(Photo: Jayachandran/Mint)
(Photo: Jayachandran/Mint)

How an Israeli craft seeks to capture the moon

  • With its steadily widening orbits, SpaceIL’s spacecraft Beresheet will attempt to land on the moon on 11 April, nearly two months after blasting off
  • If it successfully lands on the moon, Beresheet will be the first privately funded spacecraft—and Israel is the fourth country—to achieve that feat 

With less of a fuss than I might have expected, a remarkable competition ended last year. The interesting thing about it? There was no winner. In fact, the competition ended precisely because there was no longer a chance that there could be a winner.

Perhaps you know that I’m referring to the Google Lunar X Prize competition. It was intended as a catalyst for private space exploration, as opposed to efforts by national agencies like Isro or Nasa. Specifically, the challenge was to land a rover on the moon, have it travel 500m and send home images and video. The first privately funded team that could do this by 31 March 2018—a deadline that had been extended twice—would win the prize, $30 million.

Several teams from around the world entered the competition. In early 2017, five finalists were announced: among them, Team Indus from Bengaluru. But in January 2018, the competition was shut down because none of these five would be able to meet the 31 March launch deadline.

Competition or not, some of the teams decided to keep their efforts going. Two of those are Team Indus and SpaceIL from Israel—and on 21 February, SpaceIL’s moon lander shot into space atop a Space X Falcon 9 rocket. So if all goes well, SpaceIL’s craft, Beresheet (Hebrew for “genesis" or “in the beginning"), will soon land on the moon.

Though what’s interesting is what “soon" means here. In 1969, the USA’s Apollo 11 took about four days—launch to landing—to reach the moon. In 1970, the USSR’s Luna 17 landed there about a week after taking off. In 2013, China’s Chang’e 3 took five days to reach and settle into orbit around the moon, and landed a week later. Beresheet, by contrast, will only attempt to land on the moon on 11 April, nearly two months after blasting off. So while the moon is just over 350,000km away when it is closest to us, by the time Beresheet lands there, it will have travelled about 6.5 million km.

Why so long, and what will Beresheet be doing for all those weeks?

One reason for the long travel time is that Beresheet shared its ride into space with an Indonesian satellite and an experimental American craft. This is space exploration Uber Pool-style, then, which is also how SpaceIL’s co-founder Yonatan Winetraub described it just before the launch. The big advantage: it costs significantly less than a dedicated rocket would—and especially for a small private effort, price matters a great deal. The downside is that while a dedicated rocket might have set Beresheet directly on course for the moon, this Falcon 9 launch only put Beresheet into orbit around the earth, like it did the two ride-sharers.

What happens after that, so that Beresheet can take its shot at the moon, is up to SpaceIL’s engineers.

Yet mathematically, that’s a particularly interesting part of this exercise. For there are plenty of objects—the International Space Station (ISS), satellites and assorted space junk—that orbit the earth all the time. None of them are going to shoot for the moon.

What makes Beresheet different, even as it orbits the earth for several weeks? The difference is that Beresheet’s orbits will get steadily wider, and that’s entirely by design.

To understand this, think of something we’ve all done as kids—tie a stone to a string and whirl it around our heads. If you hold the string tight and keep up the whirling, the stone will keep going round and round indefinitely at the same speed. Though you’ll agree that you have to keep up the whirling; stop, and the stone will slow and drop down.

Now imagine that at one point in each pass around your head, you let the string out a little, and then pull it back in later. When you let the string out, the stone moves further away and slows down; when you pull it back in, it moves back towards you and speeds up. You need to be careful not to let out too much string or again, the stone will slow so much that it drops down. If you don’t want this to happen, you’ll have to speed up your whirling each time you let string out.

This is a passable model of what will happen with Beresheet. Gravity keeps it in orbit, as it does every other object that whirls around the earth, acting like the string you’ve tied to your stone. But that’s where the parallel to satellites and the ISS ends. With each orbit, Beresheet will fire its engine, which sends it sailing just a bit further from our planet than on its previous orbit. Then it swings back towards the earth, picking up speed as it approaches and circles us. Fire the engine again and… well, as you can imagine, this process results in ever-larger orbits.

In effect, Beresheet uses the earth’s gravity as a slingshot, building up speed and increasing its distance from the earth with each orbit. Our own Mangalyaan did much the same a few years ago, as I explained in my column “Mars, here we are". Eventually, Mangalyaan’s orbits got so elongated that it could effectively escape earth’s gravity and set a course for Mars. Over the next three weeks, Beresheet’s orbit will elongate similarly, until it spans close to 400,000km.

When that happens, the orbit will have ranged past the moon. That is, the expansion of Beresheet’s orbits has been calculated precisely so that it will intercept the moon at a particular convenient time about three weeks from now. The precision is necessary because the moon has an elliptical orbit which takes it from a low of about 350,000km to a high of about 410,000km from us. So it makes sense to plan this encounter with the moon for the point in its orbit when it is closest to us. As you read this on 1 March, the moon is about 400,000km away; on 20 March, it will have swung substantially closer, to just 359,000km away.

Aside: If you remember, there was mention of a “supermoon" on 19 February — because on that day, the moon was under 357,000km away, its closest approach till 2024. Beresheet did not launch in time to take advantage of that proximity, which is why it is shooting for 20 March.

What will begin on 20 March is, as Beresheet’s Israeli team describes it, the process of “lunar capture". I like the bravado of that term, the very vision of a tiny spacecraft “capturing" a gigantic rock out there in space. Of course it’s more accurately described as a “Beresheet capture", because it is Beresheet that will eventually start orbiting the moon. Then again, we would say that only because Beresheet is much smaller than the moon. In truth, when we think of one object orbiting a second, we might equally well describe the phenomenon as the second object orbiting the first, whatever their sizes are. So “lunar capture" fits after all.

So on 20 March, Beresheet will fly beyond the moon on its elongated earth orbit. By then, the craft will feel the moon’s gravity more strongly than the earth’s, and so we can say its lunar capture is effectively underway. By 4 April, it will be in a tight orbit around the moon. A week later, on 11 April, Beresheet will actually attempt to land on the moon, an exercise that should take about 20 nerve-wracking minutes. Not least because it will be in free-fall for the final few metres. If it does successfully land, it will be the first privately funded spacecraft—and Israel just the fourth country—to achieve that feat.

Once on moon ground, Beresheet will seek to learn something about our neighbour’s magnetic field. Moon rocks that the Apollo missions brought back were magnetized: how did that come about? An interesting puzzle, but really, everything about this effort is impressive. You could even say the length of the mission showcases exactly that. For as we follow the little spacecraft’s convoluted path to the moon, we actually see all the required science and mathematics turn into reality: the orbits, the capture, all of it.

And that’s probably why another of SpaceIL’s co-founders, Kfir Damari, said this about their mission: “Our goal is to show (children) that (space flight is) not magic—it’s something they can understand. If they can understand that, and if they can meet engineers and hear their story and see that they come from all different kinds of backgrounds, they can understand that they themselves can be those who will build the next spacecraft."

Beresheet won’t be bringing back moon rocks—in fact, it won’t be returning to earth at all. But what it stimulates here on earth might be even more valuable than rocks.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His latest book is Jukebox Mathemagic: Always One More Dance. His Twitter handle is @DeathEndsFun

Close
×
My Reads Logout