[extropy-chat] Cassini-Huygens

[Amara Graps]

I should have been much more specific in my previous message.
The transmitter signal was tracked from Earth from the Huygens probe
and it tracked it for the ~2.5 hours of its descent. That's finished
now. Here is the full sequence of data acquisition and return today.
I don't think any data 'quicklook' data analysis is expected until
late tonight (~11pm CET).

The first Huygens data is expected at ESOC in Darmstadt at about 5:15pm CET.

http://www.esa.int/SPECIALS/Cassini-Huygens/SEMQOI71Y3E_0.html

6.51 	Timer triggers power-up of onboard electronics

Triggered by a pre-set timer, Huygens's onboard electronics power up
and the transmitter is set into low-power mode, awaiting the start of
transmission.

11.13 	Huygens reaches 'interface altitude'

The 'interface altitude' is defined as 1270 kilometres above the
surface of the moon where entry into Titan's atmosphere takes place.

11.17 	Pilot parachute deploys

The parachute deploys when Huygens detects that it has slowed to 400
metres per second, at about 180 kilometres above Titan's surface. The
pilot parachute is the probe's smallest, only 2.6 metres in diameter.
Its sole purpose is to pull off the probe's rear cover, which
protected Huygens from the frictional heat of entry.

2.5 seconds after the pilot parachute is deployed, the rear cover is
released and the pilot parachute is pulled away. The main parachute,
which is 8.3 metres in diameter, unfurls.

11.18 	Huygens begins transmitting to Cassini and front shield
released

At about 160 kilometres above the surface, the front shield is
released.

42 seconds after the pilot parachute is deployed, inlet ports are
opened up for the Gas Chromatograph Mass Spectrometer and Aerosol
Collector Pyrolyser instruments, and booms are extended to expose the
Huygens Atmospheric Structure Instruments.

The Descent Imager/Spectral Radiometer will capture its first
panorama, and it will continue capturing images and spectral data
throughout the descent. The Surface Science Package will also be
switched on, measuring atmospheric properties.

11.32 	Main parachute separates and drogue parachute deploys

The drogue parachute is 3 metres in diameter. At this level in the
atmosphere, about 125 kilometres in altitude, the large main parachute
would slow Huygens down so much that the batteries would not last for
the entire descent to the surface. The drogue parachute will allow it
to descend at the right pace to gather the maximum amount of data.

11.49 	Surface proximity sensor activated

Until this point, all of Huygens's actions have been based on clock
timers. At a height of 60 kilometres, it will be able to detect its
own altitude using a pair of radar altimeters, which will be able to
measure the exact distance to the surface. The probe will constantly
monitor its spin rate and altitude and feed this information to the
science instruments. All times after this are approximate.

12.57 	Gas Chromatograph Mass Spectrometer begins sampling
atmosphere

This is the last of Huygens's instruments to be activated fully. The
descent is expected to take 137 minutes in total, plus or minus 15
minutes. Throughout its descent, the spacecraft will continue to spin
at a rate of between 1 and 20 rotations per minute, allowing the
camera and other instruments to see the entire panorama around the
descending spacecraft.

13.30 	Descent Imager/Spectral Radiometer lamp turned on

Close to the surface, Huygens's camera instrument will turn on a
light. The light is particularly important for the 'Spectral
Radiometer' part of the instrument to determine the composition of
Titan's surface accurately.

13.34 	Surface touchdown

This time may vary by plus or minus 15 minutes depending on how
Titan's atmosphere and winds affect Huygens's parachuting descent.
Huygens will hit the surface at a speed of 5-6 metres per second.
Huygens could land on a hard surface of rock or ice or possibly land
on an ethane sea. In either case, Huygens's Surface Science Package is
designed to capture every piece of information about the surface that
can be determined in the three remaining minutes that Huygens is
designed to survive after landing.

15.44 	Cassini stops collecting data

Huygens's landing site drops below Titan's horizon as seen by Cassini
and the orbiter stops collecting data. Cassini will listen for
Huygens's signal as long as there is the slightest possibility that it
can be detected. Once Huygens's landing site disappears below the
horizon, there's no more chance of signal, and Huygens's work is
finished.

16.14 	First data sent to Earth

Cassini first turns its high-gain antenna to point towards Earth and
then sends the first packet of data.

Getting data from Cassini to Earth is now routine, but for the Huygens
mission, additional safeguards are put in place to make sure that none
of Huygens's data are lost. Giant radio antennas around the world will
listen for Cassini as the orbiter relays repeated copies of Huygens
data.

-- 

Amara Graps, PhD
Istituto di Fisica dello Spazio Interplanetario (IFSI)
Istituto Nazionale di Astrofisica (INAF), Roma, ITALIA
Amara.Graps at ifsi.rm.cnr.it