I wrote earlier this week about the latest successful test of Starship and the capture of the Super Heavy booster by grabbing arms of the landing tower. This was quite a feat, but it should not eclipse what was perhaps even bigger space news this week – the launch of NASAs Clipper probe to Europa. If all goes well the probe will enter orbit around Jupiter in 2030.

Europa is one of the four large moons of Jupiter. It’s an icy world but one with a subsurface ocean – an ocean that likely contains twice as much water as the oceans of Earth combined. Europa is also one of the most likely locations in our solar system for life outside Earth. It is possible that conditions in that ocean are habitable to some form of life. Europa, for example, has a rocky core, which may still be molten, heating Europa from the inside and seeding its ocean with minerals. Chemosynthetic organisms survive around volcanic vents on Earth, so we know that life can exist without photosynthesis and Europa might have the right conditions for this.

But there is still a lot we don’t know about Europa. Previous probes to Jupiter have gathered some information, but Clipper will be the first dedicated Europa probe. It will make 49 close flybys over a 4 year primary mission, during which it will survey its magnetic field, gravity, and chemical composition. Perhaps most exciting is that Clipper is equipped with instruments that can sample any material around Europa. The hope is that Clipper will be able to fly through a plume of material shooting up geyser-like from the surface. It would then be able to detect the chemical composition of Europa material, especially looking for organic compounds.

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It is now generally accepted that 66 million years ago a large asteroid smacked into the Earth, causing the large Chicxulub crater off the coast of Mexico. This was a catastrophic event, affecting the entire globe. Fire rained down causing forest fires across much of the globe, while ash and debris blocked out the sun. A tsunami washed over North America – one site in North Dakota contains fossils from the day the asteroid hit, including fish with embedded asteroid debris. About 75% of species went extinct as a result, including all non-avian dinosaurs.

For a time there has been an alternate theory that intense vulcanism at the Deccan Traps near modern-day India is what did-in the dinosaurs, or at least set them up for the final coup de grace of the asteroid. I think the evidence strongly favors the asteroid hypothesis, and this is the way scientific opinion has been moving. Although the debate is by no means over, a majority of scientists now accept the asteroid hypothesis.

But there is also a wrinkle to the impact theory – perhaps there was more than one asteroid impact. I wrote in 2010 about this question, mentioning several other candidate craters that seem to date to around the same time. Now we have a new candidate for a second KT impact – the Nadir crater off the coast of West Africa.

Geologists first published about the Nadir crater in 2022, discussing it as a candidate crater. They wrote at the time:

“Our stratigraphic framework suggests that the crater formed at or near the Cretaceous-Paleogene boundary (~66 million years ago), approximately the same age as the Chicxulub impact crater. We hypothesize that this formed as part of a closely timed impact cluster or by breakup of a common parent asteroid.”

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Of every world known to humans outside the Earth, Mars is likely the most habitable. We have not found any genuinely Earth-like exoplanets. They are almost sure to exist, but we just haven’t found any yet. The closest so far is Kepler 452-b, which is a super Earth, specifically 60% larger than Earth. It is potentially in the habitable zone, but we don’t know what the surface conditions are like. Within our own solar system, Mars is by far more habitable for humans than any other world.

And still, that’s not very habitable. It’s surface gravity is 38% that of Earth, it has no global magnetic field to protect against radiation, and its surface temperature ranges from -225°F (-153°C) to 70°F (20°C), with a median temperature of -85°F (-65°C). But things might have been different, and they were in the past. Once upon a time Mars had a more substantial atmosphere – today its atmosphere is less than 1% as dense as Earth’s. That atmosphere was not breathable, but contained CO2 which warmed the planet allowing for there to be liquid water on the surface. A human could likely walk on the surface of Mars 3 billion years ago with just a face mask and oxygen tank. But then the atmosphere mostly went away, leaving Mars the dry barren world we see today. What happened?

It’s likely that the primary factor was the lack of a global magnetic field, like we have on Earth. Earth’ magnetic field is like a protective shield that protects the Earth from the solar wind, which is charged so the particles are mostly diverted away from the Earth or drawn to the magnetic poles. On Mars the solar wind did not encounter a magnetic field, and it slowly stripped away the atmosphere on Mars. If we were somehow able to reconstitute a thick atmosphere on Mars, it too would slowly be stripped away, although that would take thousands of years to be significant, and perhaps millions of years in total.

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The state of modern science and technology is truly amazing, much more so than the fake stuff that people like to spread around. Gravitational waves have opened up an entirely new type of astronomy, a way to explore the universe through very subtle ripples in spacetime produce by powerful gravitational events. Einstein predicted the existence of gravitational waves in 1916, but it took decades to develop the technology to actually detect them. Their existence was inferred from neutron star observations in 1974, but they were not directly detected until 2015, almost a century after their prediction.

The Laser Interferometer Gravitational Wave Observer (LIGO) uses the interference at the intersection of two lasers at right angles to each other to detect tiny fluctuations in spacetime. Each laser travels through an arm 4 kilometers long. It is sensitive enough to detect changes 1/10,000 the diameter of a proton.

