The opportunity to study the Jovian sys-

tem in unprecedented detail came cour-

tesy of a gravitational assist for the

Cassini spacecraft as it swept past Jupiter on

30 December 2000. The flyby at a distance of

about 9.7 million km accelerated the spacecraft

by about 2 km per second and put it on course

to reach its ultimate destination, Saturn, in July

2004. Cassini’s observations began in early

October 2000 and, apart from a short hiatus in

mid-December, data was expected to be

returned until well into March 2001. The inter-

ruption came when increased friction in one of

the spacecraft’s reaction wheels led to a tempo-

rary switch-over to hydrazine-fuelled thrusters.

Mission managers suspended imaging and

other scientific observations that required

pointing the spacecraft until 28 December.

One of the most impressive observational

efforts involved simultaneous studies of

Jupiter’s aurora from Cassini, Galileo and the

Hubble Space Telescope, over two periods in

December and January this year. The goal was

to combine Cassini’s measurements of the

velocity, temperature, pressure, density and

magnetic field direction of the solar wind with

ultraviolet images obtained by the Space Tele-

scope Imaging Spectrograph, in order to deter-

mine how the solar wind influences the giant

planet’s aurorae. The Galileo spacecraft also

participated in the campaign by taking mea-

surements inside the magnetosphere.

In addition, the January opportunity allowed

the science teams to compare HST images of

the day-side aurora with images of Jupiter’s

night-side aurora from Cassini. It was the first

time that Jupiter’s south-pole aurora had ever

been imaged from the planet’s night side. These

images support theories that the aurorae origi-

nate in electrical currents that connect Io and

Jupiter along magnetic field lines.

Pictures of the magnetosphere

Jupiter’s interaction with its space environment

also came under scrutiny from the Magneto-

spheric Imaging Instrument (MIMI) aboard

Cassini. MIMI includes an Ion and Neutral

Camera, a spectrometer and a high-energy par-

ticle detector. MIMI enables scientists to see

images of the planet’s particle-filled magneto-

sphere and study its underlying dynamics.

These images will eventually provide a large-

scale look at the compression and expansion of

the magnetosphere as it is buffeted by the solar

wind. “These images, when combined with the

other MIMI measurements, demonstrate the

ability of the camera to capture not only the

shape and dynamics of the magnetosphere but

also elements of its chemical composition,”

said Stamatios Krimigis of the Applied Physics

Lab at Johns Hopkins University. “They reveal

that the particles we’re detecting – primarily

hydrogen, but also oxygen, sulphur and

sulphur dioxide – are spewed from volcanoes

on the Jovian moon Io and spun out into

Jupiter’s magnetosphere, where they are

trapped, energized and accelerated to high

velocities. Then, when collisions with other

particles provide them with an electron, they

become neutral and are able to escape the mag-

netosphere. And that’s when we can detect

them with our camera.”

More traditional imaging with Cassini’s Imag-

ing Science Subsystem captured many thou-

sands of pictures of the cloud-covered planet,

its dusky rings, and assorted satellites. One new

picture – the best yet taken of the small moon

Himalia – is the first to show one of Jupiter’s

outer moons as more than a star-like dot. Cassi-

ni was generally too far away to obtain high-

resolution views of the four Galilean moons,

but its instruments were able to obtain images

as they passed through Jupiter’s shadow. Cassi-

ni shot the first movie ever made of the volcanic

moon Io in eclipse, showing bright spots of hot

lava and changes in auroral glows.

Galileo goes for Ganymede

Not to be outdone, the venerable Galileo

turned its attention to searching for aurorae on

Ganymede, the largest moon in the solar sys-

tem, during the spacecraft’s 29th orbit of

Jupiter. The venerable orbiter’s passage to

within 2300 km of the moon’s icy surface took

place on 28 December 2000, when Ganymede

was traversing the shadow of Jupiter. Other

Cassini sequences revealed changing atmo-

spheric conditions as the planet rapidly rotat-

ed. By comparing images of the same portion

of the planet taken in daylight and darkness, it

was possible to identify small areas that were

producing lightning.

Although Cassini approached almost along

Jupiter’s orbital plane, its imaging system could

clearly distinguish the mottled cloud patterns

near Jupiter’s north pole compared with the lat-

itudinal belts and zones nearer the equator.

