When Europe's comet chaser Rosetta swings by Earth tomorrow for a critical gravity assist, tracking data will be collected to precisely measure the satellite's change in orbital energy. The results could help unravel a cosmic mystery that has stumped scientists for two decades.
Since 1990, scientists and mission controllers at ESA and NASA have noticed that their spacecraft sometimes experience a strange variation in the amount of orbital energy they exchange with Earth during planetary swingbys. The unexplained variation is noticed as a tiny difference in speed gained or lost during the swingby when comparing that predicted by fundamental physics and that actually measured after the event.
Just last night, I found an interesting question in my inbox, "Out of interest, what is the closest that Rosetta gets to Planck during the fly-by?"
It was from Stuart Lowe, member of the Planck Low Frequency Instrument team at Jodrell Bank Observatory in the UK.
We asked Trevor Morley, one of the Flight Dynamics specialists at ESOC working on Rosetta, if he could help. He sent these minimum separation distances from Rosetta:
Planck : 1.225 million km at 03:39 CET on 12 November.
Herschel: 1.456 million km at 07:53 CET on 12 November
That's quite a distance, but tiny compared to what Rosetta's already covered since the start of her mission. By the time the swingby has been completed, this figure will be almost 4500 million km!
After the 86-second thruster burn on 22 October, Rosetta has lined up on a near-perfect Earth approach trajectory. Flight dynamics and mission control experts at ESOC, ESA's European Space Operations Centre, have determined that today's manoeuvre slot will not be used (click on 'Full story' for details). -- 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
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
Just got a note from Flight Dynamics - the number crunching experts here at ESOC - on their calculations for Thursday's trajectory correction manoeuvre (TCM).
They’ve started their calculations for the manoeuvre design. They’re now waiting to receive the latest DSN tracking data from NASA. Next step: they’ll spend time today to optimise and finalise the manoeuvre calculations.
After that, the team will begin generation of the commands that need to be uploaded to Rosetta for the TCM. Once the commands are in, we will be able to tell you the precise start and end times of Thursday's TCM burn.
As of now, the preliminary estimate (change possible) is a thruster burn of 6 secs.
Rosetta Spacecraft Operations Manager (SOM) Andrea Accomazzo confirms that the manoeuvre will make use of the 4 axial thrusters located at each corner of the Rosetta main body. These point directly along the central axis of the spacecraft.
Each of Rosetta's 24 thrusters can generate a force of 10 Newtons, about the same as you would experience if you were holding a large bag of apples on Earth. At launch in 2004, over half the spacecraft's mass comprised propellant (approx. 1670 kg)!
Click on 'Full story' to view a short YouTube clip on Rosetta ESB3. -- Daniel
On Friday, 13 November, 2009, at 07:46 UT, ESA's Rosetta satellite will make her third (and final) swing-by of Earth, picking up a gravity assist from our bulky home planet and altering trajectory as she enters the next stages of the 10-year journey to Comet 67 P/Churyumov-Gerasimenko.
The ESA Flight Dynamics team have predicted Rosetta's altitude at the point of closest approach (perigee height) on 13 November to be 2481 km, slightly higher than her first swing-by in March 2005 (1954 km) and lower than her second swing-by on 13 November 2007 (5295 km). We'll update this figure as we get closer to the swing-by date itself...
The geographical point of closest approach (the point on the Earth's surface over which she'll make closest approach) is 109°E and 8°S - just off the coast of the Indonesian island of Java.
As of 18:00 UTC (19:00 CET) Friday, 16 October, Rosetta was 0.1558 AU (astronomical unit - equal to the Earth-Sun distance, or 149 598 000 kms) from the Earth, a distance which is, of course, quickly decreasing.
In fact, at that time, she will be approaching Earth at 10.79 km/second. In comparison, a modern passenger jet aircraft travels about 14 km/minute, so Rosetta is travelling over 46 times faster!
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When Europe's comet chaser Rosetta swings by Earth tomorrow for a critical gravity assist, tracking data will be collected to precisely measure the satellite's change in orbital energy. The results could help unravel a cosmic mystery that has stumped scientists for two decades.
