Space diary:

FLY AND BYE
Kamal Lodaya

Pluto
Four years ago, the American space agency Nasa's spacecraft {New Horizons} flew by Pluto. As it went by it kept photographing the planet and its five moons. Once it had gone past, it sent the pictures back to Earth. That took a long time, almost a year. So we got pictures of the dwarf planet Pluto.  (At one time it was called a planet before astronomers discovered many small bodies of its size.) See the back inside cover.
Pluto is 5 billion (500 crore) km from Earth. How does one get that far?  Launched in January 2006, it took more than a year to cover about 60 crore km to Jupiter. The remaining 450 crore km to Pluto were covered at a faster speed, in a little over 8 years. How come?
The spacecraft took a path which made use of Jupiter's gravity. Roughly speaking, Jupiter picked up the approaching {NH} and flung it out of the solar system. But since all this is the effect of gravity, it can be calculated from the equations set up by British scientist Isaac Newton in the 18th century. Nasa's space engineers had pointed the spacecraft in such a way that when Jupiter changed its path, it now was travelling pointed exactly at Pluto! Imagine the accuracy this must have required. 
Also, the new path pointed not at Pluto itself, but was such that when the spacecraft would cross Pluto's orbit around the Sun, Pluto itself would arrive as well at that same point its orbit.

Farther than Pluto
What happens once the spacecraft goes past Pluto? Well, it just gets thrown out of the solar system. That seems a bit of a waste, with all the money spent on this mission.
Nasa's scientists wanted to make full use of the {NH} spacecraft.  Pluto is in the outer reaches of the solar system, a part which is called the Kuiper belt, where there are many small bodies which go around the Sun.  For example, it is from the Kuiper belt that Halley's comet comes charging into the inner solar system every 76 years. (It is next expected in 2061.)
Pluto is about 2400 km in diameter (in comparison, our Moon is 3400 km), a dwarf planet. It can be seen from Earth in a large telescope. The other bodies in the Kuiper belt are mostly much smaller, even less than 100 km in diameter. The Hubble space telescope, which takes pictures from above Earth's atmosphere, was used in 2014 to search among the hundreds of tiny dots visible in the direction that {NH} was headed for, beyond Pluto, to see if any of them were bodies in the Kuiper belt. (Most of them were stars, so very far outside the solar system.)
Nasa zeroed in on three candidates and then picked one, which is called 2014 MU69, and decided that {NH}'s path could be slightly corrected using its onboard fuel to aim for it. It was given the nickname {Ultima Thule}, which is not an official name. After three and a half more years of travel, {NH} arrived at Ultima on January 1 this year. As it did for Pluto, it took pictures as it passed by and flew on. This one shows Ultima, photographed by {NH} as it was approaching, from more than a lakh km away.  Better pictures will arrive from {NH} over the next one and a half years.

Targeting Ultima
This was incredibly more difficult than reaching Pluto. Pluto is tiny compared to Jupiter and one cannot use its gravity to change {NH}'s path.  Everything has to be done with the limited fuel available on the spacecraft.
Since Ultima is more than 6 billion km away, it is very hard to see from Earth. But when {NH} was beyond Pluto, Ultima was "only" 2 billion km away!  So it was {NH}'s camera, called {Lorri}, which took pictures with its zoom lens. Using these pictures, sent back to Earth, the mission engineers did course correction, making sure that the spacecraft would pass less than 2500 km away at its closest. They also verified that Ultima does not have moons, or a dust cloid around it (like a comet might have). At {NH}'s speed of 14 km per second, even a dust grain hitting it could severely damage its equipment.

Imaging Ultima
Since Ultima is 6.5 billion km from Earth, light, which travels at 3 lakh km per second, takes about 6 hours to reach from there to here.  So there was no chance of first seeing Ultima, deciding what to look at, which part of it had interesting features and zoom in on them, and so on.  No. The entire sequence of operations, of pointing and photographing, took place in two days and it was pre-programmed and loaded for execution on {NH}'s computers in December, a few days before the flyby. After that {NH} would have to do everything on its own.
Imagine, what if you made a mistake? Ultima is so small that mission planners were worried that they might point the camera at the wrong place and get a dark picture showing nothing! What if something which you had not thought about came up? There was a risk that the whole mission could go astray. But space is so empty, that the mission scientists took the chance. They succeeded.

