Published by the Association Pro ISSI No 12, June 2004

Ten YearsHubble Space Telescope

INTERNATIONAL SPACESCIENCEINSTITUTE

Give me the material, and I willbuild a world out of it!

Immanuel Kant (1724–1804), thegreat German philosopher, beganhis scientif ic career on the roof ofthe Friedrich’s College of Königs-berg, where a telescope allowedhim to take a glance at the Uni-verse inspiring him to his f irstmasterpiece, the “Universal Nat-ural History and Theory of Heav-en” (1755). Applying the Newton-ian principles of mechanics, it isthe result of systematic thinking,“rejecting with the greatest care allarbitrary fictions”. In his later Cri-tique of the Pure Reason (1781)Kant maintained that the humanintellect does not receive the lawsfrom nature, but rather dictatesthem upon it , since our mind re-quires a priori rules in space andtime, in cause and in effect, inorder to understand nature. Andthe result of his cosmological stud-ies conf irms the strength of hisempirical thinking. Sapere aude:“dare use your mind,” used he tosay to his critics.

In his Universal Natural Historyhe wrote: “Nature, on the imme-diate edge of creation, was as rawand undeveloped as possible. Onlyin the essential properties of theelements, which made up thechaos, can we perceive the sign ofthat perfection, which nature hasfrom its origin, since its being is aconsequence arising from the eter-nal idea of the Divine understand-ing”. Can one better describe thedawn of this world?

The present issue of Spatium isdedicated to a scientist who lived a

century after Kant and a telescopebuilt another century later. TheHubble Space Telescope has revo-lutionised our understanding ofthe cosmos much the same asKant’s theoretical reflections did.Observing the heavenly processes,so far out of any human reach,gives men the feeling of the cos-mos’ overwhelming forces andbeauties from which ImmanuelKant derived the order for a ra-tional and moral human behav-iour: “the starry heavens above meand the moral law within me. . .”.

Who could be better qualif ied torate the Hubble Space Telescope’simpact on astrophysics and cos-mology than Professor Roger M.Bonnet, the former Director ofScience at the European SpaceAgency? It was under his guidancethat in the frame of a joint NASA /ESA programme the Hubble SpaceTelescope was created, leading to one of the many marvellousachievements of the EuropeanSpace Agency’s science programme.R. M. Bonnet, now the Interna-tional Space Science Institute’s Ex-ecutive Director, gave a fascinat-ing lecture on the Hubble SpaceTelescope to the Pro ISSI audienceon 6 November 2003. We are in-debted to Professor Bonnet for hiskind permission to publish here-with a slightly revised version ofhis lecture.

Sapere aude!

Hansjörg SchlaepferZürich, June 2004

2SPAT IUM 12

Impressum

SPATIUMPublished by theAssociation Pro ISSItwice a year

Association Pro ISSIHallerstrasse 6,CH-3012 BernPhone +41 (0)3163148 96Fax +41 (0)3163148 97

President Prof.Heinrich Leutwyler,University of BernPublisherDr.Hansjörg Schlaepfer,legenda schläpfer wort & bild,CH-8185 WinkelLayoutMarcel Künzi,marketing · kom-munikation, CH-8483 KollbrunnPrinting Schellenberg Druck AGCH-8330 Pfäffikon

Front cover: The beautiful Eski-mo Nebula is an intricate struc-ture of shells and streamers of gasaround a dying Sun-like star 5000light-years away. The disc of mate-rial is embellished with a ring ofcomet-shaped objects, their tailsstreaming away from the central,dying star. The planetary nebulabegan to form about 10,000 yearsago, when the dying star started to expel an intense ‘wind' of high-speed material out into space.(Credit: Space Telescope ScienceInstitute, STScI)

Editorial

INTERNATIONAL SPACESCIENCEINSTITUTE

3SPAT IUM 12

The Man. . .

Edwin Powell Hubble was born inMarshf ield, Missouri, USA, onNovember 29, 1889. In 1898, hisfamily moved to Chicago, wherehe attended high school. EdwinHubble was a f ine student and aneven better athlete, but he alsofound time to study and earn anundergraduate degree in mathe-matics and astronomy. He thenwent to Oxford University, wherehe did not continue his studies inastronomy,but instead studied law.

In 1913, Hubble returned fromEngland and was called to the bar-rister, setting up a small practice inLouisville, Kentucky; but it didn'ttake long for him to realise that hewas not happy as a lawyer, and thathis real passion was astronomy.He therefore studied at the YerkesObservatory and received in 1917the doctorate in astronomy fromthe University of Chicago. Fol-lowing a tour of duty in WorldWar I, Hubble was employed at theMount Wilson Observatory inCalifornia, where he devised aclassif ication system for the vari-ous galaxies he observed, sortingthem by content, distance, shape,and brightness. It was during thesestudies that he noticed the redshifts in the emission of light fromgalaxies, which he correctly inter-preted as moving away from eachother at a rate proportional to theirdistance. From these observations,he was able to formulate in 1929the so-called Hubble's Law, al-

lowing astronomers to determinethe age of the universe, and prov-ing that the universe was expand-ing.

