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Mission update Ups and downs Cluster goes up but will planetary programmes go down? Peter Bond reports on space mission plans and achievements. I f at first you don’t succeed, try, try, try again. Many times during the past four decades this particular piece of popular wisdom has been applied to space exploration: the European Space Agency’s Cluster mission proved to be no exception. Four years after the loss of the first Cluster quartet in a launch explosion, the rebuilt satellites have been successfully placed into their operational orbits by dual Soyuz launches from Kazakhstan. The first pair, now called Salsa and Samba after a pan-European competition won by UK civil servant Raymond Cotton, lifted off on 16 July, a day late after minor technical problems on the pad caused by heavy rain and thunderstorms. The two remaining Cluster spacecraft, Rumba and Tango, were launched on 9 August and joined their sister spacecraft a week later. Each spacecraft carries 11 identical instruments. These will study electric and magnetic fields and waves in the plasma surrounding the spacecraft, investigate the relative abundance of electrons, protons and helium nuclei, and determine their three-dimensional distribution. Instrument check-out and commissioning are now well under way, and the first scientific data are expected to be returned in late November or early December. As the first four-spacecraft flotilla to fly in close formation through the magnetosphere, Cluster will make a unique contribution to scientific understanding of the processes taking place in the Earth’s magnetosphere and its interaction with the solar wind. Their 19 000 ×119 000 km elliptical orbits will carry them almost one third of the way to the Moon. As a consequence of these unusual orbits, they will either travel far down the magnetotail or shuttle between the magnetosphere and interstellar space. And by flying in tetrahedral formation as they cross major boundaries and regions of interest such as the polar cusps and the magnetopause, the quartet will also provide the first detailed three-dimensional view of the charged particles and electromagnetic waves that pervade near-Earth space. Once it becomes operational, Cluster will join the Solar and Heliospheric Observatory (SOHO) to complete the first Cornerstone of ESA’s Horizons 2000 long-term scientific programme. Both missions also represent an important example of solar–terrestrial science co-operation between NASA and ESA. See http://sci.esa.int/cluster/ or http://international October 2000 Vol 41 Cluster satellites Tango and Rumba pictured as they separated after their successful launch. This is the first colour picture taken in space without a human hand present on the shutter, thanks to the tiny Visual Monitoring Camera that is part of Cluster’s equipment. (ESA.) .gsfc.nasa.gov/International/Missions/Cluster_ II/Cluster_II.html. Further good news for space physicists was the agreement by ESA to fund the Ulysses mission for an extra 33 months. At its meeting in Paris on 5–6 June, ESA’s Science Programme Committee agreed to continue operating the spacecraft from the end of 2001 to 30 September 2004. However, since Ulysses is a joint mission, NASA’s approval is also required for the mission extension. So far, NASA has approved funding until December 2002 and a decision on further funding is expected in mid-2001. If that decision is positive, Ulysses will remain in operation as sunspot activity gradually declines after this year’s solar maximum. Such an extension would allow Ulysses to make the first-ever set of high-latitude observations over a full solar cycle. See http://sci.esa.int/ulysses. As if to emphasize the importance of such studies, an X5-class solar flare, one of the most powerful flares of the current solar cycle, triggered a proton storm in the neighbourhood of our planet on 14 July. The bright flash of the solar flare near the centre of the Sun’s disk was seen by SOHO’s ultraviolet telescope EIT at 10:12 UT. The flare’s intensity peaked at 10:24, and half an hour later SOHO’s LASCO instrument detected a full halo coronal mass ejection (CME) heading towards the Earth. Next, a burst of energetic particles from the solar explosion hit SOHO and resulted in a snowstorm in the imaging instruments that continued for some hours. Travelling more slowly than the energetic particles, the interplanetary shock wave driven by the gas of the CME reached SOHO at 14:19 UT on 15 July. The spacecraft’s CELIAS solar wind instru- ment registered a jump in the wind speed from 500 to 800 km s–1, increasing to over 900 km s–1 an hour later. The CME slammed into the Earth’s magnetic field half an hour after it struck SOHO, provoking auroral displays that peaked in the early hours of 16 July. Satellite operators and electric-power engineers reported many side-effects. The main casualty was the Japanese ASCA X-ray observatory which went into safe mode and began to spin out of control. With its solar panels receiving less sunlight and increased atmospheric drag rapidly lowering its orbit, ASCA is probably lost for good. Numerous other scientific spacecraft also suffered transient malfunctions as a result of this solar storm. For example, ACE temporarily