Paleo Exo-Earths

As we discover Earth clones around nearby stars, they will be of a variety of ages.   SIM will be able to locate several Earth-sized terrestrial worlds in our stellar vicinity.  Its data will provide the initial characterizaton of these planets since SIM will be able to determine the mass, as well as the orbit, of each world.  Those findings from SIM will provide the targets for follow-on missions that will analyze the light reflected from each Earth twin.  These data will enable us to determine whether that planet is habitable, and in what geologic age that planet happens to be in. 
    The image below shows how the Earth appeared during various time steps, back to 750 million years before present.  During those eras, the Earth's environment was sometimes different from that of today.  In fact, sometimes quite different.   For example, the Earth has experienced numerous Ice Ages during which ice caps covered significant fractions of the continents.  At the extreme, Earth was essentially blanketed by ice during several episodes that are referred to as "Snowball Earth."
   So, as we venture out to study nearby Earth-like worlds, both virtually and, one day, in person, we should expect those terrestrial worlds to be in any of a number of geologic phases.  Statistically, most may be in the "Pre-Cambrian" stage.  On the Earth, this refers to the time before multi-cellular life evolved.  This stretches from 500 million years ago back to the Earth's origin at 4,500 million years ago.  It is thought that one-celled life developed at anytime from 4 billion to 2 billion years ago.  Therefore, many of our New Earths may be "populated" by algae! 
 

   There is another possibility.  Some of the Earth clones may be in "future" geologic ages; perhaps, post-civilization.  We do not know the lifespan of a typical intelligent species, such as human beings.  It is quite possible that there are the ruins of extinct civilizations scattered across the Milky Way.  However, we won't know until we look, and SIM is the first step in that long voyage of exploration. 

The Need for An Exoplanet Astrometry Mission

   Even thought the 2010 Astro Decadal Report's Executive summary does not recommend SIM as a mission to be pursued, there are other voices in the astronomy community that differ.

  The first document that differs with the Astro 2010 Executive proposal comes from one of the Focus Panels of the Decadal Survey itself!  Take a look at the EOS Panel Report. On page 6-7, the panel reports on previously endorsed projects.  It notes that SIM was re-endorsed by the 1990 Decadal Survey recommends it as a candidate for NASA's Exoplanet mission. 

 On page 6-34, the EOS Panel is even more explicit.  To quote the report, "The panel is enthusiastic about one such program, SIM Lite.." 

  With a recommendation such as this from the Panel that was charged with surveying the ExoPlanet mission outlook, it is surprising, to say the least that the Executive Summary essentially ignored SIM.

 _ _ _ _

   On another front, a recent presentation by M. Fridlund, titled "European Roadmap for Exoplanets," has a very important observation in its summary.   This page, "Conclusive Recommendations," makes it quite clear that an Astometric Exoplanet mission is on its list of 3 "High Priority" missions.  Here is the relevant quote from that report -

"We can identify 3 "high priority" missions types: 
    An astrometric mission to find terrestrial planets"

  It appears that the author is somewhat baffled as to the "demise" of SIM.  I think that the same can be said for many Exoplanet researchers. 

To Know

   Even 28,000 years ago, humanity sought to know.   Recently, attention has focused on the drawings in Chauvet Cave in France.  Here is an example of some of the artwork that can be found there.

   The vast collection of art in Chauvet Cave shows that our ancestors of 28,000 years ago were much like us.  They were exploring their world and in the process art was "invented."  In the intervening gulf of time between them and us, mankind also "invented" science as a way to make sense of the world.  In our era, as we pursue our explorations, the tools that we develop to practice science can also be viewed as a form of art.  The technology that has been developed for the SIM astrometry mission is one of the best modern examples of that.  The various devices, such as the Astrometric Beam Combiner, that are at the heart of SIM, are truly works of art.  The represent the best efforts of our species to know the world. 

