A report on Giotto's earth swingby 
Author Message
 A report on Giotto's earth swingby
Just 24 hours ago, earth was visited by a spacecraft that had left here on an
Ariane rocket exactly 5 years ago: GIOTTO. The following article (which I've
just decided to spontaneously submit to Sky & Telescope, so please do not
distribute it yet!) summarizes what I've learned on a nice news conference
at the European Space Operations Center at Darmstadt, 3 hrs after the swingby:

         Earth Swingby: Giotto on its way to Comet Grigg-Skjellerup!

It was an absolute first in space history: a spacecraft returning to earth from
the depths of the solar system, to receive a gravity assist for a new job. On
2 July 1990 at 10:01:18 Universal Time, precisely five years after its launch,
the European Giotto spacecraft approached earth to 22730 km over Australia,
raced past earth in a wide hyperbola with a speed of 6.3 kilometers per second,
and changed its orbit from one inside earth's to one outside - to meet comet
Grigg-Skjellerup on 10 Juli 1992 at 15:30 Universal Time +/- a few minutes.

One week after its spectacular encounter with Halley's comet on 14 March 1990
Giotto had been put into a peculiar orbit that went 6 times around the sun
while earth performed five orbits. On 2 April 1986 Giotto had been put into
hibernation and was on its own for the next 4 years. Amazingly it was on the
very first attempt on 19 February 1990 that the spacecraft replied to a wake-up
call transmitted by NASA's Deep Space Network 70-meter antennas. At this point
Giotto was still 102 Million km from earth, and only its Low Gain Antenna was
working. It took another six tense days until the s/c could be commanded to
also turn its parabolic High Gain Antenna towars earth, and a full two months
more until the detailed checkout of the spacecraft systems and science instru=
ments could begin. Giotto was too hot, initially, only 0.75 AU from the sun,
with the thermal systems badly damaged in the Halley coma, but in April it was
cool enough to work with.

As it turned out, the essential systems of the s/c were in remarkably fine
state despite their 5 years in space (and a nominal lifetime of just 13
months). But several redundancies were lost, and no mistakes can be afforded
now. The results from the 11 scientific experiments were mixed. Completely out
of work are the Neutral Mass Spectrometer and regrettably also the Halley
Multicolor Camera, the only imaging system on board. Its electronics and
mechanics are well, but a part of the baffle tube is stuck in front of the
optics - all images returned are just black.

Two other instruments are fine but cannot be used at Grigg-Skjellerup: for the
Ion Mass Spectrometer, the flyby velocity (14km/s instead of the 68 km/s at
Halley's) is too slow to get results, and the entrance window of the PIA dust
analyser points into a direction that was useful only at Halley's. Seven
instruments, however, are fully or almost fully at work! There is the Energetic
Particles Analyser, the Optical Probe Instrument, the Magnetometer (all three
just as new), the Johnstone Plasma Analyser and the Reme Plasma Analyser (both
partially damaged) and the Dust Impact Detection System, which can still count
dust particles. The radio science is well, also, since it needs just a strong
radio signal to get information from doppler shifts and the like.

This means a shift of interest for Giotto from the questions asked at Halley's:
the main interest now is in plasma physics. As the Giotto scientists summarize
in a recent report to ESA's Science Programme Committee, there will be
 > characterisation of the changing features of the solar-wind flow (by JPA),
 > observation of cometary pick-up ions and anomalous acceleration (JPA,EPA),
 > determination of electron densities (RPA),
 > observation of upstream waves, determination of the locations of the various
   boundaries (bow shock, ionopause, cometopause etc.), and observation of the
   magnetic pile-up region and cavity immediately around the nucleus (MAG),
 > determination of the dust spatial density and size distribution and the
   optical properties of the dust grains (DID and OPE),
 > and the detection of discrete gaseous emissions, as well as the determi=
   nation of combined dust and gas densities (OPE and Radio Science).
The ESA committee was impressed enough to vote for the GIOTTO EXTENDED MISSION
on 13 June; while the roughly 7 million $ spent for reactivating Giotto have
been taken from ESA's science budget, the approx. 10 million needed for
executing the Extended Mission will have to be collected from ESA's member
states separately. It is an 'Optional Programme', no country is forced to pay
for Giotto's second appointment, but the changes for financing are quite good.

