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Saturday, June 27, 2015

New Horizons spacecraft to be closest with Pluto on 14th July

REALTIME UPDATES:

UPDATE:
On 30th June NASA fired thrusters aboard the spacecraft for 23 seconds.
This increased the craft velocity by  27 centimeters per second. Seems to be a minor value.... but what it means is that:
- velocity increase is about one-half mile per hour
- this would now  position the craft 12,500 kilometers above Pluto’s surface at the closest approach
- Had this correction not been done  New Horizons would have arrived 20 seconds late and 184 kilometers off-target from the spot where it will measure the properties of Pluto’s atmosphere.



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It has been a long story,  started some 10 years ago.
On 19th Jan 2006, NASA launched a spacecraft named 'NEW HORIZONS' to study the most ambiguous of the Planets - ( In fact there is a debate whether it is a Planet at all !! ) - the PLUTO.

Its ambiguity rises from the fact that it was discovered just in 1930 and it is estimated to have a solar period of 248 years ..  so we have not even seen 1/3rd of its orbit to ensure that what we have thought to be its orbit is accurately correct or not.


The orbit of Pluto that we know is different than all the other planets of Solar System as shown in the image here on left. One can also see that the spacecraft 'New Horizons' is travelling perpendicular to the path of Pluto indicating thereby how critical the imaging time is.

One can notice the orbit does not lie in the same plane as the other planets. Another point is that Pluto is the only planet which crosses its orbit with its neighboring planet Neptune for some time.









Before we see the 'hurdles', here ( image on Right ) is what has been planned as the 'ideal' image plan.


There are 15 images planned.
Lat/Long Location of images on the surface of planet are marked as green rectangles. Notice the small overlap at the edges so that a contiguous large area is covered without breaks in between for study.

That is the starting point in gauging the difficulty of imaging Pluto by the spacecraft when it flies nearest to Pluto on 14th July 2015.

So starting difficulty: You DO NOT know whether what you are going to image is going to be in front of your camera or not. BTW that 'within field of view  of camera' is going to be a rectangle of 100Kms X 150 Kms at a distance of some 13000 Kms from the Spacecraft. The spacecraft does not take a single 'click' preceded by 'say cheese',  It has to continuously rotate the camera to follow the object during that split second period so that there is no 'shake' in the image.

This operation has to be precisely timed so that it occurs exactly when the spacecraft is seeing the planet at its center of field. You start a little early , you will see either empty space or just the entry of planet in the field of view ..   no useful image in both the conditions.
You start imaging late..  again you will see an empty space through which Pluto might have passed a while ago.
In fact with the current knowledge base, predicted Pluto trajectory can have an uncertainty  of +/- 450 seconds.
BTW, we are talking only about the direction  in which it enter/exit the field of view, it may so happen that  the planet can pass sideways.

Did I hear someone say : 'Why not wait till you see the entry of Pluto in the field, and then, 'click'?'
Again a small problem.  If the spacecraft 'sees' Pluto when will Earthling know it? The spacecraft would be  5000 million kms from Earth at the time of encounter with Pluto. That means the signal from spacecraft would take 4 1/2 hours to reach Earth. If we wait for the signal and then give command then two way delay would mean that the spacecraft would act after 9 hrs by which time Pluto would have moved about 450000 kms! So this strategy  would not work.

Real basic problem is like this:


Figure on left shows the journey of  New Horizons starting from Earth. The detailed diagram shows how Pluto will travel below
the spacecraft in a cross fashion. Try to imagine how difficult  it is if you have to send a spacecraft that would travel 5 billion kms and would cross over the path of Pluto at the EXACT moment when the planet is just below it!!

It is not that you don't have any way of correction. But the problem is exact modelling the movement of Pluto. Since you don't have sufficient data to predict precisely the long term motion of Pluto, you have to do short term precise predictions using some actual near real time measurements and try to modify your prediction algorithms with some biases based on actual estimates.

Towards this end, NASA has started imaging the planet for last few days. They have taken references of Pluto against known stars and are trying to refine the prediction algorithm.

Based on these observations scientists have two opportunities to correct the path of spacecraft.. 1st on 30th June and 2nd and final chance on 4th July.

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UPDATE:
On 30th June NASA fired thrusters aboard the spacecraft for 23 seconds.
This increased the craft velocity by  27 centimeters per second. Seems to be a minor value.... but what it means is that:
- velocity increase is about one-half mile per hour
- this would now  position the craft 12,500 kilometers above Pluto’s surface at the closest approach
- Had this correction not been done  New Horizons would have arrived 20 seconds late and 184 kilometers off-target from the spot where it will measure the properties of Pluto’s atmosphere.


(  ALL IMAGES, EXCEPT THE LINE DIAGRAM ARE FROM NASA WEBSITES )

Sunday, November 9, 2014

Physical encounters of Philae with Comet !!

There is a running commentary at end:

( NOTE:  All timings shown here are at Spacecraft end. Rosetta will be about 511 million kms from Earth and one-way light time between Rosetta and Earth would be about 28 minutes 20 seconds. So  Earth will see these incidents 28 minutes 20 seconds late )

A table showing timeline of Earth Reporting in Indian Standard Time ( IST ) is here