LightSail 2 deployed its photo voltaic sail 5 months in the past, and it’s nonetheless in orbit across the Earth. It’s a profitable demonstration of the potential of photo voltaic powered spacecraft. Now the LightSail 2 group from The Planetary Society has launched a doc outlining the outcomes of the mission to date.
The idea of the photo voltaic sail has been round for some time, as much as Johannes Kepler, in actual fact. In 1607, Halley's comet handed over our heads, and Kepler observed how the tail of the comet was transferring away from the Solar. He rightly believed that daylight was accountable. In a letter to Galileo who’s considerably well-known in astronomy circles, Kepler mentioned, "Present ships or sails appropriate for the celestial breezes, and there can be some who will courageous this void." Fairly cool.
In fact, Kepler had no method of figuring out how proper he was. However now, because of The Planetary Society and others, we’re doing it.
The Planetary Society is a pioneer within the discipline of photo voltaic sail. LightSail 2 is in actual fact their third spacecraft with photo voltaic sail, following within the footsteps of LightSail 1, and their authentic precursor, Cosmos 1, which didn’t attain orbit when its rocket launch failed. A 3rd spacecraft with photo voltaic sail, known as LightSail three, will attain the liberation level Solar-Earth L1 if all goes effectively.
As one of many first photo voltaic sail spacecraft, LightSail 2 teaches us beneficial classes concerning the potential and limits of photo voltaic sail. On January 10, the Planetary Society launched a doc outlining a few of these classes. The doc is titled "Orbit and Perspective Efficiency of the LightSail 2 Photo voltaic Sail Spacecraft".
LightSail 2 captured this picture of the Gulf of Oman and the Persian Gulf on December 14, 2019. The sail seems barely curved because of the 185-degree fish-eye digicam lens. The picture has been corrected in colour and a part of the distortion has been eliminated. Picture Credit score: The Planetary Society
LightSail 2 slowly succumbs to pull and will get nearer to Earth. Throughout its deployment, orbital modeling predicted that it could fall to Earth a couple of yr after the deployment of its sails. However the spacecraft is in a excessive Earth orbit about 720 km (447 miles), a lot greater than different satellites and spacecraft just like the Worldwide House Station, which orbits 400 km (249 miles).
There’s comparatively little knowledge on the atmospheric density at this altitude and the ensuing decay of the orbit, so the one-year prediction was not exact. However because of LightSail 2, we now know that the atmospheric drag at this altitude is robust sufficient to drag LightSail 2 in direction of Earth. One motive is that the spacecraft shouldn’t be all the time the photo voltaic sail.
Throughout every 100-minute orbit, LightSail 2 spends solely about 28 minutes capturing photo voltaic photons, and that is the one time it will possibly change its trajectory. The remainder of the time is spent both in eclipse, or by transferring straight in direction of the Solar, or by adjusting its orientation. These 28 minutes of precise crusing time are usually not sufficient to totally counter atmospheric drag.
It's simply one of many issues The Planetary Society realized from its LightSail 2 mission. However inside these orbits, there are different variables.
The group in contrast the efficiency of the LightSail 2 when it was randomly oriented and when it was actively oriented for photo voltaic sail. They found that when the spacecraft was randomly oriented, the semi-major axis of its orbit decreased by 34.5 meters per day. When actively oriented, the identical measurement solely decreased by 19.9 meters per day. However there are lots of variations in its orbit, and typically the tiny spacecraft has elevated its orbit by 7.5 meters per day.
The video exhibits a single orbit for LightSail 2. Discover the crimson and blue traces superimposed on the spacecraft. The crimson line signifies the route of the Solar and the blue line is the route of the native magnetic discipline. When approaching the Solar, the spacecraft plucks its sails, and when it’s actively crusing, it turns its sails to seize the photons of the Solar. The Solar-z angle adjustments from about 90 levels to about zero levels.
