Juno and General Relativity

Abstract

The recently approved Juno mission will orbit Jupiterfor one year in a highly eccentric (rmin = 1.06RJup,rmax = 39RJup) polar orbit (I = 90 deg) to accuratelymap, among other things, the jovian magneticand gravitational fields. Such an orbital configurationyields an ideal situation, in principle, to attempt a measurementof the general relativistic Lense-Thirring effectthrough the Juno’s node which would be displacedby about 570 m over the mission’s duration.Conversely, by assuming the validity of general relativity,the proposed test can be viewed as a direct,dynamical measurement of the Jupiter’s angular momentumS which would give important informationconcerning the internal structure and formation of thegiant planet. The long-period orbital perturbations dueto the zonal harmonic coefficients J`, ` = 2, 3, 4, 6of the multipolar expansion of the jovian gravitationalpotential accounting for its departures from sphericalsymmetry are, in principle, a major source of systematicbias. While the Lense-Thirring node rate is independentof the inclination I, the node zonal perturbationsvanish for I = 90. In reality, the orbit injectionerrors will induce departures i from the idealpolar geometry, so that, according to a conservativeanalytical analysis, the zonal perturbations may comeinto play at an unacceptably high level, in spite ofthe expected improvements in the low-degree zonalsby Juno. A linear combination of , the periJove !and the mean anomaly M cancels out the impact ofJ2 and J6. A two orders of magnitude improvementin the uncanceled J3 and J4 would be needed to reducetheir bias on the relativistic signal to the percentlevel; it does not seem unrealistic because the expectedlevel of improvement in such zonals is three orders ofmagnitude. More favorable conclusions are obtainedby looking at single Doppler range-rate measurementstaken around the closest approaches to Jupiter; numericalsimulations of the classical and gravito-magneticsignals for this kind of observable show that a 0.2−5%accuracy would be a realistic goal.