meeting of the European Planetary Science Congress, Division for Planetary Sciences, States that Venus possibly had stable temperatures and was home to “fluid water” for 2 to 3 billion years. This was until more than 700 million years ago, and then a drastic transition took place that reshaped the planet and resurfaced around 80 percent of it.

“We hypothesize that Venus may have had a stable climate for billions of years,” says planetary scientist Michael Way from NASA’s Goddard Institute for Space Studies.

“It is possible that the near-global resurfacing event is responsible for its transformation from an Earth-like climate to the hellish hot-house we see today.”

This period of Earth-like climatic stability with the presence of water in Venusian oceans may have lasted for up to 3 billion years, the researchers think, based on several simulations of what the ancient planet’s climate conditions may have been like.

Across a number of hypothetical scenarios run at different points in history contemplating both deep (310-metre deep) and shallow (10-metre deep) affected oceans, and a water-world situation where an imaginary sea covered all of an ‘aquaplanet’ Venus ,the results suggest that Venus could have supported liquid water, with moderate surface temperatures on the planet of 20 to 40 degrees Celsius (68 to 104 Fahrenheit).

life on venus

Image credit: Pixabay

At least, this would have been the case as far back as 4.2 billion years ago, right up until about 700 million years ago. Somewhere around that time, something mysterious has happened on Venus, and eternally since the planet has been incredibly hot, with a toxic, heat-trapping ‘greenhouse effect’ atmosphere dominated by carbon dioxide and nitrogen.

“Our models show that there is a real possibility that Venus could have been habitable and radically different from the Venus we see today,” way says.

“This opens up all kinds of associations for exoplanets found in what is called the ‘Venus Zone,’ which may result in host liquid water and temperate climates.”

The findings were presented at the EPSC-DPS Joint Meeting 2019 in Geneva, Switzerland.

 

Mass of Neutrinos that has perplexed the concepts of Physics has been narrowed down

Mass of Neutrinos, the perplexing concept of Physics has been narrowed down

An enormous experiment to pin down the mass of one of the most perplexing particles in the Universe has placed a cover on how massive the neutrino really might be.

What was once considered massless, is now thought that the mass of the particle weighs no more than one electronvolt. It may not be an accurate response, but it brings us one step closer to a satisfactory solution to one of the greatest secrets of modern physics.

Neutrinos are odd. They are among the Universe’s most abundant particles, yet challenging to identify. Because of their unique characteristics, they communicate very little with ordinary matter.

Billions of neutrinos are currently zipping through your body. You can see why it’s called’ particles of the ghost.’

After years of testing of their plant in Germany, the Karlsruhe Tritium Neutrino (KATRIN) test started its test campaign to calculate the resting mass of the neutrino last spring.

At a meeting in Japan earlier this month, officials produced their first batch of results.

The findings have still not been released, and while there’s a long way to go, the researchers have divided estimates that were previously considered as possible, down from the previous upper limit of around 2 electronvolts to just 1.

Unlike units of pounds and kilograms, this measurement isn’t an easy one to picture. MIT physicist Joseph Formaggio and leading member of the KATRIN experimental group suggests starting tiny and then going more diminutive.

“Each virus is made up of roughly 10 million protons,” Formaggio said to MIT News writer, Jennifer Chu.

“Each proton weighs about 2,000 times more than each electron inside that virus. And what our results proved is that the neutrino has a mass less than 1/500,000 of a single electron!”

As it happens, nobody is astounded that the base mass of a neutrino may be so inconceivably low. When they were first recommended as part of the Standard Model of particle physics, it was assumed the particles didn’t have any mass at all.

This assumption was empirically challenged during the late 1990s by the results of a landmark experiment demonstrating neutrinos streaming from the Sun changed form in a way that meant their mass couldn’t be zero.

So if it’s not zero, what is it? For more than two decades, various experiments have done their best to constrain the limits on just how big or small it might be.

The main issue is that neutrinos do not interact with other particles. The only interaction they have is with the kind of particles we build measuring tools from via the nuclear force.

“Neutrinos are strange little particles,” says physicist Peter Doe from the University of Washington.