Portions of Jupiter’s ring system were also

imaged by Cassini (galileo.jpl.nasa.gov, ciclops

.lpl.arizona.edu/ciclops/ and www.jpl.nasa.gov/

pictures/jupiter).

Even before the Jupiter Millennium Mission

got under way, scientists had announced sever-

al startling new results. Magnetic field readings

taken by the Galileo spacecraft during a series

of close approaches to Ganymede provided

strong circumstantial evidence for the existence

of liquid water under the surface. A thick layer

of liquid, salty water beneath Ganymede’s icy

crust would be the best way to explain some of

the data, according Margaret Kivelson, princi-

pal investigator for Galileo’s magnetometer.

The evidence is more difficult to interpret than

on Europa and Callisto since Ganymede has a

strong magnetic field of its own, instead of just

a secondary field induced by Jupiter’s magnet-

ism. However, Kivelson believes a melted layer

several kilometres thick, beginning within

200 km of Ganymede’s surface, would fit the

data if it was about as salty as Earth’s oceans.

“It would need to be something more electri-

cally conductive than solid ice,” she said. In

addition, infrared spectrometer studies of

Ganymede suggest that, in the past, mineral-

rich water may have emerged from below or

melted at the surface, according to a study of

infrared reflectance measured by Galileo.

“They are similar to the hydrated salt minerals

we see on Europa, possibly the result of brine

Mission update

2.29April 2001 Vol 42

Asolar

systemfocus

Solar system exploration has been

grabbing the headlines over the

past few months, with the first

triple spacecraft observations of

Jupiter followed by the first

landing on an asteroid.

Peter Bond reports.

1: This composite image captured by Cassiniduring the Millennium Jupiter Flyby shows, in truecolours, Io and its shadow in transit against thedisk of Jupiter. The spacecraft was 19.5 million kmfrom Jupiter. (NASA/JPL/University of Arizona.)

making its way to the surface by eruptions or

through cracks,” said Thomas McCord (Uni-

versity of Hawaii, Honolulu).

High-resolution images of Ganymede also

hint at how the water or slushy ice may have

surfaced through the fractured crust. Pictures

taken as Galileo passed within 809 km of

Ganymede on 20 May 2000 reveal details of

Arbela Sulcus, a relatively smooth, bright lin-

ear feature that may have formed by complete

separation of Ganymede’s icy crust. Studies

indicate that natural radioactivity in

Ganymede’s rocky interior should provide

enough heating to maintain a stable layer of

liquid water between two layers of ice, about

150 to 200 km below the surface. This con-

trasts with Europa, where interior tidal flexing

caused by Jupiter’s gravity provides much of

the internal heat.

No lightning on Venus

Cassini’s contribution to solar system studies

has not been confined to a six-month overview

of Jupiter. According to Donald Gurnett (Uni-

versity of Iowa), the spacecraft failed to detect

high-frequency radio waves commonly associ-

ated with lightning on Venus. During Cassini’s

flybys of the planet on 26 April 1998 and 24

June 1999, the radio- and plasma-wave science

instrument searched for impulsive high-

frequency (0.125 to 16 MHz) radio signals, but

failed to detect them. “If lightning exists in the

Venusian atmosphere, it is either extremely

rare or very different from terrestrial light-

ning,” said Gurnett. “If terrestrial-like light-

ning were occurring in the atmosphere of

Venus within the region viewed by Cassini, it

would have been easily detectable.”

A similar search during Cassini’s Earth flyby

on 18 August 1999 detected continuous light-

ning at rates up to 70 impulses per second.

Despite the Cassini results, Gurnett does not

rule out the possibility that some type of low-

frequency electrical activity may exist at Venus

because radio signals cannot penetrate the

ionosphere at frequencies below about 1 MHz.

“Because clouds over Venus are at very high

altitudes of 40 km or more, it is likely that

lightning at Venus, if it exists, is primarily

cloud-to-cloud,” he said. “Terrestrial cloud-to-

ground lightning is generally more intense than

cloud-to-cloud so it is possible that the absence

of impulsive high-frequency radio signals dur-

ing the Venus flybys could be owing to the

dominance of very weak cloud-to-cloud light-

ning at Venus.” Another possibility is cloud-to-

ionosphere discharges – rather like the

“sprites” on Earth that travel up from a cloud

to the ionosphere. Sprites have a low frequen-

cy and are very difficult to detect.

