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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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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Asteroid of the Half-Month: 2685 Masursky

I went long on the last two, so this one will be shorter. We go to 2685 Masursky, which got a distant flyby from the Cassini mission on its way to Saturn. Cassini passed 1.6 million km (1 million miles) from the asteroid in Jan 2000, and saw it as ‘a dot’. Still, this is an opportunity to demonstrate just how much scientists can learn from ‘just a dot’!

The low number indicates that Masursky had its orbit determined a while ago, before our sky searches really unleashed the asteroid hordes. After brief but indeterminate sightings in the ’70s (and even an old one in the ’50s), Masursky finally got enough of a dataset to become a numbered asteroid in the early ’80s. This was in the film era- remember film?

Masursky’s orbit shows that it’s in the Eunomia family- that is, an asteroid swarm, moving together in space. The numerous family members, and their close orbits, are not felt to be an accident- 15 Eunomia likely split into a family by collisions. Masursky is a bit off, not in the family ‘core’, but not that far. And that was pretty much it for almost twenty years, until Cassini.

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Asteroid of the Half-Month: 89 Julia

Wow, what a summer. Two missions aside, the IAU just had its big meeting.

Our Solar System is a vast place, with almost a million known bodies, and a few million more lurkers, mostly in the smaller (sub-km) size bins. They formed from hundreds of planetesimals, themselves formed from smaller chunks. Some of the planetesimals crashed to bits; some of those re-formed with other fragments. We trace this jigsaw puzzle with chemistry (elemental and isotopic tracers) and dynamics (perturbation and impact studies).

If two puzzle pieces are the S-type (stony) asteroids and C-types (carbonaceous chondrites), then solving the puzzle includes the relationship between the two, and between them and other Solar System pieces. We already saw that there’s no true line between wet asteroids and depleted comets. We saw at 88 Thisbe that the C-complex actually includes a wide range of bodies and materials. Let’s look next to… asteroid 89 Julia, a ‘carbonaceous stony’!

In recent years, we’ve gotten much better at classifying asteroids (not that we’re there yet). We have also collected far more meteorites, clearly pieces of astero-cometary materials. Meteorites are a bewildering variety of classes and subclasses. What chemicals and their mineral forms link which meteorites to which SSSBs (Small Solar System Bodies)?

In the past decade, we have not only found a key link, fitting two (or more!) puzzle pieces, but there’s some chance we hold a linked sample: a lucky bit of 89 Julia itself.

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Asteroid of the Half-Month: 88 Thisbe

Most small bodies fall in two camps: neutral (grey) or “red.” This runs through the Main Belt, even the Kuiper Belt past Neptune, and among comets. Granted, red isn’t like a stop sign, more of a tint in grey. Still, the division’s there. There aren’t green or blue asteroids… except the rare B-types. Large asteroid 88 Thisbe is B, and a crucial puzzle piece.

Most asteroids are C- or S-type, the above greys and reds respectively. Lesser classes like E- and D- fit the division too, for various reasons. In the S-types, iron content, especially when weathered by space exposure, adds a faint redness. C-type is named for measurable carbon contents; this overwhelms iron with an absorbent but neutral color. The C-bodies are thought to be more primitive, formed in cooler temperatures than S-. Carbonaceous meteorites, thought to be C-fragments, have mineral levels like the Sun, and are thought to represent the levels present as our Solar System formed from a cloud- the “presolar nebula.”

When the types were first offered in the late ’70s, C- and S- were obvious, but there were misfit bodies. Various planetary scientists claimed lesser classes, or simply U- (unknown) or X-type. Meanwhile, meteoriticists were subdividing the carbonaceous meteorites. As typing grew firmer in the early- to mid-’80s, it grew clear that C- and S- were groups or supertypes, needing subdividing too. The “C group” or “C complex” covers classical C-bodies, and the lesser B-, F-, and G-types, also thought to be carbonaceous, but varying somehow.

Studying any of the C complex informs us of the other types, by comparison, and counterexample too. This is “comparative planetology”- no part of the Solar System is boring if you want to solve the jigsaw puzzle. Since C bodies inform us of carbonaceous meteorites and the Solar System’s birth, fitting in bodies like 88 Thisbe tells us where we came from, and how we got here. Since carbonaceous bodies are also wet, they’re where we can go.

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Asteroid of the Half-Month: 25143 Itokawa Part IV

I had posted that the Hayabusa project was groundbreaking for Japan. That it was an ordeal in flight, lucky to survive. And that it did valuable work, even in space before any samples of Itokawa were returned. But Hayabusa did return a sample- a gift that keeps on giving.

More words may have been written/”uttered” about Hayabusa/25143 Itokawa than any other asteroid… aside from my posts, three feature films were made about the mission, and played in theaters, homes, etc. Even if you don’t count mass media, the ongoing Hayabusa Symposia- conferences presenting Itokawa research- have no shortage of topics to discuss. The first solicitation (for a chance to do experiments on Itokawa particles) went out to the world’s scientists in 2012; now (2018) we are nearing the 5th Symposium for Itokawa results.

There’s a Hayabusa 2, of course, now at 162173 Ryugu. Less obvious is that NASA’s OSIRIS-REx mission (to 101955 Bennu) is ‘Hayabusa 2.5’ and private space companies are developing ‘Hayabusas: The Next Next Generation.” NASA’s * of *, Jim Green, has billed the coming years as “the Decade of Sample Return”, with sampling missions to Mars and a comet being designed. In the nearer term, Japan’s own MMX (Martian Moons eXplorer) was just approved as a ‘Hayabusa 3.’ It is to bring back material from Mars’ moon Phobos.

Let’s not get ahead of ourselves; Itokawa particles may be tiny, but our results from them go wide. First is that Itokawa material matches one type of meteorite, the LL chondrites. Studying the meteorites alone is like picking up a bottle on a beach, and eventually calling oneself a glass engineer. You have nothing but the broadest inferences about that bottle’s factory and processes.

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