If at first you don’t succeed, try, try, tryagain. Many times during the past fourdecades this particular piece of popular wis-

dom has been applied to space exploration: theEuropean Space Agency’s Cluster missionproved to be no exception. Four years after theloss of the first Cluster quartet in a launchexplosion, the rebuilt satellites have been suc-cessfully placed into their operational orbits bydual Soyuz launches from Kazakhstan.

The first pair, now called Salsa and Sambaafter a pan-European competition won by UKcivil servant Raymond Cotton, lifted off on 16July, a day late after minor technical problemson the pad caused by heavy rain and thunder-storms. The two remaining Cluster spacecraft,Rumba and Tango, were launched on 9 Augustand joined their sister spacecraft a week later.

Each spacecraft carries 11 identical instru-ments. These will study electric and magneticfields and waves in the plasma surrounding thespacecraft, investigate the relative abundanceof electrons, protons and helium nuclei, anddetermine their three-dimensional distribution.Instrument check-out and commissioning arenow well under way, and the first scientificdata are expected to be returned in lateNovember or early December.

As the first four-spacecraft flotilla to fly inclose formation through the magnetosphere,Cluster will make a unique contribution to sci-entific understanding of the processes takingplace in the Earth’s magnetosphere and itsinteraction with the solar wind. Their19 000 ×119 000 km elliptical orbits will carrythem almost one third of the way to the Moon.As a consequence of these unusual orbits, theywill either travel far down the magnetotail orshuttle between the magnetosphere and inter-stellar space. And by flying in tetrahedral for-mation as they cross major boundaries andregions of interest such as the polar cusps andthe magnetopause, the quartet will also pro-vide the first detailed three-dimensional viewof the charged particles and electromagneticwaves that pervade near-Earth space.

Once it becomes operational, Cluster willjoin the Solar and Heliospheric Observatory(SOHO) to complete the first Cornerstone ofESA’s Horizons 2000 long-term scientific pro-gramme. Both missions also represent animportant example of solar–terrestrial scienceco-operation between NASA and ESA. Seehttp://sci.esa.int/cluster/ or http://international

.gsfc.nasa.gov/International/Missions/Cluster_II/Cluster_II.html.

Further good news for space physicists wasthe agreement by ESA to fund the Ulysses mis-sion for an extra 33 months. At its meeting inParis on 5–6 June, ESA’s Science ProgrammeCommittee agreed to continue operating thespacecraft from the end of 2001 to 30 Septem-ber 2004. However, since Ulysses is a joint mis-sion, NASA’s approval is also required for themission extension. So far, NASA has approvedfunding until December 2002 and a decisionon further funding is expected in mid-2001. Ifthat decision is positive, Ulysses will remain inoperation as sunspot activity gradually declinesafter this year’s solar maximum. Such an exten-sion would allow Ulysses to make the first-everset of high-latitude observations over a fullsolar cycle. See http://sci.esa.int/ulysses.

As if to emphasize the importance of suchstudies, an X5-class solar flare, one of the mostpowerful flares of the current solar cycle, trig-gered a proton storm in the neighbourhood ofour planet on 14 July. The bright flash of thesolar flare near the centre of the Sun’s disk wasseen by SOHO’s ultraviolet telescope EIT at10:12 UT. The flare’s intensity peaked at 10:24,and half an hour later SOHO’s LASCO instru-ment detected a full halo coronal mass ejection(CME) heading towards the Earth. Next, aburst of energetic particles from the solarexplosion hit SOHO and resulted in a snow-storm in the imaging instruments that contin-ued for some hours. Travelling more slowlythan the energetic particles, the interplanetaryshock wave driven by the gas of the CMEreached SOHO at 14:19 UT on 15 July.

The spacecraft’s CELIAS solar wind instru-

ment registered a jump in the wind speed from500 to 800 km s–1, increasing to over900 km s–1 an hour later. The CME slammedinto the Earth’s magnetic field half an hourafter it struck SOHO, provoking auroral dis-plays that peaked in the early hours of 16 July.Satellite operators and electric-power engineersreported many side-effects.

The main casualty was the Japanese ASCAX-ray observatory which went into safe modeand began to spin out of control. With its solarpanels receiving less sunlight and increasedatmospheric drag rapidly lowering its orbit,ASCA is probably lost for good.

Numerous other scientific spacecraft also suf-fered transient malfunctions as a result of thissolar storm. For example, ACE temporarilylost output from its solar-wind velocity mea-suring sensor, SOHO suffered a small but per-manent degradation in solar panel output, andthe solid-state power amplifier on Wind expe-rienced a power loss of 25%. The Danish Oer-stedt satellite went dead but was recovered afew days later. The X-ray/gamma-ray spec-trometer on the NEAR Shoemaker spacecraft,which is in orbit around Eros, shut down fortwo days. Several spacecraft experienced prob-lems with their star trackers, including the Ger-man CHAMP, which was launched on 15 Julywhile the event was still under way.

