We asked Kristin Wirth, Rosetta Science Operations Manager, based at ESAC - ESA’s European Space Astronomy Centre, Spain, to give some tips to amateur astronomers interested in tracking Rosetta from the ground. Kristin is leading an observation campaign from ESA’s Optical Ground Station on Tenerife, Spain, and she readily provided a tonne of information.
Kristin told us that for an observer on Earth the movement of Rosetta relative to the stars is slow, except for a few hours around perigee passage - Earth closest approach (CA) on 13 November (7:45 UTC/ 8:45 CET). Rosetta will be approaching from the southern part of constellation Cetus and receding from Leo.
The path of Rosetta through the constellations around CA is as follows (times are in UTC, 13 November):
7:00 Sculptor
7:20 Piscis Austrinus
7:30 Capricornus
7:40 Sagittarius
7:50 Ophiuchus
8:00 Serpens Caput
8:10 Bootes
8:30 Coma Berenices
At closest approach the distance of Rosetta from the surface of the Earth is about 2500 km. Rosetta is flying fast: at 00:00 UTC on 13 November, the distance to Rosetta will be 260 000 km.
During approach, the Sun-Earth-Rosetta angle will be roughly 140º, so that observers on the ground will be looking at the illuminated solar arrays. We can expect a magnitude of 17 - 12 mag. This is about the same brightness of Pluto (which is about 1,150 times fainter than naked-eye visibility, according to Wikipedia).
But during recession, the Sun-Earth-Rosetta angle will be about 70º so that we will be looking at the dark side of the solar arrays and the magnitude will be much weaker.
Kristin’s observations at the ESA OGS on Tenerife will finish before Rosetta sets over Tenerife, at 04:20 UTC on 13 Nov.
This means that even before closest approach, Rosetta will only be a weak source of light and a ‘serious’ telescope will be necessary – but still within amateur range. Kristin recommends a 300 mm aperture.
She has also provided tips for observing from Central Europe and the Canary Islands (she will be at Teide Observatory, much to the ire of some of her colleagues :) ). Her tips will follow in a separate post.
As I type this, the Rosetta Flight Control Team are in a training exercise that simulates a trajectory correction manoeuvre (TCM) at 6 hours before Earth closest approach on 13 November.
This slot is crucial as it would be the last time that the team had the chance to make any corrections to the spacecraft’s trajectory before it closes in on Earth for the swingby; it is not likely to be needed.
The simulation runs for most of the day, with pre- and post-briefings held before and after and focuses on the procedures that would be necessary to make use of the TCM.
The Sims Officer can throw any number of 'real' problems at the team; so far today they have had to deal with loss of a star tracker (a navigation device that helps determine where Rosetta is pointing by viewing the star field) and a faulty reaction wheel (a device that spins so as to cause Rosetta to spin or slew).
Sometimes, the Sims Officer even calls in and announces, "OK - the SOM is sick and can no longer take part," meaning that Spacecraft Operations Manager Andrea Accomazzo has to leave the rest of the team to function without his (considerable) know-how!
But it's all part of preparing for any eventuality. -- Daniel
Michael Khan is a Mission Analyst here at ESA's ESOC Establishment in Germany. He maintains a blog that we read from time to time to find out rather cool things about ESA and other missions.
The first (and most important) trajectory correction manoeuvre (TCM) executed last Thursday was extremely accurate, and Rosetta is nicely lined up for the approach to Earth. After a preliminary analysis, Trevor Morley, lead Rosetta flight dynamics specialist here at ESOC,has told us that closest approach to Earth is now expected at 07:45:40.1 UTC (08:45:40.1 CET) on 13 November.
Last week's TCM burn lasted 86 seconds and provided an 8.8 cm/second change in orbital velocity - and the thruster performance was very good.
Bonus: Nice chart showing projection of Rosetta's approach trajectory included in this post.
The mission was launched in 2004, and will reach comet 67/P Churyumov-Gerasimenko in 2014.
Animation showing Rosetta's journey from launch until it reaches its final destination, including all major milestones along the way. Move cursor over the buttons on top to play, stop or zoom in. Click on 'select events' to select the event at which you want to see Rosetta's journey begin.
