Cold atom gravimetry for planetary missions

Highlight

• New concept of instrument to carry out gravity measurements at Mars and at Venus.

• Gravity field measurements at Mars and Venus.

• Cold Atoms Interferometry.

Abstract

Cold Atom Interferometry (CAI) is a promising new technology for gravity missions, enabling measurements with a potential error level that is several orders of magnitude lower compared to classical electro-static accelerometers. Whereas the latter typically suffer from high noise at low frequencies, with biases and scale factor instabilities, cold atom interferometers give an absolute measurement and are highly accurate over the entire frequency range. Especially for planetary missions, drift-free cold atom interferometry can be highly beneficial, because it does not need any on-board calibration. In this work we present the improvement of using a CAI instrument, with respect to classic Doppler-tracking technique, to retrieve the gravity field of Venus and Mars. In order to estimate the performances with many parameters (orbit altitude, mission duration, sensitivity) a scalar scale factor is proposed to fit a simulated CAI instrument on Earth orbit to other celestial bodies. The spherical harmonic degree strength of the gravitational field retrieval is estimated and the results presented here agree with Fast Error Propagation Tools.

Introduction

In Earth Observation, measuring the gravitational field from space can help us understand geoscience questions about e.g. geological processes, ocean circulation and climate change. There are several past and on-going gravity missions around Earth. GRACE (Tapley et al., 2004) and the currently flying GRACE-FO (Kornfeld et al., 2019) mission consist of two spacecraft with low-low Satellite-to-Satellite Tracking (SST) using microwave or laser ranging to monitor time-variable gravity. To distinguish non-gravitational effects, the satellites contain on-board accelerometers. With those combined measurements, gravity field parameters are retrieved. The Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite (Floberghagen et al., 2011), launched in 2009, exploits high-low SST using GPS and, for the first time, Gravity Gradiometry (GG) with a set of 3-axis electro-static accelerometers with a focus on (pseudo-) static gravity recovery.

Cold Atom Interferometry can be used in two ways for gravity field measurements (Table 1). The first way is a gravity gradiometer GOCE-like concept: A CAI gravity gradiometer suppresses the common vibration effects on the instrument as the inertial reference of the two clouds of atoms is the same. Currently, the mass of a CAI GG is 260 ​kg with Components Of The Shelf (COTS) sub-systems but it is expected to be reduced to 50 ​kg in ongoing studies (Carrazet al., 2014; Trimeche et al., 2019). The second is a hybrid CAI accelerometer concept (targeted mass: $<$10 ​kg) of correcting drift errors of electrostatic accelerometers that are used in a satellite-to-satellite ranging concept for measuring non-gravitational accelerations (Abrykosov et al., 2019).

A CAI gravity mission shows promising improvement on the knowledge of the Earth gravity field, and is considered as a potential candidate for future mission, together with constellation of SST spacecrafts with laser ranging links (Haagmans et al., 2020). The performance of the instrument required is at the state of the art for Earth mission but is not so stringent for other celestial bodies. In fact for other celestial bodies than Earth, there was only one dedicated gravitational mission: NASA’s lunar GRAIL mission (Zuber et al., 2012). Two spacecraft in a GRACE-like formation, including SST and radio ranging from Earth, were used for gravitational field mapping of the Moon. Other gravitational survey of celestial bodies has been so far retrieved from radio science experiments (Visser, 1007; Kaula, 1966a; Mazarico et al., 2012; Iess et al., 2010; Smith et al., 2009; Anderson et al., 1998). Such technique allows a rough measurement of the geoid and relies on the knowledge or estimation of any non gravitational forces applied on the spacecraft, as solar radiation pressure and atmospheric drag. An onboard CAI instrument could potentially either improve the knowledge of these non gravitational forces, or directly measure the gravitational potential. Regarding the actual performances of such devices with no need of calibration (Geiger et al., 2003), it appears that a gravity mission utilising CAI technology could potentially improve by orders of magnitude current gravitational models of celestial bodies other than Earth or the Moon.

In order to evaluate the performances of a Space Geodesy Mission, End-to-End simulation tools are required but time consuming, especially if several parameters are changed (e.g. orbit altitude, duration of the mission, planet studied, …). Using existing End-to-End simulations (Trimeche et al., 2019) as input we present here a very fast but quite accurate method to discriminate the different parameters using a scalar scale factor. A more realistic simulation using Fast Error Propagation Tools (Visser, 1007) is then applied to confirm the scenario chosen.

After a brief description on Cold Atom Interferometry technology in Section 2, we will introduce our model algorithm to transpose performances of a CAI space mission from Earth gravity retrieval to other planets already seen as space mission candidates (A-Envision M5 proposal), as Venus or Mars in Section 3. We will present in Section 4 the performances estimated and in Section 5 we will then discuss the validity of such model with standard modelisation such as Fast Error Propagation Tools (Visser, 1007).