Sending data back from Ultima
After {NH} had flown by Ultima on January 1, it turned its antenna back towards Earth and started sending the pictures it had taken and the other data that its instruments collected. Remember again that Ultima is very, very far, and the spacecraft has precious little fuel left to power its batteries. Pointing to Earth precisely is very difficult. So getting back all that data will take more than a year, even more than it did for Pluto.  But we will get it all. Meanwhile {NH} has first sent some advance images (one of which we saw above) which give us an idea what Ultima looks like.
The signal that Ultima sends uses a power of about 15 watts. As it travels 6 light-hours and 6.5 billion km to Earth, the signal becomes weaker and weaker. By the time it reaches us it is around a femtowatt (see the box for what this is). {New Horizons} uses a very high frequency band which is different from what is used for cellphones, TV and GPS, so all our earthly communication cannot clutter up the space signals. Nasa's {Deep Space Network}, which receives this signal, uses clever techniques to boost this signal so that it can be "heard" loud and clear.

It's a snowman!
We saw above the first large-size picture of MU69 (Ultima Thule) sent by {New Horizons}. In our solar system, apart from the nice round planets, we have seen plenty of strange shapes. Saturn's rings were one of the first surprises, discovered in the 17th century. (With his small telescope, the Italian scientist Galileo observing it wondered whether the planet had two "handles" on its sides.) The European {Rosetta} spacecraft found the nucleus of comet Churyumov-Gerasimenko (CG for short) looks like a duck!  Recently the {Hayabusa} spacecraft sent to the asteroid Ryugu by the Japanese space agency Jaxa, and the {Osiris-Rex} spacecraft sent to the asteroid Bennu by the American Nasa, found that these asteroids are diamond-shaped rather than round.
{New Horizons} has discovered Ultima to be the first body in our solar system which is a "contact binary". It is two bodies which have collided with each other and stayed that way as they rotate around each other and go around the Sun. It is thought that comets like CG may have formed from contact binaries, but no one knew for sure. Ultima provides the clearest evidence.

Next season
Now Nasa scientists are thinking, well, now {NH} just goes out of the solar system. That seems a bit silly, with all the money spent on the mission. So can we target something else? The Kuiper belt has many many bodies, and {NH} is only halfway through it so far. Can we find, target {NH} and fly-bye another one over the next 5 years and 3 billion km?

BOX 1: Units of power
As we said above, the signal is sent from {New Horizons} towards Earth using a power of 15 watts. The light bulbs we use at home are around 60 watts. A home tubelight is typically around 40 watts. A cellphone uses around 3 watts. The "beam" which is sent from the spacecraft is very focussed, it only covers an angle of one-third of a degree (as you know, 90 degrees make a right angle). As this beam travels 6.5 billion km, it spreads out farther and farther, so that the amount received, even in the large dish antennas of the {Deep Space Network} of Nasa, is very small.  At reception the signal has about a femtowatt of power.
A thousand femtowatts make a picowatt. A picowatt is a very very small unit of power. For example, a single cell in your body uses about one picowatt of power (from chemical energy stored in the cell compounds). You have around 40 trillion cells in your body.
A trillion is a thousand billion, or one lakh crores. A watt is a trillion picowatts, so a thousand trillion femtowatts.
END OF BOX 1----

BOX 2: What can we do from Earth?
Once it was decided that {New Horizons} would visit the Kuiper belt object (KBO) MU69, astronomers came up with ingenious ideas to view it.  It turns out that at present the KBO is visible (to huge telescopes like Hubble) in the constellation Sagittarius. Sagittarius is also where the Milky Way is thickest, and there we are looking at the centre of our galaxy, with a huge number of stars. After its orbit was calculated, it was found that MU69 would pass in front of many unnamed Milky Way stars.
Such events are called "occultations". Amateur and professional astronomers fanned out over several places on Earth to photograph and record these observations. The picture shows an occultation on 17 July 2017, captured by telescopes at several locations in Argentina.  These telescopes can be of moderate size, but the observer's camera, mounted on the telescope, should be able to record the exact time that the background star "blinked off". Because of the difference in locations, the light from the star went out at very slightly different times, which was measured using very precise clocks. When plotting all the data together, the "snowman" shape of Ultima becomes visible as a hole inside the tracks of light made by the stars! See the picture, that should make the explanation clearer. Astronomers are hopeful that this kind of collaboration between professionals and amateurs can now be used to outline "shapes" for other far-off objects inside the Kuiper belt, which can barely be made out as dots with our biggest telescopes.

[pic: en.wikipedia.org/wiki/(486958)_2014_MU69#/media/File:20170808-MU69Chart.jpg]
END OF BOX 2----

BOX3: On new horizons, sent as a separate article.