Astrometry

   Let's consider the art of Astrometry.  This is what the SIM mission is all about.  Astrometry is the method by which one determines the positions of stars, and other celestial objects, in the sky.  It is an ancient practice.  The most famous of the ancients who devised star maps is the Greek astronomer Hipparchus who lived in the 2nd century B.C.
   However, stars are not stationary.  They "drift" through space, or more properly, are in orbit about the center of the Milky Way galaxy.  Modern astrometry utilizes the measurement of stars' positions as they change with time.  In other words, modern astrometry produces a very accurate record of a star's motion through space.  If a star is a solitary object, with no planets or sibling stars, then it should be traveling in an absolutely straight line.  However, if the star has companions, then their mass will cause the path of the star to follow a curved path.  The larger the mass of the companion, and the closer it is to the central star, the more pronounced the curvature.
   Let's look at one famous example of this technique, the discovery of Sirius B, the White Dwarf star that orbits around Sirius A.  While we can easily see Sirius A on any winter's night, Sirius B, while massive, is very dense and is invisible to the naked eye.  In 1844, Friedrich Bessel noticed that the path of Sirius deviated from a perfect straight line.  He attributed this to the presence of an unseen companion.  The existence of Sirius B was confirmed about 20 years later when it was detected visually.
   The diagram below shows the orbits of Sirius And B, as well as the curved paths of both stars.  One can imagine a straight line passing between those curved paths.  That straight line is the path of the center of gravity, or barycenter, of the system.

   SIM uses the technique of interferometry to determine the positions of stars with unprecedented accuracy.

  The image above shows how the Sun moves around its barycenter, or center of gravity.  One can see how complicated would be the curved path of our Sun through space.

Milestones Met

NASA HQ set a series of engineering milestones before the SIM Team in 2000.  Those milestones were met, or exceeded in each case.    

    Each of these milestones was designed to "prove" that SIM would be able to actually chart stars with an unprecedented accuracy.  That precision is the key to achieving SIM's multiple goals, including the detection of Terrestrial ExoPlanets.

   The 1st Four Milestones were to be met before SIM would be allowed to proceed to Phase B in 2003.  The SIM team successfully achieved that.  By 2005, the SIM project had checked off all of the 8 milestones set before it.  Since 2005, the team worked on reducing engineering risk further.

   What we are faced with, in these times of extreme budget pressures is a choice of whether NASA should, once more, walk away from a partially complete program.

Contact with Destiny

     We are on the threshold of a new era.  With SIM, we will have the capability, for the first time in history, to find New Worlds in our stellar neighborhood.  By New Worlds, I mean planets such as our Earth, planets of the right size, and orbiting their parent stars at the right distance.  

     The urgency of continuing the SIM project is best expressed in the words of the late Carl Sagan in his book, Contact.  On page 36, the protagonist of the novel, Dr. Arroway, is arguing the case for SETI, the Search for ExtraTerrestrial Intelligence.  Those words, I believe, apply equally to SIM.  Here is the relevant quote  -

  "This is the time to be optimistic.  If we lived in any previous time of human history, we could wonder about this all our lives, and we couldn't do a thing to find the answer.  But this time is unique. ....Wouldn't you be ashamed of your civilization if we were able to listen and didn't have the gumption to do it?"  


    SIM will not listen.  Rather, it will look.  However, the message is the same.  This is a special time in which we now live.  As a species, we are poised to find, and map, nearby Earths.  This is not the time to pause, to turn our backs on the challenge that we face.  With the data from SIM, we will be able to answer the question of whether there are Worlds, like our own, circling the nearest stars.  As Sagan wrote in Contact,       

      "Can you think of a more important question?"  

Insight Through Precision

    The SIM Interferometer observatory will be a highly capable tool not only for the discovery of Exoplanets. Because of its ability to locate stars with stunning precision, SIM will also revolutionize several fields of Astrophysics.  Those of you who are amateur astronomers may not realize that the properties of Main Sequence stars are not fully understood.  One of the main reasons for this is the lack of data concerning stars' Masses and Distances.  SIM will change all of that.
  
     The SIM project has put out a book titled, "SIM Lite Astrometric Observatory."  It goes into detail on the science and technology of the SIM space observatory.  Here is a link to Chapter 8, which discusses the contributions of SIM to our basic knowledge of how stars work.

Stellar Maps

   To pick one of the target areas that will benefit from SIM, let's consider O and B giant stars.  For the first time, with SIM's data, we will be able to know the distances to a statistically significant number of these stars.  This will allow, for the first time, accurate models of how these stars evolve.