Is our solar system similar to other solar systems? That’s actually a complex question with many layers. We know that there are different types of stars, varying mainly on their mass and age. We have a yellow sun, but a system around a red, orange, or blue sun is likely to be very different. We also know that at different relative locations in the galaxy the composition of the gas clouds out of which stellar systems form can be very different. One specific difference is known as “metallicity” – which refers to the amount of elements heavier than hydrogen or helium. Older stars were formed before a lot of heavier elements were made, so they have lower mellacity. This feature also varies within our galaxy, with higher metallicity closer to the center. And different galaxies have different mettalicity.

But what should we expect from a stellar system with a yellow sun at a similar location in our own galaxy? If the known variables are the same, should we expect the compositions of elements to also be roughly the same? This gets to the deeper scientific question of how typical our system is. Can we assume that the rest of the universe is similar to our tiny little corner of it? To counteract the hubris of humanity in thinking that we are somehow special, scientists try to follow the principle to assume that we are ordinary. But is that assumption always correct?

How can we even answer this question for stars that are light-years away? The primary method that we use is spectral analysis  (spectroscopy) – an awesomely powerful tool that allows us to identify specific elements and chemicals simply from analyzing the light we see from it. You can do a spectral analysis in a lab on a sample, or you can do it with telescopes on distant objects. The method is actually fairly simply. You use a prism to spread out the light into its color spectrum (like a rainbow). You can then analyze the emission lines or absorption lines which are like a signature.

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Amid the current crisis there is some good news and significant progress – America is returning to crewed spaceflight after a 9 year gap. Scheduled for May 27th is the first crewed mission of the Space X Dragon capsule, which will send two astronauts to the ISS,  Bob Behnken and Doug Hurley. Technically this is the last test flight of the Crew Dragon capsule (the mission is called Demo-2). Last March the Demo-1 mission sent an uncrewed Dragon capsule to the ISS. The two astronauts will remain for an “extended” stay on the ISS, and then return in the capsule, splashing down in the Atlantic and recovered by a Space X recovery vessel.

If successful this will mark the return of America’s ability to send astronauts into orbit. It will also mark the first time a commercial company has done so, and is a significant milestone in the commercialization of space flight. The launch will be done in cooperation with NASA, lifting off from Pad 39A, which is the same one that launched Apollo and the Space Shuttle. The capsule will also be lifted to the ISS by a Falcon 9 rocket, which is also made by Space X. This is the rocket that can land again vertically and be reused.

There has been some back and forth on whether or not the Crew Dragon capsules themselves can be reused. Initially Musk predicted that the capsule could be reused many times, reducing the cost of getting astronauts into space. Then in 2018 they quietly backed away from this goal. The reason is that after a salt-water landing, it is time consuming (a year) and expensive to service the capsule for reuse. In order for capsule reuse to be practical you need a dry landing, which was the original plan of Space X. Apparently that has proven technologically difficult, so Space X is settling for salt-water landings, which means no reuse. However, the Crew Dragon capsule can more easily be refurbished and reused for Cargo Dragon missions without astronauts. Therefore, they will be used for this purpose. Space X has reused multiple Cargo Dragon capsules multiple time.

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This is an interesting idea that will probably not be actually implemented (although not impossible) but does raise some important points. A paper explores the viability of using urea from human urine as an agent in lunar concrete. Why is something like this even being considered?

The overwhelmingly dominant factor of building anything on the Moon is that it costs about $10,000 to put one pound of anything into Earth orbit, and more to take it to the Moon (although most of the energy would be used just getting into orbit). This is why it is a high priority for NASA to reduce the cost of getting stuff into space. Elon Musk has also made this a priority and SpaceX is geared mainly toward this purpose. Even if they reach their goal of reducing the cost by 10 fold, to about $1000 per pound, that still adds up when you are trying to build an entire Moon base. One solution is to use as much native material as possible.

Let’s talk a bit about the lunar regolith. The term regolith just refers to any loose material on top of the rocks on a world’s surface. The Earth has regolith, we call it dirt, sand, or soil. The lunar regolith is the result of micrometeors pulverizing the lunar surface for billions of years. In most locations the regolith extends down 4-5 meters, but can be as deep as 15 meters in places. Because of the absence of natural erosion from wind, water, or biological activity, the lunar regolith remains sharp and pointy. So the Moon is basically covered with a deep blanket of fine but jagged dust.

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What does an extinct mollusk have to do with the Moon? This is one of those amazing science stories that ties together multiple disciplines and lines of evidence into one elegant narrative. In this case a detailed analysis of a 70 million year old mollusk shell has given scientists a critical piece of information that will help them model the Earth-Moon system.

