Bottom of the World, Top of the Rankings

Lots of anniversaries lately. While it’s still technically summer, let’s contemplate Antarctica. Yet, school has started, so let’s learn of… the entire Solar System. That’s right, if you want to know more about space, go to Antarctica. There are multiple reasons, but meteorites are #1.

Antarctica, as modern Solar System science has taught us, is not merely the top source of meteorites. It isn’t just the source that outnumbers all other sources combined. The fruitful regions of Antarctica have yielded more than twice as many meteorites as the entire rest of the world, combined. We just learned of this in the 1969-1970 field season, when by pure luck a Japanese research expedition brought in the first bounty of space samples.

Before the JARE team (Japanese Antarctic Research Expedition) left, senior scientist Masao Gorai stated that Antarctica’s rocks were now boring, and that he would like a meteorite. (He later claimed he wasn’t being completely serious.) The team, in the course of other fieldwork, happened upon three unusual rocks in one day. Suspecting some might turn out to be meteorites, they took notes and carefully stored them. By the end of the field campaign in a few weeks, they had nine suspected meteorites. Much to Gorai’s surprise, every one of the nine was extraterrestrial, and not fragments of one object, either.

Antarctic specimens now dominate the meteorite collections of multiple countries. This “treasure trove” has given us lunar and martian rocks, and rare ones that get degraded or destroyed in other areas of the Earth. No exaggeration: Antarctica upended planetary science.

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No, X Will Not Kill You: 2020 PA

Feels like I just posted the last near miss. In the time it took me to multitask around and check again, another small body (very small, in this case) made a very, very close approach.

The Near-Earth Object (NEO) provisionally designated “2020 PA” made a flyby of just 58,300 km (36,500 miles) from us. That flyby was Aug 1- the same day we spotted 2020 PA. That’s right, had its flyby turned out to be of zero distance- that is, an impact, not a miss at all- we would have had negligible warning time. At best, our response would be to issue a notice to a certain area to take shelter. In NEO circles, this is the “civil defense” response.

Fortunately, 2020 PA did miss, by more than the altitude of our communications satellites (22,300 miles above us). And the object is, within measurement limits, roughly 6 meters across. That’s just not scary for an incoming chunk. If it were a carbonaceous chondrite, a weak petrology, there would barely be any pebbles or gravel from it surviving through our atmosphere and reaching the surface. The threat would be from an airburst and shockwave, but less than the Chelyabinsk event. If 2020 PA were an iron body and quite strong, there would be a decent-sized hole in the ground. The most likely case is that 2020 PA is a stony asteroid, and an “impact” would actually be a hail of gravel, including a few larger bits. It is exactly because the object is just 6 meters across that we just spotted it when we did.

The significance of 2020 PA is more academic and scientific. We first spotted it via the Catalina Sky Survey, using a modest telescope like a good research university might have. As more and more glass is aimed up, we will be seeing such bodies regularly. They’re there; it’s only…

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Small Target Call: Haya 2-plus?

Tomorrow, Jul 27, asteroid 2020 OO1 will make a close pass by us. How close? Well within Earth’s sphere of influence- the zone where our gravity dominates, compared to the pull of, e. g., the Sun. 2020 OO1 will miss us by about 683,000 km (426,000 miles), a good bit farther than the orbit of the moon. Even if it were on a collision course, that disaster wouldn’t make much of a film. 2020 OO1 is a small object, appearing at absolute magnitude 26.4 in telescopes. Depending on whether the body is shiny or dark, mag 26.4 means 10 to 30 meters across, respectively. If we make certain other assumptions on its properties, that would be a Chelyabinsk-class explosion, give or take, with meteorites surviving to the ground.

~683,000 km is close, but not weird. NEOs (Near-Earth Objects) get this close several times per year that we know of. (The last was just Jul 22.) As mag = 26.4 is quite dim, we’ve only been finding bodies like this recently, with fairly big telescopes in thorough search programs. A few times a year, a NEO that we know of comes inside our moon’s orbit. Again, they’re usually the tiny, common members of the population. Truly dangerous impactors- hundreds of meters to a kilometer- are not common, brighter than mag 26, and easier to spot.

What goes around comes around. The Hayabusa 2 probe, having (apparently) taken samples of carbonaceous chondrite Ryugu, is thrusting toward Earth for a Dec 12 arrival. The main Hayabusa craft will deploy a reentry capsule with the Ryugu particles, then fire its thrusters to miss Earth. And then? Assuming Earth return, capsule deployment, and the divert maneuver are all smooth (unlike Hayabusa 1), an extended mission is in consideration. A perfectly viable spacecraft can operate further, and there are no shortage of NEOs (making certain assumptions on properties) to visit. Candidates announced so far are 2001 AV43, 2001 CC21, and/or 1998 KY26. These are small objects, not unlike 2020 OO1.

