Measuring gravity with the GRASS instrument on the Hera mission
Authors: Ritter Birgit , Karatekin Özgur , Carrasco José A , Tasev Elisa , Alavés Mañogil Higinio , Noeker Matthias , Van Ransbeek Emiel , Noiset Guillaume , Berk Senel Cem 
Published: September 2022

&lt;p&gt;With the gravimeter for small solar system objects (GRASS), absolute surface accelerations in the order of nano-g can be measured. It is an innovative and extremely compact sensor that will fly as part of ESA&amp;#8217;s Hera mission onboard the Juventas CubeSat to the binary asteroid system Didymos. In 2027, Juventas will land on the secondary body of the system, Dimorphos, and GRASS will hence measure the local gravity vector and its temporal variations at the landing site. Apart from the direct mass determination, these measurements will help in synergy with other instruments to constrain the geological substructure as well as the surface geophysical environment.&lt;/p&gt;
&lt;p&gt;The instrument is currently under development at the Royal Observatory of Belgium, funded by the Belgian PRODEX office and in cooperation with EMXYS in Spain.&lt;/p&gt;
&lt;p&gt;The average gravitational force expected on Dimorphos&amp;#8217; surface is around 4.5 x 10&lt;sup&gt;-5&lt;/sup&gt; m s&lt;sup&gt;-2&lt;/sup&gt; (or 4.5 mGal, Figure 1). Apart from the self-gravitation of the body, centrifugal forces and the acceleration due to the main body of the system contribute to the surface acceleration. The temporal variation of the signal is driven by the dynamical state of Didymoon with respect to Didymain and the related librations.&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;Figure1: Modeled surface gravity in the Didymos system (left) and on Dimorphos (right). Note that only a shape model for the primary body exists.&lt;/p&gt;
&lt;p&gt;The table below lists the science objectives of the instrument for Dimorphos.&lt;/p&gt;
&lt;table&gt;
&lt;tbody&gt;
&lt;tr&gt;
&lt;td&gt;
&lt;p&gt;&lt;strong&gt;Objective&lt;/strong&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;td&gt;
&lt;p&gt;&lt;strong&gt;Measurement&lt;/strong&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td&gt;
&lt;p&gt;S#1 &lt;strong&gt;Local subsurface inhomogeneities and global mass of Dimorphos.&lt;/strong&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;td&gt;
&lt;p&gt;Determination of local gravity vector at landing location with accuracy of &lt;1% in direction and amplitude.&lt;/p&gt;
&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td&gt;
&lt;p&gt;S#2 &lt;strong&gt;Dimorphos dynamical state&lt;/strong&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;td&gt;
&lt;p&gt;Investigation of surface acceleration variations due to rotation kinematics, tides and orbital dynamics. Measurements as for S#1, but for several locations along the orbit of Dimorphos around Didymos.&lt;/p&gt;
&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td&gt;
&lt;p&gt;S#3 &lt;strong&gt;Global gravity solution, interior structure and surface mass transport&lt;/strong&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;td&gt;
&lt;p&gt;Synergy of data with other instruments (radar, radio, CubeSat decent, star trackers) to obtain holistic view of gravity and interior.&lt;/p&gt;
&lt;/td&gt;
&lt;/tr&gt;
&lt;/tbody&gt;
&lt;/table&gt;
&lt;p&gt;The gravimeter measurement system consists of monitoring the displacement and deflection of a flat spring due to a gravitational field by a capacitive transducer. Modulation of the measured g-vector by rotation allows the rejection of the zero-g bias. In addition, no levelling is required. The gravimeter will be calibrated in-situ by using electrostatic force to compensate acceleration forces.&lt;/p&gt;
&lt;p&gt;Two orthogonally aligned gravimeter axes, each with a rotating sensor head, enable finally the reconstruction of the full 3D gravity vector. Figure 2 shows a CAD drawing with dimensions (left) and the vibration test model of the gravimeter (right).&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;
&lt;p&gt;Figure 2: CAD drawing of the two-axes gravimeter (left) and a picture of the assembles GRASS vibration test model (right)&lt;/p&gt;
&lt;p&gt;We will present the scientific background and application of the GRASS instrument on Dimorphos, the current instrument status, its implementation and first test results and give an outlook on future application of the instrument for other small planetary bodies.&lt;/p&gt;