It is interesting to note here that asearly as 1917, Albert Einstein hadalready introduced his general the-ory of relativity, and produced amodel of space based on that theo-ry, claiming that space was curvedby gravity, therefore that it must be able to expand or contract; buthe found this assumption so farfetched, that he revised his theory,stating that the universe was static.Following Hubble's discoveries, heis quoted as having said that sec-ond guessing his original f indings

was the biggest blunder of his life,and he even visited Hubble tothank him in 1931.

After World War II, Edwin Hub-ble returned to the Mount Wilsonand Mount Palomar observatoriesand continued his studies until hisdeath on September 28,1953. Dur-ing his life, Hubble had tried toobtain the Nobel Prize, even hir-ing a publicity agent to promotehis cause in the late 1940s, but allthe effort was in vain as there wasno category for astronomy. Withor without a Nobel Prize he willforever be remembered as the fa-ther of observational cosmologyand as a pioneer of the distant stars.

Figure 1Edwin Hubble around 1920

TenYearsHubbleSpaceTelescope *)

Roger M.Bonnet, International Space Science Institute,Bern

*) Pro ISSI lecture, Bern,November 6, 2003

. . .and theTelescope

Its History

In the 1970s, NASA and ESA tookup the idea of a space-based tele-scope. Funding began to f low in1977. Later, it was decided to namethe telescope after Edwin Hubble.Although the Hubble Space Tele-scope (HST) was downsized laterto a 2.4 m primary mirror diame-ter from the initial 3 m, the projectstarted to attract significant atten-tion from astronomers.

The precision-ground mirror wasfinished in 1981 and the assemblyof the entire spacecraft was com-pleted in 1985. The plan called fora launch on NASA’s Space Shuttlein 1986 but just months before thescheduled launch, the Challengerdisaster caused a 2-year delay ofthe entire Shuttle programme.HST was finally launched on 24April 1990. Soon after, the tensionbuilt up as astronomers examinedthe f irst images through the newtelescope’s eyes. It was soon re-alised that its mirror had a seriousf law: the mirror edge was too f latby a mere fiftieth of the width of ahuman hair, enough of focusingdefect to prevent it from takingsharp images.

Fortunately, the HST was the firstspacecraft ever to be conceived asserviceable. That made it possiblefor engineers and scientists at the

Space Telescope Institute in Balti-more (USA) to come up with acleverly designed corrective opticspackage that would restore thetelescope’s eyesight completely.A crew of astronauts includingClaude Nicollier carried out therepairs necessary to restore the tel-escope to its intended level of per-formance during the first HubbleServicing Mission (SM1) in De-cember 1993. This mission cap-tured the attention of both as-tronomers and the public at largeto a very high degree: meticulous-ly planned and brilliantly execut-ed, the mission succeeded on allcounts. It will go down in historyas one of the great highlights ofhuman space f light. Hubble wasback in business!

The European contribution cov-ered a nominal 15% stake in themission including the Faint ObjectCamera, the first set of two solarpanels that powered the spacecraftas well as a team of space scientistsand engineers at the Space Tele-scope Science Institute (STScI)in Baltimore. Contraves Space ofZurich provided the Primary De-ployment Mechanism, assuringthe deployment of the solar panelseither by a command from the on-board computer or manually by anastronaut.

The Spacecraft

The Hubble Space Telescope is along-term, space-based observato-ry. Observations are carried out inthe visible, infrared, and ultravioletlight. It circles the Earth in 96minutes on a circular orbit 600 km

above the ground, inclined 28 deg.to the equator. Hubble’s orbitabove the Earth and its excellentstability allow astronomers tomake the high resolution observa-tions that are essential to opennew windows to planets, stars, andgalaxies. Furthermore, access tothe infrared and ultraviolet lightwindow can only be achievedfrom space, because the atmos-phere prevents it from reachingthe ground.

At the heart of the HST are a 2.4 mprimary mirror, f ine guidance sen-sors and gyroscopes as well as acollection of four science instru-ments that work from the near in-frared through the visible to ultra-violet light. There are two camerasand two combined camera /spec-trographs. Power for the comput-ers and the scientif ic instrumentsonboard is provided by large solarpanels.

The telescope uses an elaboratesystem of attitude controls to se-cure its stability during observa-tions. The spacecraft has a point-ing stability of 0.007 arcsec,whichis equivalent to a 1 coin in Parisseen from Bern. A system of reac-tion wheels manoeuvres the tele-scope into place and its position inspace is monitored by gyroscopes.The f ine guidance sensors areused to lock onto guide stars to en-sure the extremely high pointingaccuracy needed to make very ac-curate observations.

4SPAT IUM 12

The Hubble Space TelescopeServicing Concept

As stated above, the Hubble SpaceTelescope was built around a revo-lutionary servicing concept fromthe very beginnings. The conceptaimed at allowing not only repairmissions in case of faulty compo-nents, but also to replace subsys-tems once technologically moreadvanced instruments or comput-ers have become available. In addi-tion, this concept allowed the mis-

sion to be put on track after the re-pair of the initial f law of the pri-mary mirror.

