This evening, an Indian PSLV rocket will launch from...

This evening, an Indian PSLV rocket will launch from Sriharikota, India, carrying a gigantic haul of satellites into space. Up to 104 satellites are perched on top of the vehicle — the largest crop of satellites to ever launch on a single rocket at once before.
The largest crop of satellites to ever launch on a single rocket
A big chunk of those space probes — 88, in fact — belong to Planet, a US private imaging company with high ambitions of continuously monitoring the Earth from space. The satellites that Planet is sending up tonight are called Doves, and together they’re considered part of Flock 3p. The Doves are the company’s signature miniaturized space probes that can take pictures of Earth at relatively high resolutions. Planet already has more than 50 of these Dove satellites in orbit at the moment, and tonight’s launch marks the 15th time the company has launched Dove probes into space.
But this launch is extra special for Planet. “Eighty-eight satellites from a single system on a single launch would be a record there,” Mike Safyan, director of launch and regulatory affairs at Planet, tells The Verge. “And with those satellites in orbit, and if you include the other satellites we have in orbit, we will then also have the biggest fleet of Earth-imaging satellites — and of satellites in general — in human history.”

The reason that Planet can fit so many satellites on a single rocket is because its spacecraft aren’t very big. Traditional aerospace companies may work for a full year on just one Earth-imaging satellite that’s about the size of a bus, before launching the vehicle into space. Planet takes a different approach: each Doves is a modified version of a Triple CubeSat, a type of standardized satellite that measures about 4 inches wide and nearly 12 inches long. The small size of these satellites allows Planet to put multiple spacecraft on a launch at a time, and the company now has a fast growing constellation of Doves in orbit. “It’s almost like each individual satellite isn’t as important as the full system,” says Safyan.

Most of Planet’s satellites launch as secondary payloads, meaning they piggyback on other rocket rides. The Doves usually launch along with a much larger satellite and take up any extra space on the rocket that’s leftover. Ironically, the Dove satellites launching tonight are also considered a secondary payload, even though there are so many of them. The PSLV rocket’s main purpose is to launch India’s Cartosat 2D, a high-resolution Earth observation satellite. But the rocket has enough room to carry the 88 Doves, as well as 15 additional satellites from countries all over the world.
The Doves are launching into a type of orbit known as a Sun-synchronous orbit, a path that takes satellites over the Earth’s poles. Sun-synchronous satellites are special since they cross over the same areas of the planet at the same time each day. “You have the same conditions under which you’re imaging each day — consistent shadow angles and predictability of orbit,” says Safyan. Planet already has 12 Doves in this Sun-synchronous orbit, so when the 88 launch, the company will have a clean 100 satellites in this orbital region.

Since the satellites are secondary payloads, the Cartosat 2D will be deployed into orbit first, followed by the rest of the spacecraft in a timed sequence. The Doves will ride up into space inside of a deployer box, which works a bit like a Jack-in-the-box. Each Dove is packed inside a small room inside the box with a door on the end. When the time comes to deploy the Dove into space, the door opens and a loaded spring pushes the satellite outside the box into space. The Doves will be deployed about every 10 to 20 seconds. Planet will spend the next few months spacing them all out in their orbits, before the Doves can start imaging Earth full time.

Tonight’s launch is scheduled for 10:58PM ET. Check back later to watch the launch live.

Source: http://www.theverge.com/2017/2/14/14601938/india-pslv-rocket-launch-satellites-planet-doves