Germany produces a CHAMP CHAMP, the CHAllenging Minisatellite Pay-load, is intended to study the Earth’s gravityand magnetic fields to very high accuracy. Itslow circular orbit, starting at 454 km altitudeand decaying during the mission to below300 km, together with the greatly advancedinstrumentation flown on CHAMP, promise anorder of magnitude improvement in magneticfield measurements compared to its MAGSATpredecessor. CHAMP will also use anadvanced Digital Ion Drift Meter to produceglobal maps of the ionospheric electric field. Itslong mission lifetime and precessing orbitshould allow scientists to study the dependenceof the electric field patterns on magnetic activ-ity, on local time, on season and on the solarcycle phase. The low-altitude near-polar orbit,the continuous GPS data that will determineprecisely the satellite’s orbit, and onboard mea-surement of non-gravitational orbit perturba-tions should also provide an improvement ofup to two orders of magnitude in accuracy inthe determination of the broad to mesoscalestructures of the Earth’s gravity field. Thismajor breakthrough is expected to lead to newinsights and applications in geodesy, solid-Earth physics and oceanography.

CHAMP is managed by the Geo-ForschungsZentrum (Earth Research Centre)of Potsdam. International partners on the mis-sion are NASA, the French Centre National

Mission update

5.25October 2000 Vol 41

Ups and downsCluster goes up but will planetary programmes go down? Peter Bondreports on space mission plans and achievements.

Cluster satellites Tango and Rumba pictured asthey separated after their successful launch. Thisis the first colour picture taken in space without ahuman hand present on the shutter, thanks to thetiny Visual Monitoring Camera that is part ofCluster’s equipment. (ESA.)

des Études Spatiales (CNES), and the US AirForce Research Laboratories. See http://op.gfz-potsdam.de/champ/.

NASA gives precedence to MarsOn 10 August, NASA Associate Administratorfor Space Science Ed Weiler announced plansto launch not one, but two large scientificrovers to Mars in 2003, in preference to a sci-entific orbiter. An airbag cocoon, similar to theone used on the 1997 Mars Pathfinder mission,will be employed during the landing phase.

The Delta II launches from Cape Canaveralare currently planned for 22 May and 4 June2003. They will take advantage of a veryfavourable planetary alignment between Earthand Mars. After a seven-and-a-half monthcruise, the first rover should enter Mars’ atmos-phere on 2 January 2004, with the second roverbouncing to a stop on the Martian surface 18days later. The identical 150 kg vehicles willhave far greater mobility than the Sojournerrover on Mars Pathfinder and will be able totravel up to 100 metres a day. Surface opera-tions will last for at least 90 sols (Martian days).

Cornell University has been selected to pro-vide the scientific payload and lead the scienceteam. Six scientific instruments have beenselected. They include a panoramic camera, aninfrared device called Mini-TES, a microscopiccamera, a Mossbauer spectrometer to identifyiron-bearing minerals and an alpha-proton-X-ray spectrometer to measure the concentra-tions of most major elements. The packagealso includes a rock abrasion tool to exposefresh rock surfaces for study.

The landing sites have yet to be selected, but,if all goes according to plan, the rovers will beexploring very different locations. During thenext two to three years, an intensive search forpotential touchdown sites will be conductedusing the flood of data from Mars Global Sur-veyor and additional information from theMars 2001 Orbiter.

“We are thinking about localities where thereis evidence of surface processes involving whatwe might call ‘past’ water on Mars,” said JimGarvin, Mars programme scientist at NASAheadquarters. “To have two rovers drivingover dramatically different regions of Mars atthe same time, to be able to drive over and seewhat’s on the other side of the hill – it’s anincredibly exciting idea,” said Weiler. “I thinkeveryone on Earth who has ever dreamed ofbeing an explorer on an alien planet will wantto go along for the ride as we explore the sur-face of Mars.”

In a press statement announcing the decision,the agency declared: “Given the high priorityNASA and the administration assign to theSpace Science programme overall, and to thetimely exploration of Mars, the agency propos-es that Space Science cover any additional costsof the first rover mission, and that the bulk ofthe cost for the second lander is reallocatedfrom programmes outside Space Science.” Seehttp://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 thetight schedules for its ambitious Mars pro-gramme, Weiler and his colleagues are drawingback from some of the agency’s other pioneer-ing proposals. In May, it was widely reportedthat NASA was considering delaying a numberof high-profile missions. Since then, the post-ponement of the launch of the proposedEuropa Orbiter from November 2003 untilJanuary 2006 has been confirmed. Reasonsquoted included uncertainties about the typeand cost of the launch vehicle, and delays atthe Dept of Energy in development of anadvanced plutonium power source.