Since SM1, three other ServicingMissions have been carried out:during SM2 in 1997 two new in-struments were installed. In Ser-vice Mission SM3A (1999) manyof the spacecraft’s crucial techni-cal systems were exchanged. Dur-ing SM3B in 2002 the telescopewas again equipped with new sci-ence instruments. In the wake of

the Columbia accident, however,and as a f irst consequence of thestrategic redirection of the USscience programme, NASA hascancelled the next servicing mis-sion, which was foreseen in 2003.At this moment in time it is not yet clear whether the telescopewill be serviced again. This maylead to a much shorter service lifeas compared to the initial plans,which foresaw an end of missionin 2010.

5SPAT IUM 12

Improving Hubble: Servicing

ServicingMissions

SM1 SM2 SM3B SM4

STIS (= 40x Original Spectrographs)

NICMOS(1–2 µm) NICMOS / NCS COS (= 20x STIS)

ACS (= 10x WFPC2)

WFPC2 WFC3 (= 25x NICMOS)

Figure 2 shows the servicing mission planning

6SPAT IUM 12

Figure 3: The famous photograph of Claude Nicollier as a happy space worker on the f irst Hubble Space Telescope ServicingMission (Credit: NASA)

The Instruments

Hubble’s science instruments cur-rently include two cameras, twoimaging spectrographs and a suiteof fine guidance sensors:

The Wide-Field PlanetaryCamera 2 (WFPC-2)

The WFPC-2 is Hubble’s work-horse camera. It records imagesthrough selection of 48 colour f il-ters covering a spectral range fromthe far ultraviolet to visible andnear-infrared wavelengths. It hasproduced most of the pictures thathave been released as public out-reach images over the years. Its res-olution and excellent quality havemade it the most used instrumentin the first ten years of Hubble’slife.

The Space Telescope ImagingSpectrograph (STIS)

The STIS is a versatile dual-pur-pose instrument consisting of acamera and a spectrograph operat-ing over a wide range of wave-lengths from near infrared to theultraviolet.

The Near Infrared Camera andMulti-Object Spectrometer(NICMOS)

The NICMOS can take imagesand make spectroscopic obser-vations of astronomical targets. Itdetects infrared light between 0.8 µm to 25 µm.

The Advanced Camera forSurveys

This camera replaced the initialFaint Object Camera built by theEuropean Space Agency, whichwas returned to ESA after the Ser-vicing Mission S3B.

The Fine Guidance Sensors(FGS)

The HST has three Fine GuidanceSensors on board. Two of them areused to point and lock the tele-scope onto the target and the thirdcan be used for astrometry, makingvery precise position measurementsto establish stellar distances andinvestigate stellar binary systems.

7SPAT IUM 12

Figure 4 shows the Hubble Space Telescope after release from its f irstservicing mission.

Scientific Highlights

From Solar SystemScience. . .

Thanks to its unrivalled opticalresolution power, HST providesvisual information about the SolarSystem on a near continuous basis(as opposed to planetary f ly-by’s).The following examples showsome of the most striking resultsin the f ield of Solar System re-search.

8SPAT IUM 12

Figure 5 shows Jupiter’s aurora with the footprints of its moons Io,Ganymede and Europa. Since Jupiter possesses a strong magnetic f ield,the footprints of its moons become visible. Io produces particles from vol-canic eruptions. The ionised gas becomes trapped by Jupiter’s magneticf ield. As the particles spiral along the magnetic f ield lines, they hitJupiter’s atmosphere near the poles (the magnetic f ield threads the northand south poles similar to Earth’s magnetic f ield). Generally, the particlesfollow the moon’s f ield lines to the poles with some straggling particlesproviding the lagging lines. HST discovered this phenomenon because,unlike the earlier Voyager flybys, it can observe the planets continuouslyon timescales ranging from minutes to years. (Credit: STScI, John Clarke)

PillanVolcano

Io

Figure 6: Transit of Io above Jupiter. HST is not only able to show theJupiter moons, but also the shadows they cast onto the surface of Jupiter.On the left image, Jupiter’s moon Io is seen on the left from Jupiter. In thecentre and the right images Io, together with its shadow is seen before theplanet. Even Io’s volcanic activity can be identif ied with the 400 km highgas and dust plume emerging from its silhouette (right, bottom image).(Credit: STScI, J. Spencer)

9SPAT IUM 12

Figure 7 captures a cosmic ca-tastrophe, when in 1994 thecomet Levy-Shoemaker came nearJupiter,which by virtue of its giantgravitational f ield not only attract-ed the comet but also broke it intoparts. This f igure shows Jupiterbeing hit by two of Levy-Shoe-maker’s comet fragments and thesubsequent disturbances in its at-mosphere (from bottom to top).(Credit: STScI)