lost output from its solar-wind velocity measuring sensor, SOHO suffered a small but permanent degradation in solar panel output, and the solid-state power amplifier on Wind experienced a power loss of 25%. The Danish Oerstedt satellite went dead but was recovered a few days later. The X-ray/gamma-ray spectrometer on the NEAR Shoemaker spacecraft, which is in orbit around Eros, shut down for two days. Several spacecraft experienced problems with their star trackers, including the German CHAMP, which was launched on 15 July while the event was still under way. Germany produces a CHAMP CHAMP, the CHAllenging Minisatellite Payload, is intended to study the Earth’s gravity and magnetic fields to very high accuracy. Its low circular orbit, starting at 454 km altitude and decaying during the mission to below 300 km, together with the greatly advanced instrumentation flown on CHAMP, promise an order of magnitude improvement in magnetic field measurements compared to its MAGSAT predecessor. CHAMP will also use an advanced Digital Ion Drift Meter to produce global maps of the ionospheric electric field. Its long mission lifetime and precessing orbit should allow scientists to study the dependence of the electric field patterns on magnetic activity, on local time, on season and on the solar cycle phase. The low-altitude near-polar orbit, the continuous GPS data that will determine precisely the satellite’s orbit, and onboard measurement of non-gravitational orbit perturbations should also provide an improvement of up to two orders of magnitude in accuracy in the determination of the broad to mesoscale structures of the Earth’s gravity field. This major breakthrough is expected to lead to new insights and applications in geodesy, solidEarth physics and oceanography. CHAMP is managed by the GeoForschungsZentrum (Earth Research Centre) of Potsdam. International partners on the mission are NASA, the French Centre National 5.25 Mission update Cometesimals. The Hubble telescope discovered a small armada of “mini-comets” left behind from what some scientists had prematurely thought was a total disintegration of the explosive comet LINEAR. The solid nucleus of comet LINEAR was reported “missing in action” following its passage around the Sun on 26 July. For the first time, astronomers are getting a close-up view of the dismantling of a comet’s nucleus as it approaches the Sun and warms up. The results support the popular theory that comet nuclei are really made up of a cluster of smaller icy bodies called cometesimals. (NASA, Harold Weaver, the Johns Hopkins University, the HST Comet LINEAR Investigation Team, and the University of Hawa.) des Études Spatiales (CNES), and the US Air Force Research Laboratories. See http://op.gfzpotsdam.de/champ/. NASA gives precedence to Mars On 10 August, NASA Associate Administrator for Space Science Ed Weiler announced plans to launch not one, but two large scientific rovers to Mars in 2003, in preference to a scientific orbiter. An airbag cocoon, similar to the one used on the 1997 Mars Pathfinder mission, will be employed during the landing phase. The Delta II launches from Cape Canaveral are currently planned for 22 May and 4 June 2003. They will take advantage of a very favourable planetary alignment between Earth and Mars. After a seven-and-a-half month cruise, the first rover should enter Mars’ atmosphere on 2 January 2004, with the second rover bouncing to a stop on the Martian surface 18 days later. The identical 150 kg vehicles will have far greater mobility than the Sojourner rover on Mars Pathfinder and will be able to travel up to 100 metres a day. Surface operations will last for at least 90 sols (Martian days). Cornell University has been selected to provide the scientific payload and lead the science team. Six scientific instruments have been selected. They include a panoramic camera, an infrared device called Mini-TES, a microscopic camera, a Mossbauer spectrometer to identify iron-bearing minerals and an alpha-proton-Xray spectrometer to measure the concentrations of most major elements. The package also includes a rock abrasion tool to expose fresh rock surfaces for study. The landing sites have yet to be selected, but, if all goes according to plan, the rovers will be exploring very different locations. During the next two to three years, an intensive search for potential touchdown sites will be conducted using the flood of data from Mars Global Surveyor and additional information from the Mars 2001 Orbiter. 