Astronomers around the world are eagerly

awaiting the resumption of observations

with the Hubble Space Telescope after a

remarkably trouble-free servicing mission in

early March. This fourth visit to the HST by

astronauts since its launch in April 1990 saw the

replacement of a failing power control unit,

followed by the installation of two modern solar

arrays, the new Advanced Camera for Surveys

(ACS) and an experimental cooling system for

the Near-Infrared Camera and Multi-Object

Spectrometer (NICMOS).

After the longest rendezvous engine-firing in

Shuttle history, the six-day service began on 3

March with the capture of the giant telescope

by Columbia’s robotic arm. Safely docked in

the Shuttle’s payload bay, the ageing pair of

solar arrays was replaced by a smaller, more

powerful third-generation version during two

lengthy spacewalks. The new panels use galli-

um arsenide solar cells to deliver at least 20%

more power than the old arrays, even though

they are only about two-thirds as large. NASA

also claimed that the smaller arrays will reduce

atmospheric drag on the telescope, while

allowing scientists to use all of the science

instruments on board simultaneously.

The most nerve-racking episode in the entire

mission came on 6 March when the HST was

switched off for 4 hours 25 minutes in order to

enable the astronauts to replace its power

system. This unprecedented shutdown – the

first since the telescope was launched – paid off

when the original unit launched with the tele-

scope 12 years ago was successfully replaced

and the HST’s functions fully restored.

The team of spacewalkers then proceeded to

improve HST’s vision. EVA 4 saw astronauts

Newman and Massimino install the ACS in

place of the old Faint Object Camera, the last

piece of hardware on the telescope provided by

the European Space Agency. NASA claimed

that the ACS will enable astronomers to see

further into the universe and with greater

clarity and speed than ever before.

The final spacewalk involved an effort to

revive the telescope’s infrared camera, which

had been dormant since 1999, by installing a

mechanical cryocooler in place of its original

cryogenic cooling system. Engineers were

Mission update

3.22 June 2002 Vol 43

Refinementsand revelations

Peter Bond summmarizes recent

discoveries about Martian

radiation, possible new forms of

matter and a sooty black comet –

not to mention the nearly new

Hubble Space Telescope.

1: The Tadpole Galaxy, in facttwo galaxies in collision,shown by HST’s AdvancedCamera for Surveys (NASAand the ACS Science Team).

2: ACS image of the ConeNebula (NGC 2264), showinggas and dust in a turbulentstar-forming region (NASAand the ACS team).

hoping that the new neon-gas cooling system

would lower the temperature of NICMOS to

enable it to resume observations.

After Hubble was released on 9 March, there

were several weeks of monitoring the newly

installed components, drying out the telescope

and calibrating the instruments. By early April,

engineers and scientists were declaring the

HST largely “healthy and fit”. Early imaging

with the ACS was described as “excellent”, but

the experimental mechanical cryocooler for

NICMOS was giving cause for concern after

shutting down for no apparent reason. Since its

restart it had been cooling the camera’s detectors

more slowly than planned, so delays in observa-

tions with the infrared camera were expected.

Faint fuzzies

While they await the HST’s reawakening,

astronomers have plenty of previous data from

the premier space observatory to analyse. One

recent result reported in a paper submitted to

the Astronomical Journal is the discovery of an

entirely new class of objects by Jean Brodie and

Soeren Larsen (University of California, Santa

Cruz). This latest work, based on observations

at the W M Keck Observatory in Hawaii,

confirmed and extended their initial report of

the discovery, which was published in the

Astronomical Journal in December 2000.

During a search for globular star clusters,

Brodie and Larsen found evidence for “faint

extended clusters” in the archive of Hubble

Space Telescope images. These objects, much

larger and fainter than typical globular clusters,

were first detected in the lenticular galaxy

NGC 1023. “It is no surprise that these objects

had never been seen before, because they are

very faint,” said Brodie. “In all the data archives

from the Hubble Space Telescope, there are only

four galaxies for which we have good enough

observations to be able to detect them.”

Of those four galaxies, the researchers found

evidence for faint extended clusters in two

(NGC 1023 and NGC 3384), but ruled them

out in the others. Based on the HST images,

Brodie and Larsen determined the size and

brightness of these objects and their distribution

in the galaxies. They also noted that the light

from the objects is predominantly red in colour,

indicating that they contain relatively old stars.