Figure 8 shows the planet Saturn in the infrared light. The picture is a combination of three images fromHubble’s NICMOS instrument and shows the planet in reflected infrared sunlight. Different colours indicatevarying heights and compositions of cloud layers generally thought to consist of ammonia ice crystals. The eye-catching rings cast a shadow on Saturn’s upper hemisphere. The bright stripe seen within the left portion of theshadow is infrared sunlight streaming through the large gap in the rings known as the Cassini Division. Two ofSaturn’s many moons have also put in an appearance, Thetys just beyond the planet’s disk at the upper right, andDione at the lower left. (Credit STScI, E. Karkoschka)

Figures 9 (top) and 10 (bottom): HST renders invaluable services to mission planners when it comes to ex-ploring planets and moons in the frame of future landing missions. In the case of the joint NASA / ESA missionCassini / Huygens it was of primordial importance to get as much information on Titan and its atmosphere inorder to design properly the details of the lander mission for Huygens.The Near Infrared Camera and Multi-Ob-ject Spectrometer (NICMOS) was best suited to penetrate Titan’s opaque atmosphere containing nitrogen andcarbon compounds and to provide information about its surface. Figure 9 above shows Titan seen by the NIC-MOS from four different directions in a distance of approx 1.5 billion km. This is approximately equivalent toobserving a football in Zurich from Berne. The brighter areas are believed to be icy continents in oceans ofmethane and ethane, and of course scientists are eager to place Huygens on a beautiful site along one of Titan’sbeaches in January 2005, as sketched in Figure 10. (Credits: Figure 9: STScI, Figure 10: ESA)

10SPAT IUM 12

11SPAT IUM 12

Figure 11 depicts one of the close-by cradles of formation of stellar systems in the Orion nebula. The twodisk images show the two distinct disk types: a disk silhouetted against the background nebula (top left) and adisk inside the nebula whose outer parts are also ionised creating a glowing, teardrop-shaped bubble of ionisedgas around it. These disks are the precursors of new planetary systems. Hubble Space Telescope’s images of thesedisks were the first direct pictures confirming the general planet-creation scenario that has been stipulated byImmanuel Kant in 1755 and Pierre-Simon Laplace some years later. (Credit: STScI, C. Robert O’Dell)

. . .to Future Solar Systems. . .

Of course, the Hubble Space Tele-scope is instrumental not only indiscovering the secrets of the SolarSystem but of other solar systemsthat exist around other stars. In theinterstellar space, where dust andgas is extremely thinly distributed,irregularities of the density ofmatter eventually cause the higherdensities to generate slightly in-creased gravitational f ields, whichin turn slowly attract additionalmatter from the ambient space,leading to a further increase of thegravitational forces. An avalancheprocess is initiated whereby moreand more mass is concentrated intime spans of 10,000 of years.After around 1 million year a pro-toplanetary disk is formed, which,due to the angular momentum ofthe former dust cloud, rotatesaround an axis perpendicular tothe future solar system’s eclipticplane. After a further 100 millionyears, when the mass of the newstar has exceeded a certain criticalthreshold, the nuclear fusionprocess in its core is ignited, burn-ing hydrogen into helium therebyreleasing enormous amounts ofenergy, causing the new star toshine. From the debris remainingin the disk, planetesimals of in-creasing mass are formed and afteranother 1 billion years the planetshave gathered the majority of mat-ter circling around the central star.If there is a planet not too close tothe central star and not too faraway it may contain liquid wateron its surface that may become anew shelter of life in the cosmicvoid. (See also Spatium 6 for fur-ther details).

12SPAT IUM 12

5.6 billion miles

Diameter of Neuptume’s Orbit

HR 4796 A

Star

Figure 12 shows the capability of central obscuring of the NICMOSInstrument. By eclipsing the light of the central star like a solar eclipse butwith an opaque mask, the faint light of rings of matter around HR 4796Abecomes visible. This is the very place where planets will form. Note thedimension of this protoplanetary disk being in the order of 20 billion km,about twice the diameter of the Solar System. Rings seen in disks aroundmain sequence stars are most easily interpreted as gravitational sweepingby small companions to the stars i. e. planets. The only way that we knowhow to produce rings is to have gravitational sweeping of the matter bycompanion bodies. (Credit STScI)

13SPAT IUM 12

Figure 13: The STIS aboard HSTis a powerful spectrograph. Thisfigure shows schematically howthe sodium abundance in the at-mosphere of the planet HD209458b was detected. The STISwas used to observe the star be-fore, during, and after the planetpartially eclipsed the star. As-tronomers compared the depth ofthe eclipse at the wavelengths oftwo absorption lines of sodiumwith the depth at adjacent wave-lengths free of absorption. Sincethe sodium in the atmosphere ab-sorbed more starlight, the eclipsedepth at those wavelengths wasslightly greater than in the adjacent wavelengths. The versatility and power of HST made it possible to performsuch pioneering measurements. (Credit: STScI)

rela

tive

flux

1.000

0.995

0.990

0.985

–0.15 – 0.10 –0.05 0 0.05 0.10 0.15time from center of transit [days]

