<a class="mw-redirect" href="https://en.wikipedia.org/wiki/Explorer_program" id="mwAVw" rel="mw:WikiLink" title="Explorer program">Explorer program</a>: <i id="mwAV0"><a href="https://en.wikipedia.org/wiki/Explorer_4" id="mwAV4" rel="mw:WikiLink" title="Explorer 4">Explorer 4</a></i> is launched.