Using LIGO many gravitational wave events have been detected, all involving the merger of massive bodies – some combination of neutron stars and black holes. A new study, however, uses computer simulations to predict another potential source of gravitational waves – collapsars.

What are collapsars? They result from the death of rapidly spinning large stars, 15-20 solar masses. When they run out of fuel to keep their cores burning they rapidly collapse under their massive gravity, and then they explode from all that matter crashing into itself. This results in the formation of a black hole at the core, surrounded by a lot of mass that is leftover. This mass swirls rapidly around the black hole and is quickly consumed, within minutes. This large rapidly moving mass is what causes the gravitational waves – at least that is what is predicted by the current model. d

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Astronomers have discovered multiple “pits” on the surface of the moon – these look superficially like craters, but on closer inspection are actually vertical pits. There has been considerable speculation that these pits might be cave openings. Now, an analysis of data from the Lunar Reconnaissance Orbiter from 2010 reveals that at least one of these pits is in fact a cave opening.

The pit is located in the Mare Tranquillitatis, near the site of the Apollo 11 landing (the Sea of Tranquility) and is therefore called the Mare Tranquillitatis Pit (MTP). It is 100 meters across and 130-170 meters deep. The study used radar data at a downward angle which was able to image the sides of the bottom of the pit, showing that there is a possible conduit there for an underground cave system. This conduit is at least tens of meters long, but could be much larger.

While this is exciting, it’s not surprising. One hypothesis is that these lunar pits are “skylights” of underground cave systems, carved out by lava tubes when the Moon was more geologically active. If true, then it’s possible that they are extensive, and can also be quite large. Why is this so exciting?

There are two main reasons NASA and others are interested in lunar caves. One is geological – such cave systems might be billions of years old, and therefore can preserve lunar rocks by protecting them from the radiation and micro meteors that pummel the lunar surface. When we send astronauts back to the Moon (or even just highly capable robots) they could explore these caves and are likely to make some interesting discoveries about the Moon.

But the second application is the most intriguing. I have deliberately buried the lede here, partly because I suspect most readers know where this story is going. Such lunar caves could be ideal locations for future lunar bases. This is for the same reason they are good locations to do some geological investigations on the Moon – the caves are protected. This is something that science fiction shows give very short shrift to, and for this reason perhaps is greatly underappreciated by the public. Space is a very dangerous place, and not just because it’s largely a cold vacuum. Space is full of radiation, and stellar systems are full of fast-moving debris.

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Commenter Lal asks in the topic suggestions:

“Media reports that light has been travelling from that distant galaxy for 13 and a half billion years, which I assume is true, but this neither represents the original nor the current distance to that galaxy in terms of light years. I would be interested to know where we lie in the expanding universe compared to these distant galaxies.”

This is a good question, and is challenging to grasp. We need experts who have been thinking about this for decades and who actually understand what’s happening and who can explain it well. Here, I think, is an excellent discussion of this very question. I’ll give a quick summary, but for those interested, you may want to read the full article.

The basic background is that, according to modern cosmological theory, which includes the Big Bang, the universe was a singularity – one point that contained all of spacetime and all matter and energy – about 13.7 billion years ago. This point underwent rapid expansion, at first very rapid, called the inflationary period. Then it continued to rapidly expand, although at a much slower pace, although this rate of expansion has been increasing over time due to dark energy. What happens to the universe when it expands? It’s important to note first that the universe is not expanding into space – space-time itself is expanding.

Matter in the universe gets less dense and hot as the universe expands. At first matter was too hot for particles to exist. Once it cooled enough for protons and neutrons to exist, they mostly formed into hydrogen, but that was still too hot to hold onto electrons so the matter was all plasma. That eventually cooled enough for hydrogen (and some helium and a tiny bit of lithium) atoms to exist – at about 380,000 years after the Big Bang. Since then the matter in the universe has continued to cool and become less dense. However, it was also able to form stars, galaxies, heavier elements, and then lots of interesting things like people.

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As of this writing, there are 5,573 confirmed exoplanets in 4,146 planetary systems. That is enough exoplanets, planets around stars other than our own sun, that we can do some statistics to describe what’s out there. One curious pattern that has emerged is a relative gap in the radii of exoplanets between 1.5 and 2.0 Earth radii. What is the significance, if any, of this gap?

First we have to consider if this is an artifact of our detection methods. The most common method astronomers use to detect exoplanets is the transit method – carefully observe a star over time precisely measuring its brightness. If a planet moves in front of the star, the brightness will dip, remain low while the planet transits, and then return to its baseline brightness. This produces a classic light curve that astronomers recognize as a planet orbiting that start in the plane of observation from the Earth. The first time such a dip is observed that is a suspected exoplanet, and if the same dip is seen again that confirms it. This also gives us the orbital period. This method is biased toward exoplanets with short periods, because they are easier to confirm. If an exoplanet has a period of 60 years, that would take 60 years to confirm, so we haven’t confirmed a lot of those.