3 hours

1� � 10 – 4

Ingress Eclipse Egress

HST detects additionalsodium absorptiondue to light passingthrough planetaryatmosphere as planettransits across star

Additional absorptiondue to planetaryatmosphere

Normal absorptionspike depth

from star

Wavelength [nm]

Sun-like star

Gas-giant planetorbits its sun in 3.5 Earth days (orbit not in scale)

Additional lightabsorbed by planetaryatmosphere

Light absorbedby planet itself

Brightnessof

star

Time

Duration of transit

. . .and the Death of Stars

While the birth of stars is a slowprocess their death is a spectacularshow. A star with sufficient masscontent shines thanks to the nu-clear fusion of its light elements.When this fusion process comes toan end, no heat generating fusionprocess keeps the star shining any-more, it will, therefore, collapseunder its own gravitational f ieldand release thereby a lot of gravita-tional energy causing its relics par-tially to be merged to heavier ele-ments, which are then scatteredinto space. Together with fresh hy-

drogen these heavier atoms maybecome the seeds of new genera-tions of stars whose planets even-tually may support life. When ourSolar System formed around 4600million years ago it also collectedthe relics of such former genera-tions of stars, that at the end oftheir lives had produced heavy ele-ments, which today are found onEarth and in our own bodies. (seealso Spatium 3 for further infor-mation).

Figure 14: The shapes of planetary nebulae may become extremely complex like in the case of the Red SpiderNebula. It is still not understood how such complex structures may evolve. But in any case, nature is beautiful,when it explodes. (Credit: STScI)

Figure 15 shows the hourglassnebula through the light scatteredby the gas expelled by the dying starat an earlier epoch. The shapes ofdying stars show an astonishingvariety of colours and forms de-pending on the properties of theformer star. This HST snapshot ofMyCn18, a young planetary neb-ula, reveals that the object has in-tricate patterns of “etchings" in itswalls. A planetary nebula is theglowing relic of a dying, Sun-likestar. The results are of great inter-est because they shed new light onthe poorly understood ejection ofstellar matter that accompaniesthe death of these stars. Accordingto one theory on the formation ofplanetary nebulae, the hourglassshape is produced by the expan-sion of a fast stellar wind within aslowly expanding cloud, which isdenser near its equator than nearits poles. (Credits: RaghvendraSahai and John Trauger, JPL)

16SPAT IUM 12

Figure 16: The Eight-Burst Nebula NGC 3132 is another striking example of a planetary nebula. This expand-ing cloud of gas, surrounding a dying star, is known to amateur astronomers in the southern hemisphere as the“Eight-Burst" or the “Southern Ring" Nebula. The name “planetary nebula" refers only to the round shape thatmany of these objects show when examined through a small visual telescope. In reality, these nebulae have littleor nothing to do with planets, but are instead huge shells of gas ejected by stars as they near the ends of their life-times in fact probably ingesting any planet which they might have around them. NGC 3132 is nearly half a lightyear in diameter, and at a distance of about 2000 light years it is one of the nearer known planetary nebulae. Thegases are expanding away from the central star at a speed of 15km /s. This image clearly shows two stars near thecentre of the nebula, a bright white one, and an adjacent, fainter companion to its upper right. (A third, unrelatedstar lies near the edge of the nebula.) The faint partner is actually the star that has ejected the nebula. This star isnow smaller than our Sun, but extremely hot. The flow of ultraviolet radiation from its surface makes the sur-rounding gases glow through fluorescence. The brighter star is in an earlier stage of stellar evolution, but in thefuture it will probably eject its own planetary nebula. (Credit: STScI)

17SPAT IUM 12

Figure 17: While the nebula of Figure 14 was from a single star, this f igure shows a binary system consisting oftwo near-by stars. In this case, the explosion becomes very complex albeit symmetrical in shape. This f igureshows the so-called “ant nebula" (Mz 3) resembling the head and thorax of a garden-variety ant.This HST imagereveals the “ant's" body as a pair of fiery lobes protruding from the dying star. Scientists using Hubble would liketo understand how a spherical star can produce such prominent, non-spherical symmetries in the gas that itejects. One possibility is that the central star of Mz 3 has a closely orbiting companion that exerts strong gravita-tional tidal forces, which shape the outflowing gas. For this to work, the orbiting companion star would have tobe close to the dying star, about the distance of the Earth from the Sun. At that distance, the orbiting companionstar would not be far outside the hugely bloated hulk of the dying star. It is even possible that the dying star hasconsumed its companion,which now orbits inside it. A second possibility is that, as the dying star spins, its strongmagnetic f ields are wound up into complex shapes. Charged winds moving at speeds up to 1000 km per secondfrom the star, much like those in our Sun's solar wind but millions of times denser, are able to follow the twistedfield lines on their way out into space. These dense winds can be rendered visible by ultraviolet light from the hotcentral star or from highly supersonic collisions with the ambient gas that excites the material and make it to flu-oresce. (Credit: STScI)

