Creating a safe environment in Low Earth Orbit (LEO) is crucial for the long-term success of space ventures. Recently, NASA astronauts working at the International Space Station (ISS) faced a couple of challenges in outer space caused by chunks of space junk. The astronauts had to decrease the ISS altitude to avoid colliding with a defunct Pegasus rocket upper stage last week. They also had to shelter-in-place inside their spacecraft and delay a spacewalk on a separate occasion due to a new debris cloud that was caused by a Russian Anti-Satellite Test (ASAT) during which it fired a missile to destroy one of its old Soviet-era satellites in mid-November. The United States Department of Defense (DoD) says it is working to catalog thousands of new pieces of debris caused by the missile strike. Some pieces are too small that cannot be traced. Tracking debris is important because it could potentially pose a danger to astronauts at the ISS and operational satellites in orbit.
SpaceX is building the world’s most advanced Starlink broadband satellite constellation capable of avoiding orbital collisions autonomously. SpaceX founder Elon Musk shared that the company had to shift some Starlink satellite orbits to avoid collision with the space debris caused by Russia’s ASAT test. “We had to shift some Starlink satellite orbits to reduce probability of collision. Not great, but not terrible either,” he stated. SpaceX understands the importance of keeping a safe space environment and has established a Starlink Constellation Orbital Space Safety Plan (detailed below) that is outlined in a document to the U.S. Federal Communications Commission (FCC). The Starlink constellation currently has approximately 1,892 satellites out of 12,000 SpaceX has FCC permission to deploy. The satellites operate at an altitude of around 540 to 550-kilometers in LEO.
When the company announced its bold plans to blanket the Earth with thousands of internet-beaming satellites, the company faced opposition from competitors and some astronomer critics claiming that SpaceX’s satellite constellation could pose a threat to space environment if the satellites collide with each other, spacecraft, satellites, or space junk in orbit. Some said it could cause uncontrollable cascading collisions in outer space, known as the ‘Kessler syndrome’. SpaceX is taking all necessary steps to avoid that scenario.
The Starlink satellites are equipped with an autonomous maneuvering capability to avoid collisions with space debris and other spacecraft by utilizing inputs from the U.S. Department of Defense’s debris tracking system. Each satellite can use their on-board krypton-powered ion thrusters to move. This propulsion system can also be used to deorbit a satellite if it stops working. To avoid contributing to space junk the satellites are designed to easily burn up in Earth’s atmosphere. The company also strategically opted to operate the satellites at low altitudes below ~600-kilometers because it enables dead satellites to deorbit in less than five years because of the strong atmospheric drag in low altitudes. Satellites operating at higher altitudes can take decades to decay in orbit.
STARLINK CONSTELLATION ORBITAL SPACE SAFETY PLANS
SpaceX's Starlink Constellation Orbital Space Safety Plan has collision mitigation measures for every possible scenario. Collision avoidance between Starlink satellites vs. other Starlink satellites, is referred to as ‘Starlink-on-Starlink.’ The company said the satellites are programmed to follow specific instructions in orbit. “Starlink constellation orbits are ‘passively’ deconflicted. Each satellite gets assigned a station-keeping slot,” SpaceX wrote to the FCC, “Every slot is passively deconflicted (via orbit design) against all other slots in the constellation. While satellites remain in their station-keeping slots (via station-keeping burns) they are guaranteed to avoid conflicts with other Starlinks that are also in their slots.” The company said that the “’Active’ collision avoidance system is the second line of defense” and that “the vast majority of Starlink collision avoidance maneuvers are against orbital debris, or 3rd party satellites; not other Starlinks.”
SpaceX also shared the Starlink collision mitigation plan to avoid space junk, known as ‘Starlink-on-Debris.’ “Space is populated with existing debris, tracked by the [United States Military] 18th Space Control Squadron. Starlink utilizes an automated collision avoidance system, ingesting data from the 18th,” the company told the Commission. “Satellites can autonomously evaluate risk and plan avoidance maneuvers, without human input. Humans are still present in an oversight role, as an added measure of safety,” SpaceX outlined in a graphic that details the interaction, pictured below.
Source: SpaceX FCC
SpaceX also has a plan for collision avoidance between ‘Starlink-on-Other-Operator’. “Starlink is committed to being a responsible member of the space community. Though Starlink collision avoidance is automated, there are always humans on-call to coordinate and promptly respond to any external operator inquiries,” the company said, “When a maneuverable Starlink satellite sees a conjunction with another satellite: Without intervention, Starlink satellites will assume maneuver responsibility. If another operator prefers to maneuver instead, Starlink satellites can be commanded to remain ballistic for the span of the conjunction event.” The company noted that “due to the lack of industry-standard automated maneuver-responsibility arbitrage methods, Starlink satellites currently default to taking maneuver responsibility for conjunction events with other operators.”
SpaceX also has a collision avoidance plan for ‘Starlink-on-Human Spaceflight.’ “Starlink trajectories are deconflicted with the ISS on a ‘Macro’ scale, designed to avoid Starlinks showing up in ISS screenings,” the company said. “[It] Prevents introducing unnecessary hazards to ISS and human spaceflight operations. Avoiding ISS by a wide margin makes it so that no additional NASA operational actions or dedicated monitoring is necessary. The Starlink team works closely with NASA to coordinate if necessary for cargo and crew visiting vehicles,” it stated.
Featured Image Source: SpaceX