While the plasma investigations are basic research with sometimes little
interest in the comet itself (it'll just serve as a small, gas-emitting body
thrown into the solar wind), the dust studies also have an engineering side.
The inactive Grigg-Skjellerup is much more like the preferred target
comets of future space missions than the active Halley. Thus it offers the
last opportunity to study the dust env used up,
and the technicians and scientists will be on many other projects already
(even in 1990 is proved hard to reunite the teams of 1986). But in about 50
years from now, Giotto will once more return to the vicinity of earth: it
might be caught (if somebody will still care then) and end up in a space
museum...


                  (I'll be on vacation within 12 hours, though, til 28 July)



Sat, 19 Dec 1992 23:01:55 GMT
 A report on Giotto's earth swingby
Just 24 hours ago, earth was visited by a spacecraft that had left here on an
Ariane rocket exactly 5 years ago: GIOTTO. The following article (which I've
just decided to spontaneously submit to Sky & Telescope, so please do not
distribute it yet!) summarizes what I've learned on a nice news conference
at the European Space Operations Center at Darmstadt, 3 hrs after the swingby:

         Earth Swingby: Giotto on its way to Comet Grigg-Skjellerup!

It was an absolute first in space history: a spacecraft returning to earth from
the depths of the solar system, to receive a gravity assist for a new job. On
2 July 1990 at 10:01:18 Universal Time, precisely five years after its launch,
the European Giotto spacecraft approached earth to 22730 km over Australia,
raced past earth in a wide hyperbola with a speed of 6.3 kilometers per second,
and changed its orbit from one inside earth's to one outside - to meet comet
Grigg-Skjellerup on 10 Juli 1992 at 15:30 Universal Time +/- a few minutes.

One week after its spectacular encounter with Halley's comet on 14 March 1990
Giotto had been put into a peculiar orbit that went 6 times around the sun
while earth performed five orbits. On 2 April 1986 Giotto had been put into
hibernation and was on its own for the next 4 years. Amazingly it was on the
very first attempt on 19 February 1990 that the spacecraft replied to a wake-up
call transmitted by NASA's Deep Space Network 70-meter antennas. At this point
Giotto was still 102 Million km from earth, and only its Low Gain Antenna was
working. It took another six tense days until the s/c could be commanded to
also turn its parabolic High Gain Antenna towars earth, and a full two months
more until the detailed checkout of the spacecraft systems and science instru=
ments could begin. Giotto was too hot, initially, only 0.75 AU from the sun,
with the thermal systems badly damaged in the Halley coma, but in April it was
cool enough to work with.

As it turned out, the essential systems of the s/c were in remarkably fine
state despite their 5 years in space (and a nominal lifetime of just 13
months). But several redundancies were lost, and no mistakes can be afforded
now. The results from the 11 scientific experiments were mixed. Completely out
of work are the Neutral Mass Spectrometer and regrettably also the Halley
Multicolor Camera, the only imaging system on board. Its electronics and
mechanics are well, but a part of the baffle tube is stuck in front of the
optics - all images returned are just black.

Two other instruments are fine but cannot be used at Grigg-Skjellerup: for the
Ion Mass Spectrometer, the flyby velocity (14km/s instead of the 68 km/s at
Halley's) is too slow to get results, and the entrance window of the PIA dust
analyser points into a direction that was useful only at Halley's. Seven
instruments, however, are fully or almost fully at work! There is the Energetic
Particles Analyser, the Optical Probe Instrument, the Magnetometer (all three
just as new), the Johnstone Plasma Analyser and the Reme Plasma Analyser (both
partially damaged) and the Dust Impact Detection System, which can still count
dust particles. The radio science is well, also, since it needs just a strong
radio signal to get information from doppler shifts and the like.

This means a shift of interest for Giotto from the questions asked at Halley's:
the main interest now is in plasma physics. As the Giotto scientists summarize
in a recent report to ESA's Science Programme Committee, there will be
 > characterisation of the changing features of the solar-wind flow (by JPA),
 > observation of cometary pick-up ions and anomalous acceleration (JPA,EPA),
 > determination of electron densities (RPA),
 > observation of upstream waves, determination of the locations of the various
   boundaries (bow shock, ionopause, cometopause etc.), and observation of the
   magnetic pile-up region and cavity immediately around the nucleus (MAG),
 > determination of the dust spatial density and size distribution and the
   optical properties of the dust grains (DID and OPE),
 > and the detection of discrete gaseous emissions, as well as the determi=
   nation of combined dust and gas densities (OPE and Radio Science).
The ESA committee was impressed enough to vote for the GIOTTO EXTENDED MISSION
on 13 June; while the roughly 7 million $ spent for reactivating Giotto have
been taken from ESA's science budget, the approx. 10 million needed for
executing the Extended Mission will have to be collected from ESA's member
states separately. It is an 'Optional Programme', no country is forced to pay
for Giotto's second appointment, but the changes for financing are quite good.

