• Introduction
    • EPOXI



      EPOXI is an innovative mission that re-purposed the Deep Impact spacecraft to carry out two additional science investigations. By re-directing the Deep Impact spacecraft, which was already in orbit around the Sun, EPOXI’s new assignment was to search for planets around other stars and observe another comet.

      EPOXI is a combination of the names for the two components of the new mission: Extrasolar Planet Observations and Characterization (EPOCh) and Deep Impact eXtended Investigation (DIXI), a flyby of comet Hartley 2.


      Mission Management

      Michael A'Hearn
      Michael A'Hearn,
      Principal Investigator

      Michael A'Hearn of the University of Maryland, College Park, is EPOXI's principal investigator and the leader of the DIXI science team. L. Drake Deming of NASA's Goddard Space Flight Center, Greenbelt, MD, is EPOXI's deputy principal investigator and leads the EPOCh investigation. NASA's Jet Propulsion Laboratory, Pasadena, CA, manages the project. Ball Aerospace & Technologies Corporation, Boulder, CO, built the spacecraft.

  • Science Objectives

      The EPOCh goals are to:

      Observe multiple transits of giant planets in orbit around other stars, to improve our knowledge of those planets

      Find additional planets, down to Earth-size, from a direct search for transits and from perturbations to the giant planets transits

      Search for rings and moons associated with the giant planets

      Detect light reflected from the giants planets, to learn about clouds and the atmosphere

      Characterize the Earth as an extrasolar planet analog


      DIXI's objectives are to:

      Understand the structure, composition, and formation history of cometary nuclei to learn more about the origin of the solar system

      Understand how features of the cometary nucleus relate to features of the coma

      Understand diversity within a cometary nucleus and between the nuclei of different comets

      Size comparison of comets Tempel 1 and  Hartley 2, imaged by the Deep Impact/EPOXI spacecraft.

      Size comparison of comets Tempel 1 and Hartley 2, imaged by the Deep Impact/EPOXI spacecraft.  Hartley 2 is 1.2 miles long.
      Credit: NASA/JPL-Caltech/UMD/McREL



  • Details

      The Deep Impact spacecraft launched from Cape Canaveral Air Station, Florida, on January 12, 2005. Six months later, on the 4th of July, the mission carried out an incredibly complex experiment in space, excavating beneath the surface of a comet to reveal its interior. Traveling at 23,000 miles per hour, the larger "flyby" spacecraft released a smaller "impactor" spacecraft into the path of comet Tempel 1. Earthlings witnessed the brilliant release of dust upon impact 83 million miles away, as images from both spacecraft were shown in near real-time on NASA TV and the Internet.

      While the impactor was vaporized, the flyby spacecraft continued orbiting the Sun. In July 2007, NASA funded a proposal to re-purpose the Deep Impact spacecraft and give it a new assignment.

      The EPOCh observations occurred between January and August of 2008. The DIXI flyby of comet Hartley 2 took place on November 4, 2010. A primary focus of the investigation is to compare Hartley 2 with comets observed by other spacecraft to help determine which cometary features are primordial and which are the result of subsequent evolutionary processes.


      Spectacular image of comet Tempel 1
      Spectacular image of comet Tempel 1 taken 67 seconds after impact with Deep Impact’s copper probe.
      Credit: NASA/JPL-Caltech/UMD
  • Results
    • The EPOCh observations of seven previously discovered transiting extrasolar planets were captured by the spacecraft’s High Resolution Instrument (HRI). The goal of these photometric observations was to measure the quantity of light.  A total of 198,434 images were exposed. 

      The HRI also looked at the Earth from millions of miles distant as though it were an extrasolar planet to provide data that could characterize Earth-type planets for future missions.  This is part of EPOXI's scientific mission to explore the planets of other stars, to learn more about what an Earth-like planet looks like from such a distance that no details can be discerned. By learning what the Earth looks like from a great distance and how the spectra shows our oceans and large land masses, we can learn more about how to design instruments to investigate Earth-like planets from even greater distance.

      The DIXI flyby of comet Hartley 2 captured spectacular images of this small, peanut-shaped active comet.  The images show bright plumes of material spewing from the surface and are clear enough for scientists to link jets of dust and gas with specific surface features. The photos reveal a cometary snow storm created by carbon dioxide jets spewing out tons of golf-ball to basketball-sized fluffy ice particles from the comet's rocky ends.

      At the same time, a different process caused water vapor to escape from the comet's smooth mid-section. The smooth area of comet Hartley 2 looks and behaves like most of the surface of comet Tempel 1, with water evaporating below the surface and percolating out through the dust.

      Carbon dioxide appears to be a key to understanding Hartley 2.This information sheds new light on the nature of comets and even planets, showing that Hartley 2 acts differently than the four other comet nuclei that have been imaged by spacecraft. The finding that carbon dioxide is powering the many jets could only have been made by traveling to a comet, because ground based telescopes can't detect CO2 and current space telescopes aren't tuned to look for this gas.

      More detailed analysis will be needed to determine whether the difference in out gassing between the smooth and rough regions of the comet is the result of a mixing of dry-ice rich clumps with dry-ice poor clumps during the comet's formation 4.5 billion years ago or whether the difference is due to more recent evolutionary changes.


      EPOXI image showing the Moon
      EPOXI image showing the Moon beginning its transit in front of Earth.  These distant Earth images will help characterize extrasolar Earth-like planets.
      Credit: NASA/JPL-Caltech/UMD/GSFC

      View of comet Hartley 2
      View of comet Hartley 2 taken by EPOXI from a distance of 435 miles.
      Credit: NASA/JPL-Caltech/UMD
  • Noteworthy
    • EPOXI’s observations of the Moon not only unequivocally confirmed the presence of hydroxyl and water on the lunar surface, as detected by the Moon Mineralogy Mapper instrument on the Chandrayaan-1 spacecraft, but also revealed that the entire lunar surface is hydrated during at least some portions of the lunar day.

      Contrary to prevailing scientific opinions that there is probably no water on the Moon or that if there is, it would be in cold, shadowed craters at the poles, the new findings suggest a cycle in which layers of water only a few molecules thick form, dissipate, and reform on the surface every day. They suspect that hydrogen ions from the Sun are carried by the solar wind to the Moon where they interact with oxygen rich minerals in the soil and rock to produce the water and hydroxyl molecules that are formed in the morning and then evaporate by mid-day.

      This white-light image of the Moon
      This white-light image of the Moon was taken by EPOXI as part of the Earth-Moon flyby calibration of the instruments in Dec. 2007, when the spacecraft was three times the Earth's distance from the Moon.
      Credit: NASA/JPL-Caltech/UMD/GSFC