Let’s start with the Moon – astronomers know that the Moon is moving farther away from the Earth at a constant rate, 3.82 centimeters per year. We can precisely measure this because the Apollo missions left corner reflectors on the surface of the Moon, and we can shoot lasers off those reflectors and measure the round-trip travel time. Because scientists have also precisely measured the speed of light, we can use this round-trip time to calculate the exact distance between the laser on Earth and the reflector on the Moon.

Why is the Moon moving away from the Earth? In a word – tides. Tidal forces from nearby large objects causes a bulge to form. We are most familiar with this phenomenon because of the bulge in the ocean caused mostly by the Moon (and to a lesser degree the Sun) which we experience locally as a rising and falling of the sea. The tidal bulge on the Earth is slightly ahead of the Moon in its orbit, because the Earth is spinning faster than the Moon. This leading bulge tugs slightly on the Moon, accelerating it into a higher orbit farther from the Earth. This represents a transfer of momentum from the Earth to the Moon via gravity, which not only moves the Moon farther away, but slows down the rotation of the Earth (and the conservation of angular momentum is obeyed).

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We can quibble about whether or not the Big Bang should be considered an explosion, or whether it happened “in the universe.” It was the expansion of spacetime that is the universe. In any case, astronomers have detected what they think is the biggest explosion (at least discovered so far) since the big bang –  in the Ophiuchus galaxy cluster, 390 million light years from us. The explosion is essentially a bubble with a diameter the size of 15 Milky Way galaxies – about 1.5 million light years across. That’s five time bigger than the previous record holder.

Astronomers first suspected something was going on when they discovered a big X-ray bubble. They report:

It was discovered in the Chandra X-ray image by Werner and collaborators, who considered a possibility of it being a boundary of an AGN-inflated bubble located outside the core, but discounted this possibility because it required much too powerful an AGN outburst.

An AGN is an active galactic nuclei – more on that below. So they initially discounted it because it was too big, but they then followed up with radio observation, and found an identical radio bubble, confirming that this was a real fossil of an ancient explosion, centered around an AGN. So what’s going on here?

Well, astronomers are not sure. The do not know exactly what may have caused some a massively energetic event. But let’s give some background on AGNs – these are supermassive black holes (SMBH) in the centers of galaxies. Most galaxies have them, including our own. But some supermassive black holes are more super massive than others – getting up to billions of solar masses. More importantly to their activity, some of the black holes are feeding, which means that gas and dust are actively swirling around the event horizon forming an accretion disc and then plunging into the incredible gravity well of the black hole. All that gravity represents an unimaginable amount of energy, and when that gas and dust falls in it swirls around at relativistic speeds – near the speed of light.

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The Mars InSight lander is yielding data, and the first slew of papers reporting early results. The two big stories so far is that Mars has more seismic and magnetic activity than previously thought.

One open question is how much tectonic activity there is on Mars. Earth has at least 15 tectonic plates, all moving with respect to each other. When two plates rub up against each other, building up and then releasing energy, this is the major source of Earthquakes. Both Mercury and the Moon, which are smaller and therefore cooled much faster than Earth, have single crust plates. That doesn’t mean they have no seismic activity, because they are also shrinking as their cores continue to cool.

Mars is still a bit of an open question in terms of tectonic activity. It appears likely that Mars does have a tectonic plate system, but much simpler than Earths with fewer plates, and they are moving much more slowly. But this still can allow for some seismic activity. There are other sources of activity as well, such as shrinking and settling. Information on seismic activity from InSight was anticipated to help better understand the geological activity on Mars.

What they have found so far is:

“We identify 174 marsquakes, comprising two distinct populations: 150 small-magnitude, high-frequency events with waves propagating at crustal depths and 24 low-frequency, subcrustal events of magnitude Mw 3–4 with waves propagating at various depths in the mantle.”

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This weekend I was at the AAAS (American Association for the Advancement of Science) meeting in Seattle talking about science communication. The meeting often creates a pulse of science-news reporting, base on all the presentations and lectures there. One talk I didn’t get to see was by Dr. Anthony Beasley, director of the US National Radio Astronomy Observatory in Charlottesville, Virginia. He argued that the search for extraterrestrial intelligence (SETI) should “come in from the cold” and be incorporated into every aspect of astronomy. Let me go over the reasons why I completely agree.

First, doing so would be a great boost to SETI itself. For example, private funding has recently allowed a SETI project using the VLA (Very Large Array) which the project managers argue will increase the power of SETI by 10-100 fold. Taking SETI from an isolated project here and there to the mainstream of astronomy would certainly greatly magnify the power of SETI searches, and therefore increase the probability of achieving a positive result.

Further, as Seth Shostak has pointed out to us during interviews on the SGU, SETI research does a lot of non-SETI astronomy. Of you are scanning the skies with radio telescopes looking for signals that may be intelligent in origin, you are also gathering a lot of information that can be used for other purposes. So even if SETI never detects such a signal, the effort will not have been wasted. A lot of non-SETI astronomy will still have been done. The broader point is that, by combining SETI with other projects, astronomers are efficiently using equipment and data. What this means is that the question of SETI vs other projects is a false dichotomy. We can do both.