Their analysis shows that the faint extended

clusters seem to be about the same age as

globular clusters, but they look and behave

very differently. Whereas globular clusters are

typically 15 to 20 light years in diameter, “faint

fuzzies” range from 50 to 100 light years

across. They are also much less luminous than

globular clusters and they are associated with

the disk of their host galaxy, whereas most

globular clusters are associated with the halo

or spheroidal component, moving in random

orbits around the galaxy.

“The association with the disk is significant

because it means that they probably formed in a

very different way from globular clusters,”

Larsen said. To confirm these observations, and

in particular to rule out the possibility that these

were background objects and not part of the

galaxy itself, the researchers obtained spectra

with the Keck I telescope in December 2001.

“At this point, we don’t know how common

these clusters are or if they occur exclusively in

lenticular galaxies,” Brodie said. “It may be that

in the past, other dwarf galaxies have interacted

with NGC 1023 and been drawn into the disk,

giving rise to the faint extended clusters. Over

time, the stars in the clusters would redden as

they aged. That doesn’t explain why the clusters

are so big, but it is an interesting possibility.”

Chandra’s new form of matter

The HST has also collaborated with the

Chandra X-ray Observatory to identify two

small stars that are puzzling astrophysicists.

The satellite observations suggest that the

stars, known as RXJ1856.5-3754 and 3C58,

may be made of a new form of matter.

By combining Chandra and HST data,

astronomers found that RXJ1856 radiates like

a solid body with a temperature of 700 000 °C

and has a diameter of about 11 km. This is too

small to reconcile with standard models for neu-

tron stars. “Taken at face value, the combined

observational evidence points to a star com-

posed not of neutrons, but of quarks in a form

known as strange quark matter,” said Jeremy

Drake (Harvard-Smithsonian Center for Astro-

physics), lead author of a paper in the 20 June

issue of the Astrophysical Journal. “Quarks,

thought to be the fundamental constituents of

nuclear particles, have never been seen outside a

nucleus in Earth-bound laboratories.”

Observations by Chandra of 3C58 also yield-

ed startling results. Patrick Slane and Steven

Murray, also of CfA, and David Helfand

(Columbia University, New York), failed to

detect the expected X-radiation from the hot

surface of 3C58, a neutron star believed to have

been created in an explosion witnessed by Chi-

nese and Japanese astronomers in AD1181. The

team concluded that the star has a surface tem-

perature of less than one million degrees Celsius,

which is far below the predicted value. “Our

observations of 3C58 offer the first compelling

test of models for how neutron stars cool, and

the standard theory fails,” said Helfand. “It

appears that neutron stars aren’t pure neutrons

after all – new forms of matter are required.”

Drake cautioned that alternative explana-

tions may yet be found. For example, the

observations of RXJ1856 could be interpreted

as a more normal neutron star with a hot spot.

Such a model is under consideration by Fred

Walter (State University of New York), one of

the astronomers who discovered the unusual

Mission update

3.23June 2002 Vol 43

Space shorts

� HESSI NOW RHESSI. A NASA space-

craft recently launched to capture the first

high-resolution movies of solar flares in

X-rays and gamma rays has been renamed

the Reuven Ramaty High-Energy Solar

Spectroscopic Imager spacecraft (RHESSI).

Previously known as HESSI, the spacecraft

has been renamed in honour of Reuven

Ramaty, who died in 2001 after a long and

distinguished career in the Laboratory for

High Energy Astrophysics at the NASA

Goddard Space Flight Center. A new movie

released by NASA features one of the first

flares recorded by RHESSI, which occurred

on 20 February in the Sun’s southern hemi-

sphere. The moderately powerful flare,

classified as M2.4 by the National Oceanic

and Atmospheric Administration, arose

from an active region designated AR 9830

(www.gsfc.nasa.gov/topstory/20020320

hessixray.html).