In 1990, using a Galileo spacecraft instru-

ment similar to Cassini’s, Gurnett detected sev-

eral small impulses that were interpreted at the

time as being indicative of lightning. However,

Galileo’s flyby was some 60 times further from

Venus than Cassini’s.

Even nearer to Eros

NASA’s NEAR Shoemaker has become the first

spacecraft to touch down on an asteroid. It

was also the first time that the United States

has beaten the former Soviet Union to a land-

ing on another celestial body.

On the morning of 12 February, NEAR’s

thrusters were fired to brake the spacecraft,

initiating its gradual descent from a circular

orbit 35 km above the centre of mass of the

near-Earth asteroid 433 Eros. Over the next

four-and-a-half hours, as it drifted down

towards the surface, four more engine firings

reduced NEAR’s orbital velocity from 32 km/h

to just 6 km/h – a fast walking pace. During

the final 5 km of the descent, its camera cap-

tured the highest resolution images ever

obtained of an asteroid. The last of the 69 pic-

tures, taken from an altitude of 120 m, showed

features as small as a centimetre across. The

remarkably detailed views revealed a landscape

of fractured boulders, a football-field-sized

crater filled with dust and a mysterious area

where the surface appears to have collapsed.

The thrusters were still firing when the space-

craft reached the surface. At first, mission con-

trollers at Johns Hopkins University Applied

Physics Laboratory believed that the space-

craft, barely constrained by Eros’s weak gravi-

tational field, had bounced, but they later

revised their estimates to a short hop. NEAR

came to rest only about 200 m from the pro-

jected landing site, a rock-strewn plain close to

a huge saddle-like depression known as

Himeros. Eros had become only the fifth celes-

tial body to be touched by a spacecraft, fol-

lowing the Moon, Mars, Venus and Jupiter.

“With the spacecraft just about out of fuel and

our science objectives met, this is a great way

to end a successful mission,” said NEAR Mis-

sion Director Robert Farquhar, before the suc-

cessful maneouvre. “It’s all bonus science. It’s

never been tried before and it certainly is a

complicated set of manoeuvres, but at this

point the only real risk is not taking one.”

Contrary to expectations, NEAR continued

to operate after the historic landing. Initial

concerns that no telemetry would be available

proved unfounded, so plans to move the space-

craft to another resting place were shelved.

Delighted mission controllers decided to con-

tinue NEAR operations in order to gather

unique data on the surface and subsurface

composition of the asteroid from its gamma-

ray spectrometer. Transmission of the science

data after landing finished on 28 February. The

unexpected bonus of such surface data should

help mission scientists document the composi-

tion of the asteroid in greater detail.

During its five-year mission, NEAR Shoe-

maker provided the most detailed survey yet of

a small celestial body. It began a year-long orbit

of Eros on 14 February 2000 and collected 10

times more data than originally planned. The

flood of new information included a detailed

shape-model derived from more than 11 million

laser pulses; radar and laser data about Eros’s

weak gravity and solid but cracked interior;

X-ray, γ-ray and infrared measurements of its

composition and spectral properties; and about

160 000 images covering all of the 34 km long

asteroid’s boulder-covered, cratered, dusty ter-

rain. “We have answered the questions we had

when the orbit began,” said NEAR Project Sci-

entist Andrew Cheng. “We now know that

Eros is a solid body of uniform composition,

made of material probably older than the

Earth. But we also found many other things we

didn’t expect to see.”

“On the tiny fraction of the surface we’ve

seen at high resolution, we noticed strange

processes we haven’t seen on the Moon or any-

where else,” added Joseph Veverka, NEAR

imaging team leader (Cornell University). “For

example, some boulders seem to have just dis-

integrated on the surface. We’ve also seen that

some of the fine surface material moves down-

hill, filling low areas and creating flat surfaces

in craters, even with Eros’s low gravity.” For

more information see the NEAR Web site at

near.jhuapl.edu/.