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Search Status July 2020

Oh, what an [accounting period] it’s been. As of Jun 28, 2020 our census of the Solar System is at:

-959226 total asteroids, 4173 comets
-23163 Near-Earth Objects (NEOs), including 110 comets
-2095 of those Potentially Hazardous Asteroids (PHAs)

Sure, we’ve had a weird lap around the Sun since the last Status. Airborne contagion aside, there was all the action in Chile. This is a hot potato for astronomy in general (observatories all over the Atacama), the LSST as far as this blog is concerned, and of course the ascent of Man overall. Chile has been a peaceful democracy for decades now, with natural resources including global commodities (e.g., the copper you’re using right now, in multiple forms for multiple things). And yet why hasn’t Chile developed further than their current economy… but I digress.

The pace of late 2019 and early 2020 is not bad, actually. The LSST (now the Vera Rubin Observatory) hadn’t yet taken first light, so any holdup in construction and commissioning can’t show up in the stats yet. Meanwhile Pan-STARRS, and more recently ATLAS are going strong with two Hawaiian telescopes- each. (Two more ATLASes (Chile and South Africa) appear to be firmly booked.) Thus, multiple comets have been discovered with close pass possibilities …no, no sirens needed. Possibility only, close only.

The sky beckons. We can monitor telescope output online, no disease necessary. Astronomers don’t even pass around pink eye anymore.

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50th Landi-versary: Murchison

1969, in retrospect, was the year of the Allende meteor event and its recovery– overall, 2 tons of primitive material from the Solar System’s birth. It’s the single largest carbonaceous chondrite meteorite, which we classify as a CV chondrite. Another groundbreaking event happened 50 years ago: the Murchison meteorite, the largest of the CI or CM (aqueous) meteorites.

Fragments recovered near Murchison, Australia total far less than Allende- about 100 kg. This is still huge: CV carbonaceous chondrites are actually just ~1 percent carbon compounds, and only a bit of that is native carbon (graphite, tiny diamonds, etc.). We call these meteorites ‘carbonaceous’ since the ‘ordinary chondrites’ have even less- a fraction of a percent carbon. At the birth of the Solar System, carbon tended to form gases (carbon oxides and hydrides, i. e., organics) which tend to blow away instead of forming solid objects. Despite the fact that the forming Solar System had appreciable carbon, a lot dispersed to the galaxy. But CI and CM carbonaceous chondrite meteorites have more carbon- ~2 or so percent. The CI/CMs had never been heated, not to magma temperature, and not to serious oven temperatures, either. They now retain more light chemicals- organics, sulfates/sulfides, and water.

This is why 100 kg is big and important. Allende and similar chondrites are overwhelmingly rock and other durable minerals. They hold up, even when plunging through our atmosphere in a fiery display. CI/CM chondrites aren’t rock so much as clay. Water exposure has broken down most of their rock into layers or particles; when heated and stressed by atmospheric entry, the particles/layers often disperse instead of landing. Landing and Earth weather then keep damaging them. CI/CMs are thus rare, sought-after examples of the early Solar System.

And then came Murchison…

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50th Landi-versary: Allende

2019 saw lots of hoo-ha. Marketing, promotion, and hype designed to sway the vulnerable masses. And let’s not forget spin, sleight-of-hand, and overall scapegoating about Mexicans.

Here’s one (…ish) Mexican export that needs no hype (among scientists): the meteorite that fell on and around Pueblito de Allende, February 8, 1969. And I mean around: the meteor broke up in a fireball, visible from the southwest US too. Fragments fell on a strewnfield covering 50 square km. Not only was the event witnessed, leading to a swift gathering and curation, but the strewnfield was in a desert. The Allende meteorite, already an extremely primitive object, was recovered with minimal Earth exposure and weathering. We then had a sample, later dated to over 4.5 billion years in history.

Furthermore, the fragments were extensive in another way. They totaled roughly two tons, by a clear margin the largest carbonaceous chondrite known. The average meteorite is not even fist-sized. Enough Allende material exists that some is passed around as a reference sample.

I’ve touched upon the Allende meteorite before, but it bears repeating…

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Search Status July 2019

The forward progress of humanity advancing, as of Jun 29:

-794832 total small bodies in the Solar System, 4096 total comets
-20417 of them Near-Earth Objects (NEOs), including 107 official comets; some transitional objects
-Of those, 1974 Potentially Hazardous Asteroids (PHAs)

On one hand, the Pan-STARRS project successfully added a Pan-STARRS 2 telescope. We’ve also begun regular Zwicky Transient Facility (ZTF) operations, the smaller ATLAS, etc.

On the other hand, observed phenomenon don’t get officially catalogued (as above) ’til their orbits are determined quite securely. This may take a year or three. Having search telescopes around the world (and GAIA, off-Earth) and follow-up ‘scopes is great for tracking. But the rules are set on official asteroid numbering and naming; we must be confident that a small body doesn’t come back around, a few years later, and be mistaken for a new one.