� LIFT OFF FOR GRACE. The US–German

twin spacecraft known as the Gravity

Recovery and Climate Experiment

(GRACE) were launched into low Earth

orbit from Russia’s Plesetsk Cosmodrome

on 17 March. Over the next five years,

GRACE will generate monthly gravity

maps of the Earth that are up to 1000

times more accurate than those currently

in use. Data from GRACE will be used to

investigate a wide variety of scientific phe-

nomena, ranging from shallow and deep

ocean currents, water movement on and

beneath Earth’s surface, and the movement

and changing mass of ice sheets, to sea-

level heights and changes in the structure

of the solid Earth. The gravity field is cal-

culated from measurements of the distance

between the satellites as they follow their

polar orbits some 220 km apart. Regions

of slightly stronger gravity will affect the

lead satellite first, increasing by a tiny

amount its distance from the trailing satel-

lite. An extremely sensitive microwave

ranging system will measure the constantly

changing distance between the satellites. By

combining that data with precise position-

ing measurements from GPS instruments,

scientists will be able to detect minute vari-

ations in Earth’s surface mass below and

corresponding variations in the planet’s

gravitational pull. The performance of

GRACE will be validated over a period of

six months, after which the mission will

enter its observational phase. GRACE is

the first launch in NASA’s Earth System

Science Pathfinder programme (www.csr

.utexas.edu/grace and www.dlr.de/grace).

star with ROSAT in 1996. However, the hot-

spot model requires a very special orientation

of the star with respect to the Earth to explain

the absence of pulsations. The probability of

such an orientation is quite small (chandra

.harvard.edu and chandra.nasa.gov).

Mars Odyssey ‘finds water’

Initial science data from NASA’s Mars Odyssey

spacecraft, which began its mapping mission in

late February, are delighting scientists. Among

the reported findings is the possible identifica-

tion of significant amounts of frozen water.

New images taken by the thermal-emission

imaging system show the temperature of the

surface in remarkable clarity and detail during

both the Martian day and night. The thermal

infrared images, which will be used to study

Martian surface mineralogy, are 30 times sharp-

er than previously available, while the camera’s

visible-light images will fill a gap in resolution

between Viking Orbiter and Mars Global

Surveyor pictures. Targets for the camera in

mid-March included the candidate landing sites

for NASA’s twin 2003 Mars exploration rovers.

However, the most significant early result has

come from the gamma-ray spectrometer instru-

ment suite, which shows the presence of signif-

icant amounts of hydrogen in Mars’s south

polar region. The high hydrogen content can be

explained by the presence of water ice, though

the amount of ice cannot yet be quantified.

The detection of hydrogen is based both on

the intensity of gamma rays emitted by hydro-

gen, and by the intensity of neutrons that are

moderated by hydrogen. The neutron intensity

was observed by the high-energy neutron detec-

tor and the neutron spectrometer. Scientists

believe the instrument should be able to detect

hydrogen from ice up to a metre below the Mar-

tian surface, rather than simply on the ground

or in the atmosphere, but further analysis will

be conducted to confirm the interpretation.

“The preliminary assessment of the gamma-

ray spectrometer data indicates the likely

presence of hydrogen in the upper few feet of

the Martian surface, as sampled at spatial

scales approximately 400 miles across,” said

Jim Garvin, lead scientist of NASA’s Mars

Exploration Programme. “Further analysis and

another month or so of mapping will allow for

a refined interpretation,” he added.

“There is the equivalent of at least several

percent water south of 60° latitude,” said

William Boynton, principal investigator of the

GRS instrument. The neutron detectors in the

gamma-ray spectrometer suite continue to

refine the maps of near-surface hydrogen and

study surface changes as the Martian northern

winter comes to an end.

Meanwhile, measurements made by the

Martian radiation-environment experiment

during Odyssey’s cruise to the Red Planet have

cast a cloud over supporters of human

missions to Mars. The data suggest that the

daily dose of radiation experienced by astro-

nauts on their way from Earth to Mars would

be more than twice the dose endured by astro-

nauts on the International Space Station.

Odyssey’s radiation experiment stopped com-

municating and was switched off in August

2001. It resumed data gathering on 13 March

after troubleshooting efforts successfully re-

established communications with the instru-

ment. Engineers believe the problem may be

related to a memory error in the experiment’s

onboard software (mars.jpl.nasa.gov/odyssey

and grs.lpl.arizona.edu/results/presscon1/).

