#VLTJupiter 1: Jupiter from the VLT

Over the past decade, I've been exceedingly fortunate to have won time on the European Southern Observatory's (ESO) Very Large Telescope (VLT) to observe all four of the giant planets in the mid-infrared.  These observations, sampling thermal emission from 5 microns in the M band, to 10 microns in the N band, and 20 microns in the Q band, can reveal the three-dimensional temperatures, winds, gaseous composition and cloud structure, from the churning, convective tropospheres all the way into their stable stratospheres.  Many of my papers and projects have relied on this exquisite VLT dataset.

But despite all of that, I've never been able to visit VLT in person.  It's been an item on my bucket list for over ten years.  My observations were always in 'service mode', meaning that I'd design them in a piece of software (i.e., which filters to use, where to point, and for how long), then upload them to the ESO office to be put into a queue.  Those queued observations are then executed by supporting astronomers on the mountaintop, in between time-critical 'visitor mode' runs.  The trouble with this approach is that it's been hard to control the exact timing of the observations, and the data were usually taken within 1-4 weeks of when I actually wanted them.  For most datasets, that's been absolutely fine.  But now that the Juno spacecraft is in orbit around Jupiter, completing close flybys of the gas giant once every 53 days, the precise timing of my observations has become more critical.  So today (May 22), I'm writing this blog post from 2300m up on Cerro Paranal in the astronomer's residence, waiting to begin a visitor-mode run tomorrow night.

A map of the ESO sites in Chile - we flew from Santiago to Antofagasta, before a bus journey south along the B-710 to Cerro Paranal.  Cerro Armazones, the future home of the ELT, can be seen as a flattened mountain in the distance to our east.

The Juno Connection

The Leicester team are a part of an international network of planetary astronomers supporting NASA's Juno spacecraft in its exploration of Jupiter.  Juno has now completed twelve close passes around Jupiter known as perijoves, coming in over the north pole, sweeping within a few thousand kilometres of the equator, then exiting high over the south pole.   The thirteenth such encounter is on May 24th at 05:40UT, approximately 48 hours from now.  Now, Juno has been doing some tremendous science, but its science questions are focussed, and there are gaps in capability within the payload.  The Earth-based programme has been providing support in three broad areas:

1. Spatial context:  Juno has a close-in view of dynamic phenomena and weather systems as it flys north-south across Jupiter, and can't always see the wider context of the zone or belt in which that weather system is embedded.  In contrast, our data can view the whole planet at once, providing the global perspective.
2. Temporal context:  Jupiter's weather and auroral processes can evolve significantly between the perijoves, with new storms erupting, vortices interacting, belts fading and expanding, and aurora shifting and changing in response to a variety of processes in the magnetosphere and solar wind.  The Earth-based programme catalogs these changes over short timescales (between perijoves) and long timescales (multiple years) to better interpret Juno's observations.
3. Spectral context:  Juno does a fabulous job in the ultraviolet, visible (with imaging), 2-5 µm region (with imaging and spectroscopy) and 1.3-50 cm microwave region.  But there's nothing in the X-ray, 0.9-2 µm infrared, 5-25 µm thermal infrared, and the sub-centimetre range below 1.3 cm.  An army of Earth-based facilities, including Chandra, Hubble, VLT, Subaru, IRTF, VLA and ALMA, have been used over the past two years to plug this gap.  That's not to mention the amateur astronomers observing on a nightly basis.

Back in 2015, several representatives of these telescopes travelled to the ESO headquarters in Vitacura, near Santiago, for a week-long workshop on collaborations between ground-based astronomers and space missions.  I wrote about that meeting here.  As a result of this workshop, I led a white paper, addressed to the ESO Director of Science, that emphasised the timely nature of VLT and ALMA observations during the Juno mission, making the three points outlined above.  I like to think that they helped to smooth the way for the ESO proposals that ensued over the following three years, during which time we've been successful in winning service mode runs for Jupiter every semester.  These data will be useful for the Juno mission, but also for my work studying the long-term evolution of Jupiter's meteorology and climate.

And that brings me full circle to this silent mountaintop in northern Chile.  While we're working up here, I'm going to try to keep a record (who know's whether I'll ever have the chance to return!), to give a flavour of what it's like.  And that starts with the journey.

Current location - 12 km from the sea, 2.3 km in the air, on top of Cerro Paranal.  Cerro Armazones is off to our east.