Mars mission accomplished

Another NASA spacecraft passed a notable

landmark recently. The Mars Global Surveyor,

which has collected more information about

the red planet than all previous missions com-

bined, completed its primary science mission at

the end of January. “By any conceivable mea-

sure, the scientific impact of Mars Global Sur-

veyor has been extraordinary,” said Arden

Albee, Global Surveyor Project Scientist. “In

some aspects, we now have better maps of

Mars than we do of Earth.” Mars Global Sur-

veyor’s extended mission has been approved

until April 2002. The spacecraft will continue

to study the climate, surface topography and

subsurface characteristics of the planet and the

data will be used to select landing sites for

future missions. The orbiter was launched on 7

November 1996 and arrived at Mars on 12

September 1997. Although its primary map-

ping mission was delayed until March 1999, it

has since collected data for a full Martian year.

Some of the most significant findings of the

mission include:

� signs of recent liquid water at the surface;

� images of layered rocks that point to wide-

spread ponding of water or lakes on Mars in its

early history;

� the first good estimate of the amount of

water currently trapped in both Martian polar

Mission update

2.30 April 2001 Vol 42

caps combined: about one-and-a-half times the

amount of ice in Greenland;

� topographic evidence for a south pole –

north pole slope that controlled the transport

of water and sediments, and confirmation of a

flat northern plain that has been proposed as

the possible site of an ancient ocean;

� the detection of bands of highly magnetized

crust in the southern hemisphere;

� the first reliable models of the crustal struc-

ture of Mars, including the detection of ancient

impact basins and possible channels buried

beneath the northern plains;

� identification of the mineral hematite, indi-

cating a past surface-hydrothermal environ-

ment that may have provided a suitable envi-

ronment for the evolution of early life on Mars;

� significantly better understanding of atmo-

spheric dynamics, including cyclonic storms,

and the daily and seasonal behaviour of carbon

dioxide and water ice clouds;

� extensive evidence for the role of dust in re-

shaping the recent Martian environment with

dust devils, dust storms, dunes and sand sheets.

By 31 January 2001, the spacecraft had made

8505 orbits of the planet and completed more

than 58 000 images, 490 million laser-altimeter

measurements and 97 million spectral studies

(mars.jpl.nasa.gov/mgs/).

One of the most recent findings from Mars

Global Surveyor data has involved the patch-

work magnetic field discovered earlier in the

mission. It seems that ancient asteroid or comet

impacts wiped out part of the crustal magnet-

ism some four billion years ago. Although Mars

now lacks a global magnetic shield like that of

the Earth, strong localized magnetic fields

embedded in the crust appear to be a significant

barrier to erosion of the atmosphere by the

solar wind. This conclusion emerges from a new

map of the limits of the planet’s ionosphere. The

new data show that, where local surface mag-

netic fields are strong, the ionosphere reaches to

a higher altitude, indicating that the solar wind

is being kept at bay.

The findings suggest that these crustal fields

could have played an important role in the past

evolution of Mars’ atmosphere. If much of the

planet’s atmosphere has been stripped away by

the solar wind, the maps show where this effect

has had the most impact. “Finding these demag-

netized and very ancient crater sites helped us

date when the dynamo turned off, which was a

big help, because now we know when, in our

models, to turn on erosion by the solar wind,”

said David Mitchell of UC Berkeley.

Between February 1999 and April 2000,

Mars Global Surveyor mapped the position of

the ionopause. Electron reflectometer data

were used to determine when the spacecraft,

orbiting at about 400 km altitude, was inside

or outside the planet’s ionosphere. This is pos-

sible because the energy spectrum of ionos-

pheric electrons differs from that of solar wind

electrons. The final map, averaged over this

time period and thousands of orbits, represents

the probability at any given point that the

spacecraft was within or outside the iono-

sphere. Mitchell found that the ionosphere was

highest over the strong crustal magnetic fields,

where it probably extended hundreds of kilo-

metres above the spacecraft’s orbit.

Mars’ crustal magnetism remains a mystery. It

is nearly as strong at the surface as the Earth’s

magnetic field – a few tenths of a Gauss, com-

pared to a third of a Gauss on Earth – and

arrayed in east–west bands of alternating polar-

ity, extending for over 1000 km north to south

(www.berkeley.edu/news/media/download/).

