Download Ups and downs

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
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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.
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“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
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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).
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