Today, the knowledge of the solar radiative interior is obtained by the solar acoustic modes. Thanks to the latest modes detected by SoHO the sound speed has been determined down to 0.06 Ro with a resolution of 3%. This profile is used to improve the solar model and its deviations from a static vision. The rotation profile is now clearly established down to the limit of the core (Garcia et al. 2004). In order to progress toward the core and reduce the uncertainties in the radiative region, gravity modes should be measured. Recently, Turck-Chieze et al. (2004) have identified some patterns using GOLF data during the last solar minimum, that can be interpreted in terms of gravity modes. These candidates, with an amplitude of ~2 mm/s, are at the limit of the signal-to-noise ratio and are difficult to follow when the activity increases. Their research will continue until the end of the SoHO lifetime in 2008 during the next solar minimum. In the best case, only a few mixed and gravity modes will be detected with SoHO. This is the reason why a French-Spanish collaboration is now building a prototype of a new spatial instrument, GOLF-NG, that will be tested during the Summer 2005 in the Observatorio del Teide. GOLF-NG will directly address the problem of the solar convective background noise to improve the g-mode detection.