18SPAT IUM 12

Figure 18: One of the most spectacular images obtained by the HST is this sequence of the V838 Monocerotiscollected over a time span of about six months. Calculations of the speed of the shells showed that they movewith the speed of light. This is obviously an illusion. The visible shells are in fact different parts of dust cloudsreleased by the central dying star in the course of its life. They are illuminated by short bursts of light released bythe star and spreading with the velocity of light. This effect is called a “light echo." The red star at the centre ofthe eyeball-like feature is an unusual erupting super giant called V838 Monocerotis, located about 20,000 light-years away in the winter constellation Monoceros (the Unicorn). During its outburst the star brightened to morethan 600,000 times our Sun's luminosity. The outer circular feature is slightly larger than the angular size ofJupiter on the sky. For several more years its diameter will increase as ref lected light is scattered from moredistant portions of the nebula. Eventually, when light starts to be released from the backside of the nebula, thelight echo will give the illusion of contraction, and finally it will disappear probably by the end of this decade.The black gaps around the red star are regions of space where there are holes in the dust cloud. (Credit: STScI)

May 20, 2002 September 2, 2002

October 28, 2002 December 17, 2002

19SPAT IUM 12

Figure 19 shows the Whirlpool Galaxy perpendicular to its galactic plane.We clearly see the spiral arms,wherebillions of stars that build up this galaxy are concentrated. The red stars are young starting stars, while the bluedots are large massive stars. New pictures from Hubble are giving astronomers a detailed view of the Whirlpoolgalaxy's spiral arms and dust clouds, which are the birth sites of massive and luminous stars. This galaxy, alsocalled M51 or NGC 5194, is having a close encounter with a nearby companion galaxy, NGC 5195, just off theupper edge of this image. The companion's gravitational influence is triggering star formation in the Whirlpool,as seen by the numerous clusters of bright,young stars,highlighted in red. (Credit: STScI)

Beyond the Milky Way

One of the major contributions ofEdwin Hubble was the observa-tion of galaxies beyond the MilkyWay. Our galaxy is a f lat disk witha diameter of approx.100,000 lightyears. We find ourselves in one ofseveral spiral arms far outside itscentre. The large aperture of theHST as well as its (now) nearlyperfect optical quality allows ob-servations of distant objects out toregions, which have never beenobserved before. Since light travelswith a limited speed of 300,000km /s we observe distant objectsnot in their present state, but at anearlier state in the past. The fur-ther they are the earlier can welook into the past. In addition it al-lows us to observe galaxies likeours but from very different an-gles.

Figure 21 shows the ESO 510-G13 galaxy, whose galactic plane seems to be bent. HST has captured an imageof this unusual edge-on galaxy, revealing remarkable details of its warped dusty disk. In contrast, the dust andspiral arms of normal spiral galaxies, like our own MilkyWay, appear flat when viewed edge-on. (Credit: STScI)

Figure 20 shows Gomez’s Hamburger, a Sun-like star nearing the endof its life. The hamburger buns are light scattered from the central star,which itself is obscured by a large band of dust in the middle. (CreditSTScI)

21SPAT IUM 12

Figure 22 shows a ring galaxy called the Hoag’s Object. It is a young galaxy,which has not yet developed itsspiral arms.A nearly perfect ring of hot, blue stars pinwheels about the yellow nucleus. The entire galaxy is about120,000 light-years wide,which is slightly larger than our Milky Way. The blue ring,which is dominated by clus-ters of young, massive stars, contrasts sharply with the yellow nucleus of mostly older stars.What appears to be a“gap" separating the two stellar populations may actually contain some star clusters that are almost too faint tosee. Curiously, an object that bears an uncanny resemblance to Hoag's Object can be seen in the gap at the one o'clock position.The object is probably another ring galaxy.

Ring-shaped galaxies can form in several different ways. One possible scenario is through a collision with anoth-er galaxy. Sometimes the second galaxy speeds through the f irst, leaving a “splash" of star formation. But inHoag's Object there is no sign of a second galaxy, which leads to the suspicion that the blue ring of stars may bethe shredded remains of a galaxy that passed nearby. Some astronomers estimate that the encounter occurredabout 2 to 3 billion years ago. The galaxy is 600 million light years away in the constellation Serpens. (Credit:STScI)

22SPAT IUM 12

Figure 23 shows a cosmic catastrophe. This impressive image shows two colliding galaxies called NGC 3314,which lie about 140 million light years from Earth, in the direction of the southern hemisphere constellationHydra. The bright blue stars forming a pinwheel shape near the centre of the front galaxy have formed recentlyfrom interstellar gas and dust. In many galaxies, interstellar dust lies only in the same regions as recently formedblue stars. However, in the foreground galaxy, NGC 3314a, there are numerous additional dark dust lanes that arenot associated with any bright young stars.A small, red patch near the centre of the image is the bright nucleus ofthe background galaxy, NGC 3314b. It is reddened for the same reason the setting Sun looks red.When light pass-es through a volume containing small particles (molecules in the Earth's atmosphere or interstellar dust particlesin galaxies), its colour becomes redder.