5.26 “We are thinking about localities where there is evidence of surface processes involving what we might call ‘past’ water on Mars,” said Jim Garvin, Mars programme scientist at NASA headquarters. “To have two rovers driving over dramatically different regions of Mars at the same time, to be able to drive over and see what’s on the other side of the hill – it’s an incredibly exciting idea,” said Weiler. “I think everyone on Earth who has ever dreamed of being an explorer on an alien planet will want to go along for the ride as we explore the surface of Mars.” In a press statement announcing the decision, the agency declared: “Given the high priority NASA and the administration assign to the Space Science programme overall, and to the timely exploration of Mars, the agency proposes that Space Science cover any additional costs of the first rover mission, and that the bulk of the cost for the second lander is reallocated from programmes outside Space Science.” See http://www.athena.cornell.edu, http://www.jpl .nasa.gov/facts/mars03rover.pdf, http://www .jpl.nasa.gov/facts/mars2001.pdf. As NASA prepares to go all-out to meet the tight schedules for its ambitious Mars programme, Weiler and his colleagues are drawing back from some of the agency’s other pioneering proposals. In May, it was widely reported that NASA was considering delaying a number of high-profile missions. Since then, the postponement of the launch of the proposed Europa Orbiter from November 2003 until January 2006 has been confirmed. Reasons quoted included uncertainties about the type and cost of the launch vehicle, and delays at the Dept of Energy in development of an advanced plutonium power source. The aftermath of last year’s two Mars mission failures became more apparent on 4 August, when NASA’s Solar System Programme Director, Carl Pilcher, admitted that the Pluto-Kuiper Express is in serious jeopardy. “The reasons are multiple, including general budget pressure and a sense that much of the Space Science budget cannot be touched‚ to deal with these pressures,” wrote Pilcher. He went on to state that under the original budget, which is the funding still included in NASA’s current five-year plan, the agency does not have the financial resources to support launches of the Pluto-Kuiper Express, Europa Orbiter and Solar Probe, even if they are delayed until 2004, 2007 and 2008 respectively. By implication, at least one of these may well be axed from the Solar System programme. Only the day before, as if to emphasize the parlous state of NASA’s Space Science funding, the agency announced that funding for the Extreme Ultraviolet Explorer spacecraft will cease at the end of fiscal year 2000. The reason quoted was the mission’s low ranking on a “science per dollar basis” from a senior review panel. EUVE completed its primary mission in January 1996 and has since been operated as an extended mission. Successes for small missions If some future space science missions are threatened with the axe, current examples of the “faster, cheaper, better” policy are showing how success can be achieved despite major setbacks. NASA’s experimental Deep Space 1 probe – left for dead after a star tracker navigation system failure in late 1999 – was revived in June after JPL engineers uploaded new software that enabled it to use its Integrated Camera and Imaging Spectrometer for orientation. After three-weeks of navigation trials, DS1’s experimental ion engine was brought to full thrust on 28 June, just in time for a planned rendezvous with comet 19P/Borrelly in September 2001. “The ion propulsion engine on Deep Space 1 has now accumulated more operating time in space (over 200 days) than any other propulsion system in the history of the space programme,” said John Brophy, manager of the October 2000 Vol 41 Mission update The Hubble Space Telescope has found a swarm of newborn brown dwarfs throughout the Orion Nebula’s Trapezium cluster, about 1500 light years from Earth. This near-infrared image shows 50 or so of these not-quite-stars in the centre of the Trapezium cluster. (NASA; K L Luhman, HarvardSmithsonian Center for Astrophysics, G Schneider, E Young, G Rieke, A Cotera, H Chen, M Rieke, R Thompson, Steward Observatory, Univ. of Arizona.) NASA Solar Electric Propulsion Technology Applications Readiness project at JPL. The previous record for ion propulsion, 161 days, was held by NASA’s Space Electric Rocket Test 2, which was launched into Earth orbit in 1970. See http://nmp.jpl.nasa.gov/ds1/. Meanwhile, at the end of July, the NEAR Shoemaker spacecraft moved back into a 50 km orbit around Eros after spending several weeks orbiting just 35 km from the asteroid’s centre. In effect, this meant that it came anywhere between 19 and 30 km from the surface of the peanut-shaped object. From this close range, NEAR’s camera was able to resolve linear features and boulders inside craters, while the magnetometer searched for signs of a magnetic field. Changing illumination conditions mean that the spacecraft is now able to study the asteroid’s south polar region which was invisible when it first arrived in February. See http://near.jhuapl.edu/. However, while Eros is being examined in exquisite detail, delays in modifying the M-V launch vehicle have forced the Japanese Institute of Space and Astronautical Sciences (ISAS) to change the target of its Muses-C mission. The new objective is 1998 SF36, a small nearEarth asteroid that orbits the Sun once every 1.5 years. Launch is set for November or December 2002, with arrival at the asteroid in September 2005. The ISAS-built spacecraft will stay for three months while a NASA-built mini-rover wanders over the pristine surface. A sample will be returned to Earth in June 2006. See http://www.jpl.nasa.gov/facts/muses.pdf. A smashing time for comet LINEAR Another of the solar system’s smaller inhabitants, comet LINEAR (C/1999 S4), caused quite a stir this summer. The first sign of things October 2000 Vol 41 to come came between 5 and 7 July when the Hubble Space Telescope and the