The aftermath of last year’s two Mars mis-sion failures became more apparent on 4August, when NASA’s Solar System Pro-gramme Director, Carl Pilcher, admitted thatthe Pluto-Kuiper Express is in serious jeopardy.

“The reasons are multiple, including generalbudget pressure and a sense that much of theSpace Science budget cannot be touched‚ todeal with these pressures,” wrote Pilcher. Hewent on to state that under the original budget,which is the funding still included in NASA’scurrent five-year plan, the agency does nothave the financial resources to support launch-es of the Pluto-Kuiper Express, Europa Orbiterand Solar Probe, even if they are delayed until2004, 2007 and 2008 respectively. By implica-tion, at least one of these may well be axedfrom the Solar System programme.

Only the day before, as if to emphasize theparlous state of NASA’s Space Science funding,the agency announced that funding for theExtreme Ultraviolet Explorer spacecraft willcease at the end of fiscal year 2000. The reasonquoted was the mission’s low ranking on a“science per dollar basis” from a senior reviewpanel. EUVE completed its primary mission inJanuary 1996 and has since been operated asan extended mission.

Successes for small missionsIf some future space science missions are threat-ened with the axe, current examples of the“faster, cheaper, better” policy are showing howsuccess can be achieved despite major setbacks.

NASA’s experimental Deep Space 1 probe –left for dead after a star tracker navigation sys-tem failure in late 1999 – was revived in Juneafter JPL engineers uploaded new software thatenabled it to use its Integrated Camera andImaging Spectrometer for orientation. Afterthree-weeks of navigation trials, DS1’s experi-mental ion engine was brought to full thrust on28 June, just in time for a planned rendezvouswith comet 19P/Borrelly in September 2001.

“The ion propulsion engine on Deep Space 1has now accumulated more operating time inspace (over 200 days) than any other propul-sion system in the history of the space pro-gramme,” said John Brophy, manager of the

Mission update

5.26 October 2000 Vol 41

Cometesimals. The Hubble telescope discovered a small armada of “mini-comets” left behind from what some scientists had prematurely thought was a totaldisintegration 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 thepopular theory that comet nuclei are really made up of a cluster of smaller icy bodies called cometesimals. (NASA, Harold Weaver, the Johns Hopkins University, theHST Comet LINEAR Investigation Team, and the University of Hawa.)

NASA Solar Electric Propulsion TechnologyApplications Readiness project at JPL. Theprevious record for ion propulsion, 161 days,was held by NASA’s Space Electric Rocket Test2, which was launched into Earth orbit in1970. See http://nmp.jpl.nasa.gov/ds1/.

Meanwhile, at the end of July, the NEARShoemaker spacecraft moved back into a50 km orbit around Eros after spending sever-al weeks orbiting just 35 km from the aster-oid’s centre. In effect, this meant that it cameanywhere between 19 and 30 km from the sur-face of the peanut-shaped object.

From this close range, NEAR’s camera wasable to resolve linear features and bouldersinside craters, while the magnetometer searchedfor signs of a magnetic field. Changing illumi-nation conditions mean that the spacecraft isnow able to study the asteroid’s south polarregion which was invisible when it first arrivedin February. See http://near.jhuapl.edu/.

However, while Eros is being examined inexquisite detail, delays in modifying the M-Vlaunch vehicle have forced the Japanese Insti-tute of Space and Astronautical Sciences (ISAS)to change the target of its Muses-C mission.The new objective is 1998 SF36, a small near-Earth asteroid that orbits the Sun once every1.5 years. Launch is set for November orDecember 2002, with arrival at the asteroid inSeptember 2005. The ISAS-built spacecraft willstay for three months while a NASA-builtmini-rover wanders over the pristine surface. Asample will be returned to Earth in June 2006.See http://www.jpl.nasa.gov/facts/muses.pdf.

A smashing time for comet LINEARAnother of the solar system’s smaller inhabi-tants, comet LINEAR (C/1999 S4), causedquite a stir this summer. The first sign of things

to come came between 5 and 7 July when theHubble Space Telescope and the ChandraX-ray observatory observed a brief, violentoutburst as a piece of its crust broke away, likea cork popping off a champagne bottle. Theeruption spewed a great deal of dust into spacewhich dramatically increased the comet’sbrightness by about 50% in less than fourhours. Two days later, the comet’s brightnesswas back to normal. Hubble recorded theentire event and captured an image of thechunk of material jettisoned from the nucleusas it floated away along the comet’s tail.

