BepiColombo mission: Estimation of Mercury gravity field, rotation and relativity parameters
- Paper number
IAC-06-A3.P.2.05
- Author
Mrs. Noelia Sanchez Ortiz, DEIMOS Space, Spain
- Coauthor
Mr. Miguel Bello Mora, DEIMOS Space S.L., Spain
- Coauthor
Mr. Rudiger Jehn, European Space Agency/ESOC, Germany
- Coauthor
Mrs. Laura Martin, DEIMOS Space S.L., Spain
- Year
2006
- Abstract
Mercury’s main characteristics are still not well known. Mercury, which is closer to the Sun than any other planet or known natural satellite, plays an important role in constraining and testing theories on planetary formation. Mercury is a difficult target for ground-based observations and space missions due to its proximity to the Sun. Earth-based observations are normally performed in front of a strong background.
ESA has defined the BepiColombo mission to Mercury as a cornerstone within its Horizon 2000+ program. BepiColombo scientific objectives range from the determination of the gravity field of the planet to the testing of some aspects of the relativity theory. In order to assess if these mission objectives can be achieved, a complete estimation tool with complex measurement and dynamic force models is required. To study this kind of estimation problems a dedicated software tool, the General Relativity Test Tool by Analyzing the Orbit of BepiColombo (GRETCHEN) was developed.
BepiColombo’s science objectives regarding gravity field and rotation state include determination of the gravity field up to degree 20 with Signal to Noise Ratio (SNR) of 10, determination of the gravity field up to degree 4 with a SNR of 1000 or better, determination of the tidal Love number k2 to an accuracy of 1% or better, determination of the libration amplitude with an accuracy of 10% of its value (3 arcsec.) and determination of the obliquity up to 3.7 arcsec. Testing General Relativity to a level better than 1.0E-5 by measuring the time delay and Doppler shift of radio waves and the precession of Mercury’s perihelion, determining the oblateness (J2) of the Sun to better than 1.0E-8 and also the time variation of the gravitational “constant” G with a precision of about 3.0E-13 years-1 are the science objectives concerning Gravitational Theories. For the study of General Relativity, relativistic terms were included in the dynamic equations, and they are given in the so-called parameterized post-Newtonian (PPN) formalism. Some parameters define this formalism: γ, that provides a term in the acceleration of Mercury around Sun, an additional delay in the range measurements and a relativistic Doppler shift; β, that has second order effects in the light propagation and it is supposed to have a strong correlation with J2 of the sun gravity field; α1 and α2 , that measure whether or not the theory predicts post-Newtonian preferred-frame effects; and the Nordtvedt parameter, η, which describes a possible violation of the strong equivalence principle.
In order to evaluate whether the objectives of the BepiColombo mission can be achieved, a set of simulation cases were performed with the GRETCHEN software. Preliminary results show the successful achievement of the scientific target. Some of the simulations were performed in order to assess the maximum accuracy for relativity parameters if they are determined independently from the gravity field and rotation state estimation. It was confirmed that these two experiments can be decoupled without compromising the overall accuracy.
- Abstract document