| A Giant Leap or a Distant View? |
| Tuesday, 26 April 2005 | |
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Virginia Hooper, Alistair Crosby and Louisa Dunlop investigate the pros and cons of funding for manned exploration of the Solar System versus continued support of the Hubble Space Telescope and other unmanned, space-borne observatories Kennedy did it in 1961. George Bush Sr did it in 1989. Last year, George Bush did it once again, and heralded the start of a great journey to send men to the moon and then on to Mars. Just days later, it was announced that the famous Hubble Space Telescope would not receive its planned lifeline. We examine both Hubble’s achievements and the new plans for space exploration and ask: does Nasa have its priorities right?
Eye in the SkyLaunched in 1990, the Hubble Space Telescope is a 2.4-metre optical reflecting telescope that operates from ultra-violet to near infrared wavelengths. It orbits the Earth every 95 minutes from its position 600 kilometres above the surface. The simple modular design of the telescope has helped it to keep pace with innovation by allowing new components to be added with ease. Regular servicing every few years means that astronauts can add instruments and replace or repair outdated equipment. If servicing stops then Hubble will rapidly fall into disrepair. The most pressing requirement is for new gyroscopes. These allow the telescope to orientate itself and point towards objects of interest. Hubble has six gyroscopes in total and uses three at any one time. Unfortunately, only four of these are now operational. In an attempt to extend the useful life of the telescope, programmers have experimented with using just two gyroscopes at a time. Initial tests have worked well, and this new measure should keep the telescope working for an extra year — until the end of 2008. Eventually, a robot will be dispatched to de-orbit Hubble, which will then crash safely into the ocean. Hubble provides the deepest glimpses into our visible universe by virtue of its vantage point above the atmosphere. This frees it from the distortion caused by air motions at optical wavelengths, which also makes stars twinkle. Turbulence in the atmosphere causes packets of air of different densities to mix.These refract the light from stars by different amounts, producing the ‘twinkling’. Hubble’s position gives an uninterrupted view of the infrared spectrum. Infrared radiation is the most important part of the spectrum for viewing distant galaxies, because the further an object is from Earth, the more the light from it is ‘redshifted’ — shifted towards the red end of the spectrum — by the Doppler effect. Although Earth-bound telescopes can see in the infrared, they are at a disadvantage because at these wavelengths the atmosphere is relatively opaque and absorbs this radiation before it reaches the surface. For this reason, most Earth-based infrared telescopes are positioned on mountain tops, high in the atmosphere and above cloud cover, in order to reduce the opacity. Hubble’s ability to see more clearly and much further has numerous advantages, one of which is that it has enabled scientists to calculate the age of the universe. Because the universe is expanding, the further a galaxy is away from us, the faster it moves away and the redder its light becomes. To visualise this, imagine the surface of a balloon with dots drawn all over it. The galaxies in the universe move apart in the same way as the dots on the balloon move as it is being blown up. An equation known as Hubble’s Law relates the distance to and the redshift of a distant galaxy, in a way that depends on the age of the universe. Hubble measures the redshifts of certain objects called Cepheid Variables, whose distances astronomers can find straightforwardly. Cepheid Variables are pulsating stars that are so luminous — up to 10,000 times brighter than the sun — that they can be seen up to 65 million light years away. The pulsation is due to physical changes in the size and surface temperature of the star over the course of just a few days. The fluctuation period is roughly similar for all Cepheids of the same brightness. We know how bright a star should be by measuring how long it takes to pulse. If we compare the observed luminosity of a Cepheid as seen by Hubble with its modelled luminosity, we can calculate the Cepheid’s distance from the Earth. Hubble has been able to detect Cepheids up to 10 times further away than any ground-based telescope. Using its data, astronomers obtained an age of approximately 13 billion years for the universe. This new figure solves one of the great problems in astronomy as previous estimates had suggested that the universe was younger than its oldest stars!