“They’re so universal, and there’s so much we can learn once we determine this value.”

 

New photos emerge of the bizarre substance found on the surface of the moon

New photos emerge of the bizarre substance found on the surface of the moon

The lunar exploration program of China has recently published pictures that offer us a glimpse of the strange material found on the far side of the moon.

Yutu-2, the Chang’e-4 mission’s lunar rover, caught notice last month after his drive team spotted something extraordinary while roving close to a tiny crater. Our Space, which announced the results on Aug 17th, used terms that can be translated as “gel-like”, as per the Chinese-language science outreach publication. This concept triggered widespread interest and speculation among lunar researchers.

Scientists have now seized a look at that curious material, thanks to a post released over the weekend by Our Space via its WeChat social media account. Along with new images of the stuff on the moon, the post details how the  Yutu-2 team carefully approached the crater to analyze the specimen, despite risks.

Last month, the team behind the lunar rover claimed it had found a “colored mysterious substance” as described in the rover’s diary, which claimed that the material’s shape and the color are significantly different from the surrounding lunar soil.”

The pictures were taken by the obstacle-avoidance camera of the rover, which ensures that nothing is bumped into by the rover. The red and green shapes in the above picture are the most presumable artifacts of the accounts of the rover’s Visible and Near-Infrared Spectrometer.

So what exactly is it? Give it a look…

 

The best guess we have so far is that it’s just a piece of lunar glass that was formed during a meteor impact.

The Chinese researchers will likely release more images and spectrometer readings of the mysterious substance in the future.

It will have to at the very least wait for the Sun to rise again to continue its investigation. And that’s roughly a week from now.

 

Researchers discover galaxies undergoing dramatic transitions

Researchers discover galaxies undergoing dramatic transitions

We tend to think of the galactic system occurrences as those that occur uncommonly slowly compared to our short human life. It’s not always the case, though.

Six galaxies have just experienced an enormous transformation in just a matter of months in a moving way. They have moved from relatively peaceful galaxies to active quasars-the brightest of all galaxies, blasting vast quantities of radiation into the Universe.

This is not only incredibly amazing, but these occurrences could assist resolve a long-standing discussion about what generates the light in a specific galaxy type. In reality, they may show a sort of galactic nucleus activity that was earlier unknown.

The six galaxies began as galaxies of the low-ionization nuclear emission-line region (LINER); in terms of brightness, it’s kind of like being a galactic particle.

A third of all known galaxies are brighter than those with dormant supermassive black holes in the center, but not as bright as active galaxies (known as Seyfert galaxies), whose supermassive black holes are cosmic.

Now, the most brilliant of such active galaxies are quasar galaxies; indeed, they are among the most colorful objects in the Universe. The light and radio emissions we see are triggered by black hole material, called an accretion disk.

That disk includes dust and gas swirling at tremendous speeds like water running down a drain, creating enormous friction as it is pulled by the black hole’s substantial gravitational force in the center. This friction generates intense heat and light; large spray tanks emit radio waves from the polar regions of the black hole.

But when a team of astronomers led by the University of Maryland astronomer Sara Frederick walked through the first nine months of automated sky survey information from the Zwicky Transient Facility, they discovered six LINER galaxies doing something strange.

changes in galaxies

Image credit: Pixabay

“We first believed we observed a tidal disruption event for one of the six objects, which occurs when a star goes too close to a supermassive black hole and gets shredded,” Frederick said.

Frederick and her colleagues want to understand how a previously quiet galaxy with a calm nucleus can suddenly transition to a bright beacon of galactic radiation. To learn more, they performed follow-up observations on the objects with the Discovery Channel Telescope, which is operated by the Lowell Observatory in partnership with UMD, Boston University, the University of Toledo and Northern Arizona University. These observations helped to clarify aspects of the transitions, including how the rapidly transforming galactic nuclei interacted with their host galaxies.

 

jupiter flash

Astronomers reveal the mystery about the asteroid which smashed into Jupiter

On August 7, 2019, a large space rock slammed through Jupiter. It was a rare flash of light and was bright enough to be detected through telescopes. Texas-based astronomer Ethan Chappel detected it.

