Project Icarus: Exploring Exoplanet Biosignatures
The Icarus interstellar probe would carry out an assortment of science during the journey to another star.Image credit:
Project Icarus is an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination. Initiated by the Tau Zero Foundation and British Interplanetary Society, and managed by Icarus Interstellar Inc., a non-profit group of scientists dedicated to interstellar spaceflight, Icarus is working to develop a spacecraft that can travel to a nearby star.
Ian Crawford, Reader in Planetary Science and Astrobiology at Birkbeck College, University of London, and Lead Designer for the Icarus "Scientific Objectives" module, discusses the science that could be carried out during an interstellar journey and some of the science priorities when the Icarus probe arrives in the target star system.
The Icarus study is tasked with designing an interstellar space vehicle capable of making in situ scientific investigations of nearby stars. In an earlier article I discussed possible target stars for Icarus. Here, I describe some of the scientific motivations for this ambitious project.
There can be little doubt that science — especially in the fields of astronomy, planetary science and astrobiology — will be a major beneficiary of the development of rapid interstellar spaceflight as envisaged by the Icarus project.
In its long history astronomy has made tremendous advances through studying the light that reaches us from the cosmos, but there is a limit to the amount of information that can be squeezed out of the analysis of starlight and other cosmic radiation. Already we can identify areas where additional knowledge will only be gained by making in situ observations of distant astronomical objects, and this will require specialised scientific instruments to be transported across interstellar space.
The scientific objectives for an Icarus-style interstellar probe can be divided into the following broad categories, which we propose to be in order of increasing priority:
1. Science to be conducted en route during the cruise phase between the stars;
2. Astrophysical studies of the target star itself, or stars if a multiple star system is selected;
3. Planetary science studies of any planets in the target system, including moons, asteroids and comets of interest;
4. Astrobiological/exobiological studies of any habitable (or inhabited) planets or moons which may be found in the target planetary system.
Science Conducted During the Cruise Phase
By definition, any interstellar vehicle will have to traverse the interstellar medium between the solar system and the target star.
Key measurements that could be made from an interstellar vehicle, and which would add enormously to our understanding of interstellar processes, would include in situ determinations of density, temperature, gas-phase composition, ionisation state, dust density and composition, interstellar radiation field and magnetic field strength, all as a function of distance between the sun and the target star system.
Determining some of these properties of the local interstellar medium will also be important for longer term planning of later interstellar missions — for example, quantifying the impact hazard posed by interstellar dust grains.
In addition to studies of the interstellar medium conducted en route, there are other astronomical and physical measurements that could make use of the Icarus vehicle as an observing platform during the cruise phase.
These include using the long baseline opened up between Icarus and the solar system to extend the trigonometrical distance scale to extragalactic objects, test theories of gravity (including the possible detection of gravitational waves), and perhaps search for evidence of dark matter in between the stars.
In spite of the scientific interest of these observations, however, scientific investigations conducted en route are a relatively low priority when it comes to the choice of target. This is not because such investigations are scientifically unimportant, but because they can largely be conducted regardless of what the particular choice of target star.
It is true that in some directions the local interstellar medium is of more interest than others, but this is unlikely to be a scientific driver for a vehicle as complex and costly as an Icarus-type starship.
Study of the Target Star(s)
Astrophysical studies of the target star will have a higher priority. Although all potential targets for Icarus will be nearby stars (probably closer than 15 light-years), about which much can be learned from astronomical observations from the solar system, detailed studies of, for example, photospheric structure, magnetic properties, and stellar wind, would clearly benefit from the possibility of in situ observations.
From this perspective, higher priority might be given to rare or unusual stars. Examples might include an early-type star and/or a white dwarf, both of which could be achieved by selecting either Sirius (8.6 light-years away) or Procyon (11.4 light-years away) as a targets, as both have white dwarf companions.
There will also be great astrophysical interest in making close-up observations of brown dwarfs (the closest known of which are in the Epsilon Indi system at a distance of 11.8 light-years), or a nearby red dwarf (which, although not rare, are perhaps the least understood class of main sequence stars owing to their intrinsic faintness).
Last, but not least, we should not underestimate the scientific importance of making observations of another main-sequence G-type star such as Alpha Centauri A (4.4 light-years away) or Tau Ceti (11.9 light-years away) to enable direct comparisons with the sun.
Therefore, from an astrophysical viewpoint, there is certainly plenty of science that could be achieved with an Icarus-style starship.
However, interesting and important as these astrophysical considerations are, by themselves they are unlikely to be the main scientific drivers for an interstellar space mission. In part this is because foreseeable advances in astronomical techniques will enable us to continue to refine our understanding of the astrophysical properties of nearby stars without having to physically leave the solar system.
Planetary Science and Astrobiology
There seems little doubt that the presence of a planetary system will greatly increase the scientific priority of a potential target star.
This is because there are many aspects of planetary science which can only be addressed by making measurements in close proximity, including the landing of scientific instruments on planetary surfaces. We can be sure of this because, over the last half century, in situ spacecraft observations have completely revolutionised the study of the planets and moons of the solar system, providing information that could never have been obtained telescopically from the surface of the Earth or its immediate vicinity.
If this is true of the planets in our own solar system then it stands to reason that it will be true of other planetary systems also.
In addition to their intrinsic planetary science interest, the habitability of any such planets will be of compelling scientific interest.
I think we can be reasonably confident that, long before we are ready to build an Icarus-type starship, astronomical observations will be able to establish a hierarchy of priorities among any planets which may be detected around the nearest stars: (i) planets where plausible biosignatures are detected in an exoplanet's atmosphere; (ii) planets that appear habitable (e.g. for which there is spectral evidence for water and carbon dioxide) but for which there is no explicit evidence of life being present; and (iii) planets which appear to have uninhabitable surfaces (either because of atmospheric compositions deemed non-conducive to life or because they lack a detectable atmosphere) but which might nevertheless support a subsurface biosphere.
Thus, when planning an interstellar mission with astrobiology/exobiology in mind, we are likely to have a priority list of target systems prepared well in advance.
The highest priority of all would be given to extrasolar planets for which spectral evidence of an indigenous biosphere is detected by astronomical observations. In such a case, definitive proof of the existence of indigenous life, and follow-up studies of its underlying biochemistry, cellular structure, ecological diversity and evolutionary history, will absolutely require in situ measurements to be made.
This will necessitate the transport of dedicated scientific instruments across interstellar space, and would be the strongest possible scientific justification for building an Icarus-style starship. It also implies a mission architecture which will permit deceleration of the vehicle at the target system, as the kinds of measurements required are unlikely to be possible while flying through the system at a substantial fraction of the speed of light!
An interstellar space vehicle such as Icarus will yield considerable scientific benefits. These include studies of the interstellar medium conducted en route, and astrophysical observations of the target star(s).
However, it seems clear that when it comes to selecting a final target star the presence of a planetary system, and especially the presence of habitable or inhabited planets, will trump all other scientific motivations. Although, as described in my earlier article, our knowledge of planetary systems within 15 light-years of the sun is still patchy, over the coming decades advances in astronomical techniques are likely to give us a much more complete inventory of nearby planets.
So, by the time we are ready to actually build a starship such as Icarus, we will have a very good idea where to send it!
A more detailed discussion of the issues covered in this article can be found in Ian Crawford's paper on the scientific case for interstellar space probes published in the Journal of the British interplanetary Society, vol. 62, pp. 415-421, (2009). Download PDF