Hubble has also allowed research into ‘dark matter’, the most elusive but most abundant form of matter in space. This, together with the even more mysterious ‘dark energy’, makes up more than 90% of the mass of the universe. Exactly what they are remains uncertain, and it is only recently that Hubble has provided conclusive evidence for their existence, which can only be explained if there were more mass in the universe than can be accounted for by conventional methods. The presence of dark matter can be inferred from the gravitational lensing of far galaxies: gravity deflects passing light rays from objects behind the galaxies and focuses them into bright curves imaged by Hubble. The mass needed to produce such an effect is 10 times more than the mass associated with the visible galaxy, suggesting the presence of additional, dark, matter. Astronomers showed in 1998 that the expansion rate of the universe was increasing with time, using Hubble observations of exploding stars called ‘type IA supernovae’. It is thought to be dark energy that causes this acceleration. With its incredible resolution and the far-reaching field of view of its ‘Near Infrared Camera and Multi-Object Spectrometer’, Hubble has witnessed some remarkable events.These include images of the Comet Shoemaker-Levy 9 smashing into Jupiter, dust storms on Mars, and recording the births and deaths of stars. This constant stream of information is of vital importance to astronomers trying to understand the universe. As we have seen, Hubble will fail within the next few years and its supply of data to the astronomical community will cease, making new research more difficult. Not only the purely scientific impact of Hubble will be missed, but also the inspirational and cultural influence of this modern icon. Images from Hubble have permeated into popular culture and increasingly define our mental picture of the cosmos. Hubble was never intended to last indefinitely and the scientific community welcomes the prospect of a new and more sophisticated telescope. But astronomers will have to wait until at least 2011 for the launch of Hubble’s successor, the James Webb Space Telescope. This significant gap after Hubble’s final years is due to a change in priority: the US is redirecting Nasa’s focus to manned rather than robotic exploration of the near planets with a view to eventually setting a man on Mars by 2035. The first step will be to send men back to the moon by 2020 at the latest. This will be achieved by phasing out the much-criticized ISS and the space shuttle and concentrating resources on developing new technologies to allow manned explorations beyond the first few hundred kilometres of space (low Earth orbit). These plans come at enormous expense without any significant increase in the overall budget. Funds are to be squeezed from other areas; for instance, current plans are for Nasa’s research into the Earth’s environment to have its funding cut by $1.1 billion between 2005 and 2009. Return to the MoonBut it was Bush’s vision of sending men to the moon and thence to Mars that really succeeded in grabbing the headlines. However, can the White House really justify such a grand commitment? The official report of the President’s Commission on Moon, Mars and Beyond implies that crews carry out better science than probes, and that human habitation in space is both feasible and desirable. Is what it says true? The Commission advocates astronauts operating observatories on the moon, but it would surely be better — not to mention cheaper — to position those telescopes in space, well away from any human activity. In order to be as sensitive as possible, telescopes need to be kept cold and free from vibration, and it is difficult to see how an expensive and noisy manned presence nearby would help. Hubble’s successor, for instance, will be stationed 1.5 million kilometres from the Earth and will be kept at a temperature just 35 degrees above absolute zero. It is true that astronauts would be much better than robots at making detailed investigations of the Martian surface, but robots are cheaper, safer, and improving rapidly in capability. Unmanned missions can be prepared much more quickly than manned ones, and planners can then use data from previous missions to select new research priorities. A manned mission, by contrast, would be decades in the making and could be seen as redundant before it even left the ground. At today’s prices, the Apollo Program (1963–72) to send men to the moon cost $100 billion. Mars is much harder to get to, yet Nasa’s current robotic rovers cost less than 1% of the price of Apollo. They have been able to crawl more than 7 kilometres over the surface of Mars, and provide detailed field evidence of how parts of it were once covered by water. The case for government investment in human habitation and mining in space is more shaky. Proponents of space exploration sometimes argue that we might one day run out of places to live on Earth, but living in space would be profoundly expensive. Presently, it costs $10,000–$30,000 per kilogram to put a load into space, a figure that has barely fallen in 20 years. Launching five space shuttles requires as much energy as released by the bomb that destroyed Hiroshima. A recent UN report estimated that by 2300 the world’s population will level out at about 9 billion — this is 50% higher than today, but does not necessarily mean that inhabiting space is the only, or the best, option. Finally, for those who still dream of space, we may soon see the emergence of a private market in space travel. In June last year, Burt