Typically talking, the photo voltaic sail can not overcome atmospheric drag, however that’s not actually why these spacecraft are designed. Their potential lies in interplanetary journey, freed from atmospheres and planetary eclipse results. NASA's NEA Scout (Close to Earth Asteroid Scout) spacecraft will spend two years propelled by a photo voltaic sail to succeed in an asteroid, though it can obtain preliminary propulsion from chilly gasoline propellers.
The height and perigee of LightSail 2 has elevated and decreased within the 5 months since its deployment. Instantly after deployment, the spacecraft lifted its peak, making it the primary solar-powered spacecraft to take action. On the similar time, the perigee has decreased. It skilled a development reversal on the finish of October, and a return in December.
This graph exhibits the height and perigee of the LightSail 2 orbit as reported by space-track.org since July eight, 2019. The deployment of the sail happened on July 23, 2019. Credit score Picture: The Planetary Society
There are a number of the explanation why the orbit goes by means of these cycles. First, the Earth is an Oblate spheroid, not a sphere. Which means its diameter on the equator is about 42 km (26 miles) bigger than on the poles. This causes the spacecraft to bear precession, or wobble.
The second motive for the height / perigee cycles of LightSail 2 is the trail of the Earth across the Solar. This motion adjustments the angle between the Solar and the apex and perigee positions of the spacecraft.
LightSail 2 is a superb demonstration spaceship, but it surely has limits. One in every of them is its single pulse wheel. The spacecraft makes use of this wheel to orient itself parallel or perpendicular to the rays of the Solar, relying on whether or not it’s its sails or whether or not it’s actively crusing. Initially, the bottom crew did it manually, which was not efficient. Now they’ve automated the method, and the spacecraft is doing higher consequently.
However all through this course of, the group realized one in every of their beneficial classes. Frequent adjustments in sail orientation give a major enhance to the spacecraft. One of many fundamental technical challenges is to handle this momentum.
One other lesson issues photo voltaic power. Photo voltaic sails are strictly reserved for photo voltaic sails. LightSail 2 has very small photo voltaic panels that meet the spaceship's meager power necessities.
Its preliminary design offered for small photo voltaic panels on either side of the craft, however the panels on one aspect have been eliminated to accommodate the particular mirrors obligatory for the laser vary to search out its precise distance from Earth. However now that there’s solely photo voltaic power on one aspect, these panels are typically hidden by the sails. This results in voltage drops. The group was capable of work round this to some extent, by managing the spacecraft's power consumption and its angle management mode. However it is a good lesson for future photo voltaic crusing spaceships.
The photo voltaic cells on one aspect of LightSail2. The Spherical and Vivid Factor is a mini-DVD containing an inventory of members of the Planetary Society, an inventory of Kickstarter contributors and names and pictures of the Society's “Selfies to House” marketing campaign. Picture Credit score: Jason Davis / The Planetary Society
The LightSail 2 group has additionally added one other mode to the spacecraft which it calls the photo voltaic pointing mode.
The solar pointing mode will hold the spacecraft's photo voltaic sail dealing with the solar all through its orbit. This may restrict the reorientation of the spacecraft to scale back the impact of frequent orientation adjustments which give the spacecraft problematic impulse by the impulse wheel. It additionally aids in battery charging by photo voltaic cells, though it doesn’t cut back orbital degradation.
This picture of the LightSail 2 photo voltaic sail exhibits the southern tip of Madagascar. Picture Credit score: The Planetary Society
The brand new mode will even contribute to the spacecraft's pointing accuracy and provides it a extra constant begin angle for on / off thrust maneuvers.
The Planetary Society intends to carefully monitor the orbital degradation of the spacecraft to see what impact the sails themselves have. That is largely for different groups who’re learning how drag sails can be utilized to intentionally desorb orbit a spacecraft.
They will even proceed to take pictures. The principle motive for the photographs is to watch the situation of the sails, however they’re additionally an eye fixed sweet.
Yow will discover out extra about LightSail 2 on The Planetary’s Society web site. It’s a non-profit firm, so you may register if you wish to be a part of their mission. It’s a good way for residents to contribute.