The Trump Administration's policy on Deep Space Exploration is currently defined in terms of the executive action
Space Policy Directive-1, signed by President Trump on 11 December, 2017. It reads:
SUBJECT: Reinvigorating America's Human Space Exploration Program
Section 1. Amendment to Presidential Policy Directive-4.
Presidential Policy Directive-4 of June 28, 2010 (National Space Policy), is amended as follows:
The paragraph beginning "Set far-reaching exploration milestones" is deleted and replaced with the following:
"Lead an innovative and sustainable program of exploration with commercial and international partners to enable human expansion across the solar system and to bring back to Earth new knowledge and opportunities. Beginning with missions beyond low-Earth orbit, the United States will lead the return of humans to the Moon for long-term exploration and utilization, followed by human missions to Mars and other destinations;".
Sec. 2. General Provisions. (a) Nothing in this memorandum shall be construed to impair or otherwise affect:
(i) the authority granted by law to an executive department or agency, or the head thereof; or
(ii) the functions of the Director of the Office of Management and Budget relating to budgetary, administrative, or legislative proposals.
(b) This memorandum shall be implemented consistent with applicable law and subject to the availability of appropriations.
(c) This memorandum is not intended to, and does not, create any right or benefit, substantive or procedural, enforceable at law or in equity by any party against the United States, its departments, agencies, or entities, its officers, employees, or agents, or any other person.
(d) This memorandum shall be published in the Federal Register.
Donald J. Trump
You can find this document in the Federal Register. No other official policy by the Trump Administration relating to deep space or NASA has been enacted as of December 2017, nor has any funding been allotted to fulfil what is discussed is Directive 1.
Tangential to deep space however, Trump did reestablish the existence of the National Space Council in June 2017 as per Executive Order 13803, and escaping from policy for a minute, Trump has expressed passing interest in deep-space exploration:
“Well, we want to try and do it [send humans to Mars] during my first term or, at worst, during my second term, so we’ll have to speed that up a little bit, OK?”