THE ASTEROID PROBE EXPERIMENT (APEX) WOULD BE THE FIRST MISSION TO DIRECTLY EXPLORE THE INTERIOR STRUCTURE OF A NEAR-EARTH OBJECT WHILE ALSO CHARACTERIZING ITS ROTATIONAL AND SURFACE PROPERTIES. IT WILL INVESTIGATE THE ASTEROID (99942) APOPHIS A PARTICULARLY INTRIGUING OBJECT THAT WILL BE INFLUENCED BY TIDAL FORCES DURING ITS CLOSE APPROACH TO THE EARTH IN APRIL 2029 AND THAT HAS AN EXTREMELY SMALL PROBABILITY OF IMPACTING THE EARTH IN 2036. APOPHIS IS ALSO CHARACTERIZED BY A SLOWLY TUMBLING ROTATION STATE THOUGHT TO BE EXCITED BY TIDAL FORCES EACH TIME THE ASTEROID ENCOUNTERS THE EARTH. DISCOVERED IN 2004 APOPHIS IS AN SQ-TYPE ASTEROID ~325 M DIAMETER. WHEN FIRST DISCOVERED IT WAS THOUGHT TO HAVE A 2.7% CHANCE OF IMPACTING THE EARTH IN 2029. THE IDENTIFICATION OF APOPHIS AS A POTENTIALLY HAZARDOUS ASTEROID PROMPTED THE RECOGNITION THAT A DEEPER UNDERSTANDING OF THE PHYSICAL PROPERTIES OF NEOS WAS NECESSARY TO BE ABLE TO MITIGATE AGAINST AN IMPACT DETERMINE THEIR ORIGIN AND EVOLUTION AND TO UNDERSTAND THE EFFECTS OF AN IMPACT WITH THE EARTH. NASA HAS BEEN CHARTERED TO DEFINE 90% OF THE NEO POPULATION>140 M BY 2020 AND THE NRC (2010) HAS RECOMMENDED DETECTING ALL OBJECTS>30-50 M. WHILE DEFINING THE POPULATION OF OBJECTS VIA ASTRONOMICAL METHODS WILL ACHIEVE ONE OF THE GOALS SET BY NASA UNDERSTANDING WHAT COULD BE DONE TO MITIGATE AGAINST A PAH AND COMPREHENDING THE INFLUENCE OF SUCH AN OBJECT IMPACTING THE EARTH REQUIRES DETAILED IN SITU INVESTIGATION OF THEIR PHYSICAL PROPERTIES. SUCH INVESTIGATIONS MUST INCLUDE ASSESSMENTS OF A PAH S INTERNAL AND SURFACE PROPERTIES. DIRECT OBSERVATION OF HOW TIDAL INTERACTIONS WITH THE EARTH COULD CHANGE A PAH ROTATIONAL STATE IS IMPORTANT AS CHANGES IN ROTATION RATES CONTRIBUTE TO THE EVOLUTION OF PAHS ORBIT AND INFLUENCES OUR ABILITY TO PREDICT TERRESTRIAL IMPACT PROBABILITIES. SUCH DETAILED INVESTIGATIONS CAN ONLY BE ACCOMPLISHED BY SPACE MISSIONS SUCH AS APEX THAT RENDEZVOUS WITH RATHER THAN FLYBY THE OBJECT. THE APEX MISSION THEREFORE HAS THE FOLLOWING SET OF BASELINE OBJECTIVES AT APOPHIS: (1) DETERMINE THE ROTATIONAL DYNAMICS (2) ESTABLISH THE PHYSICAL DIMENSIONS (3) DETERMINE THE TOPOGRAPHY (4) DETERMINE THE INTERIOR STRUCTURE AND (5) DEFINE THE SURFACE MORPHOLOGY. ADDITIONAL OBJECTIVES INCLUDE THE CHEMICAL AND MINERALOGIC COMPOSITION AND THE SURFACE PHYSICAL PROPERTIES. THE MISSION WILL ALSO HAVE AN ADDITIONAL OBJECTIVE OF CALIBRATE INTERPRETATIONS OF EARTH- AND SPACE-BASED ASTRONOMICAL OBSERVATIONS AND EARTH-BASED RADAR OBSERVATIONS. THE MISSION WILL OBSERVE APOPHIS BEFORE DURING AND AFTER ITS ENCOUNTER WITH THE EARTH IN 2029 TO DEFINE IT CHARACTERISTICS AND TO UNDERSTAND HOW THE TIDAL FORCES OF THE ENCOUNTER INFLUENCE THE BODY ITS SURFACE AND ITS ROTATIONAL STATES. THE TIDAL FORCES WILL INDUCE DEFORMATION THAT WILL PROVIDE THE SEISMIC SOURCES FOR MEASURING THE ASTEROID S INTERNAL STRUCTURE. THESE OBJECTIVES ARE SPECIFICALLY CITED BY THE NRC (2010) STUDY AS REQUIRED TO FULLY CHARACTERIZE AN NEO AND PROVIDE THE INFORMATION NECESSARY TO DETERMINE MITIGATION EFFORTS. THE BASELINE PAYLOAD INCLUDES A PANCHROMATIC AND A SEISMOMETER; ADDITIONAL CANDIDATE INSTRUMENTS INCLUDE A MULTIBAND IMAGER A THERMAL IMAGER AND A GAMMA RAY SPECTROMETER. THE CONOPS INCLUDE A RENDEZVOUS WITH APOPHIS PRIOR TO ITS ENCOUNTER WITH THE EARTH TO CHARACTERIZE THE BODY AND DEPLOYMENT OF THE SEISMOMETER ON THE SURFACE. THE SPACECRAFT THEN CONDUCTS STATION-KEEPING OR ORBITS TO COLLECT INFORMATION AS APOPHIS ENCOUNTERS THE EARTH AND UNDERGOES TIDAL DEFORMATION. FOLLOWING THE ENCOUNTER THE SPACECRAFT WILL CONTINUE OBSERVATIONS TO FULLY CHARACTERIZE THE ROTATIONAL DYNAMICS AND ANY SURFACE CHANGES. WE WILL EXAMINE A SPACECRAFT USING SOLAR CELLS FOR POWER USING EITHER CHEMICAL OR ELECTRIC PROPULSION WITH THE ABILITY TO DEPLOY A SEISMOMETER ONTO THE SURFACE MAINTAIN A PROXIMAL POSITION WITH APOPHIS FOR SEVERAL MONTHS BEFORE AND AFTER EARTH ENCOUNTER AND TRANSMIT THE DATA TO THE EARTH.