There is also the wobble method. We can observe the path that a star takes through the sky. If that path wobbles in a regular pattern that is likely due to the gravitational tug from a large planet or other dark companion that is orbiting it. This method favors more massive planets closer to their parent star. Sometimes we can also directly observe exoplanets by blocking out their parent star and seeing the tiny bit of reflected light from the planet. This method favors large planets distant from their parent star. There are also a small number of exoplanets discovered through gravitational microlensing, and effect of general relativity.

None of these methods, however, explain the 1.5 to 2.0 radii gap. It’s also likely not a statistical fluke given the number of exoplanets we have discovered. Therefore it may be telling us something about planetary evolution. But there are lots of variables that determine the size of an exoplanet, so it can be difficult to pin down a single explanation.

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University of Central Lancashire (UCLan) PhD student Alexia Lopez, who two years ago discovered a giant arc of galaxy clusters in the distant universe, has now discovered a Big Ring. This (if real) is one of the largest structures in the observable universe at 1.3 billion light years in diameter. The problem is – such a large structure should not be possible based on current cosmological theory. It violates what is known as the Cosmological Principle (CP), the notion that at the largest scales the universe is uniform with evenly distributed matter.

The CP actually has two components. One is called isotropy, which means that if you look in any direction in the universe, the distribution of matter should be the same. The other component is homogeneity, which means that wherever you are in the universe, the distribution of matter should be smooth. Of course, this is only true beyond a certain scale. At small scale, like within a galaxy or even galaxy cluster, matter is not evenly distributed, and it does matter which direction you look. But at some point in scale, isotropy and heterogeneity are the rule. Another way to look at this is – there is an upper limit to the size of any structure in the universe. The Giant Arc and Big Ring are both too big. If the CP is correct, they should not exist. There are also a handful of other giant structures in the universe, so these are not the first to violate the CP.

The Big Ring is just that, a two-dimensional structure in the shape of a near-perfect ring facing Earth (likely not a coincidence but rather the reason it was discoverable from Earth). Alexia Lopez later discovered that the ring is actually a corkscrew shape. The Giant Arc is just that, the arc of a circle. Interestingly, it is in the same region of space and the same distance as the Big Ring, so the two structures exist at the same time and place. This suggests they may be part of an even bigger structure.

How certain are we that these structures are real, and not just a coincidence? Professor Don Pollacco, of the department of physics at the University of Warwick, said the probability of this being a statistical fluke is “vanishingly small”. But still, it seems premature to hang our hat on these observations just yet. I would like to see some replications and attempts at poking holes in Lopez’s conclusions. That is the normal process of science, and it takes time to play out. But so far, it seems like solid work.

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This is one of the biggest thought experiments in science today – as we look for life elsewhere in the universe, what should we be looking for, exactly? Other stellar systems are too far away to examine directly, and even our most powerful telescopes can only resolve points of light. So how do we tell if there is life on a distant exoplanet? Also, how could we detect a distant technological civilization?

Here is where the thought experiment comes in. We know what life on Earth is like, and we know what human technology is like, so obviously we can search for other examples of what we already know. But the question is – how might life different from life on Earth be detected? What are the possible signatures of a planet covered in living things that perhaps look nothing like life on Earth. Similarly, what alien technologies might theoretically exist, and how could we detect them?

A recent paper explores this question from one particular angle – are there conditions on a planet that are necessary for the development of technology? They hypothesize that there is an “oxygen bottleneck”, a minimum concentration of oxygen in the atmosphere of a planet, that is necessary for the development of advanced technology. Specifically they argue that open air combustion, which requires a partial pressure (PO2) of oxygen of ≥ 18% (it’s about 21% on Earth), is necessary for fire and metallurgy, and that these are necessary stepping stones on the path to advanced technology.

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One of the biggest questions of exoplanet astronomy is how many potentially habitable planets are out there in the galaxy. By one estimate the answer is 6 billion Earth-like planets in the Milky Way. But of course we have to set parameters and make estimates, so this number can vary significantly depending on details.

And yet – how many exoplanets have we discovered so far that are “Earth-like”, meaning they are a rocky world orbiting a sun-like star in the habitable zone, not tidally locked to their parent star, with the potential for liquid water on the surface? Zero. Not a single one, out of the over 5,500 exoplanets confirmed so far. This is not a random survey, however, because it is biased by the techniques we use to discover exoplanets, which favor larger worlds and worlds closer to their stars. But still, zero is a pretty disappointing number.

I am old enough to remember when the number of confirmed exoplanets was also zero, and when the first one was discovered in 1995. Basically since then I have been waiting for the first confirmed Earth-like exoplanet. I’m still waiting.

A recent simulation, if correct, may mean there are even fewer Earth-like exoplanets than we think. The study looks at the transition from a planet like Earth to one like Venus, where a runaway greenhouse effect leads to a dry and sterile planet with a surface temperature of hundreds of degrees. The question being explored by this simulation is this – how delicate is the equilibrium we have on Earth? What would it take to tip the Earth into a similar climate as Venus? The answer is – not much.

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