Rutan, an American, became the first individual to reach space in a privately funded spacecraft. Richard Branson has ordered five: prices will start at about £100,000 for a 3-hour hop, but are expected to fall as demand increases and the technology improves. But to some extent such criticism misses the point. Government space programmes have never been just about science and they have certainly never been about economics. They are about exploration and the inspiration of national pride, and Bush is correct that Nasa’s current manned programme fails to do either. But there are cheaper, more novel, and more deserving sources of inspiration than sending men back to the moon, and they include attempting to answer the biggest question of all: are we alone? It’s life, but not as we know itIn our own solar system, there are two candidates for life. The first is Mars. Mars may be cold (as low as -140°C at the poles) and have a surface atmospheric pressure one two-hundredth that of the Earth; but we know from probes in orbit that it has subsurface water ice, and it has been volcanically active within the recent geological past. There may well be hardy organisms living deep in the soil or in fractures flushed by hydrothermal fluids. Nasa’s next Mars rover, to arrive in 2010, will carry instruments to detect organic molecules in samples it collects. The second is Jupiter’s moon Europa. Europa is even colder than Mars, and has no atmosphere at all, but underneath its icy surface there appears to be an ocean: observations of Europa’s magnetic field taken by the Galileo probe in 2000 indicate a fluid conductor fairly close to the surface. Sending humans there is out of the question: one day sending a robot to slowly melt its way through the ice and look for life is not. Indeed, a working team is being set up to look at the possibility of a joint venture to Europa between the European Space Agency (ESA) and Nasa. But it is the idea of other Earths that really gets people excited. In the last few years, astronomers have realised that planets are both a lot more common and a lot more diverse than once thought: to date, 145 (decidedly non-Earth-like) planets around nearby stars have been identified. Imaging Earth-like planets around nearby stars would require unfeasibly large mirrors or lenses, because the resolution of a telescope depends on its diameter. However, instruments known as interferometers may be up to the job, and Nasa plans to start flying them in orbit within 20 years. Interferometry is a technique in which one recovers information about a source by combining several observations of the source taken far apart but at exactly the same time. It has been used by radio astronomers to produce images of bright radio sources with a hundred times the resolution of the Hubble Space Telescope. Achieving the same with light, which has a much shorter wavelength, is much trickier, but possible, as a recent experiment by the University of Cambridge ’s Cavendish Astrophysics group, COAST, showed. Their interferometer, less than 100 metres across and costing just £850,000,was able to image the surface of a star for the first time: a task far beyond even Hubble. Unfortunately, Nasa has a history of wasteful investments and overblown rhetoric. It also has some of the best engineers in the world and no shortage of worthwhile projects that won’t demand that astronauts risk life and limb. The circus that surrounds sending humans into space is both inefficient and scientifically unrewarding; George Bush should give Hubble a few more years, and then have the courage to leave manned spaceflight behind. The astronaut is yesterday’s icon. Virginia Hooper is a fourth year Natural Scientist, specialising in Geology; Alistair Crosby is a PhD student in the Department of Earth Sciences; Louisa Dunlop is a PhD student in the Department of Physics Further ReadingPresident’s Commission on Moon, Mars and Beyond A Budgetary Analysis of NASA’s New Vision for Space Exploration |
To many the most important scientific instrument of recent times, the Hubble Space Telescope has revolutionised our understanding of the universe and the processes that go on within it. Yet despite these impressive credentials, the future of this extraordinary telescope remains uncertain. In January 2004, the US government outlined an ambitious new space programme focused around manned exploration of the moon and eventually Mars. Although likely to capture the imagination of the taxpaying public, these bold endeavours will undoubtedly put pressure on other science projects. Already, seven missions including JIMO, Ulysses and Geotail have been cancelled in a bid to conserve funds. It is against this backdrop of a new vision for space exploration that the latest servicing mission for Hubble has been indefinitely postponed. The cancellation of manned shuttle servicing last year came amidst safety fears fuelled by the ill-fated Columbia service mission of 2002 when the Shuttle disintegrated on re-entry, killing all on board. Nasa now requires shuttles to be within reach of the International Space Station (ISS), to provide a refuge for astronauts in the event of a technical emergency. Hubble, however, is in orbit above the ISS so any servicing mission would be in breach of the new guidelines. A safer but more technically demanding option would be robotic servicing, but this now looks increasingly unlikely given the recent announcement of Nasa’s budget, which made no provision for future servicing missions to the telescope, either manned or robotic. 
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