But the biggest question in all of this is – is SETI itself valuable? There are two criteria that are usually brought to bear in answering this question. Mostly people focus on the probability of detecting an ET signal, with critics of SETI arguing that it is probably too small to be worth the effort of searching. Defenders of SETI often focus on the other criterion – the value to humanity if we did detect a signal. In essence SETI is like playing the lottery – the probability of winning is low but the potential benefits are high. How do we balance these two things out?

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There is more mass in the universe, stuff generating gravity, than we can see. This observation lead to the conclusion that there must be “dark matter” in the universe – stuff out there that we cannot see but is exerting its gravitational effect. At least some physicists believe that this stuff may not be a new type of matter, but may be information itself.

How do we know how much “gravity” there is? Mainly by observing the movement of stars, but essentially any observation about the large scale movement in the universe must agree with our calculations about the effects of gravity. These calculations, based on Newton and Einstein’s general theory of relativity, are extremely precise and confirmed by observation. The first one to observe a disconnect between our theories and observation was in 1933, when Swiss astronomer Fritz Zwicky observed the movement of galaxies in the Coma Cluster. When object rotate about each other their momentum is like a force flinging them apart. Gravity has to balance that force with an inward attraction. We can therefore calculate how much gravitational force their needs to be to keep a galaxy cluster together, and Zwicky calculated that there wasn’t enough, but left the question as to how open.

Then in astronomer Vera Rubin made essentially the same observation of the Andromeda galaxy. She found that the stars in the galaxy rotate at about the same velocity regardless of how close they were to the center. Calculations predicted that the closer stars should be moving much faster than the farther stars. In fact, the stars at the outer portions of the galaxy should be flying apart. She concluded there must be dark matter throughout the galaxy creating enough extra gravity to hold the fast moving stars together. In fact, there must be more than 5 times as much of this dark matter than normal matter we can see. Most of the matter of the universe is invisible and mysterious.

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(Side note: I  had a small injury to my dominant index finger which is slowing down my typing considerably. My blog posts may be shorter than usual this week.)

When I was recently in New Zealand and Australia, I tried very hard to get a good look at the Southern sky. It was a lot harder than I thought it would be. Over a period of two weeks I had really one opportunity, and I had to drive 40 minutes out of town to do it. It really drove home for me how bad light pollution has become. Depending on where you live you may spend most of your life unable to see the night sky. A true dark sky experience is also amazing – only then do most people realize what they are missing.

We are now realizing that the ability to see the night sky is a resource, one we did not collectively realize we were sacrificing for the convenience (and even safety) of lighting. No one made this decision, it evolved organically. But now we have to make a risk-reward calculation and decide how best to balance our lighting needs with preserving the dark sky as a natural resource.

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Hygiea is the fourth largest asteroid in the asteroid belt, after Ceres, Vesta and Pallas. Recent observations of Hygiea are now challenging the distinction between an asteroid and a dwarf planet.

For some context, Ceres is the largest asteroid and is also the only one that is unambiguously a dwarf planet, by current definitions. Actually, when first discovered in 1801 it was considered a planet. It was the first asteroid discovered in the asteroid belt between Mars and Jupiter, but when more were discovered it became increasingly obvious that Ceres was part of a swarm of objects in a similar orbit. In the 1850s it was then demoted to an asteroid, although was the king of the asteroids, if that is any consolation.

Then in 2006 the International Astronomical Union (IAU) created the new category of dwarf planet and redefined the threshold for being a planet. According to the IAU, in order to be a planet you have to be large enough for gravity to pull your shape into a rough sphere (called hydrostatic equilibrium), not be a moon, orbit the sun, and dominate your orbit. A dwarf planet, rather, is an object with the first three criteria, but has failed to clear out its orbit of other objects. That last bit is what caused Pluto to famously be demoted to a dwarf planet, because its largest “moon”, Charon, was considered to be too large to be just a moon. Pluto and Charon orbit each other around a center of gravity (barycenter) that is outside both worlds.

However, the Pluto-Charon situation shows what a mess the classification system is. For now the five official dwarf planets are Pluto, Ceres, Eris, Makemake, and Haumea. There are many candidate dwarf planets that await further characterization. Charon is controversial. Some astronomers argue  that Pluto-Charon should be considered a double dwarf planet system, largely because the barycenter is outside both worlds. However, other argue that the location of the barycenter is not a strict criterion, because it depends on distance. For example, the barycenter of the Jupiter-Sun system is outside the surface of the sun, because of Jupiter’s distance. For now Charon is a moon, and its “promotion” to dwarf planet is in limbo. Also, the New Horizon data has called into question whether or not Charon is in hydrostatic equilibrium, but that is a separate issue.

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