Launch: 2 March 2004
First Earth swingby: 4 March 2005
Mars swingby: 25 February 2007
Second Earth swingby: 13 November 2007
Steins flyby: 5 September 2008
Third Earth swingby: 13 November 2009
Lutetia flyby: 10 July 2010
Comet rendezvous manoeuvres: 22 May 2014
Lander delivery: 10 November 2014
Escorting the comet around the Sun: November 2014 - December 2015
End of mission: December 2015
Upon
arrival, Rosetta will continue to orbit the comet, observing what
happens as the icy nucleus approaches the Sun and then will travel away
from it. The mission is expected to end in December 2015. Subsequently,
Rosetta will again pass close to Earth’s orbit, more than 4000 days
after its adventure began. -- Amruta
15:27 UT (17:27 CEST) + a few seconds... final time to be calculated. All on track... looks like the burn used roughly 89g of fuel. Slew back to original pointing (Earth pointing) happens at 15:56 UT (17:56 CEST). The Flight Control Team and Flight Dynamics team will now do an analysis and determine the final results of the burn, which should come in a few days. We're done for the day! -- Daniel
AOS - acquisition of signal - happened at 15:15 UT (17:15 CEST - a bit early) - Rosetta is back in touch with mission controllers, as expected. -- Daniel
Rosetta's slew into correct pointing for today's thruster burn began on time at 13:51 UT (15:51 CEST). As expected, radio contact via New Norcia was lost a few seconds later (as her antenna rotated away from Earth pointing). Team are now waiting to re-acquire signal shortly after 17:21 CEST-- Daniel
Today's trajectory correction manoeuvre for Rosetta will be commanded from the Rosetta Dedicated Control Room at ESOC. Here are a few timings for key steps:
Time
UT
CEST
Event
13:51
15:51
Rosetta begins slew to correct pointing for TCM (slew takes approximately 90 mins)
13:52
15:52
Loss of contact from ESA's New Nocia station (DSA 1) - the 35m deep space station in Australia (LOS is preplanned
15:22
17:22
Acquisition of signal via New Norcia (AOS preplanned)
15:26:10
17:26:10
Start of TCM - Rosetta's 4 axial thrusters switched ON
15:27:37
17:27:37
End of TCM - thrusters OFF
Rosetta will carry out these activities autonomously; the command stack was uploaded yesterday evening. Mission controllers will primarily monitor progress and need do very little unless anything non-nominal occurs.
There is also a GO/NOGO check point prior to the TCM; at this time, the Flight Control Team will ensure that all relevant parameters are in the correct ranges. There are several, but one of the most important will be 'thruster temperature'. -- Daniel
First, let’s put the distances into perspective. Rosetta’s final destination is a comet that is located between 186 and 857 million km from the Sun as it orbits around our star. That’s an average of 3.49 AU – about 3.5 times the distance from Earth to the Sun. Even at the speed of light – the fastest speed possible in our Universe – that’s still 29 minutes from the Sun!
The spacecraft is not equipped with any sort of massive engine. It has a 32-m long solar array and 24 jet thrusters, each providing thrust equal to the weight of a bag of apples on Earth, to power its journey. These means are pretty modest! This is why Rosetta relies on a series of gravity assists to gain the energy it needs – and why the entire journey will take 10 years to complete.
For this mission, swingbys - or gravity assist manoeuvres - are crucial. Each of the four manoeuvres will use planetary gravity (from the Earth three times and Mars once) to change Rosetta’s orbital energy as well as its direction to match that of its destination comet, 67P/Churyumov-Gerasimenko, which orbits the Sun every 6.6 years.
Each of the three Earth swingbys will increase the orbital energy but the Mars swingby actually decreased it! The goal of the Mars swingby was primarily to slow Rosetta down so as to adjust her orbit to match that of Earth - called orbital phasing - for the 2nd Earth swingby
The swingby next month will be Rosetta’s fourth and last; it is also the third Earth swingby.
The ESA Flight Dynamics team at ESOC spent yesterday analysing the most recent orbit data on Rosetta and have finalised the estimates for tomorrow's trajectory correction manoeuvre. The TCM will involve switching Rosetta's four in-line (or axial) thrusters on for a pre-set length of time to kick her onto the desired approach trajectory for the Earth swingby on 13 November.
The team have estimated the necessary 'delta-v' (desired change in velocity) at 8.789 cm/seconds, with the thruster burn starting at 15:26:11 UTC (17:26:11 CEST).
Click on 'Full story' for more details (below), and/or watch an old - but still very good - NASA video on orbital mechanics from YouTube. -- Daniel
Rosetta Spacecraft Operations Engineer Sylvain Lodiot, who is SOM for one of the two ESB3 (Earth swingby No. 3) mission control teams, just rang with an update after completion of a long day of simulations.
He says everything went fairly well!
In brief:
We spent the day exercising overall mission control procedures for ESB3
One of the specific events we practiced was, what to do if we must make use of the final trajectory correction manoeuvre (TCM) slot, TCM -6h, which is available to us at 6 hours before closest approach on 13 November
We don't plan to use this TCM, but if for any reason we had to, it was necessary for us to practice generating the commands and configuring the spacecraft for such an event
The simulation engineers working with us threw in a 'fake' unexpected gyroscope failure that caused the spacecraft (actually, the Rosetta simulator) to go into a 'Low Gain Antenna Strobing mode'
We had to figure out on the spot what the problem was and then apply the correct contingency procedures; it caused a bit of sweat, but it was a good exercise in basic spacecraft operations
Sylvain said:
"Overall, today's simulations increases our confidence for the actual swingby. It's very good for us to practice the actual procedures; we've all read the manuals many times, but the best teacher is experience."
The Rosetta Flight Control Team will also conduct another simulation training on 29 October. The first live activity will take place on 22 October, when the team will command the first TCM, with Rosetta's 10-Newton thrusters burning for a preset time to bring her onto the correct approach trajectory. -- Daniel
Just received from the Flight Dynamics team: The present prediction for the time of closest approach (perigee passage) is 07:45:40 UTC (08:45:40 CET), 13 November. This should be accurate to about +/- 2 seconds. -- Daniel
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