If there are other planets in existence similar to Earth in orbit around nearby stars, are any of them habitable, and could one of them serve as a grand exploration target for the human race?
These are two of the questions which NASA will be seeking to answer with its "Origins" programme. By 2020 it plans to develop and operate astronomical observatories with far more capabilities than those of the present fleet of spacecraft. NASA also aims to expand the technology necessary to make such far-reaching missions affordable.
"Origins should not be a one-shot deal," says NASA, explaining that the programme's first target is the search for the "origin of galaxies". The next phase is the search for planetary systems, and the last will be the search for Earth-like planets around nearby stars.
As a first step, NASA will keep the first-generation Origins craft - the Hubble Space Telescope - working without any new scientific instruments beyond the planned end of its operational life (2005) after the final Space Shuttle servicing mission, due in 2002.
The second step will be the development of a successor to the Hubble and the third will be to develop the ability to deploy large infra-red (IR) interferometers in space.
Next giant leap
Other telescope projects will complete the first generation of the Origins programme. As well as the Hubble, with its newly fitted near-infra-red camera and multi-object spectrometer, there are the Wide-Field Infrared and the Far-Ultraviolet Spectroscopic Explorers.
New craft under development as part of this phase include the Space Infrared Telescope Facility (SIRTF). This will be a cryogenically cooled observatory to conduct IR astronomy from space, offering "orders-of-magnitude improvements in sensitivity over previous IR missions", says NASA. The SIRTF is due for launch in 2002, and is expected to have a 2.5-year lifetime.
NASA is preparing to take "the next giant leap in astronomical capability" with three second-generation Origins missions. With these new spacecraft, each building on experience with the earlier ones, "-astronomers around the world will get ever-sharper views of the distant universe", says NASA.
The three craft are the Space Interferometry Mission (SIM), the Next-Generation Space Telescope (NGST) and the Planet Finder (PF). The PF will be followed by a third-generation Origins mission, the Planet Mapper. The SIM, to be launched in 2003, will provide images of stars, galaxies and nebulae four times better than those being returned by the Hubble. It will also produce an improved star map.
NASA's Jet Propulsion Laboratory in California, has selected a TRW-led industry team to help design the SIM spacecraft. It will be the first space mission to use an optical interferometer as the primary instrument.
An interferometer is an instrument which works by receiving radiation - in the case of the SIM, visible light - from widely separated antennas. Instead of the large single mirror of a reflecting telescope, for example, the SIM interferometer consists of several radiation receivers, creating a larger effective aperture than that of a telescope.
Biggest challenge
SIM will revolutionise astrometry, the precise measurement of the relative positions of stars in the sky. Successive measurements of the stars by the interferometer from SIM's 900km (560 miles)-high orbit over the poles will be used to calculate distances to stars while the Earth is in orbit around the Sun. The resulting star catalogue will be 1,000 times more accurate than the equivalent generated by the European Space Agency's Hipparcos satellite, which was launched in 1989.
The SIM will also be used to measure the distance of other space objects, such as nebulae, and detect the presence of other matter around stars, such as planets, by detecting minute "wiggles" in the stars' movement caused by its gravitational influence. The measurements, within an accuracy of four microarc-seconds, are the equivalent of being able to distinguish two points of light 4,800km away, separated by the thickness of a sheet of paper.
The biggest challenge will be maintaining the stability of the spacecraft. Thermal effects and vibrational forces will have to be fully understood, as well as compensated for, as these conditions could reduce the accuracy of the instruments.
The SIM's interferometer will also produce high-resolution, high dynamic-range images of objects such as the dense cores of star clusters and, in its nulling mode, even spot a brown dwarf in close orbit around a bright star.
The high resolution will result from the interferometer's effective aperture of 9.75m, compared with the Hubble's 2.37m. In nulling mode, the SIM makes dim objects such as planets and nebulae visible by interfering with the images of bright stars, suppressing the brightness by factor of 10,000.
The SIM will be followed by the NGST. This telescope will be able to see objects 400 times fainter than those now being studied using large ground- and space-based IR telescopes, but with spatial resolution - image sharpness - comparable to that of the Hubble.
Its primary missions are to capture images of the galaxies and supernovas which were the first to appear in the universe and to search nearby stellar systems for evidence of planetary systems. It will have an extended lifetime to allow follow-on observations of newly discovered phenomena - "-to boldly go where no telescope has gone before", as NASA puts it. This has implications for the choice of IR spectrum which the NGST can observe.
"Our current understanding of how the universe was formed and evolved is remarkably well-constrained at the extreme ends of time," says NASA. In other words, there is a fairly good understanding of what the universe is like today and in the recent past, when the universe was 10-15 billion years old, from observations at all wavelengths.
"We also have a pretty good understanding of what the universe was like when it was quite young - less than about 1 million years old - based on observations of the cosmic microwave background and high-energy particle physics," says NASA - but there is a middle ground, when the universe was between 1 million and a few billion years old, which is unobserved.
"It is during this time period that the first structures we see today, namely stars and galaxies, began to form. This is where the NGST comes in," says NASA.
Three designs of the NGST have already been completed by the NASA-led study teams at the Goddard Space Flight Center, Lockheed Martin and TRW. The 2,700kg NGST will be launched in 2007 aboard a Lockheed Martin Atlas 2AS-class vehicle. It will be placed directly into solar orbit 1.5 million kilometres from Earth, at the L2 liberation point - also called the Lagrangian Point - where the gravitational pulls of the Earth and the Sun are equal. It will be equipped with a passively cooled reflector with a diameter of more than 4m, and will operate for ten years.
The third of this generation of the Origins programme will be the Planet Finder (PF), intended to detect planets going around other stars. It will be launched in 2005 and will not reach its operating position until after the NGST, launched two years later, is in place.
NASA believes that the PF should be based beyond Jupiter, outside the region of space where zodiacal dust in the Solar System could cause radio noise. The PF will consist of four IR telescopes operating together, searching for Earth-like planets and attempting to find carbon dioxide, water vapour and ozone in their atmospheres.
Favourable contrast
The detection is best achieved in the IR because of the relatively favourable contrast ratio between bright star and dim planet. This will allow the use of a nulling technique, similar to that used by the SIM's interferometer, to block out the light from the star, enabling direct detection of any planets.
Through spectroscopic analysis, the PF will be able to probe the atmospheres of detected planets to search for hints of life as we know it.
The final frontier for the Origins will be the mapping of habitable planets by a third-generation spacecraft, which is known as the Planet Mapper (PM).
This craft would provide photographs of the surfaces of Earth-like planets detected and characterised by the PF, says NASA, "-allowing us to determine whether such bodies are indeed habitable, perhaps providing destinations for future generations of explorers from Earth".
Because of the need for high-resolution imagery of such distant objects, the PM is likely to be an array of enormous telescopes, each with an aperture in the tens of square kilometres. "Such a project is beyond our planning horizons at the present," says NASA.
The PM may be "the challenge our children", as NASA puts it, "but it is important that NASA identify technological paths for the PM mission so that we can start down this long path as soon as practical."
Source: Flight International
