Carpe Umbra

July 10, 2017

We have a very different type of science flight tonight, a timed-event. This time the science focus is getting the SOFIA aircraft to be located over a specific latitude and longitude on Earth at a certain elevation at a specific time. We are flying an occultation flight. SOFIA has done this twice before capturing the shadow of Pluto as its passed in front of a distant star and created a shadow that moved across Earth at 90,000 km/hr. And we have a upcoming Triton occultation event in October later this year.

Occultation: not a typical everyday word. In fact it turns out that the word occultation is not the same as an eclipse or a transit, which I learned only recently.

An occultation occurs only when a body completely hides another as seen by the observer. The verb to occult simply means to block out.  For this July 10th event, this body 2014MU69, a Kuiper Belt Object, is going to blockout a 15th magnitude background star.

A transit is when the body passing in front of the other body only partially blocks it (like Mercury transiting the Sun and all those “transiting exoplanets” that ground-based telescopes and Kepler have been discovering).

Finally, an eclipse occurs when one body passes into the shadow of another body and disappears at least partially.

So is the Aug 21st Solar Eclipse (who’s shadow path crosses North America) an eclipse? Well, it all depends on your viewpoint.

When the Moon blots out the Sun, the Earth, by falling into the Moon’s shadow, is eclipsed.  The Sun is not eclipsed. It is correct to say is the Moon has occulted the Sun.

Thus, August 21st perhaps is more correctly categorized as a solar occultation. On the flip side, all Lunar Eclipses are real Eclipses, the Moon disappears in the shadow of the Earth (the Moon dims due to the absence of light caused by the Earth’s shadow).

The event of July 10th had a predicted ground path as described here

On board SOFIA, we had Marc Buie and Simon Porter, the experts in the computing the path of 2014MU69 who were updating this exact timeline based on the most recent HST measurements of 2014MU69 (as of Jul 4th) just the day before flight.


SwRI Scientists Simon Porter, Marc Buie, and Eliot Young will fly on SOFIA as Guest Observers to catch MU69’s shadow.

At the time of flight takeoff, SOFIA would be hunting a shadow center intercept time of 07:49:11 UTC with an interception position at Lat 16d24.2m S, Lon 175d2.4m W. The duration of the dip in the light curve would be extremely small, less than 2 seconds, as MU69 is a very tiny object (diameter is estimated to be between 10-40 km) a long way away (43 AU; or 10 AU past Pluto). A very challenging occultation to capture.

The flight planners and navigators aboard SOFIA can position the aircraft with the precision need for this measurement (within 10 km; and 1 second). I’m eager to witness first how this type of flight plan unfolds over the night.

Time to board!


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