Bakersfield Night Sky — April 1, 2017
by Nick Strobel
Last Monday the Juno spacecraft made its fifth flyby over Jupiter’s cloud tops. Juno passed just 2700 miles above the cloud tops traveling at about 129,000 mph relative to Jupiter. With Juno we have discovered that Jupiter’s magnetic fields are more complex than we expected and that the banded structure of the belts and zones of Jupiter’s clouds extend deep into its interior. We’re also getting new insights about the three million amp current system connecting Jupiter and its super-volcanically active moon, Io.
The original plan for the Juno mission was for the spacecraft to execute a couple of very large 53-day orbits and then fire its engines to change the orbits to shorter 14-day ones. When part of the plumbing for the main engine did not pressurize as expected last October, the Juno team decided to not risk having the main engine firing incorrectly and put Juno in an undesirable orbit that would not allow us to get the data we needed.
Juno will remain in the very large elliptical orbits that will bring it as close to Jupiter at its closest approach as the 14-day looping orbits, so the original science objectives will still be met but it will take a bit longer to achieve them. “Making lemonade out of lemons”, remaining in the larger orbit will allow the team to get “bonus science” of exploring the outer parts of Jupiter’s magnetosphere. That will enable us to better understand all parts of Jupiter’s magnetic field including the boundary of Jupiter’s magnetic bubble where the solar wind pushes against it.
The longer orbits will also mean that Juno spends less time within the strong radiation belts on each orbit, so Juno could last longer before its electronics give out. The original plan was to have 32 science orbits and then crashing into Jupiter in mid-October 2017. Analysis of the results by the Juno team would have continued through July 2018.
By remaining in the larger 53-day orbits, Juno will have made only 12 science orbits by July 2018. The Juno team hopes that NASA will be able and agreeable to continuing the funding for more orbits. Careful, very-precise monitoring of Juno’s acceleration on its close flybys will enable us to map Jupiter’s gravity field and that will enable us to create a picture of Jupiter’s interior all the way to its core. It takes a number of orbits to build up that map. See nasa.gov/juno for more about Juno and its unveiling of Jupiter’s secrets.
The GRACE mission has been mapping Earth’s lumpy gravity field for the past 15 years. The GRACE mission uses two spacecraft orbiting 137 miles apart from each other. The distance between them is measured super-accurately with microwave pulses. The distance between changes by a few microns as the craft pass over regions of greater mass.
Fifteen years of elegant, graceful analysis of the changes in those minute accelerations have enabled us to see what lies below our feet as never before. We can map the flow and depletion of underground water from our aquifers. We can measure the melting ice sheets as the planet warms (280 Gtons/year for Greenland and almost 120 Gtons/year for Antarctica) and how much of sea level rise is due to changing water mass vs. expansion due to warming water. We can also measure the changing motions of the mantle below the crust as the mantle adjusts to ice sheet loss and groundwater loss and the months-long relaxation after the sudden shifts of a large earthquake.
The GRACE Follow-On mission will hopefully launch next winter to continue the gravity field mapping after the original GRACE spacecraft run out of fuel this summer. See http://grace.jpl.nasa.gov for more about how GRACE works and its many discoveries.
Continuing on with the gravity theme: tickets are on sale for “Black Holes” at the William M Thomas Planetarium on April 27. The show continues to be a popular one because of its world-class visualizations of warped spacetime and black holes are just plain cool!
In our evening sky you will see Mercury low in the west shortly after sunset with orange-red Mars above it (see the second chart below). A waxing crescent Moon will be higher up in the southwest. Mercury will be visible for another week and then plunge back toward the sun. Shortly after 8 p.m., you will see Jupiter rising in the east. It will be highest about 1:25 a.m. About a half-fist width at arm’s length below Jupiter is the brightest star of Virgo, Spica. Retrograding Jupiter will move a little further west away from Spica through the rest of this month.
The first star chart below shows the southern sky at 10 p.m. Jupiter and Spica are at the far left of the chart. Higher up in the southeastern sky is Leo with the distinctive sickle shape or backward question mark at the head and front of the lion. The bright star Regulus is at the bottom of the sickle. A waxing gibbous Moon will pass just below Regulus on April 6. To the far right is very bright Sirius at the nose of Canis Major in the southwest.
Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com