Chandra X-ray observatory observed a brief, violent outburst as a piece of its crust broke away, like a cork popping off a champagne bottle. The eruption spewed a great deal of dust into space which dramatically increased the comet’s brightness by about 50% in less than four hours. Two days later, the comet’s brightness was back to normal. Hubble recorded the entire event and captured an image of the chunk of material jettisoned from the nucleus as it floated away along the comet’s tail. A week later, on 14 July, Chandra imaged the comet and detected X-rays from oxygen and nitrogen ions. Data recorded by the Advanced CCD Imaging Spectrometer showed that the X-rays were produced by collisions between the solar wind and gas from the comet. On 27 July, ground-based observers lost sight of the comet’s bright core and suggested that the nucleus had disintegrated into a pile of dust. Astronomers at the Space Telescope Science Institute quickly reprogrammed the HST to search for the missing nucleus. To their astonishment, they discovered a small swarm of “mini-comets” – at least half a dozen, each with small tails – in place of the solid nucleus. It was the first time that astronomers had been able to obtain a detailed view of the disintegration of a comet as it was warmed by the Sun. The results support the theory that comet nuclei consist of a cluster of loose clusters of ice known as “cometesimals” and dust. These fundamental building blocks seem to have grown from micron-sized grains in the early history of the solar system. See http://oposite .stsci.edu/ and http://chandra.harvard.edu. More brown dwarfs The HST has also been unveiling large numbers of elusive brown dwarfs. These “failed stars” have turned up in large numbers in the nearby star cluster IC 348 and in the Trapezium region of the Orion Nebula. Using the HST’s NICMOS camera, Joan Najita of the National Optical Astronomy Observatory in Tucson and colleagues detected about 30 brown dwarfs in IC 348. The cool brown dwarfs were distinguished from neighbouring stars by using the strength of an infrared waterabsorption band in the stellar atmospheres to determine their temperatures. “The ability to measure the temperature of each star solved several problems simultaneously,” Najita said. “In addition to helping us distinguish the cluster of brown dwarfs from background stars, we were also able to measure the masses of the brown dwarfs without having to assume their age. This greatly improved our mass estimates.” Najita’s team found that brown dwarfs are more often solitary individuals than members of binary or multiple star systems. The images also suggest that low-mass brown dwarfs are more common than high-mass ones, a trend that continues down to nearly planetary masses. “In this respect, the isolated, or free-floating, brown dwarfs found by Hubble appear to represent the low-mass counterparts of the more massive stars,” said Najita. “This suggests that stars and free-floating brown dwarfs form in the same way.” Although Hubble found that brown dwarfs are abundant, it turns out that they are not common enough to explain the missing dark matter. Najita and her colleagues conclude that brown dwarfs probably contribute less than 0.1% of the mass of our Milky Way’s halo. Meanwhile, probing deep within the Orion nebula, another team has used NICMOS to uncover a swarm of new-born brown dwarfs in a striking infrared image of the Trapezium region. About 50 brown dwarfs have been found scattered throughout the brilliant young stars that surround the central Trapezium. Ideas about brown dwarfs also took a U-turn recently with the first observation of a flare in the atmosphere of one of these objects. Chandra’s Advanced CCD Imaging Spectrometer detected the flare from brown dwarf LP 94420 during a 12-hour observation on 15 December 1999. Although ACIS detected no X-rays at all from LP 944-20 for the first nine hours, the source flared dramatically before fading away over the next two hours. The energy emitted was comparable to a small solar flare, but was a billion times greater than observed X-ray flares from Jupiter. “This is the strongest evidence yet that brown dwarfs, and possibly young giant planets, have magnetic fields, and that a large amount of energy can be released in a flare,” said team member Eduardo Martin of Caltech. Gibor Basri of the University of California, Berkeley speculated: “The flare could have its origin in the turbulent magnetized hot material beneath the surface of the brown dwarf. A sub-surface flare could heat the atmosphere, allowing currents to flow and give rise to the X-ray flare – like a stroke of lightning. “We’ve shown that older brown dwarfs don’t have coronae, but the flare tells us they still have magnetic fields and also that subsurface flares occasionally punch through into the atmosphere,” Basri said. Located in the southern constellation of Fornax, only 16 light years from Earth, LP 944-20 is one of the best studied brown dwarfs. It is believed to be about 500 million years old and has a mass that is about 60 times that of Jupiter, or 6% of the Sun’s mass. Its diameter is about one-tenth that of the Sun and it has a rotation period of less than five hours. ● Peter Bond is the RAS Press Officer (Space Science). 5.27