Collision does not mean that the single stars physically touch each other, since the distances between them are so large. But due to their gravitational f ields collisions of galaxies may lead to collisions of stars caught by thegravitational f ield of large stars in one of the galaxies. Through an extraordinary chance alignment, a face-onspiral galaxy lies precisely in front of another larger spiral. This line-up provides us with the rare chance tovisualise dark material within the front galaxy, seen only because it is silhouetted against the object behind it.Dust lying in the spiral arms of the foreground galaxy stands out where it absorbs light from the more distantgalaxy. This silhouetting shows us where the interstellar dust clouds are located, and how much light they absorb.The outer spiral arms of the front galaxy appear to change from bright to dark, as they are projected first againstdeep space, and then against the bright background of the other galaxy. (Credit: STScI)

23SPAT IUM 12

Figure 24 shows the Black Hole in the galaxy M84. HST has strongly contributed to the understanding ofthe mechanics of galaxies. Thanks to HST and new observations conducted in large ground based observatoriessuch as the Very Large Telescope of the European Southern Observatory in Chile, we know that galaxies arepowered by black holes in their centre. Their mass density is so high, that no matter, not even light can escape,hence their name. Due to their high mass they generate strong gravitational f ields, the binding force of galaxiespreventing the individual stars from escaping the common centre. In the case of M84, HST was able to providepictures of the core of the galaxy. (Credit: STScI).

The Early Universe

HST’s nearly perfect optical systemcombined with its excellent long-term stability allows to investigateregions of the universe which havenever been reached before. Some15 billion years ago, the universestarted to expand and cooled over

the next half million years. Theglowing plasma of which it wascomposed, recombined into atomsof neutral gas,mostly hydrogen andsome helium. The glow from thisera of recombination has been ob-served as the cosmic microwavebackground radiation, and is usedto study the large-scale geometry

of the universe. Over the followinghalf billion years or so, termed theDark Age, the cold gas began to as-semble into what we think are thefirst galaxies. The Dark Age endedwhen the light from the stars andthe newly formed galaxies andquasars reionised the surroundingneutral gas.

24SPAT IUM 12

Figure 25 shows the deepest portrait of the visible universe ever achieved. This historic new view is actuallybased on a million-second exposure of two separate images taken by the Advanced Camera for Surveys and theNear Infrared Camera and Multi-Object Spectrometer. Both images reveal some galaxies emerging from theDark Age, a mere 500 million years after the Big Bang. These galaxies are too faint to be seen in Hubble's pre-vious faraway looks. The HUDF field contains an estimated 10,000 galaxies in a patch of sky just one-tenth thediameter of the full Moon. Besides the rich harvest of classic spiral and elliptical galaxies, there are many strangegalaxies littering the field telling about a period when the Universe was more chaotic than today and when orderand structure were just beginning to emerge.

Getting betterwith Age

The Hubble Space Telescopestarted to be designed in 1975 andwas supposed to be launched inthe early 1980s. Due to the loss ofthe Challenger Shuttle it was

launched in 1990 only. Its technol-ogy therefore is nearly 25 years oldnow. During these 25 years tech-nological progress was impressiveand it continues probably to be so.Any non-serviceable spacecraftwould already be outdated at thebeginning of its lifetime in orbit.The ingenious concept to updatethe scientif ic instruments as wellas elements like onboard comput-

ers, tape recorders, solar panels, etc.kept HST abreast of technologicalprogress. In other words ratherthan becoming obsolete HST getsbetter with increasing age. Figure 26gives an impression of the impor-tance of the concept of service-ability for the ambitious increasein sensitivity.

25SPAT IUM 12

Telescope Sensitivity and Discoveries

SensitivityImprovementover the Eye

Year of observations

200018001600 19001700

1010

108

106

104

102

Photographic and electronic detection

Telescopes alone

Ele

ctro

nic

Pho

tog

rap

hy

Gal

ileo

Huy

gen

sey

epie

ce

Slo

w f

rat

ios

Sho

rt’s

21.

5”

Her

sche

ll’s

48”

Ro

sse’

s 72

”

Mo

unt

Wils

on

100”

Mo

unt

Pal

om

ar 2

00”

So

viet

6 m

Hub

ble

Sp

ace

Tele

sco

pe

NG

ST

Figure 26 shows the evolution of observation sensitivity as compared to the naked human eye since GalileoGalilei’s f irst astronomical observations in 1609. The striking result here is that since Galilei’s initial advance, noother advance in technology has been proportionately as great as when HST became operational. It advances thestate of the art by nearly two orders of magnitude.

