An Unexpected Journey to the Southern Hemisphere?

Artist’s rendition of New Horizons at MU69.

Normally my work life is not too exciting from an outdoors adventure standpoint. I mostly sit behind a computer writing code, writing papers, writing proposals, and writing recommendation letters. Or I’m jockeying powerpoint slides and waving my hands around at the front of large lecture-halls. Or I’m getting coffee or scrounging for left-over pizza in the fridge (actually, that happens a lot.) But every now and then, being an astronomer enables some pretty cool adventures. We have conferences (which are mostly boring) but sometimes they’re in interesting places like Venice or Hawaii and I can add on adventures. I sometimes go use large telescopes on remote mountaintops (which is cool for the first bit) and spend some daylight hours exploring the area nearby (also cool). But, very occasionally, it’s the work itself which is the adventure…

I was recently recruited to help out with an occultation observation in support of the New Horizons mission. New Horizons, of course, is famous as the plucky little space robot which flew past Pluto in July of 2015 and made all our geeky hearts go pitter-pat. But the mission is not over; yes, NH is outbound at incomprehensible speed, but the powers-that-be have figured out how to fly it past another Kuiper Belt Object (KBO, think asteroid but made out of ice) before it runs out of maneuvering fuel, electrical power, and funding. Bonus science! Bonus discovery! That KBO is known by the unwieldy name 2014MU69 (“MU69” to its friends) and New Horizons will zip past it on Jan 1, 2019. Images and spectra will be collected and our hearts will go pitter-pat all over again.

Pluto, MU69, and the Kuiper Belt.

MU69 is visible even in the best telescopes as a very faint moving dot. From this, we can determine its orbit (which is about 40 AU from the sun) and that it’s a classical KBO (probably has been orbiting in the same place since the beginning of time) and that it’s probably pretty small (based on how bright it isn’t). However, before NH arrives, scientists at the Southwest Research Institute (SWRI) want to know a few more things about MU69, like how big it is. Size is currently estimated to be in the 20-40 km range, but that estimate is based on an assumption about how reflective it is. For instance, a relatively shiny object of the observed brightness would be about 20 km in diameter while something that was black as soot might be more like 40 km across. This is interesting in its own right, but has pressing ramifications for mission planning and other subtleties.

Example of the occultation of a star by the asteroid Chariklo which, it turns out, has rings!

The way you find out how large objects are is to watch them pass in front of background stars (occultations). By timing how long the star is occulted and knowing how fast the object is orbiting, you get a physical size. If you can do this from several places, you can sometimes even suss out a shape (round vs stretched) and even whether it has a moon or ring around it or something (some asteroids have moons and rings, did you know that? Cool, huh?).

Even with all the stars in the sky, small objects are really small and occultations are pretty rare. However, a detailed search of stars along MU69’s orbital path (as viewed from Earth) shows that there actually are three occultations happening this summer! The first of these is on June 3rd when the shadow of MU69 occulting a very faint (15th magnitude) star will sweep across the southern tips of Africa and South America. I’ve been recruited to help with a “picket fence” of mobile telescopes to try to catch the occultation and pin down a size for this chunk of space ice.

Preliminary ground track for the June 3rd Occultation. Each tick mark is one minute.

I’m a cosmologist and don’t normally concern myself with objects much closer than a few dozen megaparsecs (really really far), so this is pretty far outside my professional interests. But, I know how to use a telescope, can lift 100+ pound boxes of portable telescope components, can drive a manual transmission, and am comfortable traveling to exotic places.

We’ve got two dozen of these. They’re about 2 meters tall.

This will be a massive “deployment” with two dozen complete (identical, brand-spanking-new) telescopes and four dozen astronomers on two separate continents. Details are coming together, but are still far from fixed. Sometime in late May, I’ll be jetting down to either South Africa or Argentina along with two dozen other people. Observers will be teamed up in pairs, each with a telescope, the supporting computers, cameras, batteries, and a largish vehicle to drive them around. The telescopes, even disassembled, are massive and require four large crates to transport them safely.

We’ll deploy along a north-south line across the shadow path (which we don’t know precisely where it’s going to be yet*) spaced every 10-15 km. There will be a couple nights of practice before the main event on June 2nd/3rd. Since the star is very faint, large telescopes are required. Since the shadow moves at 24 km per second across the earth and is only 20-40 km across, the entire occultation will last only 1-2 seconds! We need big telescopes (16” diameter) and very fast cameras (1/2 second exposures). Most of us won’t even capture the event, but the ground track is uncertain to the level of a few hundred km, so we need a long line of observers. Then there is uncertainty with the weather, local conditions, equipment malfunctions, and probably lion attacks.

So yeah, this should be a pretty interesting adventure! I’ll post additional pictures and stories from what should be a pretty interesting and unconventional adventure.

*For my astronomer friends, the target star is 15th magnitude and, at this point, the ground track is largely dependent on the proper motion of the background star!  This fact blows my mind.

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