In a few years, the totals will be rising dramatically. Small Solar System Bodies (SSSBs) found by Pan-STARRS/ZTF/etc. will have been tracked by GAIA, LSST, academic teams, even some well-off amateurs. If nothing else, Pan-STARRS/ZTF will spot their quarry again after another go-round. Such official, secured SSSBs will then be in the rolls above.

Last year: Search Status July 2018

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Day of Light, Trail of night…

It’s the International Day of Light! Let’s see what we can see:

-The Zwicky Transient Facility has had its first public data release: a dump of ones and zeroes from about a year of sky searching. The ZTF is a sky survey using the Palomar schmidt telescope with a new detector array. There’s a wide-field-of-view schmidt telescopes, an advanced detector with numerous “chips” in a mosaic, and a fast-slew mount. Put them together, and you’ve got a sensitive survey of objects in the Northern Hemisphere.

The ZTF is, primarily, for hunting supernovae. Exploding stars are bright enough to be seen from the other side of the universe. Each supernova, then, is a measurement of that ‘side of the universe.’ But the program all along knew it would catch asteroids too. Its first night of trials, ZTF bagged one. Depending on how many turn out to be new discoveries, the ZTF asteroid search rate may be multiple objects per night.

-The LSST mirror has reached the observatory in Chile. If the Zwicky sky survey is considered sensitive with a 48″ aperture telescope, then ponder this upcoming LSST. The Large Synoptic Survey Telescope will, in a few years, have an aperture over eight meters, or 320 inches. It will also be in the Southern Hemisphere, where our search efforts have been lagging. The amount of sky imaging that will pour out of the LSST will be so boggling, we are developing new computers just to sift through it all. It has its own, dedicated fiber optic cable leading from Chile, to university server farms in the US.

The heart of the telescope, its primary mirror, was cast and ground in Arizona. Its cargo ship steamed through the Panama Canal and down to Chile last month, and it got trucked up the mountain to the observing site, now being completed.

-All this time, Gaia is quietly taking data…

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Why The Mars Mission Is Also An Asteroid Mission

On Monday, InSIGHT (Interior exploration using Seismic Investigations, Geodesy, and Heat Transport) should land on Mars if all goes well. InSIGHT’s stated mission is to study Mars as a body, telling us of its makeup and structure. But it will also tell us of other bodies- small ones.

InSIGHT carries two major experiments, SEIS and HP3, and two less-promoted ones, RISE and a ‘weather station.’ SEIS is a seismometer, which will feel for ‘marsquakes’. HP3 is a burrowing probe, to measure the heat response of the ground. RISE will track the lander’s motion; since landers are fixed, that will tell us Mars’ motion. The weather instruments are a good thing to have anyway, and will check the SEIS and HP3 data.

In the most direct application, RISE (Rotation and Interior Structure Experiment) will weigh asteroids. That’s right, InSIGHT (like prior Mars missions) can actually weigh passing asteroids. As a body passes another, their gravities pull each other. The tug on Mars is small, but our radio measurements are very precise. Yes, we can gauge Mars being scooted one way, then back as asteroids pass. InSIGHT, solidly planted at a spot, has been wanted for this job. The rover missions span ~15 years, which would be nice. But their roving confounds our assay of Mars’ scoots. Just correcting for wheel slip in the Martian ‘dirt’ is an ongoing topic.

Today, we have the mass of Vesta, the biggest body in the inner Belt, and that of Ceres, the biggest of them all in the middle Belt. The Dawn probe orbited both; its radio gave us mass numbers out to many decimal places. As asteroids slip between Mars and Vesta, and to a lesser extent Mars-Ceres, we will doubly-constrain their gravities, and thus masses. Mass, with size, gives density. Density then tells us what that body is made of- rock, metal (more than twice as dense as rock), or ice (less than half rock’s density).

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Flight Byte: The Coming Dawn

A flight project, Dawn, is operationally done. It’s a loss, sure, but we’re looking forward to Dawn results, besides looking at Hayabusa 2 at 162173 Ryugu and OSIRIS-REx at 101955 Bennu. On slightly longer scales, InSIGHT will yield its data, but I’ll get to it shortly.

To the person on the street, these probe missions are, effectively, pretty pictures being released. The end of flight operations then feels like the end of picture taking; attention span at ease (if it wasn’t already at ease). Fortunately, we are not people on the street, nor are we ruled by our attention spans, or lack thereof. The ‘end’ of a flight is actually where it gets effective, because ugly pictures- or, arguably, non-pictures- rule our data releases.

The person on the street doesn’t ‘get’ things like spectra, or counts, or in some cases even quasi-pictures like abundance maps. Yet these are the deep datasets scientists investigate, to find deeper meaning at a Solar system body. Such investigations mean digging deeper than image pixels (including literally deeper into the target). For this reason, serious results often come later- often a year or so after a mission. Far longer than looking at a picture.

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