A week later, on 14 July, Chandra imaged thecomet and detected X-rays from oxygen andnitrogen ions. Data recorded by the AdvancedCCD Imaging Spectrometer showed that theX-rays were produced by collisions betweenthe solar wind and gas from the comet.

On 27 July, ground-based observers lost sightof the comet’s bright core and suggested thatthe nucleus had disintegrated into a pile of dust.Astronomers at the Space Telescope ScienceInstitute quickly reprogrammed the HST tosearch for the missing nucleus. To their aston-ishment, they discovered a small swarm of“mini-comets” – at least half a dozen, eachwith small tails – in place of the solid nucleus.It was the first time that astronomers had beenable to obtain a detailed view of the disintegra-tion of a comet as it was warmed by the Sun.

The results support the theory that cometnuclei consist of a cluster of loose clusters ofice known as “cometesimals” and dust. Thesefundamental building blocks seem to havegrown from micron-sized grains in the earlyhistory of the solar system. See http://oposite.stsci.edu/ and http://chandra.harvard.edu.

More brown dwarfs The HST has also been unveiling large num-bers of elusive brown dwarfs. These “failedstars” have turned up in large numbers in thenearby star cluster IC 348 and in the Trapezi-um region of the Orion Nebula. Using theHST’s NICMOS camera, Joan Najita of theNational Optical Astronomy Observatory inTucson and colleagues detected about 30brown dwarfs in IC 348. The cool browndwarfs were distinguished from neighbouringstars by using the strength of an infrared water-absorption band in the stellar atmospheres todetermine their temperatures.

“The ability to measure the temperature ofeach star solved several problems simultane-ously,” Najita said. “In addition to helping usdistinguish the cluster of brown dwarfs frombackground stars, we were also able to mea-sure the masses of the brown dwarfs withouthaving to assume their age. This greatlyimproved our mass estimates.”

Najita’s team found that brown dwarfs aremore often solitary individuals than members

of binary or multiple star systems. The imagesalso suggest that low-mass brown dwarfs aremore common than high-mass ones, a trendthat continues down to nearly planetary mass-es. “In this respect, the isolated, or free-float-ing, brown dwarfs found by Hubble appear torepresent the low-mass counterparts of themore massive stars,” said Najita. “This sug-gests that stars and free-floating brown dwarfsform in the same way.”

Although Hubble found that brown dwarfsare abundant, it turns out that they are notcommon enough to explain the missing darkmatter. Najita and her colleagues conclude thatbrown dwarfs probably contribute less than0.1% of the mass of our Milky Way’s halo.

Meanwhile, probing deep within the Orionnebula, another team has used NICMOS touncover a swarm of new-born brown dwarfs ina striking infrared image of the Trapeziumregion. About 50 brown dwarfs have beenfound scattered throughout the brilliant youngstars that surround the central Trapezium.

Ideas about brown dwarfs also took a U-turnrecently with the first observation of a flare inthe atmosphere of one of these objects. Chan-dra’s Advanced CCD Imaging Spectrometerdetected the flare from brown dwarf LP 944-20 during a 12-hour observation on 15 Decem-ber 1999. Although ACIS detected no X-raysat all from LP 944-20 for the first nine hours,the source flared dramatically before fadingaway over the next two hours. The energyemitted was comparable to a small solar flare,but was a billion times greater than observedX-ray flares from Jupiter.

“This is the strongest evidence yet thatbrown dwarfs, and possibly young giant plan-ets, have magnetic fields, and that a largeamount 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 itsorigin in the turbulent magnetized hot materi-al beneath the surface of the brown dwarf. Asub-surface flare could heat the atmosphere,allowing currents to flow and give rise to theX-ray flare – like a stroke of lightning.

“We’ve shown that older brown dwarfs don’thave coronae, but the flare tells us they stillhave magnetic fields and also that subsurfaceflares occasionally punch through into theatmosphere,” Basri said. Located in the south-ern constellation of Fornax, only 16 light yearsfrom Earth, LP 944-20 is one of the best stud-ied brown dwarfs. It is believed to be about500 million years old and has a mass that isabout 60 times that of Jupiter, or 6% of theSun’s mass. Its diameter is about one-tenth thatof the Sun and it has a rotation period of lessthan five hours. �

Peter Bond is the RAS Press Officer (Space Science).

Mission update

5.27October 2000 Vol 41

The Hubble Space Telescope has found a swarm ofnewborn brown dwarfs throughout the OrionNebula’s Trapezium cluster, about 1500 light yearsfrom Earth. This near-infrared image shows 50 orso of these not-quite-stars in the centre of theTrapezium cluster. (NASA; K L Luhman, Harvard-Smithsonian Center for Astrophysics, G Schneider,E Young, G Rieke, A Cotera, H Chen, M Rieke, RThompson, Steward Observatory, Univ. of Arizona.)