The cause of this smash was a tiny asteroid, which had a density consistent with that of meteors that are equal components of stone and iron, according to a new evaluation.

The meteor exploded in the upper atmosphere of Jupiter, about 80 kilometres above the cloud, releasing energy equal to 240 kilotons of TNT-just over half of the power from the meteor explosion over Chelyabinsk in 2013.

According to an evaluation performed by Ramanakumar Sankar and Csaba Palotai of the Florida Institute of Technology, the impactor was probably 39 feet to 52 feet (12 to 16 meters) broad, with a mass of about 408 metric tons (450 tons).

Ricardo Hueso, who is a researcher at the University of the Basque Country, also analyzed the data of the impact event and arrived at similar conclusions about the size and mass of the asteroid. Hueso remarked that since 2010, the August incident was probably the second brightest of the six Jupiter effects that were observed.

“Many of these objects hit Jupiter without being spotted by observers on Earth,” Hueso reported in a statement. “However, we now estimate 20-60 similar objects and their impact with Jupiter each year. Because of Jupiter’s large size and gravitational field, this impact rate is 10,000 times larger than the impact rate of similar objects that hit Earth.”

The new studies were aided by an open-source software program called DeTeCt, which was explicitly designed to identify impacts on Jupiter. DeTeCt was developed by Hueso and French amateur astronomer Marc Delcroix.

Ethan Chappel used DeTeCt to analyze the flash. He then contacted Delcroix and Hueso, who reached out to their connections in the amateur astronomy community to see if anyone else had observed the impact.

“This event has galvanized the amateur community, and the number of observers and the volume of data being prepared is increasing rapidly,” Delcroix said in the same statement. “DeTeCt is a fantastic showcase for professional-amateur collaboration.”

The new results about the 7th August impact were conferred on Monday, September 16 at a joint meeting of the European Planetary Science Congress and the American Astronomical Society’s Division for Planetary Sciences in Geneva.

International Space Agencies Are Going to Smash a Spacecraft Into an Asteroid

Space agencies are going to smash a spacecraft into an asteroid

The time has come to smash a spacecraft into an asteroid to test the Earth’s defence capability.

The joint spacecraft mission known as the Asteroid Impact Deflection Assessment (AIDA), will consult with specialists coming together from the United States Space Agency, NASA, and the European Space Agency (ESA).

The target asteroid is Didymos B, the smaller in the Didymos binary system. The spacecraft is the Double Asteroid Redirection Test (DART) of NASA. The main reason behind this is to test whether an impact on a spaceship can deflect the trajectory of an asteroid as a means of protecting Earth from hazardous space rocks.

“It is important that Europe plays a leading role in AIDA, an innovative mission that was originally developed by the ESA research back in 2003, remarked Ian Carnelli from ESA.

“An international effort is an appropriate way forward and, the planetary defence is in everyone’s interest,” he further added.

There are currently 850 “near-Earth asteroids” (NEAs) on ESA’s list and over 18,000 known “near-Earth objects” (NEOs), as per the reports of the agency.

Impacts of small space rocks with Earth are comparatively common, and although more substantial consequences are rarer, they can cause catastrophic damage.

The impact of an asteroid collision on Earth depends on many factors, such as the position and location of impact, and the physical properties of the asteroid.

While we have the technology available with us to mitigate such a threat from an asteroid, it has never been tested in sensible conditions.

Researchers are examining the viability of diverting an asteroid by crashing a spacecraft to see if the technique is a feasible planetary defence method.


One of two Didymos double asteroids between Earth and Mars is under consideration, which they aim to deflect the orbit of using one spacecraft’s effect.

A second observation craft will examine the site of the crash and collect information on the impact of the collision.

NASA is providing the Double Asteroid Impact Test (DART) spacecraft, which is all set to collide with its target in September 2022 at a speed of at 6.6 km/s. It is already under development.

An Italian-made miniature CubeSat called LICIACube will record the moment of impact.

ESA will launch a Hera probe in October 2024 to study the target asteroid post-impact.

The results and observations of Hera will allow researchers to transform the experiment into a technique that could be repeated were there a real threat.