Pluto proposals sought

Just before Christmas, NASA announced that

it was seeking proposals from principal inves-

tigators and institutions around the world to

develop the first mission to Pluto and the

Kuiper Belt. This Announcement of Opportu-

nity marked the first time its Office of Space

Science had solicited proposals for a mission to

an outer planet for selection on a competitive

basis. The proposals were due to reach NASA

Headquarters by 19 March 2001. “Competi-

tion has worked quite well in other NASA

space science programmes, and I expect that,

through this approach, we will see a number of

creative ideas from innovative thinkers and

organizations that have not been able to par-

ticipate in outer planet exploration before,”

said Ed Weiler, NASA Associate Administrator

for Space Science. After peer review, NASA

will select at least two proposals for more

detailed study and choose the winner in August

2001. There are no restrictions on the launch

date but the goal is to reach Pluto by 2015.

NASA will cap the cost of the Pluto mission at

$500 million (in Financial Year 2000 dollars).

The decision to solicit proposals came three

months after unacceptably large cost increases

on the Pluto/Kuiper Express (PKE) mission led

NASA to issue a stop-order on the project.

This led to numerous protests from the plane-

tary science community. The Division of Plan-

etary Sciences Committee of the American

Astronomical Society took the unusual step of

issuing a statement: “NASA has cancelled its

fourth planetary mission in the past two years.

This series of cancellations is unprecedented in

the history of NASA’s Space Science pro-

gramme… The latest is the ‘nanorover’ that

was to have been delivered to the surface of an

asteroid by Japan’s Muses-C asteroid sample

return mission… The large number of mission

cancellations and losses (by NASA) gives rise

to serious concern for the future of the US

planetary exploration programme.” �

Peter Bond, RAS press officer (space science).

Mission update

2.31April 2001 Vol 42

Space shorts� To mark the 200th anniversary of the

discovery of infrared light by William

Herschel, ESA’s Far Infrared and Submil-

limetre Telescope (FIRST) has been

renamed the Herschel Space Observatory.

� An ESA inquiry board has identified 10

recovery options to overcome a commu-

nications error on the Huygens mission

to Saturn’s moon Titan. A decision on

how to proceed will follow in a few

months. The report is at sci.esa.int/home/

huygens/index.cfm.

� After a few months of foggy vision, the

performance of the navigation camera on

NASA’s Stardust spacecraft has returned

“to nearly normal”. By heating the cam-

era’s optical path, the Stardust team was

able to boil away contaminants on opti-

cal surfaces. The camera can now detect

stars as faint as 9th magnitude, which

should allow Stardust to perform its final

manoeuvres during approach to Comet

Wild 2 in 2004. Meanwhile, Stardust

completed an Earth “gravity assist”, fly-

ing at an altitude of 6000 km on 15 Jan-

uary 2001 (stardust.jpl.nasa.gov/).

� The total lunar eclipse of 9 January

2001 attracted a lot of attention from the

ground and from a British mini-satellite,

UoSAT-12, which was built by Surrey

Satellite Technology Ltd (SSTL). Surrey

engineers pointed the satellite’s multi-

spectral camera – which normally points

Earthwards – at the Moon to capture the

eclipse from space, 40 minutes before

totality. Apart from its aesthetic aspect,

the manoeuvre was intended as a demon-

stration of the satellite’s onboard control

and imaging system capabilities (www.sstl

.co.uk/primages/UoSAT-12eclipse.jpg).

SSTL’s reputation has also been enhanced

by a $120 000 contract from NASA to

contribute to their study of the Magneto-

spheric Multiscale (MMS) mission – a

constellation of four small spacecraft fly-

ing in a tetrahedral configuration to study

the Earth’s magnetosphere and its inter-

action with the solar wind. This follows

on from the earlier Phase A study, com-

pleted by SSTL in September 2000.

� The Voyager 1 spacecraft, the furthest

spacecraft from Earth, may reach the

termination shock near the boundary

between our solar system and interstellar

space within three years. “Once we know

where the termination shock is, we’ll have

a better idea how much farther it is to the

heliopause,” said Edward Stone, Voyager

project scientist (vraptor.jpl.nasa.gov/).