While the plasma investigations are basic research with sometimes little
interest in the comet itself (it'll just serve as a small, gas-emitting body
thrown into the solar wind), the dust studies also have an engineering side.
The inactive Grigg-Skjellerup is much more like the preferred target
comets of future space missions than the active Halley. Thus it offers the
last opportunity to study the dust enviroment of such a nucleus first-hand,
before missions like Comet Rendezvous Asteroid Flyby and Comet Nucleus Sample
Return dare to actually orbit one resp. to even land and dig on one. Grigg-
Skjellerup orbits the sun every 5.1 years, with a perihelion distance of .99
and an aphelion distance of 4.93 AU: it belongs to Jupiter's familiy and often
experiences close encounters with this giant. The last one, in 1964, e.g.
altered G-S' inclination from 8 degrees to the present 21 degrees. While the
last apparition in 1987 was well observable and resulted in good astrometry
(and also lots of amateur sightings at 11th mangnitude), the 1992 apparition
will be extremely poor, with only 14mag and almost no visibility from the
Northern hemisphere. This will surely hamper astrometric measurements of its
precise position and thus the navigation for Giotto, but ESA still hopes to
approach the nucleus by less than 1000 km. The chance for a hit on the
nucleus is very remote, of course, but *if* it should happen , there would
be quite a display - according to estimations by Fred Whipple...

Giottos Swingby at earth had been prepared by 25 small attitude and orbit
maneuvers that took just 6 kg of its Hydrazine fuel, 18 kg is still left in
the tanks. The final correction on 30 June had put Giotto on such a fine
trajectory that it missed the point of closest approach to earth by just 5 km
and arrived only 2 seconds early. Only a minor orbital tweak will be required
before another two-year hiberation will begin for the spacecraft, which is
not to be reawakened before April of 1992. The gravity assist was absolutely
essential for reaching the Grigg-Skjellerup rendezvous point: it gave Giotto
an additional 3.1 km/s of orbital energy (while unmeasurably slowing down the
earth, of course) - buring all fuel on board at once would just have
resulted in a velocity increase of 16 meters per second.

And what is going to happen after the Grigg-Skjellerup adventure? There will
definitively be no third comet on the list: fuel will by then be used up,
and the technicians and scientists will be on many other projects already
(even in 1990 is proved hard to reunite the teams of 1986). But in about 50
years from now, Giotto will once more return to the vicinity of earth: it
might be caught (if somebody will still care then) and end up in a space
museum...


                  (I'll be on vacation within 12 hours, though, til 28 July)



Sat, 19 Dec 1992 23:04:13 GMT
 A report on Giotto's earth swingby

Quote:

>It was an absolute first in space history: a spacecraft returning to earth from
>the depths of the solar system, to receive a gravity assist for a new job. On
>2 July 1990 at 10:01:18 Universal Time, precisely five years after its launch,
>the European Giotto spacecraft approached earth to 22730 km over Australia,
>raced past earth in a wide hyperbola with a speed of 6.3 kilometers per second,
>and changed its orbit from one inside earth's to one outside - to meet comet
>Grigg-Skjellerup on 10 Juli 1992 at 15:30 Universal Time +/- a few minutes.

I suppose it comes down to nitpicking as to what is meant by "the depths of the
solar system", but the International Cometary Explorer (ICE, nee IUE or
something), which made a pass at comet G-Z (I'm not even going to try to
remember how to spell it) came from its stationkeeping position near one of the
Earth/Sun Lagrangian points (L1 I believe: the one between us) and made 5
flybys of the earth and/or moon to get its boost on its way to the comet.

Good article though.  Go Giotto!  (And wouldn't it be interesting to see
closeup what kind of shape that comet-beaten probe is in?)



Sun, 20 Dec 1992 02:37:16 GMT
 
 [ 3 post ] 

 Relevant Pages 

1. NASA's MESSENGER spacecraft completes succesful Earth swingby

2. Early scientific results from the Cassini/Huygens Earth swingby (Forwarded)

3. NOZOMI Succeeds in Earth Swingby

4. MESSENGER Completes Successful Earth Swingby

5. MESSENGER Lines Up for Earth Swingby

6. MESSENGER Completes Successful Earth Swingby

7. Giotto Earth Flyby On July 1

8. A night to remember: the Giotto flyby of Halley's comet (Forwarded)

9. Giotto's heritage: the past and future of comet exploration (Forwarded)


 
Powered by phpBB® Forum Software