It is important to note that the HST line in this f igure is sloped upward. Each time the spacecraft is upgraded withnew instruments its observing capabilities are increased, making HST a continuously improving facility. Theright hand bar shows the expected gains to come from the James Webb Telescope (NGST), the successor ofHubble due to launch around 2011. (Credit: STScI)

After the HubbleSpace Telescope

In sharp contrast to the spectacu-lar images HST gathered fromdying stars, its own end will be farfrom spectacular. Unlike mostspacecraft Hubble has no onboardpropulsion system, relying insteadon gyroscopes and f lywheels tomaintain stability. Thus, at the endof its lifetime, special measureswill have to be taken to prevent it from entering the Earth atmos-phere in an uncontrolled way.Initially it was foreseen to be re-trieved through a dedicated Shut-

tle mission, but, in the wake of theColumbia disaster, NASA is nowforced to concentrate the Shuttlemissions on the construction ofthe International Space Station.Therefore, plans have been pre-sented lately to de-orbit the HSTby means of a dedicated au-tonomous space tug. This space-craft is intended to launch on aDelta 2 rocket, to grapple the HSTand to guide it safely into theEarth’s atmosphere over an un-populated region. That will be theend of one of mankind’s greatestachievements . . .

Several years ago NASA initiated acompetition for the design of thesuccessor instrument, which at

that time was called the NextGeneration Space Telescope. In2003 the design by TRW Inc waschosen (Figure 27) and the missionwas baptised James Webb SpaceTelescope in honour of NASA'ssecond administrator. Unlike theHST, this instrument will not beserviceable, as it will be placed atthe Lagrange L2 point that is 1.5million km from the Earth lee-ward to the Sun. While this posi-tion is attractive, because it is inthe Earth’s shadow, hence less dis-turbed by solar radiation, it lacksservicing capabilities at least forthe time being, since it is around 5 times farther from the Earththan the current Space Shuttle’sreach.

26SPAT IUM 12

Figure 27: Artists view of the James Webb Telescope, the successor mission to the Hubble Space Telescope(Credit: TRW).

Conclusion

The unique advance permitted byHST amply sustains the Europeanand American investment in thisprogramme. The Hubble SpaceTelescope has truly revolutionisedour knowledge of:

– the creation of galaxies,– the distance scale of the uni-

verse,– giant black holes in galaxies,– the intergalactic medium,– interstellar medium chemistry,– extra solar planets,

and it is hoped that in the futureyears of its planned operation, theHST will bring us additional sci-entif ic surprises. It is an old wis-dom that leading discoveries areoften accidental: they follow theability to observe, not necessarilythe originally planned observa-tion. Hubble has dramatically con-tributed to the modern revolutionof astronomy permitted by spacetechniques and to our understand-ing of the universe.

Figure 28 shows another ringgalaxy (AM 0644-741). The Hub-ble Space Telescope not only ren-dered invaluable services to scien-tists but made us also aware of thebeauties of our world, much likethat simple telescope,which stim-ulated Immanuel Kant some 250years ago.

27SPAT IUM 12

Roger Maurice Bonnet receivedhis diploma in physics and astron-omy in 1968 from the Universityof Paris. He dedicated his doctor-ate to the imagery and spec-troscopy of the Sun. He was in-volved in the development of newtelescopes and cameras that wereflown on balloons, rockets andsatellites. In 1969 he founded theLaboratoire de Physique Stellaireet Planetaire of CNRS. There hewas engaged in the developmentof the telescope for the HalleyMulticolour Camera of the GiottoMission under the responsibilityof the Max Planck Institute at Lin-dau (Germany), which took thefirst high-resolution images of acometary nucleus. R. M. Bonnetentered the field of internationalscience policy in 1983, when theCouncil of the European Space

Agency appointed him Director ofthe Science Programme. One ofhis most prominent achievementsof that time was the formulationof the Horizon 2000 Plan. Thisprogramme consists of four largecornerstone missions and somesmaller missions, amongst which isthe European contribution to theHubble Space Telescope. Thetransparent long term planningphilosophy allowed the academicand the industrial communities totimely build up the required skillsthereby placing ESA at the secondmost important space scienceagency in the world after NASA.Later, R. M. Bonnet adopted thesame concept for the definition ofthe new Earth observation pro-gramme of ESA, known as theLiving Planet Programme.

After his departure from ESA inApril 2001 he served as DirecteurGeneral Adjoint Science at theFrench Centre National EtudesSpatiales (CNES). There he wasasked by the French governmentto chair a National Commission toanalyse and formulate the Frenchspace policy. In 2001, he was calledby the Director General of ESA toadvise him on how to formulate itsAurora programme, setting thegoal of landing humans on Marsby 2030.

At the International Space ScienceInstitute R. M. Bonnet succeededProf. J.Geiss as Executive Director

in 2003. In the same year he wasappointed President of the Inter-national Committee on Space Re-search (COSPAR). He is the au-thor of over 150 scientif ic papersand textbooks and he holds thehighest scientif ic and public serv-ice awards like for example theFrench Légion d’Honneur or theNASA Award for Public Service.

For R. M. Bonnet adventures likethe Hubble Space Telescope arethe convergence of many scientif-ic and engineering talents in re-sponse to questions, which alwayshave challenged the human mind.They realise missions thought im-possible before, which open thebrain of the young and less youngchildren to the marvellous discov-ery of the universe.

The author