Darkest object in the solar system

Mars may be wetter than expected, but it seems

that comets – or at least the surfaces of comet

nuclei – are much darker than predicted. Analy-

sis of data sent back during the flyby of Comet

Borrelly by NASA’s Deep Space 1 spacecraft on

22 September 2001 has provided new insights

into its spiky jets, dusty environment, inter-

action with the solar wind, and composition.

Most significant of all, perhaps, is the new pic-

ture of the nucleus that is emerging. Overall, the

nucleus reflects no more than 3% of incoming

sunlight, making it the darkest known object in

the solar system. “The average geometric albe-

do is black, black, black, and ranges from

extremely black to very black,” said University

of Tennessee geologist Dan Britt, who com-

pared the colour to that of photocopier toner.

Four main types of landscape have been

identified on Borrelly’s bowling-pin-shaped

nucleus. Mottled terrain covers the comet’s

opposite ends, both of which are generally

darker than Borrelly’s mid-section. These rough

areas are filled with bowl-shaped depressions,

hills, streaks aligned with the nucleus’s long

axis, and notable variations in brightness. The

mottled terrain also includes spots darker than

anything else on Borrelly, patches that reflect

just 0.7 to 1% of incoming light.

The central zone of the nucleus includes

Borrelly’s brightest and smoothest terrain and

contains dark, flat-topped “mesas” surrounded

by bright slopes. This surface is probably coated

with fresh material and is the likeliest source of

the comet’s jets. Another curious characteristic is

the small number of ridges and fractures across

the comet’s “waist” between its two lobes.

According to Jürgen Oberst (DLR Institute of

Space Sensor Technology and Planetary Explo-

ration) a three-dimensional stereo elevation

map shows that the smaller end of the nucleus

is tilted by 30º to 50º relative to the larger half.

These observations may indicate that Borrelly’s

nucleus is actually a contact binary composed

of two pieces loosely bound to each other. �

Peter Bond, RAS Press Officer (Space Science).

Mission update

3.24 June 2002 Vol 43

Space shorts

� HETE RE-ENTRY. A pair of satellites –

which died in 1996 after a Pegasus

rocket’s third stage failed to release them –

re-entered the atmosphere over China on 6

April, along with the faulty stage. NASA’s

first High Energy Transient Experiment

(HETE-1) and Argentina’s SAC-B space-

craft weighed more than half a tonne in

total, but only about 8 kg of material was

expected to survive the re-entry – chiefly

stainless steel batteries.

� CASSINI ON COURSE. Continuing its

seven-year trek to Saturn, Cassini success-

fully fired its main engine for 9.8 seconds

on 3 April to accomplish a course adjust-

ment and carry out routine maintenance

of the propulsion system. The 13th engine

burn of the mission was designed using

new procedures that will be employed for

orbital adjustments while orbiting Saturn.

Scientific instruments continued to gather

data during the procedure, which had not

been attempted during previous flight-path

correction manoeuvres. By 28 April,

Cassini had travelled 3 billion km since its

launch in October 1997. It will reach

Saturn on 1 July 2004.

� MAINTENANCE FOR VOYAGER 1.

Engineers at JPL have carried out long-

distance maintenance on one of humanity’s

far-flung ambassadors, more than 12.5bil-

lion km from home. Voyager 1’s original

attitude control system had shown increas-

ing signs of trouble over the past two years,

so the team decided to activate a backup

position-sensing system, including a Sun

sensor and star tracker that had lain redun-

dant for a quarter of a century. The system

made an unexpected lock on to the Sun

during a trial switch, so the spacecraft was

instructed to keep itself steady with gyro-

scopes during the final change-over. “After

sitting on the shelf for 25 years, it’s like

new equipment,” said Ed Massey, Voyager

project manager. “We now have the origi-

nal system as a backup if we need it,” said

flight-team member Steve Howard. The

Voyager flight team is now reduced to 14

people, compared with more than 300 in

the 1980s. Voyagers 1 and 2 completed

their tour of the outer planets in 1989 and

are now heading for the heliopause, where

the Sun’s influence ends and interstellar

space begins. Radio communications take

nearly 12 hours each way. Both spacecraft

have adequate power and communication

capabilities to continue operations for

about 20 more years if other onboard

systems hold up (voyager.jpl.nasa.gov).